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International Geology Review, Vol. 47, 2005, p. 1–23.Copyright © 2005 by V. H. Winston & Son, Inc. All rights reserved.

A Field and Chemical Study of Serpentinization—Stonyford, California: Chemical Flux and Mass Balance

JOHN W. SHERVAIS,1 PETER KOLESAR, AND KYLE ANDREASEN

Department of Geology, Utah State University, Logan, Utah, 84322-4505

Abstract

Serpentinized harzburgites and dunites in the Coast Range ophiolite near Stonyford, California,form massive, decameter- to kilometer-scale blocks in serpentinite schist; together these form ser-pentinite broken formation that grades into mélange where exotic blocks have been incorporatedinto the serpeninite schist. Whole rock geochemical data and modal reconstruction of protolith com-positions show that serpentinization here proceeded essentially isochemically for Si, Mg, and Fe,whereas other elements (Ca, Al, Cr) were lost to an aqueous flux.

Mass balance calculations based on actual primary and secondary mineral compositions showthat significant Fe, Al, and Cr may be accommodated in serpentine. The transformation of ortho-pyroxene to serpentine (bastite) releases significant amounts of silica, which forms additional ser-pentine when it reacted with MgO released by the serpentinization olivine; this reaction suppressedbrucite formation and accounts in part for the conservation of silica and magnesia documented bywhole rock geochemistry. For normal harzburgites (20–25% modal orthopyroxene), approximatelyhalf of the potential brucite was suppressed.

Volume expansion was considerable: 25–30% for pseudomorphic replacement of orthopyroxene,50–60% for replacement of olivine. The increase in volume resulted primarily from the addition ofwater to hydrate the primary silicate assemblage; loss of Al and Ca to aqueous solutions results in aslight loss of mass. Expansion is accommodated by orthogonal fractures at both the microscopic andmacroscopic scales. Subsequent movement along the macroscopic serpentinized fractures led to theformation of serpentinite broken formation, with a matrix of sheared and foliated serpentinite, andrelict blocks of massive, less serpentinized peridotite. This movement may have resulted in partfrom the volume expansion of the peridotite, as rigid, less serpentinized blocks were forced to adjustto increased volumes in their totally serpentinized selvages by differential movements that forcedthe blocks to move in the direction of least principal stress.

Introduction

THE TRANSFORMATION OF ultramafic rocks to ser-pentinite—in particular, the transformation of meta-morphic tectonites that represent oceanic uppermantle—is an important process that influences thephysical properties of oceanic lithosphere, faultingand fault mechanics, chemical flux between oceansand oceanic lithosphere, and fluid flux to the mantlewedge source of arc volcanism, to mention but a fewareas of significance. Important reviews of this pro-cess include Coleman (1971a), Moody (1976),Wicks and Whittaker (1977), and O’Hanley (1996).The pioneering work of Coleman and his associateson serpentine and serpentinization (Coleman,1971a; Coleman and Keith, 1971; Hostetler et al.,1966) was coincident with the development of platetectonics, which emphasized the importance of ser-

pentinized upper mantle in ophiolites and oceaniclithosphere (e.g., Coleman, 1971b, 1977, 2000).

The Coast Range ophiolite (CRO) in northernCalifornia consists largely of serpentinized peridot-ite, which forms a narrow belt ranging from a fewtens of meters up to almost a kilometer across (Hop-son et al., 1981; Hopson and Pessagno, in press).Along much of its length, this belt consists ofsheared serpentinite and serpentinite-matrixmélange, but in a few locales (e.g., Stonyford,Chrome, Elder Creek) thick sections of massive,unsheared, partially to wholly serpentinized peri-dotite are common (Fig. 1). This massive serpen-tinite preserves the primary textures of theharzburgite and dunite protoliths, and in many casespreserves primary phase compositions as well.

Previous work has shown that the CRO repre-sents oceanic crust and mantle that formed over asubduction zone (e.g., Evarts, 1977; Shervais and1Corresponding author; email: [email protected]

10020-6814/05/777/1-23 $25.00

2 SHERVAIS ET AL.

Kimbrough, 1985; Shervais, 1990; Stern andBloomer, 1992; Giaramita et al., 1998). More recentstudies suggest that the final stage of ophiolite for-mation was precipitated by the collision of a spread-ing center with the subduction zone, leading to themixing of MORB affinity magmas and mantle withthe suprasubduction zone magmas and mantle(Shervais, 2001; Shervais et al., 2004; Shervais etal., in press). Most of this work has focused on thecrustal section of the ophiolite, and relatively littlework has been carried out on the mantle peridotites,which are commonly serpentinized. In order to eval-uate the origin of the peridotites, however, we need

to understand the serpentinization process and itseffect on mineral and whole rock compositions.

Because the CRO near Stonyford preservesmassive serpentinite and partially serpentinizedperidotite, it is possible to study here the process ofserpentinization, and its relationship to pre-existingmineral and whole rock compositions. In particular,we can examine the question of constant volumeversus constant mass during the serpentinizationprocess, chemical flux during serpentinization, andmass balance relationships. Our work supports theconclusion that volume expansion occurs initiallywithin the serpentine mesh that forms around

FIG. 1. Geologic sketch map of California showing the distribution of Coast Range ophiolite locales. Stonyford isshown in bold.

SERPENTINIZATION IN STONYFORD, CALIFORNIA 3

primary olivine, followed by the formation of serpen-tine veinlets that form a stockwork in the partiallyserpentinized samples (Coleman and Keith, 1971;Wicks et al., 1977; Maltman, 1978). On the outcropand larger scale, volume expansion is accommo-dated by orthogonal fractures, as proposed byO’Hanley (1996; see also O’Hanley and Offler,1992), followed by pervasive shearing of wholly ser-pentinized rocks, which may preserve blocks of lesssheared or unsheared serpentinite within them. Tosome extent, the shearing may be driven by the forceof volume expansion as wall rocks are forced asideto create accommodation space for the serpentine(e.g., Coleman and Keith, 1971).

Geologic Setting

The Coast Range ophiolite near Stonyford, Cali-fornia, consists of three lithotectonic elements: (1)the Stonyford volcanic complex (SFVC), a mid-Jurassic seamount complex composed of massivesheet flows, pillow lava, hyaloclastite breccias, andchert (Shervais and Kimbrough, 1987; Shervais andHanan, 1989; Shervais et al., 2004, in press); (2)massive, partly serpentinized but generallyunsheared harzburgite and dunite; and (3) stronglysheared serpentine-matrix mélange and broken for-mation (Fig. 2). The Coast Range ophiolite separatesmetamorphic rocks of the Franciscan assemblage tothe west from unmetamorphosed sediments of theGreat Valley Series to the east. The SFVC is petro-logically distinct from other CRO localities, consist-ing largely of oceanic tholeiite, with less commonalkali basalt and high-Al, low-Ti basalt (Shervais etal, 2004; Shervais et al., in press).

The Stonyford serpentine-matrix mélange formspart of the Tehama-Colusa serpentinite mélange,which extends from Elder Creek in the north toWilbur Springs in the south (Hopson et al., 1981;Hopson and Pessagno, in press), and includes theRound Mountain mélange near Paskenta (Jayko etal., 1987; Huot and Maury, 2002). The Stonyfordmélange matrix consists of strongly sheared and foli-ated blue-green serpentinite schist, which wrapsaround phaccoidal blocks of less sheared serpenti-nized peridotite. The mélange contains exoticblocks that were incorporated tectonically into thesheared serpentine matrix along its margins.Mélange blocks along the eastern margin of theserpentinite belt comprise a wide range of igneousrocks derived from the oceanic crust of the CoastRange ophiolite, including wehrlite, clinopyroxen-

ite, gabbro, diorite, quartz-diorite, and keratophyre.These blocks are similar to lithologies that comprisethe Coast Range ophiolite near Elder Creek, about60 km north of Stonyford. Additional tectonic blocksinclude unmetamorphosed volcanogenic wackesand conglomerates correlative with the CrowfootPoint Breccia and foliated metasediments that maycorrelate with the Galice Formation (Jayko andBlake, 1986; Zoglman, 1991). Mélange blocks alongthe western margin of the serpentinite belt includeblueschist, amphibolite, and pale green metavolca-nic rocks that are distinct from basalts of the Stony-ford volcanic complex, (Zoglman, 1991). These twobelts of sheared serpentinite-matrix mélange areseparated by kilometer-scale blocks of massiveharzburgite and dunite, or by rocks of the Stonyfordvolcanic complex (Fig. 2). Locally, serpentinitemylonites preserve S-C shear fabrics that indicateexhumation of the serpentinite belt along a low-angle detachment fault with top-to-the-east shearsense (Dennis and Shervais, 1991).

The Stonyford volcanic complex appears to reststructurally on the mélange belt, as shown by out-crops beneath the crest of Auk-Auk Ridge (Fig. 2).These outcrops show that the Stonyford volcaniccomplex is separated from the underlying mélangeby a low-angle fault. As noted above, the underlyingmélange contains abundant blocks of CRO crustallithologies, suggesting that the volcanic complexwas built on a substrate of CRO prior to or during itstectonic disruption into the mélange (Shervais et al.,in press). The eastern contact of the volcanic com-plex is partly covered by younger alluvium, but ser-pentinite mélange has been mapped along thesoutheastern margin of the complex, adjacent tomassive harzburgite, between Little Stony Creekand Salt Creek (Fig. 2). The mélange here containsblocks of volcanogenic sandstone that may be cor-relative with the Crowfoot Point Breccia near ElderCreek. The presence of sheared serpentinite east ofStony Creek, in fault contact with rocks of the GreatValley series, suggests that, at least in part, themélange wraps around the eastern margin of theStonyford volcanic complex. It is clear from the out-crop patterns that these exposures of serpentinitemélange are separated by a major fault, however,that is covered by younger alluvium, so it is not pos-sible to define precisely the extent of mélange alongthe eastern margin.

Massive harzburgite and dunite form coherentblocks up to at least 5 × 2 km in size that are resis-tant to erosion and stand out as ridges aligned with

4 SHERVAIS ET AL.

the volcanic complex. Massive harzburgite under-lies Black Diamond ridge north of the volcanic com-plex and the ridges that border Salt Creek, HyphusCreek, and Little Stony Creek south of the volcaniccomplex (Fig. 2). The harzburgite that forms theseridges is locally sheared, but comprises a mappableunit that is distinct from the sheared serpentinitemélange. The sheared serpentinite-matrix mélange

typically contains a variety of exotic blocks, whereasthe massive harzburgite/dunite unit contains onlyperidotite blocks, even where shear zones are com-mon. In this respect, the massive harzburgite unitand the sheared serpentinite adjacent to it may bethought of more appropriately as a broken formation.

Figure 3 shows two views of this serpentinite bro-ken formation at the south end of Black Diamond

FIG. 2. Geologic map of the Stonyford area, showing the distribution of sheared serpentinite mélange and massiveperidotite adjacent to the Stonyford volcanic complex, with Franciscan Complex to the west and sediments of the GreatValley Series to the east. Mapping by John W. Shervais and Marchell Z. Schuman.


FIG. 3. Field photos of serpentinized peridotite, Stonyford, California. A. Sheared serpentinite broken formation containingdecameter-scale blocks of massive serpentinized harzburgite, overlain by massive harzburgite of Black Diamond Ridge along alow-angle fault contact. B. Close-up of decameter-scale blocks of massive serpentinized harzburgite in sheared serpentinitematrix. C. Massive harzburgite and dunite containing small dikes of gabbro and orthopyroxenite, Little Stony Creek section.

6 SHERVAIS ET AL.

Ridge, in which strongly sheared and foliatedserpentinite schist encloses blocks of relativelymassive, partly serpentinized harzburgite. Theserpentinite broken formation is overlain by themassive, unsheared harzburgite of Black DiamondRidge along a low-angle fault contact that representsthe shearing front between the sheared andunsheared serpentinite (Fig. 3A). Closer up, phac-coidal blocks of massive, partly serpentinizedharzburgite are seen to float in a sea of sheared,blue-green serpentinite schist (Fig. 3B). Foliation inthe serpentinite schist dips about 45º to the east,under Black Diamond Ridge and the Stonyfordvolcanic complex.

Dikes of isotropic gabbro and diorite within themassive harzburgite and dunite are relatively rarebut widely distributed. Most mafic to felsic dikes are0.3 to 0.5 m thick, and some may be seen to extendfor several meters in outcrop. These dikes are alwaysrodingitized—that is, altered by Ca metasomatismduring serpentinization to an assemblage of hydrousCa-Al silicates (zoisite, prehnite, hydrogrossular,vesuvianite, diopside). The rodingite dikes appearin the field as white to off-white, aphanitic to fine-grained rocks with a mottled texture that resemblesmafic phases in a felsic groundmass. Orthopyroxen-ite dikes are also relatively rare, though more com-mon than gabbroic dikes. The orthopyroxenite dikesare generally thin (1–3 cm) and consist of large, sub-hedral orthopyroxene with minor olivine (Fig. 3C).When present in harzburgite, it is common for thesedikes to be immediately adjacent to dunite.

Petrography

Mineral assemblages were determined both pet-rographically and through X-ray diffraction studiesof whole rock samples. X-ray diffraction analysiswas carried out with a Panalytical X’Pert Pro X-raydiffraction spectrometer with monochromatic CuK-alpha radiation, using the High Score softwareprogram to index peaks and identify minerals. Sam-ples were backloaded into sample mounts andscanned from 5º to 75º 2-theta at 0.8º/minute.Degree of serpentinization was calculated from losson ignition data, assuming a water content of 14 wt%for serpentine and of 31 wt% for brucite. Primarymodes, degree of serpentinization, and metamorphicmineral assemblages for each sample studied hereare summarized in Table 1.

The samples studied here include harzburgite,dunite, and one olivine orthopyroxenite. Primary

TAB

LE 1

. Pri

mar

y M

odes

of U

ltram

afic

Roc

ks a

nd M

etam

orph

ic A

ssem

blag

e fr

om X

-Ray

Diff

ract

ion

and

Ele

ctro

n M

icro

prob

e A

naly

sis1

Sam

ple

no.

Roc

k ty

peSe

rpen

tine,

%O

livin

e, %

Opx

, %C

px, %

Cr-

spin

el, %

Liza

rdite

Chr

ysot

ileB

ruci

teM

agne

tite

Oth

er

SFV

-14-

1H

arzb

urgi

te35

76.2

14.7

7.0

2.1

XC

linoc

hlor

e

SFV

-220

-1H

arzb

urgi

te58

73.1

16.2

7.7

3.0

X?

X

SFV

-173

-1H

arzb

urgi

te89

60.6

38.0

0.0

1.4

XX

SFV

-222

-1H

arzb

urgi

te42

78.6

13.3

7.0

1.1

XX

X

SFV

-179

-1H

arzb

urgi

te62

86.6

8.1

4.6

0.7

XX

X

SFV

-223

-1H

arzb

urgi

te58

81.7

12.3

5.3

0.7

X?

X

SFV

-142

-5B

Opx

ite V

ein

100

32.5

67.0

0.0

0.5

XSF

V-1

42-4

Dun

ite10

094

.33.

71.

20.

8X

X

SFV

-180

-1D

unite

100

99.5

0.0

0.0

0.5

XX

SFV

-142

-5A

Dun

ite10

091

.28.

10.

00.

7X

XX

1 Bol

d X

= d

omin

ant p

hase

; ? =

pos

sibl

e pr

esen

ce m

aske

d by

oliv

ine

peak

s.


modes were determined by counting 430 points onenlarged high-resolution scans of one inch diameterprobe mounts for olivine, orthopyroxene (Opx),clinopyroxene (Cpx), and Cr-spinel. Primary modesfor the harzburgites indicate ~60–87% olivine, 8–38% Opx, 0–8% Cpx, and up to 3% Cr-spinel. Pri-mary modes for the dunites indicate 91–99% oliv-ine, with ~3–8% Opx and <1% Cr-spinel (Table 1).Overall, the harzburgites represent partiallydepleted oceanic mantle formed by melting of morefertile peridotites, whereas the dunites representthoroughly depleted mantle that probably formed bydissolution of pyroxene during porous flow of apyroxene-undersaturated melt (e.g., Quick and Gre-gory, 1995). The orthopyroxenite dike consists of67% orthopyroxene, plus olivine. The dike is ~1 cmthick, and is separated from the adjacent harzburg-ite by a zone of dunite.

Serpentinization ranges from 35–100% in theharzburgites, whereas all the dunites are completelyserpentinized. X-ray diffraction studies show thatlizardite is the dominant serpentine mineral in allsamples, with minor chrysotile in three to five sam-ples. The orthopyroxenite dike consists entirely oflizardite, with no indication of accessory brucite ormagnetite. Brucite was detected in seven samples; itis absent from two samples that had high primarymodal Opx and from one sample that is only 35%serpentinized (Table 1). Magnetite was also rarelydetected by X-ray diffraction, although it can beseen petrographically in minor amounts scatteredthrough the thin sections, or decorating fracture sur-faces with a black coating (Fig. 4A), although muchof this black coating may be Fe-rich brucite (JamesBeard, person. commun., 2004).

Serpentine occurs in four textural associations:mesh-texture rims and centers, pyroxene psuedo-morphs, and veins. Olivine is commonly replaced bymesh-textured serpentine along grain boundariesand along orthogonal microfractures within singlegrains. The mesh rims form an orthogonal stockworkof pseudofibrous serpentine in which the apparentfibers are cross-sections of lizardite plates alignedperpendicular to the fracture walls. The olivinegrains enclosed by this mesh may be unaltered, ormay be replaced by finely crystalline or isotropicserpentine that does not display the characteristicpseudofibrous texture of the mesh rims (Fig. 4A).Wicks et al. (1977), and Wicks and Whittaker(1977), have shown that the pseudofibrous mesh-rims consist dominantly of lizardite 1T (± minormagnetite), whereas the massive serpentine replacing

the mesh centers consists dominantly of lizardite ±brucite; chrysotile is rare in this association. Ortho-pyroxene is pseudomorphed by lizardite 1T, whichreplaces pyroxene topotactically to form bastite(Wicks and Whittaker, 1977; Dungan, 1979a,1979b). We have confirmed this by X-ray diffractionof a microsample of bastite from the orthopyroxenitedike, which shows lizardite 1T as the only phasepresent—no brucite or magnetite is detected in theX-ray spectrum, and bastite psuedomorphs are opti-cally free of visible magnetite (Figs. 4A and 4B).

FIG. 4. Photographs of serpentinized peridotites in probemounts, field of view = 2.54 cm (one inch). A. Mesh texture inolivine, bastite after opx, SFV-220-1. B. Microfractures of lateserpentine that cross-cut earlier mesh texture and bastite,SFV-223-1.

8 SHERVAIS ET AL.

The fourth textural association, not found in allsamples, consists of subparallel to orthogonal ser-pentine-filled fractures (microveins) that transectthe samples and cut both the earlier mesh textureand bastite pseudomorphs (Fig. 4B); this is theribbon-texture of Maltman (1978). These microveinsare typically spaced ~1 to 3 mm apart and containfibrous or platy serpentine, chlorite-serpentineintergrowths (septechlorites), and possibly sepiolite.They are typically decorated with microcrystallinemagnetite, which accentuates their visibility in thinsection (Fig. 4B).

No indication has been found to support thepresence of antigorite or other high-temperatureassemblages in the Stonyford serpentinites.

Mineral Chemistry

Mineral phases were analyzed with a JEOL 8900Superprobe at the University of Nevada, Las Vegas,using 15 kV 25 nA probe current and natural andsynthetic mineral standards. In addition to primaryphases, serpentine minerals were analyzed in eachtextural association.

Primary phasesPrimary silicate phases that were not totally

replaced by serpentine include olivine, orthopyrox-ene, and clinopyroxene. The olivine ranges in com-position from Fo90.3 to Fo91.7, with about 0.02 wt%CaO (Table 2). Olivine is preserved only in theharzburgites; no relict olivine was found in the dun-

ites. Orthopyroxenes are enstatite (~En89.8 Wo1.5)and clinopyroxenes are chromian diopside (~En46.4Wo49.8). Alumina contents range from 1.1 to 2.4 wt%in pyroxenes from the depleted (fore-arc) peridotites(dunite, harzburgite) to 3.8 to 6.6 wt% in pyroxenesfrom the abyssal peridotites. Pyroxenes from thefore-arc peridotites have alumina contents similar tocoexisting bastite, but pyroxenes from the abyssalperidotites have 2× to 3× higher alumina than incoexisting bastite. All of the analyzed pyroxeneshave low concentrations of the other pyroxene, indi-cating low pyroxene solvus temperatures of around600–800ºC—well below the peridotite solidus. Cr-spinel compositions vary widely between samplesand define two distinct peridotite groups: abyssalperidotites, which contain spinels with low Cr#s(100 × Cr/[Cr + Al]) and high Mg#s (100 × Mg/[Mg+ Fe]) that plot at the undepleted end of the abyssalperidotite array of Dick and Bullen (1984), and fore-arc peridotites, which contain Cr-rich spinels thatplot within the array of depleted fore-arc peridotites(Ishii et al., 1992).

SerpentineElectron microprobe analyses of serpentine

present special challenges to evaluation and inter-pretation. The variable content of structural andadsorbed water, and the possible presence of bruciteor hollow tubes of chrysotile, result in oxide totalsthat vary from about 80 to 89 wt% (Table 3). As aresult, it is difficult to evaluate analyses for quality.In addition, it is not possible to determine Fe3+/Fe2+

TABLE 2. Average Composition of Primary Minerals in Peridotite Samples

Sample no. Rock Olivine Fo Opx Mg# Cpx Mg# Cr-Sp Mg# Cr-Sp Cr#

SFV14-1 Harzburgite 90.3 90.4 92.2 75.6 11.1

SFV220-1 Harzburgite 91.4 91.3 92.4 72.1 20.0

SFV173-1 Harzburgite 63.2 38.0

SFV222-1 Harzburgite 91.1 91.3 92.8 59.7 44.5

SFV179-1 Harzburgite 91.7 91.9 94.4 56.9 51.2

SFV223-1 Harzburgite 91.5 91.8 94.2 57.0 53.6

SFV142-5B Opxite vein 49.1 59.5

SFV142-4 Dunite 52.1 68.6

SFV180-1 Dunite 48.5 70.1

SFV142-5A Dunite 47.6 74.2

SERPENTINIZATION IN STONYFORD, CALIFORNIA 9TA

BLE

3. S

erpe

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e an

d R

elat

ed P

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Ana

lyse

s by

Ele

ctro

n M

icro

prob

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ectr

osco

py1

Sam

ple

no.:

SFV

-14-

1 SF

V-1

80-1

SFV

-142

-4

SFV

-173

-1SF

V-1

42-4

SF

V-1

4-1

SFV

-173

-1SF

V-1

73-1

SFV

-142

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V-2

22-1

SFV

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1

Para

gene

sis:

Mes

h ri

mM

esh

rim

Mes

h ri

mM

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Chr

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Bas

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D

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Har

z.H

arz.

Har

z.D

un.

Har

z.

Ana

lysi

s no

.30

166

6711

663

3213

313

154

211

6

SiO

2 41

.08

38.7

1 41

.20

40.4

5 39

.63

39.4

1 39

.70

39.5

5 38

.71

43.5

5 39

.29

TiO

2 0.

00

0.00

0.

00

0.00

0.

00

0.01

0.

01

0.02

0.

00

0.00

0.

08

Al 2O

3 1.

81

0.13

0.

05

1.49

0.

45

0.00

0.

88

2.18

1.

22

1.51

5.

47

FeO

5.

00

4.37

3.

83

5.84

5.

11

9.34

3.

99

5.61

5.

74

7.27

9.

74

MnO

0.

07

0.09

0.

07

0.09

0.

20

0.16

0.

03

0.12

0.

21

0.21

0.

21

MgO

36

.76

38.6

2 41

.07

37.6

0 38

.40

41.1

6 39

.45

35.5

1 37

.88

34.4

8 31

.77

CaO

0.

08

0.06

0.

06

0.02

0.

49

0.01

0.

02

0.03

0.

38

0.46

1.

78

Na 2O

0.

01

0.01

0.

00

0.00

0.

00

0.01

0.

00

0.00

0.

00

0.04

0.

06

Cr 2O

3 0.

01

0.03

0.

00

0.17

0.

02

0.00

1.

55

1.33

0.

69

0.49

0.

58

Tota

l 84

.85

82.0

1 86

.28

85.6

6 84

.31

90.0

8 85

.62

84.3

5 84

.85

88.0

3 89

.08

Stru

ctur

al fo

rmul

ae b

ased

on

14 o

xyge

ns

Si3.

9809

3.

8973

3.

9236

3.

9119

3.

8999

3.

7246

3.

8341

3.

8907

3.

8102

4.

1054

3.

7504

Ti0.

0000

0.

0000

0.

0000

0.

0000

0.

0000

0.

0004

0.

0004

0.

0013

0.

0000

0.

0000

0.

0057

Al

0.20

63

0.01

58

0.00

54

0.16

99

0.05

16

0.00

00

0.10

02

0.25

30

0.14

17

0.16

72

0.61

58

Fe0.

4054

0.

3681

0.

3052

0.

4723

0.

4209

0.

7380

0.

3219

0.

4614

0.

4721

0.

5729

0.

7771

Mn

0.00

56

0.00

76

0.00

52

0.00

75

0.01

68

0.01

25

0.00

23

0.01

00

0.01

76

0.01

68

0.01

67

Mg

5.30

78

5.79

52

5.82

87

5.41

99

5.63

08

5.79

82

5.67

72

5.20

62

5.55

72

4.84

42

4.51

86

Ca

0.00

83

0.00

69

0.00

57

0.00

19

0.05

19

0.00

07

0.00

17

0.00

35

0.03

95

0.04

64

0.18

15

Na

0.00

19

0.00

16

0.00

00

0.00

08

0.00

04

0.00

11

0.00

00

0.00

02

0.00

06

0.00

66

0.01

04

Cr

0.00

05

0.00

21

0.00

00

0.01

30

0.00

15

0.00

00

0.11

85

0.10

35

0.05

36

0.03

65

0.04

34

Sum

9.91

61

10.0

925

10.0

737

9.98

41

10.0

722

10.2

755

9.93

77

9.82

64

10.0

388

9.75

95

9.87

61

Mg#

92.9

94

.0

95.0

92

.0

93.0

88

.7

94.6

91

.9

92.2

89

.4

85.3

Mg/

Si1.

33

1.49

1.

49

1.39

1.

44

1.56

1.

48

1.34

1.

46

1.18

1.

20

Si +

Al

4.19

3.

91

3.93

4.

08

3.95

3.

72

3.93

4.

14

3.95

4.

27

4.37

1 Par

agen

esis

ref

ers

to te

xtur

al s

ettin

g, i.

e., m

esh

text

ure,

cor

es o

f mes

h te

xtur

e, b

astit

e ps

eudo

mor

ph a

fter

Opx

. Als

o in

clud

es ir

on r

ich

vari

etie

s an

d re

late

d ph

ases

.

10 SHERVAIS ET AL.

ratios from stoichiometry, and the oxidation state ofiron must be assumed when calculating mineral for-mulae. O’Hanley and Dyar (1993, 1998) used Möss-bauer spectroscopy to determine Fe3+ in lizarditeand chrysotile, but our analyses do not distinguishthese phases. Structural formulae in Table 3 are cal-culated on the basis of 14 oxygens, after the recom-mendations of Whittacker and Wickes (1970), withall iron as Fe2+; acceptable analyses should haveSi + Al ~4.0 and total cations ~10.0. Analyses thatfall well outside this range have been rejected asserpentine, although they may represent complexmixtures of serpentine interlayered with other tri-octahedral sheet silicates. Phases tentatively identi-fied through this process include interlayeredserpentine-chlorite and sepiolite.

Mesh textures after olivine include both meshrims and mesh centers. The mesh rims and meshcenters are generally high in MgO (37–42 wt%)and silica (38–42 wt%) and relatively low in FeO*(= 6%), Cr2O3 (= 0.2%), and alumina (= 1%). Thereis a cluster of mesh center analyses with very highMg#s (100 × Mg/[Mg + Fe] ~95) that have Cr2O3concentrations that range from 0.4 to 1.0 wt%, and afew mesh center analyses are rich in FeO* (6–9wt%), and also have higher Cr2O3 (0.2–0.5 wt%)and alumina (~1.1–1.5 wt%); these are similar incomposition to some Fe-rich serpentines after Opx(bastite).

Serpentines after orthopyroxene (bastite) are gen-erally low in MgO (~34–37 wt%) but have silicasimilar to serpentines after olivine (38–42 wt%).FeO*, Cr, and alumina are generally higher thanmost serpentines after olivine, with the exceptionsdiscussed above. The low Mg, high Cr, and high Alseem to reflect the composition of the original ensta-tites (e.g., Dungan, 1979a, 1979b; Wicks and Plant,1979). The Fe-rich serpentines after Opx are evenlower in MgO (~32–33 wt%) and silica (~35–37wt%), and also have lower Cr and Al than otherbastites (but still higher than most mesh-texturedserpentine after olivine).

Serpentine in the later microveins is generallysimilar to the mesh-texture serpentine, except wherethe veins cut former pyroxenes. Here the microveinserpentine may be high in Cr and Al, similar topyroxene pseudomorphs (Table 3).

Other secondary phases

Other secondary phases in the serpentinites wereidentified petrographically, chemically (electronmicroprobe analysis), or through X-ray diffraction

studies. These phases include magnetite, brucite,inter-layered chlorite-serpentine (septechlorites),mixed layer clays, and possibly sepiolite. Magnetiteoccurs as discrete grains, but its modal abundanceis typically low (less than 1% modally). The veryfine grained black material commonly seen decorat-ing fracture surfaces may be magnetite or Fe-richbrucite. Brucite is confirmed by X-ray diffraction inall but three samples. Two of the brucite-free sam-ples are Opx-rich (pyroxenite vein and Opx-richharzburgite), and one is less than 35% serpentinized(Table 1). Mg-chlorite forms a thin vein in oneharzburgite, but was not observed in other samples.Sepiolite was tentatively identified from a micro-probe analysis of a phyllosilicate with low MgO rel-ative to serpentine. Minerals indicative of highertemperatures and/or higher silica activities, such astalc, tremolite, or anthophyllite, were not found inany of the samples studied here.

Bulk Rock Chemistry

Major-element analyses were carried out on aPhilips 2400 X-ray fluorescence spectrometer atUtah State University, using pressed powders forboth major and trace elements. In addition, major-element compositions were calculated from primarymodes using the average composition of the primaryphases; where no primary silicate minerals remain,comparable data from sample SFV-223 were used.Primary modes were determined on enlarged scansof the probe mount, where points are assigned to pri-mary phases based on their texture and association.A total of 430 points were counted on each one-inchdiameter probe mount. Whole rock major-elementdata are compared in Table 4 to reconstructedmajor-element compositions. In all cases, wholerock analyses are recalculated to 100% anhydrousto facilitate comparison with the modal reconstruc-tions. Loss on ignition ranges from 5 wt% to 15 wt%,corresponding to 35% to 100% serpentinization;samples with LOI > 14% include brucite.

All of the samples analyzed here have XRFwhole rock compositions typical of serpentinizedultramafic rocks, with 40–45 wt% silica, 42–52%MgO, and 8.5–9.5 wt% FeO*; alumina and lime areboth less than 2 wt%, while other elements occuronly in trace amounts (all recalculated to 100%anhydrous/volatile-free). Whole rock compositionsbased on modal reconstruction are basically similar,with slightly lower iron and slightly higher aluminaand lime (Table 4).

SERPENTINIZATION IN STONYFORD, CALIFORNIA 11TA

BLE

4. W

hole

Maj

or-E

lem

ent G

eoch

emis

try

of th

e St

onyf

ord

Peri

dotit

es1

Sam

ple

no.:

SFV

-14-

1SF

V-1

42-4

SFV

-142

-5A

SFV

-142

-5B

SFV

-173

-1SF

V-1

79-1

SFV

-180

-1SF

V-2

20-1

SFV

-222

-1SF

V-2

23-1

Roc

k ty

pe:

Har

z.D

unite

Dun

iteO

pxite

Har

z.H

arz.

Dun

iteH

arz.

Har

z.H

arz.

Who

le r

ock

anal

ysis

by

X-r

ay fl

uore

scen

ceSi

O2

44.1

441

.39

39.3

145

.32

45.9

539

.37

37.5

141

.85

40.4

140

.13

TiO

20.

061

0.00

70.

006

0.07

30.

022

0.00

70.

005

0.03

50.

008

0.00

6A

l 2O3

1.72

0.31

0.17

1.23

1.01

0.33

0.21

1.05

0.47

0.35

FeO

*9.

458.

309.

689.

639.

139.

099.

298.

519.

108.

84M

nO0.

160.

140.

160.

200.

180.

150.

150.

140.

150.

14M

gO42

.00

49.0

749

.92

42.7

042

.58

49.6

251

.96

46.1

848

.24

49.2

0C

aO1.

735

0.09

50.

065

0.22

80.

092

0.71

10.

229

1.52

80.

834

0.60

9N

a 2O0.

043

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.02

90.

000

0.00

0K

2O0.

014

0.00

00.

000

0.00

00.

000

0.00

10.

000

0.00

00.

000

0.00

0P 2O

50.

000

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

0N

iO0.

363

0.35

80.

358

0.21

40.

326

0.36

70.

420

0.32

20.

351

0.35

8C

r 2O3

0.31

00.

340

0.32

70.

406

0.71

20.

350

0.23

00.

354

0.43

40.

356

Prim

ary

mod

e, p

ct.

Oliv

ine

76.2

94.3

91.2

32.5

60.6

86.6

99.5

73.1

78.6

81.7

Opx

14.7

3.7

8.1

67.0

38.0

8.1

0.0

16.2

13.3

12.3

Cpx

7.0

1.2

0.0

0.0

0.0

4.6

0.0

7.7

7.0

5.3

Cr-

spin

el2.

10.

80.

70.

51.

40.

70.

53.

01.

10.

7

Mod

al r

econ

stru

ctio

nSi

O2

41.6

939

.94

40.9

049

.83

45.1

141

.58

39.7

741

.81

42.4

042

.14

TiO

20.

034

0.00

0.00

0.00

0.00

0.00

0.00

0.03

0.00

0.00

Al 2O

32.

310.

360.

261.

521.

300.

450.

072.

610.

770.

56Fe

O*

8.72

8.50

8.33

6.57

7.46

7.84

8.54

7.77

8.07

7.84

MnO

0.13

0.13

0.13

0.14

0.13

0.13

0.12

0.12

0.13

0.12

MgO

44.9

149

.54

49.6

540

.10

44.5

448

.13

51.0

644

.51

45.8

347

.22

CaO

1.65

40.

330.

080.

460.

271.

240.

021.

801.

741.

35N

a 2O0.

062

0.01

0.01

0.01

0.01

0.01

0.01

0.10

0.01

0.01

K2O

0.00

10.

000.

000.

010.

000.

000.

000.

000.

000.

00P 2O

50.

000

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Cr 2O

30.

346

0.99

0.43

0.67

0.71

0.38

0.26

0.75

0.55

0.44

1 Fir

st b

lock

= X

RF

anal

ysis

nor

mal

ized

to 1

00 v

olat

ile fr

ee; s

econ

d bl

ock

= pr

imar

y m

ode;

thir

d bl

ock

= w

hole

-roc

k ch

emic

al c

ompo

sitio

n es

timat

ed b

y m

odal

rec

onst

ruct

ion.

12 SHERVAIS ET AL.

Chemical Flux Based on Bulk Rock Compositions

Comparison of the anhydrous whole rock analy-ses to primary compositions calculated by modalreconstruction is carried out most easily by usingisochon diagrams (Grant, 1986), which provide agraphic solution to the equations for metasomaticreplacement of Gresens (1967). Isochon diagramscompare the original unaltered composition (repre-sented here by modal reconstruction from relictmineral compositions) with the composition aftermetamorphism and metasomatic alteration (Grant,1986). In this case, the current, metasomaticallyaltered composition has been normalized to 100%anhydrous, so volume or mass changes caused byhydration have been eliminated.

Isochon diagrams for the samples studied hereare shown in Figures 5A (all elements) and 5B(expanded to show Al2O3, CaO, TiO2, and Cr2O3); anisochon with a slope of one (no gain or loss of mass)is shown for comparison. Elements that plot abovean isochon for conserved elements have beenenriched in the altered sample, whereas elementsthat plot below the conserved element isochon havebeen removed. Normally we would chose an elementlike aluminum that we believe to be immobile todefine the isochon (e.g., King et al., 2003). In thiscase, assuming aluminum to be a conserved elementdefines an isochon with a slope of 0.74, indicatingthat mass increased during metamorphism byalmost 35% (Fig. 5). There is no evidence for such alarge mass increase. In fact, an isochon slope of 0.74would require major increases in silica, magnesia,and iron, for which there is no apparent source out-side of the peridotite.

Alternatively, the correct isochon may have aslope that is defined by both magnesia and silica(which comprise about 90% of samples studiedhere) and not by alumina. In this scenario, bothmagnesia and silica cluster around the isochon withslope = 1, indicating little loss or gain of either spe-cies (Fig. 5). In general, there is a tendency for MgOto be slightly enriched, and silica to be slightlydepleted, relative to their calculated parents. Iron isslightly enriched in all samples. All of the othermajor elements (Al, Ca, Ti, Cr) are strongly depletedrelative to the unserpentinized protoliths (Fig. 5B).This is essentially the same observation as Colemanand Keith’s (1971) for the Burro Mountain peridot-ite, where they found Si, Mg, Fe, Cr, and Al to beconserved. The difference in our observations con-

cerning Al and Cr mobility may reflect our differenttechniques—Coleman and Keith (1971) used aseries of ratio plots to determine which elementswere conserved, in contrast to our use of modalreconstruction.

The mobility of Ca during serpentinization is rel-atively well known, and is the prime cause of Cametasomatism of mafic and felsic dikes to formrodingite (e.g., Coleman, 1971a). Three rodingitecompositions are presented in Table 5; all were ana-lyzed by X-ray fluorescence methods using glassdisks for major elements and pressed powders fortrace elements. All of these samples have extremelyhigh CaO (20–25 wt%), essentially double what isnormally found in the most Ca-rich basalts. Ca isalso high in cold springs that emanate from ser-pentine terrains both on land and in oceanic crust(Barnes et al., 1971, 1978; Beard and Hopkinson,2000; Früh et al, 2003).

In contrast, it is somewhat surprising to see Aland Cr among the most mobile elements. Thisimplies that these elements could find few suitablemineral hosts in the rocks and were thus mobilizedin the highly alkaline, high pH fluids that form dur-ing serpentinization at low temperature (Barnes andO’Neill, 1971; Barnes et al., 1972, 1978; Beard andHopkinson, 2000). It is well known that whereasCr3+ is relatively stable, Cr6+ may be easily mobi-lized by highly alkaline, high pH fluids (e.g., Ozeet al., 2004). In addition, Drever (1997), andWesolowski and Palmer (1992), have demonstratedthat aluminum solubility varies with pH, with a min-imum solubility at a pH of ~ 6, and increasing solu-bility with increasing alkalinity. Barnes andcoworkers have shown that cold springs that releasewater from serpentinization reactions typically havepH ~ 9–11.8 (Barnes and O’Neill, 1971; Barnes etal., 1971) .

Mass Balance Calculations

The transformation of magnesian olivine into ser-pentine is described by two idealized equilibria thatpreserve constant mass, aside from hydration:

3H2O + 2Mg2SiO4 = water olivine

1Mg3Si2O5(OH)4 + 1Mg(OH)2 (1)serpentine brucite

and


4H2O + 3Mg2SiO4 + 1SiO2 = water olivine aqueous silica

2Mg3Si2O5(OH)4. (2)serpentinite

A similar equilibria describes the transformation oforthopyroxene to serpentine:

4 H2O +3 Mg2Si2O6 = water orthopyroxene

2Mg3Si2O5(OH)4 + 2 SiO2. (3)serpentine aqueous silica

These idealized equilibria depart from realitybecause the primary phases and resulting serpen-

FIG. 5. Isochon diagrams for serpentinized peridotites, comparing whole rock XRF analysis to composition by modalreconstruction—which is assumed to represent the primary, pre-serpentinized composition. A. Isochon diagram scaledfor the major elements Si, Mg, and Fe; minor elements plot near origin. B. Isochon diagram scaled for the minor elementsCa, Al, Ti, and Cr.

14 SHERVAIS ET AL.

tine depart from their ideal compositions as a resultof solid solution. In order to understand the actualchemical exchange taking place during these trans-formations, we have performed a series of mass bal-ance calculations using the observed mineralcompositions of both primary and secondary phases,looking at both variations in the chemistry of theprimary silicate phases and variations in theobserved compositions of serpentine in differenttextural settings (mesh rim, mesh centers, bastite).The mass balance calculations were carried outusing the program GENMIX (LeMaitre, 1981), aver-age primary mineral compositions for both abyssal

and depleted phases, average serpentine composi-tions for each textural setting, plus iron-rich serpen-tine variants and endmember brucite and magnetite.Because GENMIX only considers whole phases, weused (OH) radicals instead of water in the calcula-tions; thus, we have ignored the production of hydro-gen gas that must accompany these reactions inthe Fe-bearing system (e.g., Evans, 2004). To allowfor open system behavior of some components thatwe know were not conserved, we included aqueouslime and silica as products or reactants in somecalculations.

Representative calculations are presented inTable 6. The first two models balance equilibria sim-ilar to equation (1) for olivine plus water → serpen-tine + brucite + magnetite, using either mesh rim ormesh center serpentine. These models have nearperfect solutions (residual sum of the squares <1.0)showing that 80–84 grams of olivine will form a sim-ilar mass of serpentine, plus 12–16 grams of bruciteand 3–4 grams of magnetite. The second two modelsexamine equilibria similar to equation (3) for ortho-pyroxene plus water → serpentine plus silica pluslime. In both cases, ~87 grams of Opx yield ~85grams of serpentine plus 14 grams of silica liberatedin aqueous solution. The depleted Opx (with low pri-mary alumina) has a near perfect solution (r2 < 0.3)because the bastite pseudomorph has almost thesame alumina content as the pyroxene. The abyssalOpx has higher primary alumina in excess of what isfound in the bastite; this equilibria has a poor fit (r2

= 8.6) based on large residuals for alumina. If weassume that alumina is mobile, as suggested by theisochron diagrams, this equilibria would have a bet-ter fit. We note that samples with high-aluminapyroxenes also contain septechlorite in microveins,suggesting in part an internal redistribution of alu-mina. The last two models mimic equation (2), forolivine plus silica plus water → serpentine plusmagnetite. Both examples have good to acceptablefits (r2 = 0.4 to 1.1) in which 75–78 grams of olivineplus 8–10 grams of silica from solution form ~98grams of serpentine and 1–2 grams of magnetite. Therelease of silica from the Opx reaction will drive thisequilibria and significantly enhance serpentine pro-duction and suppress brucite formation. Not shownare models using Fe-rich serpentine; these modelshave results similar to those discussed above, exceptthat magnetite production is suppressed as more Fegoes into the serpentine.

Tables 7–9 present these results with the massproportions converted to moles for comparison with

TABLE 5. Whole-Rock Analyses of Rodingites from Stonyford area, by X-Ray Fluorescence Analysis

Sample no.: SFV-150-1 SFV-209-1 SFV-163-1

Rock: Rodingite Rodingite Rodingite

Area: Salt Creek Hornet Nest Salt Creek

SiO2 45.86 47.87 46.42

TiO2 1.15 2.07 1.50

Al2O3 14.04 12.81 15.06

FeO* 8.48 8.25 9.94

MnO 0.14 0.10 0.15

MgO 6.72 2.02 5.51

CaO 23.80 22.50 21.55

Na2O 0.16 3.61 0.25

K2O 0.01 0.21 0.01

P2O5 0.10 0.31 0.12

SUM 100.46 99.74 100.51

LOI 4.21 10.34 5.28

FeO/MgO 1.26 4.09 1.81

Alkalis 0.17 3.82 0.26

Ti 6891 12405 9009

Nb 4 14 4

Zr 85 203 103

Y 30 34 34

Sr 60 610 161

Rb 1.0 4.0 2.0

Zn 64 61 80

Cu 64 52 86

Ni 67 9 81

Cr 265 32 312

V 281 323 343

Sc 51 38 46

Ba 101 124 799


the idealized equilibria, and normalized to two orthree moles each of the appropriate mafic reactant(olivine or orthopyroxene). Mass proportions weredivided by the molecular weight of each phase,calculated from its average microprobe composition,to determine the number of moles of each species.Volumes were calculated by dividing the formulamolecular weights by the unit cell volume derivedfrom X-ray diffraction cell parameters.

Equilibria describing the transformation of twomoles of olivine to serpentine plus brucite (Table 7)yield results similar to the ideal equilibria, but withsomewhat less brucite produced (0.8 to 0.99 moles)along with small amounts of magnetite (0.05 moles).When Fe-rich serpentine is used as a product, mag-netite is not produced and the amount of brucite isreduced (0.65 to 0.73 moles).

Equilibria describing the transformation of threemoles of orthopyroxene to serpentine plus silica pro-duces about 2.1 to 2.25 moles of serpentine and 1.5to 1.66 moles of silica (Table 8). Small amounts ofexcess lime is also a product, reflecting rejection ofCa from the serpentine lattice. Because the serpen-tine in bastite tends to have higher Fe, Al, andCr than serpentine after olivine, magnetite is notproduced and residuals for Al and Cr are small.

Equilibria describing the transformation of oliv-ine plus silica to serpentine, with no excess MgO asbrucite, require about 0.75 to 1.0 moles of silica forevery three moles of olivine to produce around twomoles of serpentine (Table 9). These reactions pro-duce about 30% to 40% more serpentine than equi-libria lacking silica as a reactant, along with smallamounts of magnetite. The amount of silica requiredis almost exactly the same as the amount of silicaproduced by reacting 1.5 moles of orthopyroxene tobastite. Thus, for every mole of orthopyroxene in therock, two moles of olivine can be reacted withoutproducing brucite.

A harzburgite containing about 21% modalorthopyroxene and 79% modal olivine containsabout two moles of orthopyroxene and eight moles ofolivine. Thus, about half the olivine present may betransformed into serpentine without producing bru-cite, and fixing all of the silica released by thepyroxene serpentinization within the sample. Theremaining olivine must form serpentine plus bru-cite, unless the excess magnesia is removed fromthe system by fluids or silica is added from an out-side source. The importance of excess silica fromorthopyroxene in balancing excess MgO from oliv-

ine has been noted by a number of investigators(e.g., O’Hanley and Offler, 1992).

Discussion

Serpentinites associated with the Stonyford vol-canic complex form a large portion of the CoastRange ophiolite in northern California (Hopson etal., 1981; Hopson and Pessagno, in press). Theseserpentinites may be separated into two groups,based on their internal structure and texture: mas-sive, unsheared peridotites that form resistantridges, and strongly sheared serpentine schists thatenclose decameter to kilometer scale blocks of mas-sive peridotite. Sheared serpentinites form thematrix of the serpentinite mélange and broken for-mation that characterizes much of the ophiolite beltfrom Elder Creek through Wilbur Springs. The mas-sive, relatively unsheared and only partly serpenti-nized peridotites represent essentially intact blocksof ophiolite mantle that have been incorporated intothe serpentinite broken formation through progres-sive hydration and deformation of the primary man-tle tectonites.

In the Stonyford area, exotic blocks crop out atdifferent levels of exposure, suggesting partial pres-ervation of the original stacking order of the litholo-gies. Thus, amphibolites and other high-gradeblocks are only found along the western margin ofthe mélange belt, west of the Stonyford volcaniccomplex and massive harzburgite ridges. Plutonicand volcanic blocks of Coast Range ophiolite affin-ity (other than Stonyford volcanic complex) crop outbeneath the volcanic complex, along the easternmargin of the western mélange belt (e.g., southwestof Auk-Auk Ridge, Fig. 2). The eastern mélangebelt (east of the volcanic complex and massiveharzburgite ridges) contains low-grade metasedi-ments of Galice-like affinity (e.g., Jayko and Blake,1987) as well as volcanogenic sandstones that corre-late with the Crowfoot Point Breccia near ElderCreek (Simpson-Seymore, 1999). This implies thatmost of the non-serpentinite blocks formed by pro-gressive disruption of the overlying oceanic crust,while the high-grade blocks were incorporated fromthe subjacent Franciscan complex (e.g., Jayko et al,1987).

Isochon diagrams that compare major- andminor-element compositions of serpentinized peri-dotites with their original compositions (calculatedby modal reconstruction) show that there is littlechange in the concentration of the most abundant

16 SHERVAIS ET AL.

TAB

LE 6

. Mas

s B

alan

ce R

esul

ts fo

r Se

rpen

tiniz

atio

n of

Pri

mar

y Si

licat

e Ph

ases

Cal

cula

ted

Usi

ng L

east

Squ

ares

Min

imiz

atio

n Pr

oced

ures

and

GE

NM

IX1

Mod

el 1

2

Rea

ctan

ts u

sed:

SiO

2Ti

O2

Al 2O

3Fe

OM

nOM

gOC

aON

a 2OK

2OO

HC

r 2O2

Tota

l16

.02%

wat

er0

00

00

00

00

100

010

083

.98%

Olv

_aby

39.8

10

09.

570.

1350

.46

0.02

00

00.

0110

0

Prod

ucts

use

d83

.62%

Ser

p_M

e39

.46

00.

464.

840.

0839

.83

0.07

0.01

014

0.24

98.9

912

.55%

Bru

cite

00

00

069

00

031

010

03.

82%

Mag

n0

00

92.7

10.

20

00

00

092

.91

Est

imat

ed c

ompo

sitio

nsR

eact

ants

33.4

30

08.

040.

1142

.38

0.02

00

16.0

20.

0110

0Pr

oduc

ts33

00.

387.

590.

0741

.97

0.06

0.01

015

.60.

298

.88

Diff

eren

ces

0.43

0–0

.38

0.44

0.03

0.41

–0.0

4–0

.01

00.

42–0

.19

Mod

el 5

3 R

eact

ants

use

d:Si

O2

TiO

2A

l 2O3

FeO

MnO

MgO

CaO

Na 2O

K2O

OH

Cr 2O

2To

tal

17.2

9% w

ater

00

00

00

00

010

00

100

82.7

1% O

lv_d

ep39

.54

00

8.46

0.13

51.1

50.

020.

010

00

99.3

1

Prod

ucts

use

d80

.56%

Ser

pOlA

40.3

70

0.4

4.73

0.1

38.4

20.

130

015

0.23

99.3

816

.21%

Bru

cite

00

00

069

00

031

010

03.

23%

Mag

n0

00

92.7

10.

20

00

00

092

.91

Est

imat

ed c

ompo

sitio

nsR

eact

ants

32.7

10

07

0.11

42.3

10.

020.

010

17.2

90

99.4

3Pr

oduc

ts32

.52

00.

326.

810.

0942

.13

0.1

00

17.1

10.

1999

.27

Diff

eren

ces

0.18

0–0

.32

0.19

0.02

0.17

–0.0

90.

010

0.18

–0.1

9

Mod

el 1

34

Rea

ctan

ts u

sed:

SiO

2Ti

O2

Al 2O

3Fe

OM

nOM

gOC

aON

a 2OK

2OO

HC

r 2O2

Tota

l12

.51%

wat

er0

00

00

00

00

100

010

087

.49%

Opx

_aby

53.4

60.

064.

596.

20.

1433

.74

0.64

0.03

0.01

00.

4899

.35

Prod

ucts

use

d86

.12%

Ser

pOp1

39.7

80.

011.

675.

520.

1435

.31

0.22

00

150.

8598

.513

.01%

sili

ca10

00

00

00

00

00

010

00.

87%

lim

e0

00

00

010

00

00

010

0


Est

imat

ed c

ompo

sitio

nsR

eact

ants

46.7

70.

054.

025.

420.

1229

.52

0.56

0.03

0.01

12.5

10.

4299

.43

Prod

ucts

47.2

70.

011.

444.

750.

1230

.41

1.06

00

12.9

20.

7398

.71

Diff

eren

ces

–0.5

0.04

2.58

0.67

0–0

.89

–0.5

0.03

0.01

–0.4

1–0

.31

Mod

el 1

75

Rea

ctan

ts u

sed:

SiO

2Ti

O2

Al 2O

3Fe

OM

nOM

gOC

aON

a 2OK

2OO

HC

r 2O2

Tota

l12

.81%

wat

er0

00

00

00

00

100

010

087

.19%

Opx

_dep

54.9

60

2.2

5.52

0.15

34.5

50.

960.

010.

010

0.7

99.0

6

Prod

ucts

use

d85

.41%

Ser

pOp1

39.7

80.

011.

675.

520.

1435

.31

0.22

00

150.

8598

.513

.94%

sili

ca10

00

00

00

00

00

010

00.

65%

lim

e0

00

00

010

00

00

010

0

Est

imat

ed c

ompo

sitio

nsR

eact

ants

47.9

20

1.92

4.81

0.13

30.1

20.

840.

010.

0112

.81

0.61

99.1

8Pr

oduc

ts47

.92

0.01

1.43

4.71

0.12

30.1

60.

840

012

.81

0.73

98.7

2D

iffer

ence

s0

–0.0

10.

490.

10.

01–0

.04

00.

010.

010

–0.1

2

Mod

el B

96

Rea

ctan

ts u

sed:

SiO

2Ti

O2

Al 2O

3Fe

OM

nOM

gOC

aON

a 2OK

2OO

HC

r 2O2

Tota

l14

.12%

wat

er0

00

00

00

00

100

010

07.

94%

sili

ca10

00

00

00

00

00

010

077

.93%

Olv

_aby

39.8

10

09.

570.

1350

.46

0.02

00

00.

0110

0

Prod

ucts

use

d97

.58%

Ser

p_M

e39

.46

00.

464.

840.

0839

.83

0.07

0.01

014

0.24

98.9

92.

42%

Mag

n0

00

92.7

10.

20

00

00

092

.91

Est

imat

ed c

ompo

sitio

nsR

eact

ants

38.9

70

07.

460.

139

.33

0.02

00

14.1

20.

0110

0Pr

oduc

ts38

.50

0.45

6.97

0.08

38.8

60.

070.

010

13.6

60.

2398

.84

Diff

eren

ces

0.46

0–0

.45

0.49

0.02

0.46

–0.0

5–0

.01

00.

46–0

.23

Mod

el B

13

7 R

eact

ants

use

d:Si

O2

TiO

2A

l 2O3

FeO

MnO

MgO

CaO

Na 2O

K2O

OH

Cr 2O

2To

tal

15.0

0% w

ater

00

00

00

00

010

00

100

10.5

6% s

ilica

100

00

00

00

00

00

100

74.4

3% O

lv_d

ep39

.54

00

8.46

0.13

51.1

50.

020.

010

00

99.3

1

(Tab

le c

ontin

ues)

18 SHERVAIS ET AL.

elements (Mg, Si, Fe) when corrected for hydration;minor elements (Ca, Al, Cr), however, are moder-ately to strongly depleted in serpentinites. Thus, theprocess of serpentinization must be essentially isoch-emical for the major elements, although some massloss must occur to account for the decrease in minorelements. This is consistent with the conclusions ofmany previous studies, including Hostetler et al.(1966), Coleman and Keith (1971), and O’Hanleyand Offler (1992). The alternative interpretation ofthe isochon diagrams—that alumina is conservedand therefore significant mass must have beenadded in the form of Si, Mg, and Fe (e.g., King et al.,2003)—is unrealistic because there is no viablesource for the massive amounts of Mg and Fe thatwould be needed. Although excess silica may bederived from dewatering of sediments or evenmetavolcanic rocks, the Mg and Fe required couldonly come from other ultramafic rocks. Inasmuch asall serpentinized ultramafic rocks have similar com-positional relationships (Mg-depleted ultramaficrocks have not been described), there is no way toderive the Mg and Fe needed.

Much of the Ca may be transmitted to the surfaceby cold springs that issue from the serpentinites(e.g., Barnes et al., 1978; Fruh-Green et al., 2003),but the dominant sink for Ca is clearly the rodingites(Coleman, 1971a; Coleman and Keith, 1971; Frost,1975). The sink for Al and Ti is less obvious: per-haps chloritic blackwall adjacent to the rodingites?But even though a sink is not well constrained, itseems clear from the isochon results that both Aland Ti must be mobile during serpentinization.

The mass balance results based on actual min-eral compositions show that serpentinization departsto some degree from the idealized equilibria weoften use to describe it. For olivine, these departuresresult largely from the variations in Mg/Fe ratio ofthe protolith, with more Fe resulting in less serpen-tine. For orthopyroxene, these departures resultlargely from less than ideal silica contents, and hightetrahedral alumina, which lower the excess silicaproduced, and from the high Fe, Al, and Cr contentsof serpentine psuedomorphs after Opx, whichincreases the mass of serpentine produced.

The other important aspect of mass balance forserpentinization of harzburgites is the interplaybetween excess silica produced by the serpentini-zation of Opx (equilibria [3]), and excess MgO pro-duced by the serpentinization of olivine underequilibria (1). In the absence of excess silica fromOpx, the excess MgO produced by equilibria (1)

TAB

LE 6

. (C

ontin

ued)

Prod

ucts

use

d98

.50%

Ser

pOlA

40.3

70

0.4

4.73

0.1

38.4

20.

130

015

0.23

99.3

81.

50%

Mag

n0

00

92.7

10.

20

00

00

092

.91

Est

imat

ed c

ompo

sitio

nsR

eact

ants

39.9

90

06.

30.

138

.07

0.01

0.01

015

099

.49

Prod

ucts

39.7

60

0.39

6.05

0.1

37.8

40.

130

014

.77

0.23

99.2

8D

iffer

ence

s0.

230

–0.3

90.

240

0.23

–0.1

10.

010

0.23

–0.2

3

1 Pri

mar

y si

licat

es u

sed:

aby

ssal

oliv

ine

(Fo9

0.5)

; dep

lete

d ol

ivin

e (F

o91.

5); a

byss

al O

px (4

.6%

Al 2O

3); a

nd d

eple

ted

Opx

(2.2

% A

l 2O3)

; ser

pent

ine

min

eral

s in

clud

e m

esh

text

ured

,ol

ivin

e re

plac

emen

t, an

d O

px r

epla

cem

ent (

bast

ite).

2 Res

idua

l sum

of s

quar

es =

.918

1634

; chi

-squ

are

= .0

8346

94.

3 Res

idua

l sum

of s

quar

es =

.277

9407

; chi

-squ

are

= 2.

5267

33E

-02.

4 Res

idua

l sum

of s

quar

es =

8.6

4788

4; c

hi-s

quar

e =

.786

1713

.5 R

esid

ual s

um o

f squ

ares

= .2

6642

3; c

hi-s

quar

e =

2.42

2028

E-0

2.6 R

esid

ual s

um o

f squ

ares

= 1

.136

912;

chi

-squ

are

= .1

0335

56.

7 Res

idua

l sum

of s

quar

es =

.437

977;

chi

-squ

are

= 3.

9816

09E

-02.


results in modal brucite. While modal brucite isobserved in most of the samples studied here(those low in primary Opx), it is not an abundantphase and is not present in the amounts predictedby equilibria (1). Excess silica formed during theproduction of bastite suppresses brucite formation,and enhances the production of more serpentine.In harzburgites with typical Opx modes of around20–25%, more than half of the expected brucite is

suppressed, and 30–40% more serpentine isformed.

The question of constant volume versus constantcomposition has been contentious for some time(e.g., Hostetler et al., 1966; Thayer, 1966; Colemanand Keith, 1971; Wicks et al., 1977; O’Hanley,1992, 1996). The topotactic replacement of Opxby lizardite argues for essentially constant volumerelations (Dungan, 1979a, 1979b), although the

TABLE 7. Olivine to Serpentine + Brucite Equilibria, Normalized to Two Moles of Olivine Reactant1

Water Olivine Serpentine Brucite Magnetite Delta V

3.31 OH + 2.00 olivine aby = 1.03 serpentine mesh + 0.76 brucite + 0.06 magnetite 1.51

3.33 OH + 2.00 olivine dep = 1.02 serpentine mesh + 0.82 brucite + 0.05 magnetite 1.52

3.59 OH + 2.00 olivine aby = 1.01 serpentine cores + 0.92 brucite + 0.06 magnetite 1.53

3.61 OH + 2.00 olivine dep = 1.01 serpentine cores + 0.99 brucite + 0.05 magnetite 1.54

2.44 OH + 2.00 olivine aby = 1.02 serpentine Fe + 0.65 brucite + 1.43

2.50 OH + 2.00 olivine dep = 1.01 serpentine Fe + 0.73 brucite + 1.43

1Calculated from GENMIX mass balance results. Volume change calculated from molar volumes of solid phases.

* * * * *

* * * * *

* * * * *

* * * * *

* * * *

* * * *

TABLE 8. Orthopyroxene to Serpentine Equilibria, Normalized to Three Moles of Orthopyroxene Reactant1

Water Orthopyroxene Serpentine Silica (aqueous) Lime (aqueous) Delta V

5.25 OH + 3.00 Opx aby = 1.94 bastite cores + 1.55 silica + 0.11 lime 1.23

6.13 OH + 3.00 Opx dep = 2.03 bastite cores + 1.52 silica + 0.03 lime 1.23

5.40 OH + 3.00 Opx aby = 1.97 bastite Fe + 1.66 silica + 0.08 lime 1.27

6.28 OH + 3.00 Opx dep = 2.05 bastite Fe + 1.64 silica + 1.27


* * * * *

* * * * *

* * * * *

* * * *

TABLE 9. Olivine Plus Aqueous Silica to Serpentine Equilibria, Normalized to Three Moles of Olivine Reactant1

Water Olivine Silica (aqueous) Serpentine Magnetite Delta V

4.77 OH + 3.00 Olivine aby + 0.75 silica = 1.94 serpentine mesh + 0.06 magnetite 1.62

5.20 OH + 3.00 Olivine aby + 0.97 silica = 2.03 serpentine cores + 0.06 magnetite 1.68

4.72 OH + 3.00 Olivine dep + 0.82 silica = 1.97 serpentine mesh + 0.04 magnetite 1.63

5.22 OH + 3.00 Olivine dep + 1.04 silica = 2.05 serpentine cores + 0.04 magnetite 1.69


* * * * *

* * * * *

* * * * *

* * * * *

20 SHERVAIS ET AL.

volume expansion calculated for bastite formation inour mass balance results is only 25–30%, about halfthe expansion calculated for serpentine-formingreactions from olivine (Table 5). Nonetheless, themass balance results argue against significant lossof Si and Mg during serpentinization, since both ofthese elements are easily balanced using theobserved mineral compositions. In contrast, concen-trations of elements like Al and Ti are poorly repro-duced in the mass balance calculations (as shown bythe high residuals for these elements), and other ele-ments (like Ca) can only be balanced by assumingan aqueous flux of dissolved cations. These massbalance results are supported by the isochon dia-grams, which show that Si, Mg, and (to a lesserextent) Fe must be conserved during serpentiniza-tion, whereas Ca, Al, and Cr are lost (Fig. 5). If weassume that Al is conserved (e.g., King et al., 2003),the isochon diagram requires that Si and Mg beadded, not lost—the opposite result predicted byconstant volume calculations.

O’Hanley (1992, 1996) proposed that volumeexpansion during serpentinization is accommodatedat the outcrop scale largely along orthogonal cross-fractures that force blocks apart and allow serpenti-nization to penetrate farther into the resultingblocks. These macroscopic cross-fractures maybegin as the orthogonal microfractures observed inmany of the samples studied here, which are in turnlarger-scale versions of the commonly observedmesh textures (Wicks et al., 1977). As blocksbecome separated by these orthogonal cross-frac-tures, differential movement will cause the serpen-tinite between the blocks to become sheared andfoliated. This differential movement may be entirelytectonic in origin, focusing strain within the weakestpart of the serpentinized lithosphere. Alternatively,much of the shearing and foliation observed may becaused by the expansion process itself, as blocks areforced to adjust to increased volumes in the totallyserpentinized selvages, which are expanding inthree dimensions, by differential movements thatforce the blocks to move in the direction of leastprincipal stress. This is basically the same processproposed by Fryer and Fryer (1987) to explain theemplacement of nonvolcanic (serpentinite) sea-mounts in forearc settings. Similar models havebeen proposed for other partly sheared serpentinites(e.g., Coleman and Keith, 1971).

Conclusions

Serpentinization of harzburgites and dunitesadjacent to the Stonyford volcanic complex pro-ceeded essentially isochemically for Si, Mg, and Fe,whereas other elements (Ca, Al, Cr) were lost to anaqueous flux. Volume expansion was considerable(25–30% for bastite formation, 50–60% for forma-tion of mesh textures from olivine), primarily result-ing from the addition of water. This expansion wasaccommodated by orthogonal fractures at both themicroscopic (Wicks et al., 1977; Maltman, 1978)and macroscopic scales, as proposed by O’Hanley(1992). Subsequent movement along the macro-scopic serpentinized fractures led to the formation ofserpentine broken formation, with a matrix ofsheared and foliated serpentinite, and relict blocksof massive, less serpentinized peridotite.

The distribution of exotic blocks in the shearedserpentinite mélange marginal to the broken forma-tion suggests that the original structural stackingorder is preserved, with Franciscan-derived blocksin the west and Coast Range ophiolite–derivedblocks in the east. This preserved stacking orderimplies that the serpentine mélange formed withinor adjacent to an active subduction zone, in whichthe serpentinized peridotites represent the mantlelithosphere of the hanging wall of the subductionzone (as proposed by Jayko and Blake, 1986 andJayko et al., 1987). This process has been docu-mented in active convergent margins (e.g., Fryer etal., 1995; Peacock, 1993) and represents a viableactualistic model for the formation of serpentinitemélange in the northern Coast Ranges.

Acknowledgments

This paper is dedicated to Bob Coleman, whostimulated the thought processes of generations ofstudents and colleagues, both in the field and atmeetings, on topics ranging from ophiolites to eclog-ites, and who continues to inspire us today. Dr.James Beard provided an insightful and construc-tive review. This research was supported in partby NSF grants EAR8816398 and EAR9018721(Shervais). Geologic mapping of the Stonyford ophi-olites formed part of a master’s thesis by MarchellZoglman Schuman (Zoglman, 1991) at the Univer-sity of South Carolina.


REFERENCES

Barnes, I., and O’Neill, J. R., 1971, The relationshipbetween fluids in some fresh alpine-type ultramaficsand possible modern serpentinization, western UnitedStates: Geological Society of America Bulletin, v. 80,p. 1947–1960.

Barnes, I., Rapp, J. B., O’Neill, J. R., Sheppard, R. A., andGude, A. J., 1971, Metamorphic assemblages anddirection of flow of metamorphic fluids in fourinstances of serpentinization: Contributions to Miner-alogy and Petrology, v. 35, p. 263–276.

Barnes, I., O’Neill, J. R., and Trescases, J. J., 1978,Present day serpentinization in New Caledonia, Oman,and Yugoslavia: Geological Society of America Bulle-tin, v. 80, p. 1947–1960.

Beard, J. S., and Hopkinson, L., 2000, A fossil serpentini-zation-related hydrothermal vent, Ocean Drilling Pro-gram Leg 173, Site 1068, (Iberia Abyssal Plain): Someaspects of mineral and fluid chemistry: Journal of Geo-physical Research, v. 105, B7, p. 16,527–16,539.

Coleman, R. G., 1971a, Petrologic and geophysical natureof serpentinites: Geological Society of America Bulle-tin, v. 87, p. 897–918.

Coleman, R. G., 1971b, Plate tectonic emplacement ofupper mantle peridotites along continental edges:Journal of Geophysical Research, v. 75, p. 1212–1222.

Coleman, R. G., 1977, Emplacement and metamorphismof ophiolites, in Galli, M., ed., High pressure–low tem-perature metamorphism of the oceanic and continentalcrust in the Western Alps: Rendiconti della SocietaItaliana di Mineralogia e Petrologia: Pavia, Italy, Edi-trice Succ. Fusi., p. 161–190.

Coleman, R. G., 2000, Prospecting for ophiolites along theCalifornia continental margin, in Dilek, Y., Moores,E. M., Elthon, D., and Nicolas, A., eds., Ophiolites andoceanic crust: New insights from field studies and theOcean Drilling program: Geological Society of Amer-ica Special Paper 349, p. 351–364.

Coleman, R. G., and Keith, T. E., 1971, A chemical studyof serpentinization; Burro Mountain, California: Jour-nal of Petrology, v. 12, no. 2, p. 311–328.

Dennis, A. J., and Shervais, J. W., 1991, Style and signifi-cance of sheared serpentinite fault rocks, Stonyford,northern California Coast Ranges [abs.]: GeologicalSociety of America, Abstracts with Programs, v. 23, no.5, A236.

Dick, H. J. B., and Bullen, T., 1984, Chromian spinel as apetrogenetic indicator in abyssal and alpine-type peri-dotites and spatially associated lavas: Contributions toMineralogy and Petrology, v. 86, p. 54–76.

Drever, J. I., 1997, The geochemistry of natural waters,3rd ed: Upper Saddle River, NJ, Prentice Hall, 436 p.

Dungan, M. A., 1979a, Bastite pseudomorphs after ortho-pyroxene, clinopyroxene, and tremolite, in Wicks, F.J., ed., Serpentine mineralogy, petrology, and paragen-esis: The Canadian Mineralogist, v. 17, p. 729–740.

Dungan, M. A., 1979b, A microprobe study of antigoriteand some serpentine pseudomorphs, in Wicks, F. J.,ed., Serpentine mineralogy, petrology, and paragene-sis: The Canadian Mineralogist: v. 17, p. 771–784.

Evans, B. W., The serpentine multisystem revisited:Chrysotile is metastable: International GeologyReview, v. 46, p. 479–506.

Evarts, R. C., 1977, The geology and petrology of the DelPuerto ophiolite, Diablo Range, central CaliforniaCoast Ranges, in Coleman, R. G., and Irwin, W. P.,eds., North American ophiolites: Oregon, Departmentof Geology and Mineral Industries, Bulletin no. 95, p.121–139.

Frost, B. R., 1975, Contact metamorphism of serpentinite,chloritic blackwall, and rodingite at Paddy-Go-Easy-Pass, Central Cascades, Washington: Journal of Petrol-ogy, v. 16, p. 272–312.

Früh, G. G. L., Kelley, D. S., Bernasconi, S. M., Karson,J. A., Ludwig, K. A., Butterfield, D. A., Boschi, C., andProskurowski, G., 2003, 30,000 years of hydrothermalactivity at the Lost City vent field: Science, v. 301, no.5632, p. 495–498.

Fryer, P., and Fryer, G. J., 1987, Origins of non-volcanicseamounts in a forearc environment, in Keating, B. H.,Fryer, P., Batiza, R., and Boehlert, G. W., eds., Sea-mounts, island, and atolls: American GeophysicalUnion, Geophysical Monograph Series, v. 43, p. 61–72.

Fryer, P., Mottl., M., Johnson, L., Haggerty, J., Phipps, S.,and Maekawa, H., 1995, Serpentine bodies in theforearcs of western Pacific convergent margins: Originand associated fluids, in Taylor, B., and Natland, J.,eds., Active margins and marginal basins of the west-ern Pacific: American Geophysical Union, Geophysi-cal Monograph Series, v. 88, p. 259–279.

Giaramita, M., MacPherson, G. J., and Phipps, S. P., 1998,Petrologically diverse basalts from a fossil oceanicforearc in California: The Llanada and Black Mountainremnants of the Coast Range ophiolite: GeologicalSociety of America Bulletin, v. 110, no. 5, p. 553–571.

Grant, J. A. 1986, The isocon diagram—a simple solutionto Gresens’ equation for metasomatic alteration: Eco-nomic Geology, v. 81, p. 1976–1982.

Gresens, R. L., 1967, Composition-volume relationshipsof metasomatism: Chemical Geology, v. 2, p. 47–65.

Hopson, C. A., Mattinson, J. M., and Pessagno, E. A., Jr.,1981, Coast Range ophiolite, western California, inErnst, W. G., ed., The geotectonic development of Cal-ifornia, Rubey Volume I: Englewood Cliffs, NJ, Pren-tice-Hall, p. 418–510.

Hopson, C. A., and Pessagno, E. A., Jr., in press, Tehama-Colusa serpentinite mélange: A remnant of Fran-ciscan-like Jurassic oceanic lithosphere, northern Cal-ifornia: International Geology Review.

Hostetler, P. B., Coleman, R. G., Mumpton, F. A., andEvans, B. W., 1966, Brucite in alpine serpentinites:American Mineralogist, v. 51, nos. 1–2, p. 75–98.

22 SHERVAIS ET AL.

Huot, F., and Maury, R. C., 2002, The Round Mountainserpentinite melange, northern Coast Ranges of Cali-fornia: An association of backarc and arc-related tec-tonic units: Geological Society of America Bulletin, v.114, no. 1, p. 109–123.

Ishii, T., Robinson, P. T., Maekawa, H., and Fiske, R.,1992, Petrological studies of peridotites from diapiricserpentinite seamounts in the Izu-Ogasawara-Marianaforearc, Leg 125, in Proceedings of the Ocean DrillingProgram, Scientific Results, v. 125: College Station,TX, Texas A&M University, Ocean Drilling Program,p. 445–485.

Jayko, A. S., and Blake, M. C., Jr., 1986, Significance ofKlamath rocks between the Franciscan Complex andCoast Range Ophiolite, Northern California: Tectonics,v. 5, no. 7, p. 1055–1071.

Jayko, A. S., Blake, M. C., Jr., and Harms, T. A., 1987,Attenuation of the Coast Range ophiolite by exten-sional faulting, and nature of the Coast Range “thrust,”California: Tectonics, v. 6, no. 4, p. 475–.

King, R. L., Kohn, M. J., and Eiler, J. M., 2003, Con-straints on the petrologic structure of the subductionzone slab-mantle interface from Franciscan complexexotic blocks: Geological Society of America Bulletin,v. 115, p. 1097–1109.

LeMaitre, R. W., 1981, GENMIX—a generalized petrolog-ical mixing model program: Computers and Geo-sciences, v. 7, p. 229–247.

Maltman, A. J., 1978, Serpentinite textures in Anglesey,North Wales, United Kingdom: Geological Society ofAmerica Bulletin, v. 89, p. 972–980.

Moody, J. B., 1976, Serpentinization: A review: Lithos, v.9, no. 2, p. 125–138.

O’Hanley, D. S., 1992, Solution to the volume problem inserpentinization: Geology (Boulder), v. 20, p. 705–708.

O’Hanley, D. S., 1996, Serpentinites: Records of tectonicand petrological history: Oxford, UK, Oxford Univer-sity Press, Oxford Monographs on Geology and Geo-physics, 277 p.

O’Hanley, D. S., and Dyar, M. D., 1993, The compositionof lizardite 1T and the formation of magnetite in ser-pentinites: American Mineralogist, v. 78, nos. 3–4, p.391–404.

O’Hanley, D. S., and Dyar, M. D., 1998, The compositionof chrysotile and its relationship with lizardite: TheCanadian Mineralogist, v. 36, p. 727–739.

O’Hanley, D. S., and Offler, R., 1992, Characterization ofmultiple serpentinization, Woodsreef, New SouthWales: The Canadian Mineralogist, v. 30, p. 1113–1126.

Oze, C., Fendorf, S., Bird, D. K., and Coleman, R. G.,2004, Chromium geochemistry in serpentinized ultra-mafic rocks and serpentine soils from the FranciscanComplex of California: American Journal of Science, v.304, p. 67–101.

Peacock, S. M., 1993, Large scale hydration of the litho-sphere above subducting slabs: Chemical Geology, v.108, p. 49–59.

Quick, J. E., and Gregory, R. T., 1995, Significance ofmelt-wall rock reaction: A comparative anatomy ofthree ophiolites: Journal of Geology, v. 103, p. 187–198.

Shervais, J. W., 1990, Island arc and ocean crust ophio-lites: Contrasts in the petrology, geochemistry, and tec-tonic style of ophiolite assemblages in the CaliforniaCoast Ranges, in Malpas, J., Moores, E. M., Panay-iotou, A., and Xenophontos, C., eds., Ophiolites: Oce-anic crustal analogues: Nicosia, Cyprus, GeologicalSurvey Department, Proceedings of the SymposiumTroodos 1987, p. 507–520.

Shervais, J. W., 2001, Birth, death, and resurrection: Thelife cycle of supra-subduction zone ophiolites:Geochemistry, Geophysics, Geosystems, v. 2 [papernumber 2000GC000080].

Shervais, J. W., and Hanan, B. B., 1989, Jurassic volcanicglass from the Stonyford volcanic complex, Franciscanassemblage, northern California Coast Ranges: Geol-ogy (Boulder), v. 17, p. 510–514.

Shervais, J. W., and Kimbrough, D. L., 1987, Alkaline andtransitional subalkaline metabasalts in the FranciscanComplex melange, California, in Morris, E. M., andPasteris, J. D., eds., Mantle metasomatism and alka-line magmatism: Boulder, CO, Geological Society ofAmerica Special Paper 215, p. 165–182.

Shervais, J. W., and Kimbrough, D. L., 1985, Geochemicalevidence for the tectonic setting of the Coast Rangeophiolite: A composite island arc-oceanic crust terranein western California: Geology (Boulder), v. 13, p. 35–38.

Shervais, J. W., Kimbough, D. L., Renne, P. R., Murchey,B., and Hanan, B. B., 2005, Radioisotopic and bios-tratigraphic age relations in the Coast Range ophiolite,northern California: Implications for the tectonic evo-lution of the Western Cordillera: Geological Society ofAmerica Bulletin, in press.

Shervais, J. W., Murchey, B., Kimbrough, D. L., Hanan,B. B., Renne, P. R., Snow, C. A., Schuman, M. Z., andBeaman, J., 2004, Multi-stage origin of the CoastRange ophiolite, California: Implications for the lifecycle of supra-subduction zone ophiolites: Interna-tional Geology Review, v. 46, 289–315.

Simpson-Seymore, A., 1999, Tectonic implications of theStony Creek basaltic sandstone, Late Jurassic GreatValley Series, Stonyford, California: Unpubl. M.Sc.thesis, University of South Carolina, 122 p.

Stern, R. J., and Bloomer, S. H., 1992, Subduction zoneinfancy: Examples from the Eocene Izu-Bonin-Mari-ana and Jurassic California arcs: Geological Society ofAmerica Bulletin, v. 104, p. 1621–1636.

Thayer, T. P., 1966, Serpentinization considered as a con-stant-volume metasomatic process: American Mineral-ogist, v. 51, p. 685–710.


Wesolowski, D. J., and Palmer, D. A., 1992, Experimentalstudies of aluminum solubility and speciation inbrines, in Kharaka, Y. K., and Maest, A. S., eds., Pro-ceedings of the 7th International Symposium on Water-Rock Interaction, WRI-7, v. 1, p. 185–188.

Whittaker, E. J. W., and Wicks, F. J., 1970, Chemical dif-ferences among the serpentine “polymorphs”; a dis-cussion: American Mineralogist, v. 55, nos. 5–6, p.1025–1047.

Wicks, F. J., and Plant, A. G., 1979, Electron microprobeand X-ray microbeam studies of serpentine textures:The Canadian Mineralogist, v. 17, p. 785–830.

Wicks, F. J., and Whittaker, E. J. W., 1975, A reappraisalof the structures of the serpentine minerals: The Cana-dian Mineralogist, v. 13, p. 227–243.

Wicks, F. J., and Whittaker, E. J. W., 1977, Serpentine tex-tures and serpentinization: The Canadian Mineralo-gist, v. 15, p. 459–488.

Wicks, F. J., Whittaker, E. J. W., and Zussman, J., 1977,An idealized model for serpentine textures after oliv-ine: The Canadian Mineralogist, v. 15, p. 446–458.

Zoglman, M. M., 1991, The Stonyford Volcanic Complex:Petrology and structure of a Jurassic seamount in thenorthern California Coast Ranges: Unpubl. mastersthesis, University of South Carolina, 129 p.

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