Oceanic plagiogranites as a result of interaction between magmatic and hydrothermal systems in the slow-spreading mid-ocean ridges

ISSN 0869�5911, Petrology, 2010, Vol. 18, No. 4, pp. 369–383. © Pleiades Publishing, Ltd., 2010.Original Russian Text © S.A. Silantyev, L.Ya. Aranovich, N.S. Bortnikov, 2010, published in Petrologiya, 2010, Vol. 18, No. 4, pp. 387–401.

369

INTRODUCTION

The term “oceanic plagiogranites” was proposedby (Coleman, 1977) for description of group of felsicplutonic rocks found among plutonic complexes of themodern mid�ocean ridges (MOR) and ophiolites ofpaleocollisional zones. According to the modelaccepted in 1960–1980s and based on studying ophio�lite complexes (for instance, Hess, 1962; Engel andFisher, 1975; Aldis, 1981), the MOR plagiogranitesare characterized by the following peculiarities:(1) occurrence among modern oceanic crust andophiolitic complexes, where they form intrusive bod�ies and dikes in the gabbros from upper level of the sec�tion and in sheeted dike complex; (2) derivationthrough differentiation of parental tholeiitic melts andaffiliation to the tonalite–diorite–trondhjemite asso�ciation; (3) high SiO2 content, low K2O (no more than0.8 wt %), (Ce/Yb)cn and (La/Sm)cn < 1.5–2, whichcorresponds to the final products of the AFM differen�

tiation trend of tholeiitic magma (for instance, Cole�man, 1977).

The period of 1990–2000s brought a revolution inthinking concerning the structure of the oceanic crust.It was established that the wide development of mantleperidotites and gabbros in the axial zones of Mid�Atlantic (MAR) and Southwest Indian ridges is thecharacteristic feature of the Hess�type crust developedin the slow�spreading MOR. These rocks often associ�ate with vein and dike plagiogranite bodies (Fig. 1).Unlike the postulates of canonical model, the plagiog�ranites of the slow�spreading MOR occur at all levelsof the oceanic crust section and their veins and dikesoften cut across both gabbroids and peridotites.

Recently obtained data highlight some peculiarityin the localization of oceanic plagiogranites (OPG) inthe crest zone of MAR. The MAR plutonic complexescontaining vein granitoid bodies are situated in theaxial zone of the ridge, which contains active hydro�thermal fields. A prominent example of this regularity

Oceanic Plagiogranites as a Result of Interactionbetween Magmatic and Hydrothermal Systems

in the Slow�Spreading Mid�Ocean RidgesS. A. Silantyeva, L. Ya. Aranovichb, and N. S. Bortnikovb

aVernadsky Institute of Geochemistry, Russian Academy of Sciences, Kosygina 19, 199991 Moscow, Russiae�mail: [email protected]

bInstitute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017 Russia

e�mail: [email protected]; [email protected] February 1, 2010

Abstract—The paper considers some petrological and geochemical aspects of the formation of oceanic pla�giogranites (OPG)—felsic intrusive rocks, which were found in the plutonic complexes of modern mid�ocean ridges (MOR) and ophiolites of paleo�collisional zones. Based on the multi�equilibrium clinopyrox�ene–orthopyroxene–amphibole–plagioclase geothermobarometry, typical OPG found in gabbros and peri�dotites were formed at temperatures of 820–850°C and pressure of 2–2.5 kbar. Close temperature estimates(825 ± 50°C) were obtained from literature data on Ti content in zircon, with allowance for lowered TiO2activity in the rock. Under these P–T parameters, OPG can be generated only in the presence of fluid of wateractivity (aH2O) close to 0.9. OPG and associated recrystallized gabbroids contain high�temperature horn�

blende with significant Cl content (0.5–2 wt %). In addition, the plagiogranites are characterized by partic�ular geochemical features such as extremely high Na2O/K2O (up to 135), sharp LREE enrichment

((Ce/Yb)cn and (La/Sm)cn up to 10 and 4, respectively), and elevated 87Sr/86Sr ratio relative to DMM. Allthese facts point to the key role of hydrothermal fluid, the seawater derivative, in the OPG formation. Thefluid with aH2O = 0.9 (approximately 28 wt % NaCl) could be produced from seawater due to hydration reac�

tions at the higher lower temperature horizons of oceanic crust in the course of its percolation to the OPGgeneration areas. The formation of plagiogranites in the MOR oceanic core complexes possibly reflects thefundamental feature of oceanic accretion: practically simultaneous (at the geological time scale) proceedingof exogenic (neptunic) and endogenous (plutonic) processes.

DOI: 10.1134/S0869591110040041

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is the large hydrothermal cluster including theAshadze and Logachev fields (MAR between 12° and17°N) (Batuyev et al., 1994; Beltenev et al., 2005;Shipboard…, 2007).

The origin of oceanic plagiogranites is widely dis�cussed by the world scientific community. The condi�tions of formation of these rocks that form thin veinbodies and dikes in the gabbro�peridotite substrate ofslow�spreading mid�ocean ridges imply the participa�tion of aqueous fluid in the their parental magmaticsystem. However, the composition and nature of thisaqueous fluid (magmatic or marine in origin) up tonow remain the object of hot debates.

The petrological and geochemical features of felsicrocks of the modern oceanic basins are considered innumerous works (for instance, Coleman, 1977; Dicket al., 1991; Silantyev, 1998; Silantyev et al., 2005). Aswas shown in (Silantyev et al., 2008), the fundamentalgeochemical characteristics of felsic rocks that com�pose veins and small dikes of diorite and trondhjemitecomposition in the MAR plutonic complex is theirenrichment in trace elements and LREE, which isassociated with characteristic isotopic signatures ofDMM reservoirs based on N�MORB composition.The following well established geological, mineralogi�cal, and geochemical features of OPG should be takeninto account to construct their petrological model.

(1) MAR plutonic complexes with OPG veins areconfined to the ridge axial zone, which contains activehydrothermal fields. For instance, OPG found in theMAR axis, between 12° and 15°N (Ashadze andLogachev fields) occur as vein and dike bodies in gab�broids and peridotites bearing distinct signs of high�

temperature interaction with host rocks. Both withinthe Ashadze field (12°58′–13°00′Ν) and in the area ofthe Logachev hydrothermal field (14°45′–14°49′N),the peridotites and gabbros penetrated by vein grani�toids of the considered type contain high�temperatureamphibole–phlogopite assemblage, which indicatesthe input of aqueous fluid in the OPG formation andcontact transformation of the rocks of the gabbro�peridotite complex.

(2) According to data on composition and forma�tion temperature of fluid inclusions from MAR gab�broids [for instance, (Kelley and Delaney, 1987)], thehigh�salinity fluid inclusions found in recrystallizedgabbroids of MAR were entrapped at temperatureabout 700°С. This temperature estimate of fluid�mag�matic transformation of MAR gabbro and peridotitesis only slightly lower than estimates (780–800°С)obtained using Ti�in�zircon geothermometer (Watsonand Harrison, 2005). It is noteworthy that the averagetemperature of the formation of the Archean TTGdetermined by the same method (Watson and Harri�son, 2005) accounted for 700°С.

(3) MAR gabbroids contain Cl�rich aluminoushornblende. The gneissic gabbroids contain two majortypes of amphibole: actinolite with varying Al content(Al2O3 = 1–6 wt %) and aluminous hornblende(Al2O3 > 8 wt %) corresponding in composition toedenite (or edenite–hastingsite). In terms of composi�tion, the aluminous hornblende is ascribed to the mag�matic amphiboles, though, judging from its relationswith pyroxene, this mineral undoubtedly representsthe newly formed secondary phase unrelated to themagmatic evolution of the melt parental for MORB

12

34

56

Fig. 1. Localities of finds of oceanic plagiogranites (OPG) in the oceanic core complexes (OCC): (1) Atlantis Massif, MAR, 30°N(Expedition…, 2006); (2) western side of the MAR rift valley, 15°30′N (Silantyev, 1998); (3) Ashadze hydrothermal filed, MAR,12°59′N; (4) Markov Deep, MAR, 5°N (Savel’eva et al., 2008)); (5) Central Indian Ridge, 25°S (Nakamura et al., 2007);(6) Southwest Indian Ridge, borehole 735B (Maeda et al., 2002).


OCEANIC PLAGIOGRANITES AS A RESULT OF INTERACTION 371

gabbroids. The characteristic feature of this high�tem�perature hornblende in MAR is high Cl content (up to1.2–2.0 wt %) in the anionic group. Based on theexisting concepts that magmatic products of MORhave extremely low Cl content (for instance, Jambon,1994), the high Cl content in this high�temperaturehornblende found in MAR gabbroids is presumablyrelated to its introduction with a high�salinity fluid ofmarine origin.

(4) As was shown experimentally (Koepke et al.,2004, 2007; Sisson et al., 2005), the moderate�pres�sure melting of mafic protolith, including oceanicgabbros and basalts, in the presence of aqueous fluidproduces melts that are compositionally close to thenatural OPG of MOR.

(5) Thin dike bodies and veins of OPG are observedin all petrographic rock types of the MAR plutoniccomplexes. According to the canonical model ofspreading based on plate tectonic postulates, oceanicplagiogranites are derivatives of the parental MORBmelts and occupy the same position as ophiolitic gran�itoids: at the boundary between isotropic gabbro andsheeted dike complex. However, presently availabledata on structure of real crustal sections of MOR indi�cate that oceanic granitoids in the slow�spreadingridges occupy a distinct position, being found in allrock types throughout the entire section. Similar rela�tions of felsic vein rocks with host plutonic rocks wereestablished in many MAR areas (Silantyev, 1998;Shipboard…, 2003; Shipboard…, 2007; Savel’eva et al.,2008): 6°N (Markov deep); 12°58′N (Ashadze field);15°30′N; and 30°N (Atlantis Massif).

(6) LREE enrichment and high (as compared tothe host rocks) 87Sr/86Sr ratios. The trace element vari�ations (degree of enrichment) in gabbro and peridot�ites associated with vein OPG could reflect the reac�tion interaction of these rocks, which leads to the for�mation of hybrid gabbro. The Sr and Nd isotopecomposition of the granitoid�bearing MAR plutoniccomplexes unambiguously suggests a contribution ofenriched component represented by vein OPG in themagmatic systems of MAR crest.

(7) high Na2O content in OPG of MAR.This work is devoted to some petrological and

geochemical aspects of the OPG formation, with spe�cial emphasis on estimating the physicochemicalparameters of their formation, the use of which signif�icantly refines the model schemes and provides insightinto composition and transport properties of fluid.

PETROLOGICAL FEATURES AND CONDITIONS OF OPG FORMATION

OPG typically occur as “pockets”, patches, vein�lets, and veins from few millimeters to few centimetersin size in host peridotites and gabbroids (Fig. 2) andstructurally are practically identical to the plagiogran�ite migmatites of ophiolite complexes (for instance,see Gillis and Coogan, 2002). Comparatively large

OPG veinlets contain angular xenoliths of host rocks(Fig. 2b). In peridotites, leucocratic segregationsoccur only as veins, with distinct reaction zones at thecontact (Fig. 3). Reaction relations between plagiog�ranite veinlets and host ultrabasic rocks were alsonoted in (Silantyev et al., 2008; Savel’eva et al., 2006;Gillis and Googan, 2002). The veinlets are quartz�free. This fact should be taken into account in anyconsiderations of bulk chemical variations of OPG:the lowest�SiO2 compositions (52–58% SiO2)reported in recent review (Koepke et al., 2007) weredetermined in the leucocratic veins in ultramafic rocksand high�Mg gabbro (troctolites), in which significantpart of SiO2 was spent for formation of minerals inreaction zones.

Gabbroids typically lack reaction zones, some�times showing gradual transitions from medium�grained segregations of plagiogranites to the fine�grained areas with “dispersed granitic material”

2 cm(a)

(b)

Fig. 2. Structural relation of OPG with host gabbroids(photos of samples). Shown are the gradual transitionsfrom “dispersed” granitic material to the vein segregations(a) and the presence of angular fragments of gabbro inOPG vein (b).

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(Fig. 4). Such structural relations indicate that OPGcould be formed via partial melting of gabbroids.

Major minerals of OPG are plagioclase, quartz,and amphibole (Figs. 3, 4). Sometimes, the plagiog�ranites contain epidote (clinozoisite, allanite),K�feldspar, and phlogopite; though the primary natureof epidote could not be unambiguously established inall cases (Koepke et al., 2005). Common accessoryminerals are ilmenite, Ti�magnetite, zircon (Figs. 3d,4d), and apatite. Albite, epidote, actinolite, and chlo�rite occur as secondary minerals. Fresh (weaklyaltered by secondary alterations) gabbroid samples,which contain both OPG patches and dispersed gran�itoid material represent the most suitable object forestimating the conditions of OPG formation (Figs. 2a,4a; see also Fig. 3 in Koepke et al., 2005). These sam�ples show distinct textural transformations, as well aschanges in proportions and chemistry of minerals,which are related to either melting or interactionbetween host rocks and percolating felsic melt. Amongmost important changes is the appearance of newlyformed clino� and orthopyroxene, amphibole, andplagioclase (Figs. 4a⎯4c). As compared to primarymagmatic Cpx from gabbro, the newly formed Cpxlocated near OPG veins has much higher content of

CaO and lower TiO2 and Al2O3, as well as somewhatlower ХMg = Mg/(Mg + Fe) (Table 1). At the sametime, newly formed orthopyroxene show somedecrease in CaO and Al2O3 contents and insignificantdecrease in ХMg relative to magmatic orthopyroxene(Table 1). Amphibole ascribed to the edenite–horn�blende join (Table 1) is developed at the contact withnewly formed clinopyroxene (Fig. 4c) or form inter�growths with plagioclase and quartz directly in the pla�giogranite patches (Fig. 4b). The structural position ofthis amphibole in samples distinctly indicates its for�mation during melting, which, in turn, testifies to theparticipation of aqueous fluid in this process. Thecomposition of plagioclase in OPG varies within An22–An14, whereas plagioclase in gabbro has much morecalcic composition (An40–42, Table 1). It should benoted that plagioclase crystals in OPG often showzoning, with practically pure albite rims (Fig. 4b).Albitization presumably was not related to the mag�matic stage of OPG formation, which follows fromremarkable similarity in structure and zoning patternbetween these crystals and those synthesized in exper�iments on recrystallization of plagioclase in the pres�ence of sodic�silicate solutions at moderate tempera�ture (Hovelmann et al., 2010).

0.5 mm(a) Pl

Pl

Pl

Ol

Ol

Opx Pl

Cpx

Cpx

Cpx

CpxOpx

Opx

500 µm

Ol

Zrn

Zm

Ol

Prg

Prg

Prg

Opx

Ol

HblHbl

Hbl

HblPl

50 µm 200 µm

(b)

(d)(c)

Fig. 3. Structure of reaction zone at the contact of peridotite with plagioclase veinlet.(a) General view of reaction zone; note granulation of olivine and preservation of large grain of primary orthopyroxene of peri�dotite (photo of polished thin section; crossed nicols); (b) area of reaction zone containing newly formed clinopyroxene (Cpx),orthopyroxene (Opx), and zircon (Zrn); (c) detailed structure of reaction zone; it is seen that early pargasite amphibole is over�grown by hornblende; (d) veinlet of hornblende cutting across host peridotite and plagioclase (OPG) veinlet (b, c, d are imagesin back�scattered electrons).



The recrystallization with change of mineral com�position and appearance of new phases is also observedat the contact reaction zones between peridotites andOPG veinlets. Cрх and Oрх formed by this way signifi�cantly differ from their magmatic analogues by mor�phology and composition (Figs. 3a⎯3c, Table 2),while amphibole has high�Mg sodic composition cor�responding to pargasite (Fig. 3c, Table 2) and then isreplaced by hornblende (Fig. 3c). The fact that thehornblende was formed later follows from its cross�cutting relations with leucocratic veinlet (Fig. 3d).Plagioclase in the leucocratic veinlets is much morecalcic in composition than that observed in OPGamong gabbro (An31–33, Table 2), which is possiblyrelated to the formation of pargasite amphibole at theexpense of interaction of albite molecule of the meltwith minerals of host peridotite.

The appearance of new mafic minerals accompa�nying OPG formation enables us to estimate the con�ditions of their formation using geothermobarometryof multicomponent solid solutions (Aranovich, 1991).Calculations were made using TWQ program (Ber�man, 1991) and thermodynamic dataset (Berman and

Aranovich, 1996) for coexisting Opx, Cpx, and Pl inOPG from gabbro and peridotite (these compositionsare shown in bold in Tables 1 and 2). The results of cal�culations are shown in Р–Т diagrams (Figs. 5a, 5b) asequilibrium curves of different reactions between endmembers of solid solutions at given mineral composi�tions. The coordinates of intersection points of thesecurves define the Р�Т conditions of formation of cor�responding assemblages. Important parameter defin�ing the convergence of calculations (i.e., when thearea of isopleths intersection is close to point) is silicaactivity in the rock. This parameter was used as

independent (comp. Aranovich and Kozlovskii,2009), because it was suggested that the rocks did notcontain quartz under conditions corresponding to themelt existence. Calculated Р and Т values appeared tobe close for samples shown as an example in Fig. 5:2.0–2.3 kbar and 820–840°С, whereas estimates of

significantly differ, which is well consistent with

inferred lower values of in peridotites.

aSiO2( )

aSiO2

aSiO2

(a) 0.5 mmPl

Hbl

QtzPl

Pl

Qtz

Qtz

Fsp

Hbl

Hbl

Hbl200 µm

Qtz

Pl

50 µm

llm

llm

TmZrn

Hbl

Cpx

Cpx

Hbl

Qtz

Ab

Pl40

Pl22

Qtz

(b)

(d)(c) 500 µm

Fig. 4. Relations of OPG veinlet with host gabbroids.(a) Features of medium�grained hornblende plagiogranite (lower left corner), gradually passing (towards the upper right corner)into fine�grained mass with phenocrysts of newly formed minerals and in gabbro (photo of thin section, crossed nicols); (b) equi�librium granitic texture formed by intergrowths of hypidiomorphic grains of sharply zoned plagioclase (with pure albite rims),quartz, and hornblende; (c) details of structure of fine�grained transitional zone from gabbro to medium�grained OPG; it is seenthat hornblende is confined to the newly formed clinopyroxene and that plagioclase from gabbro strongly differs from that in OPG(numbers denote An content in Pl, mol %); (d) intergrowths of fine�grained ilmenite, Ti�magnetite, and zircon in OPG vein (b,c, d, are photos in back�scattered electrons).

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The water activity in fluid during OPG for�

mation can be estimated from equilibrium conditionsof reactions with participation of amphibole. Sincethermodynamic properties of complex Amph solidsolution are less studied than properties of other min�erals used in our calculations, only several simplestreactions with the best studied end members [tremo�lite, actinolite, and Mg�tschermakite (Mader andBerman, 1992)] were used in calculating . The

calculations were conducted at given pressure (Р =2.2 kbar) and temperature (Т = 830°С). A significant

aH2O( )

aH2O

scatter in obtained , from 1.0 to 0.85 (Figs. 6a,

6b), is presumably related to the errors in describingthe thermodynamic properties of Amph. However, cal�culated equilibrium curves at given mineral composi�tion define a compact cluster around = 0.9. Tak�

ing into account the aforementioned scatter, this firstdirect estimate can be taken as characteristic for con�ditions of OPG formation.

One more widely spread method of estimate oftemperature of OPG formation is Ti�in�zircon ther�mometry (Watson and Harrison, 2005). This methodis based on experimental calibration of temperature

aH2O

aH2O

Table 1. Representative chemical composition of primary (1) and newly formed (2) minerals from gabbro and OPG

Components Cpx1 Cpx2 Opx1 Opx2 Amph Pl1 Pl

SiO2 51.25 52.40 50.60 50.06 43.01 58.54 62.06

TiO2 0.55 0.00 0.00 0.00 2.73

Al2O3 1.20 0.43 0.56 0.29 9.93 26.66 23.81

FeO 14.95 16.57 32.11 33.54 18.48

MnO 0.42 0.70 0.82 0.87 0.25

MgO 11.55 10.42 14.50 14.36 10.58

CaO 19.86 21.03 2.20 0.96 10.55 8.36 5.03

Na2O 0.32 0.39 0.00 0.00 2.30 6.88 8.28

K2O 0.00 0.00 0.00 0.00 0.21 0.00 0.43

Total 100.10 101.94 100.79 100.08 98.03 100.43 99.61

X 0.59 0.55 0.45 0.44 0.57 0.40 0.25

Note: X is the mole Mg/(Mg + Fe) ratio in the mafic minerals and Ca/(Ca + Na) in plagioclase. Compositions used for estimation of T, P,and are shown by bold. Structural relations between minerals are shown in Fig. 4.aH2O

Table 2. Representative chemical composition of primary (1) and newly formed (2) minerals from peridotite and OPG

Components Cpx1 Cpx2 Opx1 Opx2 Amph Pl

SiO2 51.08 53.79 54.79 54.68 44.66 59.97

TiO2 0.40 0.27 0.00 0.00 2.95

Al2O3 3.56 0.78 3.46 0.66 10.61 25.31

Cr2O3 1.01 0.00 0.39 0.00 0.00

FeO 5.88 6.32 9.79 18.07 8.29

MnO 0.00 0.37 0.13 0.64 0.00

MgO 16.11 16.61 31.00 26.19 16.39

CaO 19.71 21.35 1.50 0.47 11.40 7.30

Na2O 1.01 0.42 0.00 0.00 2.75 7.60

K2O 0.00 0.00 0.00 0.00 0.24 0.11

Total 98.77 99.91 101.06 100.73 97.28 100.29

X 0.91 0.80 0.85 0.69 0.86 0.34

Note: X is the mole Mg/(Mg + Fe) ratio in the mafic minerals and Ca/(Ca + Na) in plagioclase; compositions used for estimating T, P, and are shown by bold. Structural relations of minerals are shown in Fig. 3.aH2O



dependence of Ti content in Zrn in equilibrium withquartz and rutile, which is described by reaction:

TiO2 + ZrSiO4 = ZrTiO4 + SiO2 (A)

Ru + Zrn = Ti�Zrn + β�Qtz.

In compliance with revision of experimental data(Watson and Harrison, 2005) conducted by Ferry and

Watson (2007), the Ti content in Zrn ([ppm Ti]) equi�librium with Ru and Qtz is related with temperature byfollowing equation:

log([ppm Ti]) = 5.711 – 4800/T, K. (1)

If quartz and rutile are absent, equation of equilib�rium conditions of this reaction should be correctedfor lowered values of oxide activity (ai):

(2)

Since activity of SiO2 in most OPG at magmaticstage (expressed relative to β�Qtz) is typically lessthan 1 (see above, as well as Grimes et al., 2009), whilerutile is absent (i.e., is significantly less than 1),the latter term provides significant contribution in

ppm Ti[ ]( )log 5.711 4800/T K,–=

– aSiO2/aTiO2

( ).log

aTiO2

10

9

8

7

6

5

4

3

1300600

500400

300200

100

2

1

700800

9001000

11001200

1

2

3

6

57

Pre

ssur

e, k

bar

Temperature, °С

Teq = 840°CPeq = 2.0 kbaraSiO2 = 0.7

1. Cts + β�Qtz = An2. β�Qtz + Di + α�Opx = An + En3. β�Qtz + Hd + α�Opx = An + Fs5. Di + α�Opx = Cts + En6. Hd + α�Opx = Cts + Fs7. Hd + En = Di + Fs

7

6

5

4

3

1200800700600

2

1

900 1000 11002

3

6

5

1

Pre

ssur

e, k

bar

Temperature, °С

Teq = 820°CPeq = 2.3 kbaraSiO2 = 0.5

4

1. Cts + β�Qtz = An2. β�Qtz + Di + α�Opx = An + En3. β�Qtz + Hd + α�Opx = An + Fs4. Di + α�Opx = Cts + En5. Hd + α�Opx = Cts + Fs6. Hd + En = Di + Fs

(а)

(b)

Fig. 5. P⎯T diagram with isopleths of coexisting mineralsfor OPG from gabbro (a) and peridotite (b).Coordinates of intersection data points correspond to con�ditions of formation of mineral assemblages. Numbers incurves correspond to end�member reactions shown in fig�ures.

900

850

800

750

700

650

6000.40.60.81.0

1

2

2.2 kbarаSiO2

= 0.5

H2O activity

Tem

pera

ture

, °С

3

1000

950

800

750

700

650

6000.60.80.91.0

1

2

2.2 kbarаSiO2

= 0.7

H2O activity

Tem

pera

ture

, °С

3

0.7

900

850

(а)

(b)

Fig. 6. Diagram temperature�water activity for equilibriaof dehydration reactions with participation of amphiboleand coexisting minerals of gabbro (a) and peridotite (b) atgiven pressure of 2.2 kbar and silica activity (0.7 and 0.5,respectively). Abscissa of intersection points of curves withisotherms yields in fluid. Numbers in lines corre�

spond to reactions between end members: (1) 2β�Qtz +3En + 4Di + 2H2O = 2Tr; (2) 2β�Qtz + 4Hd + 3Fs +2H2O = 2fTr, (3) 4An + 3En + 2H2O = 2Ts + 2β�Qtz.

aH2O

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temperature estimate according to (2). The value of can be estimated using Ti content in magnetite

in association with ilmenite, which is fairly typical ofOPG:

2FeTiO3 = Fe2TiO4 + TiO2 (B)

Ilm Ti�Mt Ru;

(3)

The value of the Gibbs free energy of reaction (B),ΔG°(B), calculated from dataset (Berman and Ara�novich, 1996) weakly depends on temperature withintemperature range of 700–900°С and accounts for21000 J. Taking into account that at this temperatureaTi�Mt ≈ 1.2ХTi�Mt (Sack and Ghiorso, 1991), while TiO2

content in magnetite varies from 4 to 7 wt %, the esti�mation of TiO2 activity (relative to rutile) in the rockscontaining OPG according to equation (3) yields

from 0.3 to 0.4. Taking into account this correc�

tion and silica activity = 0.7 obtained above for

the OPG melt in gabbro according to equation (2) andanalytical data on Ti content in zircons from OPG(Grimes et al., 2009), we calculated the temperaturesof their formation. Results of calculations are shown ashistogram in Fig. 7. For 69 samples, the average tem�perature is estimated to be Т = 825 ± 50°С (1σ), whichis very close to estimates obtained using method ofmultiequilibrium geobarometry (Fig. 5). It should bealso noted that in general the temperatures deter�mined from zircon geothermometer define very nar�row interval of 750–900°С (Fig. 7) and estimatesobtained on the basis of both these methods are signif�icantly lower than temperatures based on amphibole–plagioclase geothermometer of Holland and Blundy(1994): 910–1020°С (Koepke et al., 2005). This dis�agreement is presumably caused by the fact that calcu�

aTiO2,

aTiO2( )ln ΔG° B( )/ 8.313T( ) aTi�Mt( )ln+[ ]– .=

aTiO2

aSiO2

lations in the work (Koepke et al., 2005) were based oncomposition of An�rich plagioclase from gabbro, noton the acid plagioclase from OPG.

It is difficult to compare directly the obtained tem�perature estimates with results of model experimentson partial melting of oceanic gabbro in the presence ofaqueous fluid (Koepke et al., 2004; Berndt et al.,2005), because experiments were conducted at Т ≥900°С in order to obtain significant amount of melt(glass). In all experiments, temperature showed posi�tive correlation with melting degree and negative, withSiO2 content in the melt. Insignificant low�tempera�ture extrapolation of experimental data (Fig. 8) yieldsmelting degree about 3–5% and SiO2 = 67–70 wt % atТ ≈ 830°С (Р = 2 kbar), which is well consistent bothwith estimated OPG fraction in the MOR crust sec�tion and with most often observed SiO2 content inOPG. Such consistency does not imply that all OPGwere formed at similar values of temperature, pressure,and water activity in fluid. However, it emphasizes theplausibility of obtained estimates and, which is ofprime importance, the necessity of sufficiently highfluid pressure for OPG formation.

In accordance with our estimates, fluid that pro�voked OPG generation should differs insignificantlyfrom pure aqueous fluid = 0.9). The question

arises as to the nature of this fluid. The recrystallizedMAR gabbroids associated with OPG contain high�temperature Cl�rich (0.5–2.00 wt % Cl) hornblende.Amphibole of this composition could be formed viadifferentiation of water and chlorine�bearing mag�matic melt. However, since Cl, as F, significantlyexpands the amphibole stability field by temperature,in this case, it should crystallize first, which is incon�sistent with its structural relations with other mineralsin samples. The participation of aqueous–salt fluids inmodern MORB�producing magmatic systems alsoseems to be doubtful, because melt inclusions in oliv�

aH2O(

950

900

850

800

750

700

650

600

Tem

pera

ture

, °С

Тav. = 825°С;MSWD = 47°(n = 69)

Fig. 7. Individual temperatures estimated on the basis of Ti content in zircon from OPG (analytical data after Grimes et al., 2009)with allowance for lowered value of TiО2 activity in the rock (equation 2 in text).



ine from MOR basalts practically do not contain chlo�rine (Sobolev and Chaussidon, 1996). This leads us toconclude that the sea water�derived high�temperatureaqueous–salt fluid took part in the OPG generation.

Our estimates of water activity during OPG gener�ation are close to = 0.9, which, assuming aque�

ous–salt composition of fluid, temperature of 830°С,and pressure of 2 kbar, and according to experimentaldata on the activity–composition relations in thewater–NaCl system (Aranovich and Newton, 1997),corresponds to = 0.9 (about 28 wt % NaCl

equiv.). Hence, this fluid must be eight times moreconcentrated than seawater (3.5 wt %). An increase insalinity of hydrothermal solutions in the MOR settingcould be caused by interaction of percolating seawaterwith rocks of oceanic crust, which leads to the forma�tion of aqueous minerals. In extreme cases, this mech�anism leads to the fluid saturation and the appearanceof water�soluble chlorides in the intergranular space ofhighly serpentinized rocks (Barnes et al., 2009).According to (Kelly and Delaney, 1987), the fluidinclusions in minerals from recrystallized gabbroids ofMAR contain high�density NaCl�rich (up to 50 wt %NaCl equiv.) brines, which are homogenized at tem�peratures more than >700°C. These finds obviously aredirect confirmation for the participation of high�tem�perature aqueous–salt fluid in the OPG–gabbro reac�tion system.

GEOCHEMICAL FEATURES OF OPG

Available data on major element composition ofOPG from the MOR oceanic core complexes (Silan�tyev, 1998; Bazylev et al., 2001; Savel’eva et al., 2003;Expedition…, 2006; Silantyev et al., 2008) indicatethat these rocks do not follow fractionation trend oftholeiitic magma parental for N�MORB in the AFMdiagram (Fig. 9). In accordance to the existing geody�namical classifications of granitoids based on traceelement variations, OPG from axial zones of Mid�Atlantic Ridge (MAR) and Southwest Indian Ridgefall in WPG (within�plate granites) field (Pearce et al.,1984) or correspond to the derivatives of ocean�islandmagmatism (Eby, 1992). Such a geochemical specificsof OPG is inconsistent with their simplified interpre�tation, which implies that they represent final prod�ucts of magmatic evolution of the MOR rift valley. Thecharacteristic feature of OPG from the oceanic corecomplexes is the high Na contents occasionally reach�ing up to 8 wt % at Na2O/К2O up to 135. It should benoted that OPG sampled in different MAR segmentsshow significant regional differences (including Na2Ocontent). These differences presumably reflect theconditions of formation of these rocks in oceaniclithosphere.

Host gabbroids contain recrystallized varieties thatoccupy in composition intermediate position betweentroctolite gabbros and gabbronorites and

aH2O

XH2O

trondhjemites. Figure 9 distinctly demonstrates sys�tematic compositional changes of the gabbroids fromthe oceanic core complexes of MOR during theirrecrystallization, which leads to the formation of leu�cocratic plagioclase enriched rocks corresponding inmajor element distribution to the trondhjemites. Thepossible genetic link between OPG and host gabbros isalso evident from REE distribution patterns of theserocks (Fig. 10). The LREE enrichment in recrystal�lized gabbro ((Ce/Yb)cn = 1.88–2.02) is practicallysimilar to that of trondhjemites ((Ce/Yb)cn = 1.27–2.38). As compared with host ophitic (normal) gab�bros, the plagiogranites reveal distinct features of sharpLREE enrichment: (Ce/Yb)cn and (La/Sm)cn in theserocks reach, respectively, 10 and 4. Gabbroids recrys�tallized at the contact with trondhjemite veins alsohave high LREE contents ((Ce/Yb)cn = 1.5–3) andpossibly represent hybrid rocks formed during interac�tion between gabbro and OPG.

Nd isotope variations in OPG of MAR correspondto those in associated basaltoids and gabbro and, thus,could indicate the contribution of DMM source in theformation of their parental melts. On the other hand,practically all available OPG samples have higher87Sr/86Sr ratio than DMM (Fig. 11). Since 87Sr/86Srratio is a sensitive indicator of degree of alteration ofthe rocks of oceanic crust during their interaction withseawater and its fluid derivatives (Silantyev and Kos�titsyn, 1990), the enrichment in radiogenic Srobserved in OPG indicates that their formation wascontributed by component, which was not related to

100

90

80

70

60

50

40

30

0120011001000900800

20

10

70

40

65

60

55

50

45

Deg

ree

of m

elti

ng,

%

SiO

2 (melt), w

t %

Temperature, °С

Degree of melting, %

SiO2 (melt), wt %

Fig. 8. Variations of melting degree of model gabbro (leftordinate) and silica content in the melt (right ordinate)versus temperature according to experimental data(Berndt et al., 2005) (symbols). Dashed curves are lowtemperature extrapolations.

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SILANTYEV et al.

MORB�generating reservoir and, presumably, repre�sented fluids of marine origin.

PROBLEM OF OPG AGE

In recent decade, a progress in application ofmethods of isotope geology for dating the rocks of oce�anic basement was related to the study of behavior ofU–Pb system in zircon extracted from gabbroids andassociated OPG. The isotopic study of zircons fromgabbroids of the axial MAR zone at 5°–6°N (Markovdeep) using laser ablation (Kostitsyn et al., 2009)showed that all samples of gabbronorites dredged inthis area from the same dredging site contain zirconswith following ages: 1.95 ± 0.05 Ma, 0.82 ± 0.02 Ma,and 0.98 ± 0.10 Ma. The U and Pb isotope compositionof zircon from this collection was previously analyzedusing SHRIMP ion microprobe (Bortnikov et al.,2005). Based on this study, two zircon generationswere distinguished in these rocks: young zircons withages of 0.760 ± 0.041 Ma and 2.39 ± 0.19 Ma (only ingabbronorites) and ancient zircons with ages of 87 ±7 Ma, 499 ± 15 Ma, 657 ± 13 Ma, 3117 ± 27 Ma, and1852 ± 41 Ma (in troctolites and gabbronorites).

The U–Pb dating and Lu–Hf isotope analysis of 77zircon grains extracted from trondhjemites (OPG)and host gabbroids in the oceanic core complex of theAshadze hydrothermal field (MAR, 12°58′N) demon�

strated that zircons both from OPG and gabbroids aredated at 0.96 ± 0.03 Ma (Kostitsyn et al., 2009). Itshould be especially emphasized that zircons fromOPG of the Ashadze filed are similar to thoseextracted from associated gabbroids in terms of traceelement distribution.

Data on isotope dating of zircon extracted fromplutonic rocks of MAR are also presented in the work(Pilot et al., 1998). These authors found zircon withages about 330 and 1600 Ma (U�Pb method) in gabbrodrilled in the intersection zone of MAR and Kanefracture zone (near 23°32′N). The time of zircon crys�tallization in the plutonic complex from the VemaFracture Zone (Central Atlantic) is estimated by U�Pbmethod (Lissenberg et al., 2009) at 90000–235000 years.SHRIMP U–Pb dating on zircon from Fe–Ti gabbroand OPG in the oceanic core complex of the South�west Indian Ridge, borehole 735B (Indian Ocean) isreported also in (Schwartz et al., 2005). The age of zir�con from this area is 10.6–13.9 Ma, which, accordingto (Schwartz et al., 2005), corresponds to the age ofmagnetic anomalies that are observed in the area ofthis ODP borehole.

Presented data on the age of zircon from OPG andassociated gabbroids indicate that the best statisticallyjustified U–Pb datings are ages of MAR granitoidswithin the range of 0.76–1.95 Ma. This age obviouslymarks the OPG�related late magmatic stage of the

Fig. 9. AFM�diagram for the rocks from the oceanic core complexes containing OPG.



evolution of the oceanic core complexes of MAR.However, the interpretation of all available datings onplutonic rocks in the crest zone of MAR will beincomplete without consideration of very ancient agesobtained by different authors (see above) for gabbroidsand OPG of Central Atlantic. The analysis of isotope�geochemical features of MAR basalts (Dosso et al.,1999) demonstrated that some segments in the axialridge zone are characterized by compositional hetero�geneity of mantle sources, which was presumablyformed prior to the opening of modern Atlantic andwas inherited from the lithosphere of the ancientGondwana supercontinent. The isotopic ages of majorstages of magmatic evolution of mantle sourcesbeneath MAR between 31° and 41°N estimated in thiswork fall within the time range of 100–300 Ma, whichis close to the age of gabbro reported in (Pilot et al.,1998). The reconstruction of possible mantle mag�matic sources beneath the MAR rift valley using com�parison of petrological–geochemical parameters ofresidual peridotites and associated basalts made it pos�sible to conclude that among mantle peridotites thatcompose the MAR axial zone north of the equatorpresumably present representatives of subcontinentallithospheric mantle, which did not participate in theformation of modern magmatic associations of rift val�ley (Silantyev, 2003). In the framework of this hypoth�esis, the fragments of continental or ancient oceaniclithosphere presumably exist beneath the MAR axialzone. In this case, ancient zircon could be derivedfrom gabbroids, which have no relation with modernoceanic lithosphere and were incorporated intoancient mantle protolith. The melting of this gabbroidmaterial with participation of high�temperaturehydrothermal fluid well explains the formation ofOPG with zircons of two age generations: xenogenic(ancient) and newly formed (young). Undoubtedly,prior to obtaining reliable data on ancient U–Pb agesof zircon from the oceanic core complexes of MOR,any geodynamic interpretations of these intriguingdatings will remain the object of discussion.

The comparison of the results of deep�water drill�ing conducted in different areas of the MAR crest zone(Shipboard…, 2003; Expedition…, 2003) with isotopedata on zircons extracted from OPG and recrystallizedgabbro of the oceanic core complexes provides theopportunity to estimate the production of graniticmelts during Hess�type crustal accretion. Drilling inthe MAR segment located north of the Ashadzehydrothermal field recovered section of typical oce�anic core complex consisting of the following rocks:troctolites (14%), olivine gabbro and normal gabbro(74%), diabases (10%), OPG (2%) (Shipboard…,2003). It is noteworthy that close estimates of relativeamount of OPG were obtained on the basis of extrap�olation of experimental data on degree of partial melt�ing of gabbros depending on temperature (Fig. 9).Since age of zircon from OPG and host gabbroids ofthe Ashadze field (MAR, 12°58′N) accounts for 0.96 ±

0.03 Ma, the similar amount of felsic rocks is presum�ably formed in the section of the oceanic crust of theslow�spreading ridges for 1–2 Ma of its existence. Theproportions of rocks that compose the oceanic corecomplexes make it possible to believe that OPG wereformed at the final stages of their magmatic evolution.Therefore, the same age data on zircons indicate thatexhumation of the oceanic core complexes of MARbegan no earlier than this time.

CONCLUSIONS

Presented data show that simultaneous combina�tion of two factors is required for OPG formation: hightemperature (820–850°С) at depths corresponding tothe boundary of lower crustal horizons and shallowmantle (1); penetration of marine hydrothermal fluidat this level (2). Such a scenario is consistent eitherwith conditions of stationary geothermal gradient nearMOR or, which seems to be more plausible, with pen�etration of hydration front into the area of the exist�ence of cooling magmatic chamber beneath the ridgeaxis. Thus, the formation of plagiogranites in theMOR oceanic core complexes presumably reflects thefundamental property of accretion of oceanic lithos�phere: practically simultaneous (at geological timescale) proceeding of exogenic (neptunic) and endoge�nous (plutonic) processes (Fig. 12).

1000

100

10

1

Ho LuTmTbEuCeLa

NdEr YbGyGdPr Sm

Roc

k/C

hon

drit

e

OPG, MAR, 15°44′NOPG, MAR, 15°30′NOPG, MAR, 12°58′NOPG, SWIR, borehole 735В

Gabbro, MAR, 15°44′NGabbro, MAR, 15°30′NGabbro, MAR, 12°58′N

Fig. 10. Chondrite�normalized REE distribution pattern(Sun and McDonough, 1989) for OPG and host normaland recrystallized gabbroids of OCC of the Mid�AtlanticRidge and Southwest Indian Ridge (for latter, data weretaken from (Maeda et al., 2002).

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SILANTYEV et al.

0.5133

0.5132

0.5131

0.5130

0.5129

0.51280.7060.7050.7040.7030.702

DMM

Gneissose gabbro

Trondhjemites of MAR

MORB

of northern MAR

143 N

d/14

4 Nd

in t

he

rock

87Sr/86Sr in the rock

Ashadze Logachev Trondhjemites of MAR

Gabbro

Trondhjemites

MAR, 15°30′NMAR, 15°44′NMAR, 5°N (Savel’eva et al., 2008)

Gabbro

0.5132

0.5128

0.5124

0.51200.7100.7080.7060.7040.702

DMM

Peridotites of MAR

MORB, North Atlantic

Northern MAR

143 N

d/14

4 Nd

in t

he

rock

87Sr/86Sr in the rock

seawatercontamination

Southern MARand Kergelen

plateau

MORB, South AtlanticBasalts, Kergelen plateau

Harzburgites, Ashadze

Dunite, AshadzeHarzburgites, LogachevDunite, Logachev

Fig. 11. Variations of 143Nd/144Nd versus 87Sr/86Sr in the plutonic rocks that compose OCC of the Mid�Atlantic Ridge.Data on isotopic composition of MORB in MAR of the northern hemisphere are given after (Dosso et al., 1991); basalts of theDiscovery and Shona rises, as well as Walves Ridge (South Atlantic) are given after (Douglas et al., 1999; Humphris and Thomp�son, 1983). Data on the basaltoids of the Kergelen plateau (Indian ocean) were taken from (Dosso and Murthy, 1980). The com�position of N�MORB⎯DMM mantle source was taken after (Workman and Hart, 2005).

Fig. 12. Main processes that determine composition of oceanic lithosphere in the MAR axial zone.Dashed line shows neptunic and plutonic processes responsible for OPG formation.

Neptunic processes

Plutonic processes

Submarine weathering

Oceanic metamorphism

Hydrothermalactivity

Hydrothermalanatexis

Partial melting

Magmatic

Mantle metasomatism

Subsolidus recrystallization

Determining parameters and agentsPlutonic processes Neptunic processes

Magmatic melts and fluidsTemperaturePressureDegree of meltingComposition of melt source

Seawater and its derivativesTemperatureW/R (water/rock mass ratio)PressureProtolith composition

interaction



The presence of OPG in the MOR core complexescan be used as marker of the highest temperaturedeeply rooted oceanic hydrothermal systems. A com�plete cycle of hydrothermal circulation, which deter�mined the juxtaposition of neptunic and plutonic pro�cesses, can be represented as following scheme(Fig. 13).

(1) Formation of hydrothermal fluid due to inter�action of seawater with host rocks; this stage is accom�panied by increase in temperature and salinity of fluidowing to hydration reaction.

(2) Interaction of high�temperature hydrothermalfluid with uncooled (or subjected to secondary heat�ing) gabbroid material. Formation of OPG.

(3) Rapid transportation of hydrothermal fluid tothe surface of oceanic floor and its mixing with seawa�ter in the discharge zone, with formation of ore chim�neys.

ACKNOWLEDGMENTS

We are grateful to E. V. Sharkov (IGEM RAS) forkindly given samples of OPG from the oceanic corecomplex of MAR at 5°N. We also thank Prof.J. Koepke (Institute of Mineralogy, Hannover Univer�sity, Germany) for cooperation, which made it possi�ble to obtain experimental data having a fundamentalsignificance for reconstructing the conditions of OPGformation. We also thank Yu.A. Kostitsyn (GEOKHIRAS) for discussion of different aspects of interpreta�tion of U–Pb dates on gabbroids and granitoids fromthe oceanic core complexes.

The work was supported by the Russian Founda�tion for Basic Research (project nos. 09�05�00008�a,09�05�00193a, RFBR�DFG 08�05�91952NNIO�a)and by Program of Presidium of RAS no. 17 “Funda�mental Problems of Oceanology: Physics, Geology,Biology, and Ecology” (theme “Mid�Atlantic Ridge:Features of Hydrothermal Interaction between Oceanand Lithosphere and Ore�Forming Processes”).

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Oceanic plagiogranites as a result of interaction between magmatic and hydrothermal systems in the slow-spreading mid-ocean ridges