A petrologic model for the constitution of the upper mantle and crust of the Koolau shield, Oahu, Hawaii, and Hawaiian magmatism

Earth and Planetary Science Letters, 62 (1983) 215-228 215 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

[3]

A petrologic model for the constitution of the upper mantle and crust of the Koolau shield, Oahu, Hawaii, and Hawaiian

magmatism

Gautam Sen * Department of Geoscienees, University of Texas at Dallas, Box 688, Richardson, TX 75080 (U.S.A.)

Received June 15, 1982 Revised version received October 26, 1982

A petrological model for the uppermost upper mantle and crust under the Koolau shield to a depth of about 60 km has been derived on the basis of petrology of the upper mantle and crustal xenoliths in nephelinites of the Honolulu Volcanic Series. Three main xenolith suites exist in the Koolau shield: dunites, spinel lherzolites, and garnet-bearing pyroxenites. On the basis of mineralogy, it is inferred that the dunites represent cumulates in shallow crustal tholeiitic magma chambers, the spinel lherzolites form a thick ( - 40 kin) layer in the upper mantle, and the garnet pyroxenite suite occurs as veins and stringers in the spinel Iherzolites at about 60 km depth.

The eruption sequence in a Hawaiian volcano, i.e., tholeiite ~ transitional basalt ~ alkali basalt, is generated by partial melting of a volatile-bearing garnet-lherzolite part of a lithospheric plate as it rides over a hot spot. If the tholeiite, transitional, and alkali basalts of Hawaiian volcanoes are generated at the same depth, then the observed sequence of lavas requires replenishment of the source area with volatiles and gradual decrease of the degree of partial melting with time. Post-erosional olivine nephelinites are produced from isotopically distinct, deeper source area, which may be the asthenosphere.

1. I n t r o d u c t i o n

I n t r a p l a t e m a g m a t i c processes tha t give rise to i s l and cha ins like Hawa i i are n o t well unde r s tood . Or ig in of the H a w a i i a n group of i s lands is gener- a l ly a t t r i bu t ed to passage of the Pacific pla te over a re la t ively fixed, a n o m a l o u s l y ho t me l t i ng zone, t e rmed a " h o t spo t" or a p lume , in the Ear th ' s m a n t l e [1]. Shaw (cited in D a l r y m p l e et al. [1]) p r o p o s e d an a l t e rna t ive hypothes i s by which the m e l t i n g occurs in the H a w a i i a n m a n t l e as a resul t of heat gene ra t i on due to shear ing of the l i tho- sphere over the as thenosphere . Mel t gene ra t i on by

University of Texas at Dallas, Geosciences Department Contri- bution No. 428.

* Present address: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90024, U.S.A.

shear hea t ing or viscous d i s s ipa t ion is an a t t rac t ive hypothes i s i n a s m u c h as it is capab le of exp la in ing ce r ta in a g e - v o l u m e - e r u p t i o n rate re la t ionsh ips observed in i nd iv idua l vo lcanoes of the H a w a i i a n Is lands . However , the shea r -mel t ing hypothes i s does no t exp la in why the me l t i ng should occur at res t r ic ted spots a n d no t wherever the pla tes are

moving . Shaw a n d Jackson [2] suggested that v iscous d iss ipa t ive hea t ing causes par t ia l me l t i ng in the a s thenosphe re which p roduces dense residues tha t s ink th rough the m a n t l e a n d create a " g r a v i t a t i o n a l ancho r " which fixes the a n o m a l o u s m e l t i ng zone in the man t l e . O ' H a r a [3] first p o i n t e d ou t that it is imposs ib le for the res idual per ido t i te to be denser t h a n rela t ively Fe- r i ch source peri- dot i te , which was s u b s e q u e n t l y c o n f i r m e d b y den- si ty m e a s u r e m e n t s on dep le ted a n d less-deple ted per ido t i tes [4]. Also, the repor ted Shaw-Jackson

0012-821X/83/0000-0000/$03.00 © 1983 Elsevier Scientific Publishing Company

216

densities for source and residual mantle rocks were measured on garnet pyroxenite and dunite suite rocks, respectively. A major finding of the present study is that both dunite and garnet pyroxenite suite rocks likely represent crystal segregations from basalt magmas, which negates the very basis of the "gravitational anchor" hypothesis.

A third hypothesis suggests that "propagat ion of a tensional fracture caused by a thermal high or incipient upwelling that reflects a concentration of heat-producing radioactive elements in the mantle gave rise to the Hawaiian chain" [1, p. 35]. According to this hypothesis, pressure-release melting occurs in the asthenospheric material which wells up into these fractures. McDougall (cited in Dalrymple et al. [1]), who was a main proponent of this hypothesis, further suggested that the anomalous heat source moves in an opposite direc- tion to that of the Pacific plate. In general, these various hypotheses point toward the presence of an anomalously hot area in the upper mantle beneath Hawaii, in spite of their differences in the magma-producing mechanism.

The reason(s) for the existence of such a hot spot remains a speculative subject. A better under- standing of such anomalous behavior of the

Hawaiian upper mantle clearly requires a knowl- edge of the constitution of the upper mantle and associated magmatic processes.

The Koolau shield forms the eastern part of Oahu (Fig. 1), and is composed of a thick tholeiitic shield (Koolau series: 1.8-2.7 m.y.), capped by minor nephelinitic lavas of the Honolulu Volcanic Series (HVS: 0.3-0.6 m.y. [5]). A large number of unaltered xenoliths of dunite, gabbro, plagioclase spinel lherzolite, spinel lherzolite, garnet- and spinel-bearing pyroxenite, websterite, and lherzolite occur in the HVS olivine nephelinites [6--8]; their detailed examination provides an opportun- ity to document the lithologic variations in uppermost upper mantle underneath a volcanic island formed by hot spot activity. Petrologic observations of a large number of these ultramafic xenoliths (details are given in Sen [8]) form the basis for a petrologic model of the constitution of the crust and uppermost upper mantle and of the possible petrologic processes related to the generation of Hawaiian lavas.

2 . T h e x e n o U t h s a n d t h e i r d i s t r i b u t i o n

Petrography, mineral chemistry and geochemistry permit classification of most of the xenoliths

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i . "" . . . . """..~. Garnet- beorir~j I ~ d o n t ~ .

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4'~...~.~..~..<~ 7 , , . . . . . ! ~. " ~ . , . J / (i )~1 ~"~ ..~..';~'Y " " ~ " , f ,,Ko,~i _ " - - - . . . . - / . / r-.

/ \. _ ,l".k ~ ,<ooo ' t ..... 2 ~ - 2 o ' -

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Fig. I. Map showing Koolau caldera and areal distribution of the xenolilh types on the Koolau shield. Location of some of the xenolith-bearing vents, and the approximate outline of the Koolau caldera are shown.

into three major suites [6-8]: dunite, spinel lherzolite and pyroxenite suites. The remaining xenoliths, including plagioclase + spinel lherzolite, a phlogopite-rich orthopyroxenite (15 modal percent phlogopite), an olivine-gabbro, and an amphibole-harzburgite (35% amphibole) have been grouped simply as "others" [8]. Of the various types, spinel lherzolite xenoliths are most abundant, and dunite xenoliths are next.

The various xenolith types show restricted distribution patterns across the Koolau shield with dunites occurring exclusively in olivine nephelinite vents in the vicinity of the Koolau crater, spinel lherzolites in the intermediate areas, and a mixture of garnet-bearing pyroxenites (Fig. 1) and spinel lherzolites in the outermost parts of the Koolau shield ([7,8]; Fig. 1). The distribution pattern shown in Fig. l is generally similar to that of Jackson and Wright [7, pp. 413-415], except that these workers had indicated the presence of garnet-bearing xenoliths in the Kaau vent, which were not observed in my detailed microscopic study of the xenoliths belonging to the Jackson collection (the Smithsonian Institution) or the Presnall collection (the University of Texas at Dallas). Garnet-bearing xenoliths seem to be restricted to the Salt Lake group of vents-- including Salt Lake, Makalopa, and Aliamanu. Also, the dunite suite xenoliths were not found to occur in the Salt Lake group of vents (contrast fig. 5 of Jackson and Wright [7]). The xenolith-distribution contours of Jackson and Wright (their fig. 3) were based mostly on their field data, whereas Fig. 1 of the present study is based on detailed microscopic and microprobe examination of a large number of xenoliths.

The variation in xenolith types across the Koolau shield is paralleled by a variation in size of the xenoliths, the xenoliths in the Koolau caldera area being the smallest (largest xenolith is 3 cm in diameter) and the Salt Lake (outer shield) xenoliths being the largest (up to 20 cm in diameter). The dunite xenoliths are more friable than lherzolite or pyroxenite, which may have been a chief contributing factor to the observed size variation of xenoliths. Another possibility is that the ascent rates of the xenoliths-bearing nephelinites near the center of the Koolau shield were different and

217

slower than those which erupted at the Salt Lake crater area. With simple assumptions of spherical shapes of the xenoliths and Newtonian viscosity of the enclosing nephelinite melt, Stoke's law can be used to estimate minimum ascent rates of the magma: v = DZ(A#)g/181~a . Using O . . . . llth = 3.33 g cm -3, pmeJt = 2.8 g cm -3, and /~a (apparent viscosity of nephelinite)= 350 poises, the minimum ascent rates (v) of nephelinite magma containing xenoliths of 3 cm (Koolau caldera area dunite xenoliths) and 20 cm (Salt Lake pyroxenite and lherzolite xenoliths) diameters (D) are calculated to be 0.7 cm s-~ and 31 cm s - i , respectively. A magma containing a few percent crystals by volume may rheologically behave more like a Bingham plastic, and therefore, the assumption of Newtonian viscosity of nephelinite magma may be invalid. In the non-Newtonian case in which Rey- nold's number is > 0.1, following Spera's [9] suggestion, the magma ascent rate:

v = 0 . 3 4 4 ~ 1 k ~ / \ 2 4Aog

where K is a dimensionless constant equal to about 5.0 and o 0 is the yield strength whose measured values fall in the range 102--103 dyne cm -2 [9]. Using o 0 = 1 0 2 dyne cm -2, the minimum nephelinite magma ascent rates are found to be 1.5 cm s-~ and for dunite xenoliths in the vicinity of Koolau caldera and 25 cm s -~ for pyroxenite xenoliths of the outer shield. From the above estimates it appears that the nephelinites took between about 50 and 270 hours (for o 0 = l 0 2 and 103 dyne cm -2) to bring the pyroxenite suite xenoliths to the surface from 60 km depth (where the pyroxenites appear to have last equilibrated in the mantle, as discussed later).

3. Petrology and petrogenesis of xenoliths

Dunite suite. Xenoliths belonging to this suite consist predominantly of olivine with trace amounts of spinel. Minor orthopyroxene, clinopyroxene, and plagioclase occasionally occur interstitially. Most dunites have olivines in the compositional range F o 8 2 _ 8 7 , with the exception of a few in the range F o 8 7 _ 9 0 (Fig. 2). Fig. 2 also indicates that

218

15 t A L K A L I BASALTS >10 .~,... Fo 35

c0, [:i!:lr, ~ P , , ,

g30-] THOLEI IT IC BASALTS ¢20] t0] a ~ , , --, , f ~ 3 p , , , , ~ , , , , 7

57 59 6t 63 65 67 69 7! 73 75 77 79 8t 83 85 87 89 91 93 M o l e O/o Fo

ITITIll White (1966) 6 ~o LHERZOLITE [~1 t5 SUITE

[ ] Shaw & Jackson (1973) 2

[ ] Leeman e/o/ (t980) 17 Ot I , ~ , I I 1

[ ] Basalt volcanism study o>, ~J PYROXENITE pr°ject ('981)4° , ~) i ~ ~ ] ~ SU]TE

[ ] Present study 11~" 0 , J , i r q - i q

t I D ~ h NITE

r i r J I , 7 79 8| 83 85 87 89 9f 93

M o l e % FO

Fig. 2. Composition of olivines in Hawaiian xenoliths and lavas.

the compositions of the dunite olivines and olivine phenocrysts of the Hawaiian lavas overlap. The spinels are Fe-rich chromites, which are compositionally akin to the spinels that occur as inclusions in olivine phenocrysts of the Hawaiian tholeiitic lavas (Fig. 3). Textures of these dunites range from porphyroclast ic to allotriomorphic granular. Olivine grains in the dunites often show well de- veloped subgrain boundaries (deformation twin lamallae).

On the basis of bulk-rock chemical composition and metamorphic texture of the dunites, Jackson and Wright concluded that the dunites represent mantle residues from partial melting. Extensive partial melting of mantle lherzolite and extraction of melts may produce a dunite residue which must have more refractory (higher Mg/Fe ) olivine than that of the source lherzolite and the olivine which crystallizes from the melt. Fig. 2 shows that olivine in the dunite suite xenoliths generally overlap in composition (i.e., in M g / ( M g + Fe)) with the

100

90

80

7O

6o

O X 40 O O 30

I 1 I I |

• 1.3unile-Suite- 1 • Spinel Lherzolite Suite u Pyroxenite Suite

HAWAIIAN - THOLEIITES

- d *

2 0 - • -

3.0 0.8 0.6 0.4 0.2 0 M g / ( M g + Fe 2 + )

Fig. 3. Composition of spinels in Hawaiian xenoliths and tholeiitic lavas [40]. Unfilled and filled squares--pyroxenite suite spinel with and without garnet rim, respectively.

olivine phenocrysts in the Hawaiian lavas. There- fore, they cannot represent residua genetically related to the Hawaiian lavas. I interpret the dunites to be recrystallized cumulates related to the Koolau series tholeiites primarily because of their mineral chemical similarity, especially spinel compositions, with the latter (Figs. 2, 3 [8,10]).

Spinel lherzolite suite. The spinel lherzolite suite rocks consist of a relatively Mg-rich olivine, orthopyroxene, chrome-diopside, and spinel (Figs. 2-4). A wide variation in mode (O148_95 , 0 p x 3 _ 4 3 ,

Cpx 2- 33, and Spt r_ l a) exists in these xenoliths [8]. Overall texture ranges from porphyroclastic to allotriomorphic granular. Deformation twin lamellae are common in olivine and orthopyroxene.

The general refractory character of the spinel

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Ci /A\ / \

/ \ / \

/ \

,f 2 Mg F.

o A 'A /

/

A I / o A A

D I V

/

Fig. 4. Composition of clinopyroxenes and garnet in Hawaiian xenoliths. Filled symbols--pyroxeni te suite, unfilled symbols- -spinel lherzolite suite. Location of Salt Lake, Kaau, and Pali vents are given in Fig. 1.

lherzolite suite is indicated by its constituent mineral phases, especially olivine (Fo87_93 , with a single olivine-Fo85.6). A partial overlap in olivine composition of the spinel lherzolite, dunite, and pyroxenite suites exists (Fig. 2).

Spinels of this suite are brown in color, in contrast to the black spinels in dunites and green spinel in the pyroxenite suite. Compositionally they are generally higher in Mg/(Mg + Fe) than spinels of the pyroxenite and dunite suites (Fig. 3). Spinel lherzolite spinels are also somewhat higher in Cr / (Cr + AI) than those of the pyroxenite suite, and noticeably lower than that of the dunite suite. Pyroxenes of the spinel lherzolite suite are also more refractory than that of the pyroxenite suite (Fig. 4).

Any parent-daughter-residue-type relationship between the spinel lherzolite suite xenoliths and the Hawaiian lavas is unlikely to exist, because of the following reasons:

(1) Seismic studies (e.g. [11]) indicate that the Hawaiian lavas probably originate at depths >~ 60 km, which is likely to be in the garnet lherzolite stability field near the garnet/spinel lherzolite transition zone.

(2) Nd isotopic studies (e.g. [12]) suggest that the source mantle rock of the Hawaiian lavas should have a LREE-depleted (chondrite-normal- ized) pattern. All, except one, spinel lherzolite xenoliths show LREE-enriched patterns (e.g. Frey [13]).

(3) Spinel lherzolite xenoliths of Oahu have

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3H e /4He ratios ( - 1.3 × 10 5) that are similar to that of the mid-ocean ridge basalts (MORB), but are generally lower than the 3He /4He ratios of most basalts and phenocrysts in lavas of Hawaii (>~ 1.7 × 10 5; e.g. [14]). The observed Mg/ (Mg + Fe) ratios of the mineral phases of the spinel lherzolite xenoliths indicate that, if they are genetically related to MORB as suggested by the rare gas isotopic data, then they must represent mantle residues from which various amounts of MORB were extracted.

Pyroxenite suite. The pyroxenite suite rocks are characterized by the following assemblage: clinopyroxene + Cr-poor spinel _+ olivine + garnet + orthopyroxene _+ phlogopite +_ amphibole. These pyroxenites typically form veinlets in spinel lherzolite in composite xenoliths (e.g. [6,7]). An extremely wide variation in modal proportions of the constituent minerals is exhibited by the pyroxenite suite rocks (O170Cpx25(Opx + Gar + Sp)5-O15sOpxlsCpx24Gar2Spl-Cpx100 + S p t r ) . Thus, based on modal proportion of the constituent phases, pyroxenite suite rocks range from lherzolite, through websterite and wehrlite, to clinopyroxenite. Clinopyroxene of the pyroxenite suite rocks is dark gray colored, in contrast to the deep green color of the spinel lherzolite clinopyroxene. Also, orthopyroxene is generally not an abundant phase in the pyroxenite suite rocks. Green-colored spinel can make up as much as 8% of a pyroxenite suite rock, although it usually occurs in trace amounts. Garnet occurs more com- monly as reaction rims around spinel grain, and to a lesser extent, as exsolution blebs in clinopyroxene. Therefore, it appears to be a secondary phase which formed only during subsolidus recrystallization of a primary spinel-bearing pyroxenite suite rock. Phlogopite and amphibole are very common accessory minerals in the pyroxenite suite rocks, in contrast to their relatively rare occurrence in the spinel lherzolite suite rocks. The overall texture of the rocks of this suite is typically allotriomorphic granular. Evidence of deformation, like deformation lamallae in olivine, is less conspicuous in the pyroxenite suite rocks. Porphyroclastic texture, which is so common in the spinel lherzolites, is typically absent in the rocks of the pyroxenite suite.

The constituent mineral phases of these pyroxenite suite rocks are relatively poorer in Mg/ (Mg + Fe) than that of the spinel lherzolite suite but overlap that of the dunite suite and phenocrysts in Hawaiian lavas (Figs. 2, 3). C r / (Cr + AI) ratio is generally lower in (a) the pyroxenite suite spinels than that of the other xenolith suites (Fig. 3), and (b) the pyroxenite suite clinopyroxenes (0.002- 0.052) than the clinopyroxenes of the spinel lherzolite (0.04-0.14) and dunite (0.06-0.17) suites.

The pyroxenite suite has been variously interpreted by previous workers as high-pressure liquids, crystal accumulates, parental mantle, and residua from partial melting (see Frey [13] for relevant references). Regardless of whether the Hawaiian lavas are primary or differentiated from primitive melts, the Mg/ (Mg + Fe) ratio of olivine phenocrysts in them would have to be less than that of the source or residual mantle rock. Note in Fig. 2 that (a) the most magnesian olivines of the Hawaiian tholeiites (Fo89_90) and of the alkali basalts ( F o 8 7 ) a r e more magnesian than that of the pyroxenite suite rocks (Fo86_ 87), and (b) the dunite suite xenoliths, which have been interpreted to be crystal segregations from the Koolau tholeiitic

2000

1500

21000 o L

t~ E i~ 500

- - - - 1 ~ - T - - T - - 1 - - T - - - -

CMAS Z,~erzol/?e

/ /

0O ... . . . ~ ~ L L _ I__ t0 20 30 40 50 60

P r e s s u r e ( K b ) Fig. 5. Pressure, temperature estimations of equilibration of the Hawaiian xenoliths examined. Dots--spinel lherzolite suite, tr iangles--pyroxenite suite. C & R - - C l a r k and Ringwood oceanic geotherm. The CMAS lherzolite solidus is after Pres- nail [41]. PL, SL, and GL represent stability fields of plagioclase-, spinel-, and garnet-lherzolite, respectively.

magmas [10], also have more magnesian olivine (Fo89_90). Thus, it is clear that the pyroxenite suite rocks cannot represent source or residua genetically related to the Hawaiian magmas (see also Sen and Presnall [15]). It is considered unlikely that the pyroxenite suite rocks with widely different modes but very similar mineral chemistry represent liquid compositions. Origin of this suite as crystal accumulates from magmas is strongly favored. Such a mode of origin of the pyroxenite suite was also suggested by Frey [13] on geochemi- cal considerations. Frey further pointed out the similarity of 87Sr/86Sr ratios of the pyroxenite suite and the Honolulu volcanic series lavas, which are different from the Koolau lavas [5], which suggests that the pyroxenote suite rocks represent crystal segregations from alkaline basaltic magmas similar to the HVS lavas. This conclusion is further supported by recent Nd isotopic data (e.g. [161).

4. Pressure and temperature of equilibration in the mantle

In general, the temperatures were estimated from the compositions of coexisting pyroxenes using Wells' [18] equation (4). Pressure estimates for the garnet-bearing xenoliths and the spinel lherzolites are based upon the (A1203)opx isopleths of Perkins and Newton [19] and Dixon [20], respectively, in the CaO(C)-MgO(M)-A1203(A)- SiO2(S ) system. Uncertainty in the P estimates of the spinel lherzolites is large and is critically de- pendent on T estimates, because of a very shallow angle of intersection of the pyroxene-isotherms and the (A1203)op x isopleths in the CMAS spinel lherzolite field [32]. Hence some of the very high P estimates (> 20 kbar) obtained in this study for some spinel lherzolites may be overestimations.

The partial overlap in P, T of the spinel lherzolites and the garnet-bearing pyroxenite suite mem- bers (Fig. 5) is interesting and not surprising, inasmuch as veinlets of the garnet-bearing pyroxenite suite do occur in spinel lherzolite. Distinctly different bulk compositions of the two suites, as indicated by a higher Cr / (Cr + A1)~pin~ I in the spinel lherzolite suite than pyroxenite suite (Fig. 3),

221

evidently have stabilized both rock types at the same P, T [21]. The significance of estimates of P and T from spinel- and garnet-bearing.xenoliths has been a matter of some debate. Boyd [22] obtained two dense clusters of P, T points, as estimated from existing pyroxene compositions, for the kimberlitic garnet lherzolite xenoliths in northern Lesotho, and drew a "kinked" geotherm through those clusters, roughly paralleling the con- tinental shield geotherm of Clark and Ringwood (cited in Boyd [22]). Irving [23] argued that the xenoliths, brought up to the surface by the including lavas, probably undergo P, T perturbations associated with tectonic activity and magmatism. Hence they may not describe a normal, "steady- state" geotherm. The Koolau mantle xenoliths seem to represent equilibrium assemblages which equilibrated under mantle P, T conditions, inasmuch as they contain mineral assemblages that are stable only in the mantle, and also because their constituent mineral phases exhibit what appears to be equilibrium partitioning of different elements between them [8]. Inasmuch as these xenoliths were brought up to the surface at different times over a period of at least 0.3 m.y. by different nephelinitic magmas [5], each of them probably records certain P, T conditions corresponding to its time of resi- dence in the mantle prior to the formation of nephelinitic magmas. Thus, the P, T points re- corded by the Koolau mantle xenoliths possibly represent a tight array of "geotherms" which existed between the time of last generation of the Koolau tholeiites and the first generation of the HVS nephelinites. In other words, I suggest that the estimated P, T conditions represent a period during which the mantle beneath the Koolau shield was cooling off.

5. Constitution of the uppermost upper mantle and crust under the Koolau shield

The model presented here (Fig. 6) for the crust and the uppermost upper mantle under the Koolau shield is based on geophysical data and inferences about the origin of the xenoliths. The presence of a thickened oceanic crust (10-20 km), made up mostly of tholeiitic basalt, gabbro and plagioclase-

222

SALT LAKE KOOLAU CALDERA ~j ~ ~ ~ S W CRATER s ~

/ '.-a~ ~

t5 L~

/

Fig. 6, Schematic model of the crust and uppermost upper mantle under the Koolau shield (see text for explanation).

bearing pyroxenite, under the Koolau shield is suggested by gravity and seismic data [24,25]. Fig. 6 also shows the presence of a large magma chamber, with its bot tom at around 15 km depth. The number, shape and size of the magma chambers are unknown, and the two magma chambers shown in Fig. 6 are schematic. The dunite suite xenoliths are suggested to have formed in such magma chamber(s). It may be pointed out that Roedder 's [26] study of the primary CO 2 fluid inclusions in the dunite olivines suggested a maximum pressure of about 5 kbar (15 km depth) for the formation of these dunites, which led me to the inference concerning existence of magma chamber(s) ar around this depth.

The high-density Koolau "plug" (6 km wide) that was found by geophysical means [24,25] probably represents the deformed (cumulate) dunites, underlain in turn by relatively Fe-rich spinel pyroxenites, which may represent "frozen" magmas along the vertical dimension of the Koolau con- duit (Fig. 6).

The thickened crust under Koolau is probably underlain by a layer of plagioclase lherzolite. Oc- currence of plagioclase lherzolite xenoliths in the Pali No. 2 vent of Oahu and common presence of

plagioclase lherzolite in ocean-floor dredge sam- ples [27] suggest the presence of a plagioclase lherzolite layer in the oceanic upper mantle. Ex- perimental studies on simple and complex systems indicate that such a lithologic type should be stable to a maximum depth of about 30 km in the oceanic upper mantle (e.g. [28]).

Beneath the plagioclase lherzolite layer is probably a transition zone, madeup of both plagioclase and spinel-bearing lherzolite, below which is a very thich layer of spinel lherzolite. The presence of such a thick layer of spinel lherzolite (Fig. 6) is suggested by the pressure temperature stability field of spinel lherzolite in phase equilibria experi- ments on simple and complex systems (e.g. [28]).

Garnet-bearing pyroxenite suite rocks are unlikely to form any layer of uniform thickness in the upper mantle beneath Oahu but are more likely to form veinlets and stringers into the spinel lherzolites, as observed in the composite xenoliths studied, and also by analogy with the occurrence of pyroxenite dikes and veinlets in spinel Iherzo- lites of Alpine massifs. The depth at which the pyroxenite suite rocks occur (Fig. 5) has been estimated from mineralogic geobarometry (see above).

Only reasonable guesses can be made about the lithologic composition of the zones that underlie the entire sequence described above, inasmuch as these zones are not represented by xenoliths examined. It seems possible, however, that xenoliths from deeper zones are present but have not been collected and examined, or that they have been significantly altered by magmatic processes such that their "depleted" or "undeple ted" mineral chemistry cannot be identified. In fact, the same problem was encountered by Jackson and Wright [13, p. 424], who stated that " N o satisfactory mantle materials from which tholeiite could be generated are found in the xenoliths in this a rea . . . " . Phase equilibrium studies, in general, suggest that at depths below 60-70 km, garnet lherzolite should be stable [28]. This material must also be the source for the Koolau series tholeiites, if the tholeiites come from a depth slightly below 60 km as suggested by seismic studies on Kilauea (e.g. [t 1]).

Assuming that the tholeiitic shield is underlain by a 5-km-thick normal oceanic crust, Jackson and

Wright [13] calculated a total volume of the shield-plus-normal oceanic crust as 45 × 103 km 3. So, deducting the volume of a 5-km-thick normal oceanic crust, one obtains a volume of about 35 × 103 km 3 for the Koolau tholeiite shield. Even if we ignore the olivine-fractionation which these tholeiites may have undergone prior to eruption [29], it appears that a large volume ( - 1.5 x l0 s km 3) of garent lherzolite must have been depleted of tholeiitic components, provided that these tholeiites are produced by approximately 30% partial melting [28]. Therefore it is very likely that enor- mous masses of depleted lherzolite and harzburgite, and possibly dunites, exist above undepleted garnet lherzolite at depths greater than 60 km beneath the Koolau shield.

Four significant differences exist between the model presented above and a widely quoted model of Jackson and Wright [13]:

(1) In the latter model, dunite and harzburgite, which were inferred by Jackson and Wright to be residua from partial melting of lherzolite, form a thick layer between 20 and 60 km depth. It was indicated earlier that (a) the dunite xenoliths are too Fe-rich to be residua, (b) harzburgite xenoliths are absent from the Koolau shield, (c) spinel lherzolite is the most abundant xenolith type, which comes from about 30-60 km depth range, and (d) Koolau magmas originated at a depth below 60 km. All these points argue against the Jackson- Wright (JW) model and favor the model presented here. In fact, the following interesting statement of Jackson and Wright [13, p. 426] goes against the suggestion of the presence of any thick dunite/harzburgite layer in the 20-60 km depth range: "we do not find abundant harzburgite xenoliths, nor do we find the (residual?) dunite xenoliths to contain the highly magnesian olivine characteristic of (spinel) lherzolite.

(2) A plagioclase lherzolite layer, shown in the present model, is absent from the JW model.

(3) Garnet pyroxenite and lherzolite, inferred by Jackson and Wright to be the source for the Honolulu series magmas, forms a layer below 100 km depth in the JW model. The following argu- ments (presented in detail earlier) certainly dis- favor the presence of such a layer in the Koolau mantle: (a) garnet pyroxenite and lherzolite (py-

223

roxenite suite) cannot represent the source for the Honolulu series magmas, (b) the pyroxenite suite forms veinlets and dykelets in spinel lherzolite in composite xenoliths, and (c) pyroxene geobarometry suggest the formation of the pyroxenite suite at around 60 km depth.

(4) Garnet lherzolite forms a layer between 60 and 100 km depths in the JW model. Although this interpretation cannot be refuted, it seems likely that rocks at these depths are more refractory, caused by fusion and extraction of magmas. Accordingly, I favor the presence of a garnet- harzburgite/dunite layer at these depths.

6. Comparison with models of "normal" oceanic mantle

Most petrologic models of the "normal" oceanic uppermost upper mantle ("normal" meaning away from mid-ocean ridges and "hot spots") are based on observations on ophiolite and alpine-type complexes, and on ultramafic rocks dredged from the ocean floor. The majority of these models includes a refractory harzburgite layer (e.g. [28]) in place of the plagioclase lherzolite layer shown in Fig. 6. Such models are based on studies of ophiolite and alpine complexes, in which harzburgite is the predominant lithic type in the ultramafic tectonite part. Although this material has been recovered from the ocean basins, Bonatti and Hamlyn [27] indicated in a recent review paper that plagioclase lherzolite comprises "a significant component of dredge recoveries of ultramafic rocks", and accordingly, they favored a model with a plagioclase lherzolite layer at the 10-25 km depth range in the oceanic upper mantle away from mid-ocean ridges. This model is very similar to the one presented above, based on the ultramafic xenoliths of Oahu. Such similarity may suggest that the uppermost mantle layers, between 10 and 60 km depth, underneath the Koolau shield were unaffected by the Hawaiian magmatic processes. If, on the other hand, models advocat- ing the presence of a thick harzburgite layer in the "normal" oceanic mantle are correct, then possible replenishment of the harzburgite layer with basaltic

224

components, followed by recrystallization, may have formed plagioclase lherzolite beneath Oahu.

7. Hawaiian magmatism: a discussion

A common sequence of eruption of different magma types occurs in each of the Hawaiian volcanoes; rapid eruption of shield-building tholeiites over a longer period (>/1 m.y.) is followed by a short period of eruption of transitional through alkali basalts from the same vent (e.g. [1]); this sequence is followed by a 2-m.y. period of erosion which is followed in turn by eruption of nephelinites through vents that are scattered all across the shield. In the Koolau shield, however, the transitional and alkalic stage did not occur, and the tholeiites were followed by the post-erosional nephelinites [7].

Phase equilibria studies suggest, in general, that nephilinites probably originate by about 5% melting of garnet lherzolite at much greater depths than tholeiites or alkali basalts [30]. It should also be pointed out that toleiites and alkali basalt lavas all have similar Pb, Sr, and Nd isotopic compositions, whereas the post-erosional nephelinites are isotopically distinct [5,12,30-33]. From the infor- mation above it may be surmised that the pre-erosional lavas of variable composition had a common mantle source region (lithosphere?) that is distinct from and shallower than that (asthenosphere?) of the post-erosional nephelinites. Alter- natively, it is possible that there was a metasomatic event with infiltration of melt from the surround- ing mantle with a different isotopic signature prior to the generation of the nephelinites. The time gap of 2 m.y. between the pre-erosional and post-erosional lavas and the eruption pattern of nephelinites (i.e., through scattered vents) also tend to suggest that the generation of the nephelinites may not be directly related to the generation of pre-erosional lavas [7]. The consistent eruption pattern in the Hawaiian islands is a rather marka- ble feature, and any petrologic model must explain this consistency.

Roedder [26] and Muenow et aL [34] have found significant amounts of CO 2 in glass-vapor inclusions in the olivines of dunites and in Hawaiian

tholeiites. Amphibole and phlogopite have been found to occur in several xenoliths of the present study. Therefore, it appears that the melting processes in the Hawaiian mantle involve the volatiles H20 and CO 2.

It appears that in order to generate tholeiitic, transitional, and alkalic basalts in that sequence from the same source mantle, it is necessary either to (a) migrate the source melting zone with depth and progressively decrease the amount of partial fusion, in which case the Di-OI-Opx pseudoinvariant point in a natural system would move with time across the Di-Ol boundary into the Ne-O1-Di part of the basalt tetrahedron [28], or (b) incorpo- rate CO 2, Na20, K20 into the source mantle (metasomatism [33]) and progressively decrease the amount of partial fusion by dropping the temperature, thus similarly expanding the orthopyroxene volume to cause progressive movement of the pseudoinvariant zone from tholeiite into the alkali basalt volume, but all at nearly the same pressure (depth). These two cases are, of course, end-member possibilities. The first model has been proposed, amongst many workers, by Presnall et al. [35] whereas the second one is discussed here. A minimum pressure of about 20 kbar ( - 60 km) is required by both models, each of which is capable of explaining (a) the gradual increase of LREE components from tholeiites to alkali basalts in a single Hawaiian volcano, and (b) and the higher N a2 0 and K20 contents in the alkali basalts (e.g. [33]). Transient reversals in overall chemical trends in these lavas can be explained by subsequent fractional crystallization and magma mixing processes, as suggested, for example by Presnall et al. [35].

8. Dynamic model

The model proposed here is dynamic in the sense that it attempts to explain generation and extraction of the pre-erosional magmas (i.e., tholeiites, alkali basalts) in the lithosphere as a function of interaction of a changing geotherm and mantle peridotite solidus, which would occur as the lithospheric plate approaches, rides over a relatively fixed hot spot and then moves away from it.

STAGE: A T (°C)

0 t200

B C

P,Kb~ i \ \ s ' ' 2 I :, , , I ho, soot "P* ~'Sz

/ , , , , ' ) T 9o! j f20 A S T H E N O S P H E R E /J] O tl~" -

~500°c . . . . " ,' T

t501 ~. 180~- / S ~ ....... P

2tO LI 1400"6 . . . . . 0 T

240 L E ~ Oceomc crust ~ Gornel IInerzolite

Piegloclose IherzoMe ~ Harzburgile/dunite Spinel Iherzollte ~ ] Partial rnelhng zone

Fig. 7. Schematic model showing possible meh generation phenomenon in the Hawaiian lithosphere associated with hot spot activity. Detailed explanation of this model is given in the text. Mature oceanic geotherm X Y rises to X Y 2 falls back to XY s as the lithosphere approaches, rides over, and then moves away from a hot spot (see text for explanation). SPI = lherzolite solidus in presence of C O 2 + H 2 0 vapor. SS2 = vapor-free lherzolite solidus. Stage C: S S I = lherzolite solidus, S~S 3 =

harzburgite or dunite solidus.

Fig. 7 schematically shows the main features of the model, in which the presence of a "hot spot" (and its adiabat) is assumed. As the rigid lithosphere moves over the hot spot, the heat flux at the basal part of the lithosphere is increased. The heat flux at the interior of the lithosphere, however, remains virtually unchanged since heat transport into the lithosphere must occur through con- duction, which is an extremely slow process. Thus, the geotherm assumes a "kinked" nature (e.g. [36]). The changing nature of the geotherm with accom- panied melting phenomenon is represented by three contrasting stages, namely, A, B, and C, in Fig. 7. Stage A represents mature oceanic lithosphere in which the geotherm intersects the solidus of CO 2 + H20-bearing mantle peridotite to produce a partial melt zone at around 100 km depth ( = 33 kbar). Stage B shows what happens when the lithosphere rides over a hot spot: the normal oceanic geotherm X Y now sweeps into a position X Y 2

owing to the additional heat flux provided by the hot spot. C represents the stage when the litho-

225

spheric plate cools off as it moves away from the hot spot: the geotherm gradually falls back to XY~ position which will be caused by a thermal lag due to sluggish rates of diffusion and advection. It should be pointed out that the change from stage A through B to C must be gradual.

Let us examine possible melting phenomenon associated with hot spot activity as illustrated above. Melting can occur in peridotite of the lithospheric plate only when its solidus is intersected by the geotherm. As suggested earlier, melting probably occurs in the Hawaiian lithosphere in the presence of CO 2 and H20. Two end-member cases, in which melting of peridotite occurs in the presence of CO 2 and H20, will be considered: in the first, melting occurs in a lithosphere in which the source is not replenished with volatiles, in the other, the source is buffered with respect to volatiles from adjacent parts of the lithosphere by metasomatism.

Case h The geotherm rises toward X Y 2 and as it barely touches the hypothetical source lithospheric peridotite solidus (coexisting with vapor containing H20 and CO2), some melting occurs as a particular area of the lithospheric plate nears the hot spot. This first melt and subsequent minor melts, which are produced as the geotherm keeps rising, will preferentially absorb CO 2 and thus enrich the volatile phase in H20 (e.g. [37]). Judg- ing from the partitioning of LREE into early liquids and from phase equilibrium, the earliest melts are likely to be alkalic in character (e.g. [38]) and should erupt earliest in the eruption sequence of the Hawaiian lavas. The reason that they are not found may be that either (a) these magmas, because of minor volumes or the nonavailability of permeable channels, cannot erupt on the surface, or (b) they were erupted but are completely covered by the later voluminous tholeiites. Only very recently, alkali basalt lavas have been reported to erupt very early in the Loihi Seamount, which is the youngest volcano in the Hawaiian Emperor chain [39]. As the amount of melting increases, the REE pattern should flatten and the melt becomes tholeiitic in composition. The Hawaiian tholeiitic melts most likely were derived from parental picritic magma by olivine-fractionation [29]. Pro- duction of picritic magma would require a large

226

amount of partial fusion ( - 3 0 - 4 0 % , [28]). It is inferred that such extensive partial fusion of the upper mantle lherzolite in Hawaii occurs when stage B is reached (Fig. 7), i.e., geotherm (XY2) makes a large intercept with the solidus SS2.

Large-scale production of picritic melts will slowly deplete the source mantle entirely of volatiles, since no volatile replenishment occurs (as assumed in this case). Total depletion of volatiles, and possible disappearance of clinopyroxene from the residuum, will cause the peridotite solidus to rise to very high temperatures ( S S 3 in Fig. 7), whereas the geotherm cannot rise any higher than X Y 2 as the lithospheric plate rides over the hot spot. Thus, melting stops completely after the production of the picritic melts.

This case would be exemplified by the Koolau shield, where no alkali basaltic cap is found after the eruption of tholeiites [7]. Nephelinites did erupt on the Koolau shield after a gap of about 2 m.y., but they appear to have been extracted from an isotopically different source layer in the mantle (asthenosphere?) which underlies the mantle source region for the tholeiites.

The pyroxenite suite rocks are the trapped crystal-liquid mushes, representing early nephelinitic magmas (HVS) which apparently did not escape from the mantle.

Case II: In this case a continuous replenishment of volatiles to their levels in the source mantle by metasomatism is assumed. All the melting steps of case I apply in this case, except that the source mantle solidus never reaches the position S S 3. After the production of picritic melts, a sequence of gradually decreasing volumes of transitional to alkali basalts will be produced as the geothermal gradient drops, owing to the movement of that part of the lithospheric plate away from the center of the hot spot.

This case is exemplified by the eruption sequence of the other Hawaiian islands, where a continuous pre-erosional sequence from tholeiites through transitional to alkali basalts occurs.

9. Summary (1) The uppermost 60 km of the upper mantle

under the Koolau shield is made up mostly of spinel lherzolite.

(2) The crust is thickened beneath the Koolau shield by the Koolau series tholeiites, and the transition zone between the thickened crust and spinel lherzolite mantle is composed of a thin layer of oceanic crust and a layer of plagioclase lherzolite.

(3) The dunites and plagioclase-bearing pyroxenites formed as early cumulates in fractionating tholeiitic magma chamber(s) which existed at a maximum depth of 15 km, i.e., possibly near the crust-mantle boundary or the petrologic Moho.

(4) The garnet-bearing pyroxenite suite formed veinlets into the spinel lherzolites at depths of

60 km appropriate to the transition zone between spinel- and garnet-lherzolite stability fields.

(5) From petrologic considerations, the earliest sequence of volcanic eruptions should be alkali basalt ---, transitional basalt ---, tholeiite in all Hawaiian volcanoes. Only recently, alkali basalt has been reported to erupt early in Loihi Seamount, the newest addition to the Hawaiian volcanic chain.

(6) The large-scale eruption of tholeiites followed by transitional to alkali basalt, as observed in most Hawaiian lavas, can be explained by different degrees of melting in a volatile-bearing garnet lherzolite mantle in which the volatiles are constantly replenished in the source region of the mantle. In the Koolau shield, where transitional and alkali basalts have not been reported to occur, generation of the tholeiites can be accounted for by melting in a similar mantle in which the volatiles have not been replenished.

Acknowledgements

The research work was supported by NSF grants EAR 76-22541A01, EAR 8018359, and EAR 7822766 to D.C. Presnall (University of Texas at Dallas), and manuscript preparation was supported by NSF grant EAR 7923615 to W.G. Ernst (UCLA). Different versions of the manuscript were read by W.C. Ernst, D.C. Presnall, S.R. Bohlen, P. Bird, G. Schubert, S. Lipshie, and C. Ross. Re- views by these persons and an anonymous re- viewer helped in improvement of the manuscript. Stimulating discussions with D.C. Presnall and J.E. Quick are gratefully acknowledged. I am

t h a n k f u l to D.A. Clague , W . G . Me l son , a n d W.P.

L e e m a n for p r o v i d i n g s o m e rock s a m p l e s a n d th in

s ec t i ons o f the la te E .D. J ackson . M o s t o f the

x e n o l i t h s of the p r e s e n t s tudy were co l l ec t ed by

D .C . Presnal l . D.J . Schulze p e t r o g r a p h i c a l l y ex-

a m i n e d a large n u m b e r o f p y r o x e n i t e su i te x e n o -

l i ths f r o m Salt Lake, M a k a l a p a a n d A l i a m a n u

ven ts . D i s c u s s i o n wi th J .R. D i x o n r e g a r d i n g calcu-

l a t i on of ac t iv i t ies o f ens t a t i t e a n d the use of his

sp ine l - l he rzo l i t e b a r o m e t e r is g ra te fu l ly a c k n o w l -

edged . I a m thank fu l to F. Spe ra a n d D. C lague

fo r p r o v i d i n g p r e - p r i n t s o f the i r pape r s . M y ap-

p r e c i a t i o n also goes to C a t h y M c C a n n a n d Vicki

J o n e s - D o y l e for the i r he lp in p r e p a r a t i o n of this

m a n u s c r i p t . T h e r e f e r ences c i t ed are the b a r e

m i n i m u m , fo l lowing the r e s t r i c t i ons i m p o s e d by

the J o u r n a l ' s pol icy . I beg fo rg iveness of those

r e s e a r c h e r s w h o feel tha t the i r w o r k s shou ld have

b e e n ci ted.

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Documents

A petrologic model for the constitution of the upper mantle and crust of the Koolau shield, Oahu, Hawaii, and Hawaiian magmatism