evolution of the lithosphere beneath oahu, hawaii: rare earth element abundances in mantle xenoliths

Earth and Planetary Science Letters, 119 (1993) 53-69 53Elsevier Science Publishers B.V., Amsterdam

[CL]

Evolution of the lithosphere beneath Oahu, Hawaii:rare earth element abundances in mantle xenoliths

Gautam Sen a, Frederick A. Frey b, Nobumichi Shimizu cand William P. Leeman d

a Department of Geology, Florida International University, Miami, FL 33199, USAb Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA 02139, USA

c Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USAd Keith-Weiss Geological Laboratories, Rice University, Houston, TX 77251, USA

Received July 27, 1992; revision accepted May 20, 1993

ABSTRACT

Rare earth element contents of clinopyroxenes in Hawaiian mantle xenoliths from Oahu were determined with an ionmicroprobe. The analyzed xenoliths are from four vents of the alkali Honolulu Volcanics (HV). Three (Kaau, Pali and Kalihi-KPK) are located close to the caldera of the extinct Koolau shield volcano, and the fourth, Salt Lake Crater (SLC), is onthe periphery of the shield volcano. Systematic differences exist in REE contents between clinopyroxenes of the KPK andSLC xenoliths: (1) KPK pyroxenes are typically zoned in REE contents whereas SLC pyroxenes are homogeneous, (2) theLREE-depleted (chondrite-normalized) patterns that characterize many of the KPK xenoliths are not found in SLCxenoliths, and (3) the convex-upward REE patterns that are characteristic of SLC xenoliths are not found in KPK xenoliths.Relative to abyssal peridotites, the LREE-depleted Hawaiian lherzolite pyroxenes (interpreted to be residual oceaniclithosphere) have higher contents of REE, Na2 O, TiO2 and FeO, and more modal clinopyroxene. These LREE-depletedHawaiian xenoliths represent deeper, less-depleted parts of the melting column, whereas the abyssal peridotites representthe uppermost, more strongly depleted part of the mantle. The spoon-shaped, LREE-enriched and convex-upward REEpatterns in the xenoliths have resulted from metasomatic enrichment of the lithosphere caused by reaction with magmas thatformed the Honolulu Volcanics. A model for the evolution of the oceanic lithosphere is presented in which fractures werethe main mode of transport of the Honolulu Volcanics. Metasomatic enrichment resulted from interaction betweenpercolating Honolulu Volcanics magmas and wallrock. The differences between SLC and KPK xenoliths are attributed tochromatographic fractionation effects: SLC xenoliths are postulated to have come from a greater depth where theyequilibrated to a larger extent with the percolating magmas than the KPK rocks.

1. Introduction

Trace element and isotopic compositions ofbasaltic lavas have suggested the existence ofgeochemically distinct reservoirs in the Earth'smantle [1-3]. Because magmas form over a rangeof depth, mix during ascent and interact withcompositionally heterogeneous wallrocks theyprovide only indirect information on the lateraland vertical compositional and mineralogical vari-ations within the mantle. This makes it difficultto equate the chemically defined end-membercomponents to physical components, such aslithosphere, asthenosphere and plumes. The study

of mantle xenoliths has a distinctive advantage inthat these rocks provide a direct window on theEarth's uppermost mantle. Because of their sen-sitivity to melting, crystallization and melt-wall-rock reaction processes, abundances of trace ele-ments are particularly helpful in deciphering thecomplexities involved in magma generation andmigration processes.

In this paper we augment our prior work [4-8,31] on Hawaiian xenoliths and present new ionprobe data on rare earth element contents ofclinopyroxenes in spinel Iherzolites, compositexenoliths (Iherzolite veined by garnet and spinelpyroxenite) and pyroxenites. In contrast to most

0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

G. SEN ET AL.

xenolith studies which provide petrological dataon a single xenolith suite from a volcano or a fewsamples from different volcanoes, this investiga-tion focuses on mantle xenoliths from four dif-ferent vents of the Honolulu Volcanics (HV) on

(a) Koolau Shield Vol

Oahu. Three of them, Kaau, Pali and Kalihi(KPK), are situated near the rim of the inferredKoolau caldera, but the fourth, Salt Lake Crater(SLC), is on the flank of the Koolau shield [4,9].Spinel lherzolites, which are the focus of our

cano Koolau

Kalihi VolcanoPali Caldera

Salt Lake Crater I

Garnet-bearingXenoliths

Spinel lherzolitesSp. Iherzolites,few Dunites, &Pyroxenites

99% Dunites

*4 No Garnet s

4 - Largely Mantle Samples

(b)5-.

3 -

3.0

3 - 6- - -

2 - 2 2

- 1 0 -0 0.4 1.2 2.0 0 0.4 1.2 2.0 0 0.4 1.2 2.0

Cr203-SLC Cr203-Kaau Cr203-PaliFig. 1. (a) Sketch showing approximate location of the four xenolith-bearing vents on the extinct and eroded tholejitic Koolau shieldvolcano. The distance from Pali to Salt Lake is about 13 km. Note how the proportions of xenolith rock types vary as a function ofdistance from the caldera. (b) Abundances of Na2 O and Cr 2 03 in clinopyroxenes of spinel lherzolites from Salt Lake Crater (SLC),

Kaau and Pali.

4-Crustal Cumulates -

0.6 1.8 3.0 0.6 1.8Na2O-SLC Na2O-Kaau

0.6 1.8

Na20-Pali

3.0

54

1 1

EVOLUTION OF THE LITHOSPHERE BENEATH HAWAII

study, are rare, altered, or absent in other HVvents. The analyzed samples were selected on thebasis of our previous petrographic and electronmicroprobe study [5] to span the entire composi-tional range of mantle-derived Hawaiian xeno-liths. Previous studies [6,8] noted that most of thespinel lherzolites are characterized by enrich-ments in highly incompatible trace elements (e.g.,greater than chondritic light REE/heavy REEabundance ratios), and inferred that these rocksrepresent LREE-depleted lithosphere that wassubsequently enriched through interaction withHV melts. The two principal objectives of thisstudy are to characterize REE abundances ofclinopyroxenes in Hawaiian mantle xenoliths andthe processes that controlled REE abundances.Our ultimate goal is to understand the geochemi-cal evolution of the oceanic lithosphere beneath aHawaiian volcano.

2. Background

The eastern part of Oahu is a deeply eroded,extinct shield volcano, the Koolau volcano. About1.8 Ma after the Koolau volcano ceased to erupttholeiitic lavas, strongly alkali lavas of the Hon-olulu Volcanics erupted through vents scatteredover the shield. These lavas contain xenoliths ofdunite, spinel lherzolite, garnet-bearing pyroxen-ite, and FELG (Fe-rich lherzolitic group, which isdefined in [7]). Vents located within the Koolaucaldera area (e.g., Ulupau, Nuuanu and Makalo-pa) contain abundant xenoliths of cumulatedunites (Fig. 1 [10]). The Kaau, Pali and Kalihi(KPK) vents near the rim of the Koolau calderacontain abundant spinel lherzolite xenoliths [4],rare dunites, and a few pyroxenites (± spinel). Arare plagioclase lherzolite xenolith has also beenfound at Pali. The Salt Lake Crater (SLC) on theflank of the shield also contains spinel lherzolitesand garnet-bearing xenoliths (i.e., FELGs andpyroxenites). Composite xenoliths, in which gar-net clinopyroxenite forms veins in spinel lherzo-lite, have only been found at SLC [4,7,11].

Major element compositions of the mineralphases, modal abundances and the textural char-acteristics of these xenoliths were described bySen [5]. Petrographically, these spinel 1herzolitestypically display coarse-grained porphyroclastic toallotriomorphic-granular textures. In terms of

55

cpx (mean, abyss)

20 cpx (mean)

t 10 opx (mean)00

6 oll

0 ' ' '

0 10 20 30 40 50 60 70 80

Modal %Fig. 2. Modal abundances (vol%) of minerals in the Hawaiianspinel lherzolites [5]. The mean abundances are also indi-cated. The frequency distribution of clinopyroxene in abyssalperidotites [12] shows that clinopyroxene is much more abun-

dant in Hawaiian spinel Iherzolites.

mode, the Hawaiian spinel Iherzolites are gener-ally similar to spinel lherzolite xenoliths fromelsewhere but are much richer in clinopyroxenethan abyssal peridotites from oceanic fracturezones (Fig. 2). Olivine compositions are typicallyFo5 9 9', and the most forsteritic olivines occur inspinel lherzolite xenoliths from the SLC. Theclinopyroxenes (1-20 vol%) are green chromiandiopsides with about 5-7% Al20 3 ; they com-monly contain exsolved spinel + orthopyroxene.Those from the SLC are also enriched in Na andCr compared to clinopyroxenes from the KPK(Fig. 1). All the spinel lherzolites contain brownMg- and Cr-rich spinel. Geothermobarometry ofthe veined xenoliths (spinel Iherzolite intruded bygarnet pyroxenite veins), garnet pyroxenites andFELGs from the SLC suggests that they lastequilibrated at pressures of 20-25 kbar, i.e. overa depth range of about 60-75 km [5].

3. Rare earth element composition of pyroxenes

A Cameca IMS-3f ion microprobe (MIT-Brown-Harvard facility, now located at WoodsHole Oceanographic Institution) was used to ana-lyze clinopyroxenes in the spinel lherzolite, pyrox-enite and some FELG xenoliths for eight rare

G. SEN ET AL

TABLE la

Rare earth elements in clinopyroxenes of Hawaiian spinel lherzolite xenoliths

Vet Sp. La Ce Nd SmNo.

Kaau 77KAPS-3(1) 12.58 13.96 7.90-3(2) n.d. 9.69 5.78-17 6.99 12.73 8.01-20(1) 3.55 8.38 5.39-20(2) 1.52 4.20 4.14-8 0.31 2.13 4.24-4 n.d. 2.05 2.13-29(1) n.d. 30.11 11.90-29(2) n.d. 24.41 9.63-26 n.d. 1.97 2.40

Pali 1 77PA-1011(1) 19.69 9.12 6.72-1011(2) 1.22 0.97 3.78-21(1) 7.43 16.60 7.33-21(2) 3.63 5.45 2.74-39(1) 0.17 0.42 2.37-39(2) 0.05 0.35 1.77-31A 1.81 3.91 3.19-2 2.82 4.17 2.70

Pal-3 n.d. 1.05 2.15

2.821.852.811.821.772.211.132.902.221.25

9.628.651.931.391.541.221.581.471.33

Pali 2 77PA11-8 5.77 14.41 9.89 3.38-IA-I n.d. 1.21 2.22 1.55-2 4.26 10.45 6.53 1.86

Kalihi 77KALI-5(Cc) 2.53 4.94 3.83 2.04-5(Cr) 9.78 23.82 16.97 4.57-5(sm) 17.34 30.25 22.09 5.53-7 4.55 6.64 5.49 2.32-10(1) 6.85 15.10 8.27 2.48-10(2) 4.88 11.18 6.31 1.83

Salt Lake 77SL-2(X) n.d. 9.16 10.60 3.30-8(X) n.d. 12.58 10.86 3.75-6(X) n.d. 14.40 7.57 2.05-5 n.d. 21.52 15.08 4.17-10 n.d. 4.61 4.02 1.49-13 n.d. 12.30 6.84 1.39

69SAL-211B(X,I) n.d. 16.43 12.54 4.0469SAL-211B(X,2) n.d. 12.86 11.11 3.58

SL-4 (Nobu) n.d. 8.61 6.85 1.71

Eu Dy Er Yb Cpx Na2O Cr203(ppm) modal% (wt%)

1 .320.611.230.660.900.550.510.970.770.62

14.486.330.740.660.540.770.650.750.64

1.180.650.57

0.841.672.171.081.100.64

1.191.380.831.510.620.471.291.160.64

5.411.614.743.323.784.372.232.191.762.73

14.7910.963.483.073.652.953.403.032.45

5.322.782.27

4.446.207.004.824.942.98

2.322.552,803.351 .961.152.321 .061.57

3.290.852.941.992.282.881.301.020.981.70

14.9211.272.191.902.361.812.142.011.44

3.121.711.37

2.693.694.402.953.151.91

1.001 .091 .681.840.990.571 .080.880.89

2.500.802.271.511.812.121.381.070.911.77

12.497.681.761.531.831.611.691.591.59

3232251515191 17713

1313101077

1.501.61

1.751.321.33

1 841.21

1.64

1 .79

1.33

0.480.45

0.590.490.72

0.750.63

0.52

0.56

0.54

1.68 0.59

2.52 151.72 230.99

2.11 172.99 173.55 172.492.56 201.55 10

1.011.071.711.70 120.680.53 130.980.900.76

1.91I .29

I 720.48

1.750.431 .77

0.630.28

0.640.64

0.660.440.63

2.41 0.91

2.34 1.43

(1) and (2) = rim and core of a grain in the same xenolith; Cc and Cr = core and rim of acoarse grain; sm = small grain; (X) = spinel lherzolite wallrock clinopyroxene in compositexenoliths.

earth elements. The operating procedures werethose of Johnson et al. [12], except that La wasincluded in this study using a basaltic glass as astandard. Three spots on each grain and threegrains per rock were usually analyzed (Tables 1and 2). The number of spots and grains for analy-sis was increased if significant heterogeneity wasnoted. Although the clinopyroxenes contain ex-solved spinel + orthopyroxene, no attempt wasmade to reconstitute the original bulk (pre-ex-solution) composition of the clinopyroxenes.However, the diameter of the ion beam (ca. 25,Am) was significantly larger than the exsolvedblebs (5-15 Am, but generally - 5 ,am) and it islikely that the analyzed compositions representbulk pyroxene compositions. Uncertainties aregenerally similar to the values quoted in Johnson

et al. [12]: +7-15% for LREE and +5-7% forthe middle and HREE.

3.1 REE abundances in spinel lherzolite clinopy-roxenes

3.1.1 Kaau, Pali and Kalihi (KPK)Clinopyroxenes from the KPK spinel lherzo-

lites have the most varied REE abundances. Their

TABLE lb

Composite xenolith 69SAL-IIOAB from the SLC

Distance Ce Nd Sm Eu Dy Er Yb

VeinWallWalWallWall

8.98 5.67 1.75 0.62 1.81 0.87 0.874.5 cm 10.5 6.04 1.64 0.55 1.26 0,74 0.791.5cm 6.93 4.05 1.16 0.39 1.16 0.54 0.581.5 cm 8.98 5.67 1.75 0.62 1.81 0.87 0.871.5 cm 4.46 2.73 1.00 0.40 1.01 0.52 0.48

56

EVOLUTION OF THE LITHOSPHERE BENEATH HAWAII

TABLE 2

Rare earth elements in clinopyroxenes in Hawaiian pyroxenite and FELG xenoliths

Vent Sp. La Ce Nd Sm Eu Dy Er YbNo. (ppm)

Pali 1 77PA-12(l) 3.36 10.77 10.74 3.16 1.13 2.41 0.89 0.56-12(2) 2.67 8.10 7.81 2.31 1.00 2.01 0.78 0.41

Salt Lake 77SL-2(Y) n.d. 23.94 18.55 5.78 2.24 3.80 1.51 1.53-8(Y) n.d. 12.22 9.69 3.07 1.19 3.05 1.58 1.59-6(Y) n.d. 13.79 9.24 2.86 1.09 2.71 1.29 1.39-7 n.d. 21.11 21.43 6.99 2.56 4.17 1.52 1.88-28A n.d. 10.10 6.52 2.00 0.73 1.02 0.39 0.27-28B n.d. 8.77 5.9 2.01 0.67 0.81 0.35 0.25

20-B (Grl) n.d. 13.42 10.36 3.71 1.35 2.02 0.66 0.5720-B (Gr2) n.d. 9.27 7.355 2.71 0.93 1.38 0.38 0.39

(1) and (2)= rim and core of a grain in the same xenolith; (Y)= pyroxenite veinclinopyroxene in composite xenoliths; GrI and Gr2 = two grains in the same rock.

chondrite-normalized profiles (Fig. 3) can bebroadly grouped into three types: a LREE-de-pleted group, a spoon-shaped (i.e., concave fromLa to Sm) group, and a LREE-enriched group.LREE-depleted pyroxenes have not been foundin the three Kalihi xenoliths studied. It is possiblethat these Kalihi samples are not representative.Such large variations, including the spoon-shapedpatterns, have been observed in other xenolithsuites (e.g., Dreiser Weiher, Germany [13] andthe Massif Central, France [14]). They are notunique to Hawaii.

Most of the KPK xenoliths are strongly zonedin LREE (Fig. 4). Similar zoning was not ob-served in the Salt Lake xenolith pyroxenes. Inzoned pyroxenes, the rims (width approximately40-100 ,m) are significantly richer in La, Ce andNd than the cores, but Sm to Yb abundances aresimilar in the rims and cores (Fig. 4). It is difficultto ascertain whether such zoning is gradual be-cause the zoning patterns were not studied indetail. However, based on the available data thezoning does appear to be concentric. In addition,sample 77KALI-5 from Kalihi not only exhibitsstrong zoning of clinopyroxene porphyroclasts, butREE profiles for rims of these grains are similarto those for small clinopyroxene neoblasts.

3.1.2 Salt Lake Crater (SLC)REE profiles for clinopyroxenes from SLC

spinel lherzolite clinopyroxene vary from convex-upward to LREE-enriched; some intermediate

patterns are broadly convex-upward with flat seg-ments at Nd to Eu and Er to Yb (Fig. 4). Thereare clear distinctions between SLC and KPK REEpatterns:

(1) The spoon-shaped patterns have not beenfound in the SLC.

(2) KPK patterns are typically flat from Sm toYb and in this respect are very similar to theabyssal peridotites [12]. In contrast, SLC patternshave a distinctive negative slope between Sm andDy (Fig. 3).

(3) The mean abundances of HREE are lowerin SLC clinopyroxenes than in KPK (Fig. 3).

Four composite xenoliths (spinel lherzoliteveined by pyroxenite) from the SLC (SL-2, -6, -8and 69SAL-11OAB) were analyzed (Figs. 5 and 6).69SAL-11OAB is the largest (23 x 12 cm); theother three are very small (4 X 3 cm). Note thatthe vein assemblage in 69SAL-11OAB is a spinelpyroxenite, whereas in the other three compositexenoliths it is garnet pyroxenite. In SL-8 and -6the vein and wallrock pyroxenes have virtuallyidentical abundances of all REE. SL-2 is distinctfrom the other two by virtue of having signifi-cantly higher REE in the vein than in the wall-rock (Fig. 5). Clinopyroxenes in the vein andwallrocks of 69SAL-lOAB are LREE-enrichedand have very similar chondrite-normalized REEpatterns, although the absolute abundances varyconsiderably (Fig. 6). Interestingly, the clinopy-roxene farthest away from the vein is the mostLREE-enriched. Individual clinopyroxenes within

57

8 G. SEN ET AL

La Ce Nd Sm Eu Dy Er Yb

06.0LIU

(- I

.1

10

0.0 10

C-

La Ce Nd Sm Eu Dy Er Yb

La Ce Nd Sm Eu

La Ce Nd Sm Eu

Dy Er Yb

Dy Er Yb

Fig. 3. Chondrite-normalized (normalizing values are from [40]) REE abundances in clinopyroxenes in spinel lherzolite xenoliths.Symbol size roughly corresponds to analytical uncertainties of determination. Also, multiple grains from a single xenolith and zonedpatterns in single grains are plotted. Thus, for example, although only three Kalihi xenoliths were studied, seven patterns for Kalihiare shown which represent measured abundances in cores and rims of zoned clinopyroxene grains in individual xenoliths. Similarsymbols do not represent a particular xenolith sample. The important features are as follows: (1) Wide variations fromLREE-depleted to LREE-enriched patterns are shown, by Kaau and Pali lherzolite c~linopyroxeines, and (2) several of the Salt Lakexenolith clinopyroxenes show a convex-upward pattern and have lower [Yb]n values. The stippled field represents calculatedclinopyroxenes that could have equilibrated with HV magmas. These were calculated using the equation (Cill' (C,]cx x [D]cPx/I,where [C,]' and [C]P represent concentration of an element i in the liquid (TI) and coexisting clinopyroxene, respectively. Drepresents the partition coefficient for that element (values used are those in [12]). Note that excluding the strongly LREE-en-

riched sample (77SL-13, r-) most SLC clinopyroxenes with convex-upward patterns are similar to the calculated pyroxenes.

1.5 cm of the contact are quite variable in REEcontents; however, like the vein they all have acharacteristic hump at Sm-Eu.

In summary, in two of the veined xenoliths(SL-8 and -6) the vein and wallrock clinopyrox-enes seem to have equilibrated in REE, whereasin the other two samples the REF content of the

clinopyroxenes in vein and wallrock are signifi-cantly different.

3.2 REE in clinopyroxenes of SL C pyroxenite andFELG xenoliths

Clinopyroxenes from FELG and pyroxenitexenoliths were analyzed for comparison with the

r00.o 10

.o100

C-)r.)0 1

C-

58

EVOLUTION OF THE LITHOSPHERE BENEATH HAWAII

10u0 . . . . . . . . . ... . .

0

LI

CW

10 I-

La Ce Nd

100

0 10

cI=LI0.

linn - . . . . .

60

CL.

La Ce Nd Sm Eu Dy Er Yb

Sm Eu Dy Er Yb

10 -

La Ce Nd Sm Eu Dy Er Yb

Fig. 4. Core and rim compositions of single zoned clinopyroxene grains from Kaau, Pali and Kalihi vents are shown. In all cases,note that the rim has a conspicuously greater abundance of LREEs but very similar Eu to Yb abundances. In the Kalihi panel the

rim of a coarse grain and a fine grain have similar REE contert-

spinel lherzolite data (Table 2 and Fig. 7). Thechondrite-normalized REE patterns are flat fromthe LREE to the middle REE, but indicate highSm/Yb ratios. The ion probe results comparewell with previously published neutron-activationanalyses of the SLC xenoliths [6].

4. Discussion

We compare REE abundances in clinopyrox-enes from Hawaiian spinel lherzolites, peridotitesfrom Zabargad Island (Red Sea [17]) and abyssalperidotites from the Indian Ocean [12] with theintention of answering the following questions:

(1) Do any of the Hawaiian xenoliths representplume material? (2) How do Hawaiian xenolithscompare with other oceanic peridotites? (3) Towhat extent was the oceanic lithosphere affectedby Koolau and Honolulu magmatism?

In the discussion that follows, we use the ex-pressions 'less depleted' and 'more depleted'peridotites to describe the relative amount ofbasaltic magma extracted from a residual peri-dotite. Some authors [e.g., 18] have used degreeof 'fertility' to indicate how much magma may beextracted from a given peridotite. In this sense,our 'less depleted' would be the equivalent of'more fertile' peridotite.

_d0

LI

C)LI

Pali

Core

77PA-21

. , . I I . . . I . . . .

Kaau

Core

77KAPS-20

. . .

59

I

I

G. SEN ET AL.

C

0.

U 10

C-

Ce Nd Sm Eu

0

.0U

0UKC-)

Dy Er Yb La Ce Nd Sm Eu Dy Er Yb

Ce Nd Sm Eu Dy Er Yb

Fig. 6. The photograph is a hand specimen of 69SAL-1lOAB,a composite xenolith with a spinel pyroxenite vein (on theleft-hand side) in a spinel Iherzolite wallrock. One polishedthin section was made from the vein, and three were madefrom the wallrock at different distances from the vein. TheREE graph shows that all clinopyroxenes have patterns with

negative slopes but with variable REE abundances.

4.1 Are Hawaiian spinel lherzolites samples of theplume?

Most models for the origin of Hawaiian lavaspostulate a hot, deep mantle plume that is amagma source as it ascends to 60 km [4,19].

0 1 s.Phase equilibrium data for samples of the upper5 Ihenzoute mantle suggest that the assemblage spinel +

olivine + opx + cpx should be stable at 60 kmu [4,20]. Therefore, based on these models the up-

permost parts of the plume are spinel peridotite.SL-2 Do the spinel lherzolite xenoliths from Oahu

represent recrystallized plume material? The Pb,Ce Nd Sm Eu Dy Er Yb Sr and Nd isotopic differences between the SLC

Fig. 5. Chondrite-normalized REE abundances in clinopyrox- spinel lherzolites and the likely plume-derivedenes from wallrock Iherzolite and garnet-pyroxenite vein in (largely) Koolau lavas do not support the hypoth-

three SLC composite xenoliths. esis that these xenoliths represent deep mantle

1016

C-

I II . .I, I.X

70 xenite

Sp. Iherzolite

SL-6

l l . I l l l l lI

100

60

EVOLUTION OF THE LITHOSPHERE BENEATH HAWAII

100 I I .I I I I I

SLC Pyroxenite & FELG

10

Ce Nd Sm Eu Dy Er Yb

-- 77SI-2(Y)8(Y)

- 6(Y)-0- 7

28A28B20-B(G1)

- 20B(G2)

Fig. 7. Chondrite-normalized REE abundances in clinopyrox-enes from SLC pyroxenites and FELGs [8].

plume material (Fig. 8 [21,22]). The few analyzedSLC spinel lherzolite xenoliths are all similar in3 He/ 4 He to Pacific MORB but in a 8 7 Sr/86 Sr vs.14 3Nd/ 14 4Nd plot all but one of the SLC spinellherzolites are isotopically similar to the HV lavasand none is like MORB or Koolau tholeiites. Thesample that plots closest to the Koolau field inFig. 8 has been interpreted as representing litho-

0.5135

0.5133

0.5131

.Z 0.5129

-. 0.5127z

n 0.5125

0.51230.702 0.703 0.704 0.705 0.706

87 Sr/86 SSr I Sr

Fig. 8. 1 4 3 Nd/144 Nd vs. 8 7 Sr/86 Sr of SLC spinel lherzolites (0)are compared with Honolulu (HV), Koolau and northernPacific MORB lavas (data sources: [2,3,21, Shimizu, unpub-

lished]).

0

U

Ce Nd Sm Eu Dy Er Yb

Fig. 9. LREE-depleted clinopyroxenes in KPK xenoliths arecompared with those in abyssal peridotites (shaded [12]) and

Zabargad peridotites (ruled [20]).

sphere modified by plume-derived Koolau mag-mas [21]. Because most of the xenoliths have Srand Nd isotopic ratios between the fields foroceanic lithosphere and the Hawaiian plume, assampled by the Koolau shield, and because of theabsence of parent/daughter-isotopic correlationin the Sr and Nd systems, Vance et al. [28]proposed that these xenoliths are samples ofoceanic lithosphere that were modified by inter-action with a spectrum of Hawaiian lavas rangingfrom alkali to tholeiitic. Additional evidence for ametasomatic process is the lack of correlationbetween Mg/Fe and Na2 O content in clinopyrox-ene [5].

Until isotopic data are available for KPK xeno-liths [M. Tatsumoto, research in progress], REEabundances constitute the best approach for eval-uating whether these xenoliths represent plumematerial. Pyroxene cores in several of these xeno-liths are relatively depleted in LREE (Fig. 9).Their similarity to clinopyroxene in abyssal peri-dotites is consistent with the xenoliths represent-ing fragments of the oceanic lithosphere. How-ever, we shall argue later that the LREE-en-riched rims of clinopyroxenes in these xenoliths(Fig. 4) reflect interaction with LREE-rich meltsthat are more like lavas of the Honolulu Vol-canics than Koolau tholeiites. Therefore, the KPKxenoliths do not appear to be related to theplume source of the Koolau shield lavas.

HV

Pac. MORBs

Koolau

61

62

4.2 Comparison of Hawaiian spinel lherzolites with

other oceanic peridotites

Because the sources of MORB are character-ized by a relative depletion in LREE it is useful

to compare oceanic peridotites that are relatively

depleted in LREE (e.g., some Hawaiian spinet

lherzolites, lherzolites from Zabargad Island in

the Red Sea [17,23] and abyssal peridotites[12,24,25]). The Hawaiian xenoliths contain more

clinopyroxene than abyssal peridotites (Fig. 2);

hence, the Hawaiian xenoliths are 'less depleted'.

Consistent with this inference, the LREE con-

tents of clinopyroxenes from the Hawaiian xeno-

liths are higher than those in clinopyroxenes from

abyssal peridotites; in fact, they fill the gap be-

tween the more depleted abyssal peridotites and

peridotites from Zabargad Island (Fig. 9). Also,

relative to clinopyroxenes from abyssal peri-

dotites, clinopyroxenes in the Hawaiian spinet

lherzolites have higher contents of Na 20 and

range to higher TiO2 abundances (Fig. 10). FeO *

(not shown in Fig. 10) is also higher in Hawaiian

xenoliths. These differences in clinopyroxenecomposition are not likely to represent the effects

of subsolidus re-equilibration because the calcu-lated temperatures [26] are similar: abyssal peri-

dotites = 925'-1180'C with a mean of 1060'C,and Hawaiian lherzolites = 950'-1250'C, with a

mean of 10750C.A likely explanation for the compositional and

modal differences between these Hawaiian spinet

lherzolites and abyssal peridotites is that they

have been depleted to varying extents by extrac-

tion of basaltic magma. The paths for residues

from fractional melting are shown in Fig. 10. Theresults for Na2 0, Sm and Yb are most important

because clinopyroxene is the dominant host for

these elements, and subsolidus recrystallizationand re-equilibration between clinopyroxene and

other phases in a spinet lherzolite are not ex-

pected to affect their abundances as strongly asmelting processes.

The Sm-Yb variation in abyssal peridotites is

consistent with fractional melting of a sourcesimilar to peridotites from Zabargad or some of

the KPK peridotites (Fig. 10a). Samples with rela-

tive LREE enrichment (i.e., SLC and several of

the Pali peridotites) are not suitable sources.Although TiO2 contents of clinopyroxene are af-

(a)

'40

Z

(b) 1.0

0.8

0.6

0F. 0.4

0.2

0.0

I

AA

2 - 02 0

0.1 - 1 10

[Sm~n 10

0 3

G. SEN ET AL.

o Abyss. Per.* Kaauo PaN

A SLC* Kalihi

--4- RFM path-1-w- RFM Path-2

~d~AiMIS

. 0 ,X ,,., ,.....

.1 I 10 100

[Smin

Sp.Lherz.Res a A

0

.1I [Sm~n 10 100

Fig. 10. Compositional comparisons between abyssal peri-

dotites, Oahu xenoliths and Zabargad peridotites (only in the

Sm-Yb the square with a diagonal). Also shown are frac-

tional melting residual clinopyroxene compositions using two

separate staring pyroxenes (RFM path 1 and RFM path 2).

See the appendix for additional information.

fected by subsolidus re-equilibration with spinet

and orthopyroxene [13], the TiO2-[Sm]1n trajec-tory of the abyssal peridotites also intersects thefield for KPK xenoliths (Fig. 10b). Again, SLCsamples with relative LREE enrichment and thenear-vertical trend for Pali xenoliths is inconsis-tent with the trend for melting residues.

In contrast to TiO2 and REE, the Na2 O con-tents of clinopyroxenes in abyssal peridotites arenot consistent with the calculated trend for

residues from fractional melting. Elthon [181 rec-ognized this problem, and he proposed that theanomalously high Na2 O contents in many of theabyssal clinopyroxenes reflects refertilization of a

I

EVOLUTION OF THE LITHOSPHERE BENEATH HAWAII

o 0.6

Kn -

0.0 1 . I800 900 1000

T[oC]

Fig. 11. Na2 O content of clinopyroxenes vs. estimated two-py-roxene temperatures. Abyssal peridotite data [12] are shownalong with standard errors of +30'C for temperature and+0.18 for Na2 O. These are very conservative values, and it islikely that the analytical uncertainties for Na2 O are even

larger.

highly depleted peridotite by addition of basalticmelt in the shallow mantle. Because these rocksrecrystallized under subsolidus conditions, as isevident from their mineral chemistry and texture[25], a possible alternative is that the Na2 O con-tents of the clinopyroxene have been increasedduring recrystallization. During subsolidus cool-ing of clinopyroxene in the spinel lherzolite sta-bility field, spinel + orthopyroxene exsolve fromclinopyroxene. Because Na2 O preferentially par-titions into clinopyroxenes the Na2 O abundancein clinopyroxene should increase with progressivecooling and exsolution. Figure 11 shows that arough negative correlation exists between Na2 Oand the temperature of subsolidus equilibrationbetween ortho- and clinopyroxenes in abyssalperidotites. Because of the relatively large uncer-tainties associated with these values, the qualityof this correlation is poor. It does neverthelesssupport the recrystallization hypothesis.

A possible interpretation of the data forHawaiian and abyssal peridotites is that clinopy-roxenes with variable but [LREE/HREE]n < 1('n' = chondrite-normalized) reflect vertical het-

erogeneity in the oceanic mantle lithosphere. Ata mid-oceanic ridge, peridotite begins to meltwithin the garnet peridotite stability field andcontinues to melt as it ascends through the spinellherzolite field into the plagioclase peridotitefield. The residue at the top of the melting col-umn (i.e., the uppermost mantle) is the mostdepleted because it has yielded basaltic magmathroughout the entire melting regime [27-29].Therefore, the uppermost mantle, represented byabyssal peridotites, should be more depleted inincompatible elements than the deeper parts ofthe oceanic lithosphere (represented by Hawaiianxenoliths). Because clinopyroxene is rapidly con-sumed during melting [27] the more depletedrocks, abyssal peridotites, also contain lessclinopyroxene (Fig. 2). We do not imply thatxenoliths from shallower depths do not exist inHawaiian lavas. Indeed, cumulate mafic and ul-tramafic xenoliths and rare refractory (Fo9 0olivine) plagioclase lherzolite xenoliths (depth ofequilibration of about 25 km) have been found atthe Pali vent [20].

We infer that the combination of Hawaiianxenoliths with (LREE/HREE)n < 1 and theabyssal peridotites is a good representation ofvertical compositional variations within the man-tle column that created the overlying oceaniccrust. The differences between abyssal and Hawaiidatasets do not necessarily translate into largedifferences in depth within the lithosphere. Al-though several models for MORB generation holdthat MORB is produced over a great depth range,the bulk of the magmas are generated within thelimited depth range 25-35 km [27,28]. Therefore,assuming that the magma extraction rate fromthe rising mantle parcel does not change greatlyover this depth range, a spinel peridotite sampledat 40 km may be significantly less depleted thanone from 35 km.

The consistent variations in clinopyroxenecomposition among the three oceanic lherzolitesuites is remarkable when it is considered thatthey represent different ages and three differentocean basins. In a very broad sense such consis-tency indicates that the major element, minorelement and REE compositions of the lherzoliticsource of the oceanic crust in these differentoceans have been very similar over tha past 90Ma (the age of the Hawaiian lithosphere).

63

G. SEN ET AL.

La Ce Nd Sm Eu Dy Er Yb

Fig. 12. Schematic model of evolution of KPK (Kaau, Pali andKalihi)-type REE enrichment patterns, based on zoning pat-terns in clinopyroxenes in several xenoliths. The startingclinopyroxene had a LREE-depleted character depicting the90 Ma old Pacific lithosphere. REE enrichment occurredsubsequently which created spoon-shaped patterns and even-

tually LREE-enriched patterns.

that such pyroxenes would be similar to the bulkof the SLC spinel lherzolite clinopyroxenes intheir REE contents and would be characterizedby convex-upward [REE]n patterns. Thus, stronglyLREE-enriched xenoliths, such as 77SL-13, couldnot have equilibrated with HV-type magmas.

One explanation for clinopyroxene with highLREE/HREE is that the clinopyroxene crystal-lized from trapped melts [25,30,31]. If this crystal-lization from a melt occurred in a closed system,the clinopyroxene will have a melt-like REE pat-tern. Therefore, clinopyroxene in some mantlerocks may reflect the effects of such processes.We evaluated this possibility by calculating mix-ing curves generated by mixing of a HV lava (alikely candidate for a trapped melt) with clinopy-roxene in a depleted lherzolite (Fig. 13a and b).

(a)

4.3 Enrichment of the lithosphere

REE zoning trends in pyroxenes from KPKxenoliths are consistent with the hypothesis thatparts of the sub-Koolau lithosphere started out asa LREE-depleted material that was subsequentlyenriched by Hawaiian magmas (Fig. 12). We ad-dress the following questions: (1) What were theenriching agents? In this connection the relativeroles of Koolau and Honolulu magmatism needto be evaluated. (2) What were the processes ofenrichment? Important constraints on the litho-sphere enrichment process are (a) the restrictionof spoon-shaped and LREE-depleted clinopyrox-enes to KPK xenoliths (Fig. 3), (b) lack of REEzoning in SLC xenoliths, and (c) variable extentof REE equilibration between vein and wallrockpyroxenes. These constraints allow us to addressquestions that range from the origin of theclinopyroxenes in these spinel lherzolites to theprocesses of magma-wallrock interaction.

The Pb, Nd and Sr isotopic similarity betweenthe SLC spinel lherzolites and HV lavas indicatethat the enriching medium was related to HVmagmas (Fig. 8). In Fig. 3 REE contents ofclinopyroxenes in equilibrium with HV magmaswere calculated using the cpx/melt partition co-efficients of Johnson et al. [121. This figure shows

(b) 20

15

5o

S5

0

0

-U-- P.1-3

---- m--- a .05

- .1

*-a- .15

-a- .25-.-..-- .3

I Mixing

Pali

Kaau

SLC Sp.Lhz.

SIC Pxte

QL..0

[Ce/Nd]n

Fig. 13. (a) A calculated mixing curve representing fractionaladdition (each tick mark represents 2% melt added) of afractionated HV melt to a LREE-depleted clinopyroxene inthe xenolith Pal-3 representing depleted residue. While someof the Kaau and Pali clinopyroxenes may resemble suchpyroxenes, all SLC and several Pali and Kaau clinopyroxeneshave much higher Ce/Nd than can be explained by such aprocess. (b) REE patterns expected for such mixtures. Thedecimal numbers represent fraction of melt added (e.g., 0.1means 10% HV added to Pal-3 clinopyroxene). The variety ofREE patterns created does not include the commonly found

spoon-shaped patterns (cf. Fig. 3).

64

.0.m a 0-

> ) 4 4 E 4) *=c

b) ) ,w JU.-~

CZ to* ~-.

;:; ~ ~ ;z~ ::~4 Zoz 4,

CZ P

.0E * q

to~ 0~.

4,4.. . . . .. . . . .*~

eg4U P 0V

40 0

0 0 t 2.t

0cU=0 U~ M > ~00

.0 :E 0. 0

M ti

00 UU U 4,

0Cj

Co M . 10

EVOLUTION OF THE LITHOSPHERE BENEATH HAWAII 65

G. SEN ET AL.

Figure 13b shows that such trapped melt crystal-lized pyroxenes would not develop the spoon-shaped patterns that are observed in the KPKxenolith clinopyroxenes. Therefore, this modelcannot explain the range of REE patterns found.

Spoon-shaped REE patterns are characteristicof ion-exchange processes [32-34]. Several au-thors have shown that in percolation-controlledfluid migration in which the fluid evolves throughion exchange with the matrix (wallrock), concen-tration 'fronts' of individual elements developwithin the migrating fluid in a 'column'. Therelative positions of these fronts depend in parton the fluid/matrix velocity, permeability, andfluid/matrix partition coefficients [32-34]. Ingeneral, the more incompatible LREE frontstravel faster than the middle REE and HREEfronts. Interaction between melts similar to theHV and wallrock can produce enrichments inLREE and depletions in HREE in the wallrocks[331. The spoon-shaped patterns which character-ize the KPK xenoliths may reflect passage of onlythe La, Ce and Nd fronts through that part of themelt column which was interacting with the wall-rocks. Preservation of zoned LREE abundancesfrom core to rim of individual grains in several ofthe KPK xenoliths shows that solid-state diffusionof these elements within clinopyroxene did noteliminate the concentration gradients across indi-vidual grains. Vasseur et al. [34] showed that in arock of highly variable grain size the smallergrains would develop uniform REE enrichmentmuch faster than the coarse grains. This is exactlywhat is seen in 77KALI-5, in which REE weredetermined on small groundmass grains as well ason coarse porphyroclasts (Fig. 4). Thus, thespoon-shaped chondrite-normalized REE pat-terns in the KPK xenoliths and the LREE zoningare consistent with a chromatographic fractiona-tion process.

The lack of spoon-shaped patterns in SLCxenoliths and their lower HREE contents arealso consistent with the ion-exchange hypothesis.Because alkali basaltic magmas, like HV, havelower HREE than tholeiitic (MORB) lavas, it islikely that the ion-exchange equilibration processwill lead to lower HREE in the matrix. Thematrix in this case is the lithosphere that initiallyequilibrated with MORB (high HREE) and sub-sequently interacted with alkali HV magma (low

REE). Figure 3 shows that the SLC lherzoliteclinopyroxenes with convex-upward REE pat-terns could have equilibrated with HV magmas.The lack of REE-zoning in SLC clinopyroxenessuggests, in contrast to KPK xenoliths, that suchequilibration reached completion beneath SLC.

Why should the bulk of the SLC Iherzolitesreach greater equilibration than KPK? One possi-bility is that large-scale porous flow occurred inthe mantle and SLC samples were close to theinput of the percolating fluids. Hence they com-pletely equilibrated with the fluid. In contrast,the LREE-enriched KPK xenoliths (and at leastone SLC sample) were distant from the fluidinput so that their compositions reflect equilibra-tion with only the rapidly migrating highly incom-patible elements, such as LREE. An alternativeview, which we prefer, is that the xenoliths pref-erentially sample metasomatized wallrocks ofmagma ascent conduits formed by magma fractur-ing. In both interpretations the greater extent ofequilibration for SLC xenoliths is consistent withthe SLC xenoliths originating from deeper mantlelevels than KPK xenoliths. Although useful geo-barometers for spinel lherzolites do not exist,there is evidence that the SLC xenoliths camefrom a deeper level. For example, garnet-bearingassemblages are only found in SLC and not inKPK vents. Pyroxene thermobarometry of suchgarnet-bearing assemblages indicates that theylast equilibrated at a depth of 60-75 km [5].Many of these garnet-bearing assemblages formveins in spinel lherzolites which must thereforealso come from such depths in the lithosphere.Because no garnet-bearing xenoliths have beenfound in KPK vents, some SLC xenoliths musthave come from deeper levels than did KPKxenoliths. The similar temperatures estimated forSLC and KPK xenoliths [5] do not necessarilyreflect a similar depth of origin because the tem-peratures are unlikely to reflect a geotherm [4,35].

5. Summary and a model

Clinopyroxenes from the sub-Koolau litho-sphere vary significantly in REE abundances. Forexample, spinel lherzolite xenoliths from the KPKvents range from LREE-delpleted (chondrite-normalized) to strongly LREE-enriched; core torim increases in LREE are common in these

66

EVOLUTION OF THE LITHOSPHERE BENEATH HAWAII

clinopyroxenes. In contrast, clinopyroxenes fromspinel lherzolite xenoliths in the SLC vent are notzoned in LREE, they have lower HREE contentsand many have convex-upwards chondrite-nor-malized REE patterns. The LREE-depletedclinopyroxene from Hawaiian xenoliths are not asdepleted in LREE as those from abyssal peri-dotites. If the oceanic upper mantle is composi-tionally zoned with depth, abyssal peridotites maybe typical of upper oceanic mantle that is highlydepleted as a result of continuous MORB extrac-tion during ascent whereas the Hawaiian xeno-liths with lesser amounts of LREE-depletion anda higher proportion of clinopyroxene may reflectdeeper sections of the oceanic mantle that haveexperienced lesser amounts of melt extraction.

In Fig. 14 we use REE data to propose aschematic model for the evolution of the oceaniclithosphere beneath Hawaii. As a result of vol-canism at a ridge axis the oceanic lithosphere iszoned in REE content, with the lower portionsignificantly less depleted ((Ce/Nd). > 0.2) thanthe upper portion ((Ce/Nd), < 0.2). The xeno-liths do not provide evidence for interaction withthe plume-derived Koolau magmas; i.e., the xeno-liths, except for one (Fig. 8), do not have thedistinctive Sr and Nd isotopic ratios of Koolaulavas and several xenoliths retain the relativeLREE depletion that is characteristic of MORB-related mantle. Earthquake data on Loihi, MaunaLoa and Kilauea suggest that shield-buildingmagmas ascend by fracturing from a depth of 60km [36,37]. Below this depth magmas are likely tobe transported in diapirs [36]. Based on the oc-currence of cumulate dunite xenoliths in HVvents in the Koolau caldera area, the plumbing

67

system for the ascent of Koolau magmas had anapproximate radial diameter of < 3 km (Fig. 1).

Several geochemical characteristics, such as Ndand Sr isotopic ratios, LREE enrichment andREE zoning, indicate that some of the KPK andall of the SLC xenoliths were residual oceanicmantle that was subsequently enriched in LREEby percolating melts related to such magmas asthe HV. The wide variety of xenoliths in HVmagmas provides evidence for ascent via deepfractures; e.g., an equilibration depth of 90 kmwas estimated for a garnet-dunite xenolith fromSLC [36]. It is likely that during the ascent of HVmagmas enrichment of wallrocks occurred byion-exchange chromatography as melts perco-lated laterally away from the main magma con-duits. The REE patterns of clinopyroxene in KPKxenoliths reflect a relatively early stage of theion-exchange process in which only the LREEinteracted with the matrix. In contrast, convex-upward REE patterns in clinopyroxene from manyof the SLC xenoliths reflect nearly complete equi-libration with HV melts. The greater degree ofequilibration for SLC xenoliths is consistent withother data that suggest SLC xenoliths were de-rived from deeper levels than KPK xenoliths.

Acknowledgements

Most (samples with the prefix 77) of the ana-lyzed xenoliths belong to Prof. D. Presnall's col-lection (University of Texas at Dallas). 69SAL-211B, -20-B and -119ABX are from the DaleJackson collection (Smithsonian Institution).Three composite xenoliths (77SL-2, -6 and -8)were donated by Professor G.P.L. Walker (Uni-

APPENDIX

Change in REE (ppm) and Na2 O and TiO2 (wt%) in clinopyroxene as a function of Rayleigh fractional melting was calculated forthe two following starting compositions:

Na2 O TiO2 Ce Nd Sm Eu Dy Er Yb

1 1.32 0.37 2.13 4.24 2.21 0.55 4.38 2.88 2.122 1.5 0.4 2.1 4.3 1.19 0.53 4.3 2.85 2.1

The equations used are A6 (for batch) and A4 (fractional melting) in Johnson et al. [12]. The Ds were calculated using Kds given inElthon [18] for Na and Ti and in Johnson et al. [12] for REEs, and the mode for 77KAPS-8 given in Sen [5].Calculation of Ps for use in the equations is complicated by the lack of experimentally determined modes of multiply saturatedpseudo-invariant points or pseudo-univariant curves at pressures greater than about 10 kbar. The Ps were calculated at moderatepressure used in Fig. 10 using the following equation from Kinzler and Grove [28]:

1.0 melt + 0.3 olivine = 0.4 orthopyroxene + 0.82 clinopyroxene + 0.08 spinel

G. SEN ET AL.

versity of Hawaii). This work was supported byNSF grants EAR-8815858 and 8903879 (Sen).Manuscript preparation was partly supported byEAR 90-18437 (Leeman). We thank SammanthaLane and Zimin Gao, ex-graduate students of theFlorida International University, for their helpwith data acquisition. We are very grateful to H.Stosch, K. Johnson and H.G. Wilshire for theirthoughtful official reviews. GS is indebted to J.-L.Bodinier for his insightful comments. These re-views have been of great help in revising themanuscript. We are entirely responsible for theviews expressed here, and these differ in severalrespects from those of the reviewers.

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