Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy

EPSL ELSEVIER

Earth and Planetary Science Letters 150 (1997) 245-260

Plume-lithosphere

interactions in the ocean basins: constraints from the source mineralogy

Comelia Class a3b, * , Steven L. Goldstein a~1 a Max-Planck-lnstitutfir Chemie, Postfuch 3060. D-55020 Ma&, Germany b Lamonf-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA Received 25 April 1996; revised 2 May 1997; accepted 8 May 1997

Abstract Trace element relationships of near-primary alkalic lavas from La Grille volcano, Grande Comore, in the Indian Ocean, as well as those of the Honolulu volcanic series, Oahu, Hawaii, show that their sources contain amphibole and/or phlogopite. Small amounts of each mineral (2% amphibole in the source of La Grille and 0.5% phlogopite plus some amphibole in the source of the Honolulu volcanics) and a range of absolute degrees of partial melting from N 1 to N 5% for both series are consistent with the observed trace element variation. Amphibole and phlogopite are not stable at the temperatures of convecting upper mantle or upwelling thermal plumes from the deep mantle; however, they are stable at pressure-temperature conditions of the oceanic lithospheric mantle. Therefore, the presence of amphibole and/or phlogopite in the magma source region of volcanics is strong evidence for lithospheric melting, and we conclude that the La Grille and the Honolulu series formed by melting of the oceanic lithospheric mantle. The identification of amphibole f phlogopite in the source region of both series implies that the metasomatism by fluids or volatile-rich melts occurred prior to melting. The presence of hydrous phases results in a lower solidus temperature of the lithospheric mantle, which can be reached by conductive heating by the thermal plumes. Isotope ratios of the La Grille and the Honolulu series display a restricted range in composition and represent compositional end-members for each island. Larger isotopic variations in shield lavas, represented by the contemporaneous Karthala volcano on Grande Comore and the older Koolau series on Oahu, reflect interaction of the upwelling thermal plumes with the lithospheric mantle rather than the heterogeneity of deep-seated mantle plume sources or entrainment of mantle material in the rising plume. Literature Os-Sr isotope ratio covariations constrain the process of plume-lithosphere interaction as occurring through mixing of plume melts and low-degree melts from the metasomatized oceanic lithospheric mantle. The characterization of the lithospheric mantle signature allows the isotopic composition of the deep mantle plume components to be identified, and mixing relationships show that the Karthala and Koolau plume end-members have nearly uniform isotopic compositions. Based on independent arguments, isotopic variations on Heard and Easter islands have been shown to be a result of mixing between deep plume sources having distinct isotopic compositions with lithosphere or shallow asthenospheric mantle. To the extent that these case studies are representative of oceanic island volcanism, they indicate that interaction with oceanic lithospheric mantle plays an important role in the compositions of lavas erupted during

* Corresponding author. E-mail: [email protected] ’ Also at: Department of Earth and Environmental Sciences,

Columbia

0012-821X/97/$17.00 0 1997 Elsevier Science B.V. All rights resewed. PI1 SOO12-821X(97)00089-7

University,

Palisades,

NY, 10964, USA.

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Science Letters IS0 (19971 245-260

the shield-building stage of plume magmatism, and that isotopic compositions of deep mantle plume sources are nearly uniform on the scale that they are sampled by melting. 0 1997 Elsevier Science B.V. Keywords:

amphibole group; phlogopite; melting; lithosphere; oceanic crust; mantle plumes

1. Introduction Most oceanic island basalts (OIB) are considered to be generated by the melting of upwelling material from boundary layers in the deep mantle [I]. Their isotopic compositions indicate long residence times outside of the convecting shallow mantle source of mid-ocean ridge basalts (MORB), and thus they carry information on the differentiation and convective history of the Earth. However, oceanic island magmatic rocks do not represent the composition of the deep mantle sources in a straightforward manner. Many oceanic islands display large isotopic and trace element variations, and until now there is no consensus as to whether they reflect intrinsic heterogeneity of the deep mantle source, mixtures of the plume source with entrained mantle, or contamination of plume-derived melts by shallow lithosphere. The possibility that lithospheric mantle contributes significantly to continental intra-plate volcanics is generally accepted (e.g. [2] and refs. therein). Beneath the oceans, the significance of lithospheric mantle contributions to hotspot- or plume-related lavas is still a matter of debate. The isotopic composition of young oceanic lithosphere is well constrained by studies of MORB. Isotopic variations from enriched ‘plume’ compositions in shieldbuilding lavas towards a ‘MORB-like’ composition in post-shield lavas, as observed in Hawaii, have been taken to indicate increasing contributions from the lithosphere (e.g. [3]). However, such an interpretation is equivocal since asthenospheric upper mantle entrained in a rising diapir would show similar isotopic features, and such a contribution has been suggested for the Galapagos Islands [4]. Although lithospheric contributions to some plume-related volcanics have been suggested, their role is not clear; a geochemical tool needs to be found to identify lithospheric mantle sources independent of their isotopic composition. Here we use a simple trace element approach to identify minor phases in magma sources. We present

evidence that lavas of La Grille volcano, Grande Comore, in the Indian Ocean are derived from sources containing amphibole. This indicates melting of the lithospheric mantle, because amphibole is stable under lithospheric temperature conditions but neither in the convecting upper mantle and nor in upwelling thermal mantle plumes. Identification and charactetization of the lithospheric end-member has important implications for the composition of aging oceanic lithosphere and deep mantle plume sources, melting relationships beneath oceanic islands, and the meaning of ocean-island basalt isotopic variations. We show that, in many cases, the main shield-building lavas in ocean islands are mixtures of plume and oceanic lithospheric melts, and isotopic compositions of deep plume sources are identifiable and nearly uniform.

2. Geological setting and results Grande Comore is the youngest island of the Comorian island chain in the western Indian Ocean. Ages of the main constructional phases increase along the chain with distance from Grande Comore [5], consistent with formation by a hotspot. Magnetic anomalies in the Somali Basin [6] indicate that Grande Comore formed on N 140 Ma old oceanic crust. Grande Comore consists of the adjacent volcanoes La Grille and Karthala. Karthala is an alkali basaltic shield volcano, while La Grille lavas are associated with a series of cinder cones aligned along fissures [7,8]. Both La Grille and Karthala have been active concurrently, although the average age of La Grille is thought to be older [8]. La Grille lavas are highly silica, undersaturated and mainly basanitic, but there are also rare alkali basalts and nephelinites [7]. They are near primary melts, as indicated by high Mg#, Ni and Cr contents (averaging 380 ppm and 730 ppm, respectively, Table l), and the presence of abundant mantle xenoliths. Olivine, commonly with chromite inclusions, is the

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Table

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Science Lmers

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150 (1997) 245-260

I

Major elements,

trace elements,

and Nd-Sr-Pb

isotope ratios of La Grille lavas

Sample

G88-6

G88-12

G7

G8

GlO

Gil

G14

G15

G16

G17a

KG3

KG4

SiO TiOt

42.98 2.17 12.60 0.20 12.52 12.45 2.91 1.31 0.49 3.12 8.24 0.18 0.03 69.2 -5 43 598 80 651 27 210 738 303 7.18 1.63 4.86 4.5 33.9 56.90 109.49 11.76 42.88 7.58 2.30 6.52 0.88 0.93 2.51 0.34 2.17 0.31

42.58 1.88 10.84 0.19 16.64 11.27 2.58 1.17 0.44 2.69 8.87 0.12 0.03 14.5 8 31 524 58 559 22 165 946 521 5.35 1.20 3.25 3.9 30.9 43.6 77 _

42.6 1 1.84 11.21 0.19 14.91 12.16 2.86 1.08 0.63 3.49 7.79 0.20 0.04 73.0 3 32 590 69 721 25 163 873 490 7.82 1.71 3.62 3.8 30.1 63.70 115.46 12.83 47.47 8.14 2.40 6.60 0.88 0.85 2.23 0.29 1.79 0.25

43.11 2.08 11.82 0.19 14.91 11.82 2.71 1.10 0.44 3.66 8.32 0.16 0.04 71.8 3 29 456 56 566 24 159 784 440 4.83 0.96 3.15 3.7 31.9 42.26 81.94 9.39 34.94 6.16 1.90 5.60 0.79 0.80 2.10 0.27 1.62 0.23

45.16 1.96 13.09 0.19 12.02 1 I .36 2.12 0.89 0.34 2.33 9.99 0.22 0.06 66.3 -6 22 362 40 524 23 128 637 313 _ _

42.72 1.88 10.98 0.19

4’7.27 1.69 15.09 0.18 7.13 1 1.92 2.81 0.55 0.26 1.73 10.26 0.13 0.00 56.4 -20 12 239 24 367 24 105 296 96 2.15 0.45 1.36 2.6 32.1 19.8 39 _

42.54 1.96 9.75 0.19 19.06 9.87 2.46 0.98 0.40 3.25 8.98 0.11 0.01 76.0 16 30 412 56 512 21

44.02 1.71 12.91 0.19 12.80 11.63 2.85 0.96 0.41 2.87

42.79 2.08 12.31 0.19 13.42

46.96 1.76 14.54 0.18 9.37 11.08 3.00 0.62 0.25 2.21 10.23 0.12 0.03 60.3 - 15 13 296 39 361 22 138 481 152 _ _ _ _ _

30.67 58.30 6.12 27.19 5.41 1.77 5.20 0.73 _

42.32 1.94 10.06 0.20 17.39 11.61 2.85 1.23 0.58 3.48 7.83 0.16 0.04 75.9 11 35 571 13 652 23 177 1178 596 6.58 1.53 4.20 4.2 29.8 56.10 107.32 11.74 42.09 6.74 2.04 5.79 0.82 _

36.03 6.25 1.94 5.59 0.82 _

2.11 0.21 1.78 0.24

1.97 0.25 1.55 0.20

2.07 0.29 1.68 0.32

AlzO? MnO MgO CaO Na,O KzO P,O, Fe,O,c Fe0 H,O co, M@ x,1, Rb Ba Nb Sr Y Zr Cr Ni Th U Td

Hf SC La Ce Pr Nd Sm ELI Gd Tb Ho Er Tm Yb LU

37 6.42 2.02 0.85 1.0 0.28 1.70 0.23

_

16.25 11.44 2.69 1.12 0.43 4.21 7.42 0.13 0.02 74.1 I 33 531 59 571 23 167 849 486 _ _ _ _ _ 45.64 86.54 9.41

8.82 0.15 0.03 69.0 -4

I I .82 3.08 1.23 0.48 3.20 8.33 0.15 0.04 70.3 3I 43 602 78

0.32 I .92 0.26

163 1124 616 4.83 1.09 3.43 4.1 27.6 40.41 76.50 8.81 33.43 6.19 1.87 5.21 0.72 0.70 1.83 0.25 1.49 0.19

27 481 50 545 25 146 514 318 5.05 0.99 2.77 3.4 31.7 42.30 79.21 8.13 32.33 5.83 1.82 5.31 0.78 0.81 2.21 0.32 2.00 0.29

2.13 0.28

20.87 41.96 4.46 18.31 3.54 1.18 3.71 0.59 0.65 1.85 0.25 1.59 0.24

19 4.39 1.50 0.69 _ _

639 26 203 715 331 1.27 1.51 4.68 5.0 33.9 54.0 99 _ 46 7.50 2.34 _ 0.96 _ _ 0.29

Sample

G87-9

G88-2

G3

G5

G12

G13

G17b

G17d

G18a

G18b

KG1

KG2

KG5

SiO, TiO, Al,O? MnO’

45.40 1.I1 13.61 0.19 11.11 11.22 3.04 0.68 0.32 3.04

43.64 2.14 13.11 0.18 12.11 12.43 2.65 1.19 0.41 2.29

46.30 1.56 12.92 0.18 12.94 9.83 2.50 0.56 0.25 2.38

42.95 1.80 11.02 0.19 16.02 11.90 2.75 0.97 0.59 3.41

43.43 1.76 12.78 0.18 13.04 11.61 2.12 0.86 0.41 4.39

42.19 2.18 11.28 0.20 14.51 12.57 3.11 1.35 0.59 4.14

43.24 2.06 12.03 0.19 12.97 11.36 3.00 1.04 0.44 2.86

43.9 I 1.64 12.13 0.18 14.37 10.73 1.96 0.52 0.37 3.18

43.40 1.67 11.22 0.18 16.24 10.52 1.89 0.71 0.40 3.11

41.79 1.80 10.92 0.17 15.86 12.55 2.69 0.64 0.7 1 3.98

43.45 2.08 12.69 0.19 12.17 11.84 2.86 1.00 0.43 2.64

43.00 2.07 12.14 0.18 13.40 11.35 3.07 1.09 0.44 2.14

42.71 1.78 10.94 0.19 15.92 11.92 2.17 0.97 0.59 1.81

MgO CaO Na,O K,O PZO, Fe,O,c

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Table 1 (continued) Sample

G87-9

G88-2

G3

G5

G12

G13

Gl7b

G17d

G18a

Gl8b

KG1

KG2

KG5

Fe0 HzG CO* Mg# X0’ Rb Ba Nb Sr Y Zr Cr Ni

9.58 0.15 0.03 64.1 -9 15 319 37 429 23 136 588 267

9.17 0.16 0.00 68.1 -6 30 478 55 548 24 163 610 260

10.21 0.13 0.11 67.5 -3 13 247 27 340 22 102 529 360

8.20 0.28 0.22 73.8 6 26 561 65 684 23 154 979 528

7.25 1.09 0.04 69.8 -3 23 476 52 536 24 145 612 319

7.63 0.24 0.03 71.7 2 40 603 78 683 25 195 934 413

9.73 0.18 0.03 67.6 -3 23 410 51 562 22 152 647 372

9.03 1.20 0.08 70.5 1 12 305 39 442 22 118 685 423

8.30 1.33 0.07 73.3 7 16 355 42 486 20 124 823 553

7.21 0.93 0.09 74.4 6 14 703 81 675 25 180 922 480

9.58 0.26 0.00 66.8 -6 24 457 47 530 24 147 619 305

10.26 0.09 0.01 68.5 -2 24 411 50 551 22 154 631 364

9.50 0.13 0.06 73.9 6 26 417 47 499 22 145 714 387

Samples7

G88-6

G88-12

G3

G7

G8

GlO

Gil

Sr/s6Sr ‘43Nd/ ‘44Nd ‘06Pb/ ‘04Pb ‘07Pb/ ‘04pb 208Pb/204Pb

0.703309 0.512847 19.192 15.579 39.044

0.703176 0.512872 -

0.703212 0.512843 -

0.703178 0.512870 19.234 15.583 39.076

0.703188 0.512873 19.211 15.576 38.993

0.703293 0.512849 19.179 15.584 39.047

0.70315 0.51287 19.21 15.59 39.09

Samples7

G12

G14

G15

G16

Gl7a

Gl8a

KG3

Sr/ s6 Sr ‘43Nd/ ‘44Nd ‘06Pb/ ‘04Pb “‘Pb/ ‘04Pb “sPb/ ‘04Pb

0.703309 0.512844 19.135 15.588 39.006

0.703219 0.512874 19.148 15.563 38.972

0.703193 0.512853 19.103 15.555 38.894

0.703283 0.512866 19.203 15.571 39.032

0.703239 0.512857 19.186 15.585 39.030

0.703120 0.512905 19.160 15.568 38.961

0.703277 0.70331 0.512855 0.51285 19.176 19.27 15.580 15.60 39.042 39.152

KG4

Major and trace elements were measured by isotope dilution (Th, U, Sm and Nd on G88-6, G7, G8 and G16) or high performance liquid chromatography (REE except G88-12 and G15) [63] at MPI, Mainz; INAA (Th and U on KG 3, REE on G88-12 and G15) or XRF (all other - 14wt%) and gives elements) at the University of Karlsruhe. Mg# = lOOMg/(Mg + 0.9 X Fe,,,). X,’ = lOO(MgO,,,, - 14wt%)/(MgO,, the percentage of olivine fractionation (negative values) or accumulation from a parental magma with assumed MgO = 14 wt%. The olivine composition was assumed to be 85 mol% forsterite, based on microprobe analyses of olivines in some La Grille volcanics. Isotope ratios were measured by static multicollection on a MAT 261 TIMS at MPI, Mainz. s7Sr/ s6Sr are normalized to ‘“Sr/ ‘s.Sr = 0.1194. Over the course of this study, NBS 987 Sr standard values were 0.710229 ir. 20 (2 (T external reproducibility, II = 9). ‘43Nd/ ‘44Nd are normalized to = 0.511860 based on measurements of the La Jolla standard, which were ‘46Nd/ ‘44Nd = 0.7219 and corrected to ‘43Nd/‘44Nd 0.511851 * 18 (20 external reproducibility, n = 12). Pb isotopes were measured on leached rock chips. Samples were corrected for mass fractionation by a factor of 0.13% a.m.u.-’ for 207Pb/ zo4Pb and “*Pb/ 204Pb and 0.12% a.m.u.-’ for ‘06Pb/ 204Pb based on 19 and 9 measurements of the NBS 982 Pb standard, respectively, during two different periods of measurements. The reproducibility of the NBS 982 standard is estimated to be 0.044% (period 1; 2a external reproducibility, n = 19) and 0.039% (period 2; 2rr external reproducibility, n = 9). All samples were measured at least twice, the listed data are averages. The total procedure blank is less than 200 pg Pb and is therefore considered negligible.

only phenocryst phase. Lavas display a large range of SiO, (42-47%) and fractionation-corrected incompatible trace element abundances (e.g. Nb varies by more than a factor of 4, Table 1, Fig. 1). In contrast to the large chemical variations, the isotope ratios of La Grille lavas show a very small variation (*‘Sr/ 86Sr = 0.7032-0.7033, 143Nd/ 144Nd = 0.51284-0.51291, 206Pb/ ‘04Pb = 19.1-19.27,

207Pb/ *04Pb = 15.55-15.6, 208Pb/ 204Pb = 38.939.15; Table 1). The combination of near-primary compositions, large trace element variations, and nearly uniform isotope ratios indicates that La Grille lavas are a cogenetic suite, formed by variable degrees of melting from a common mantle source. Therefore, trace element relationships reflect differences in the melting regime.

C. Class, S.L. Goldstein/Earth

and Planetary

The partitioning of a trace element, j, between a melt and the solid residue depends on the solid phases present. Increasing ‘compatibility’ with solid phases during partial melting is reflected by higher values of the bulk solid/melt distribution coeffi-

,

f 1997)

249

245-260

D, cients (Djsolid’melt) [9]. Methods of estimating through inversion of trace element data on lavas has been extensively discussed (e.g. [ 10-161). The method we employ compares the relative enrichments of trace elements in different fractional melts (cf. [ 15,17,18]) but utilizes the sequence of elements in conventional trace element diagrams to compare

3. Trace element variations and residual phases

”

Science Letters 150

I

,

W21 4.0

,..l

noI..,.. 1500

"

"

1200

Prl

”

”

60 60 40 20

0o

5

FH

40LNb60

;

FL

100

VI

L

o

;

FH

40LNb160

i

100

FL

Fig. 1. Element variations in La Grille, Grande Comore, used to estimate the enrichment ratios (E = CFL/CFH, discussed in the text). Selected incompatible trace elements are shown versus Nb as an index of variable degrees of partial melting (F). Brackets signify that concentrations are corrected for minor olivine fractional crystallization or accumulation [46]. CF, and CF, are estimated for low and high degrees of partial melting (F, and FH) through a double linear regression. In this suite, only Ti shows a kink in the correlation with Nb. Constant Ti for [Nb] > 60 indicates buffering during low F by a residual phase with a high (kT”/me”. At lower [Nb], it behaves as an incompatible element, and ETi (Fig. 3) is based on samples with [I%] < 60. Concentrations are wt% for K,O and TiO,, otherwise ppm. Relative values of E are related to the slopes. They can be compared directly because y axes are scaled to the same factor of 5 X C, The slopes of Ba and K,O vs. Nb, for example, are lower than Th vs. Nb. Diamonds = samples with clear alkali loss due to alteration a;d are not included in the estimate of the enrichment ratios. Almost all of the other samples have constant Rb/Nb and slightly increasing K/Zr ratios with increased melting, which shows that Rb and K contents are not affected by alteration. Only a few samples with low Rb and K contents show significantly lower Rb/Nb and K/Zr than the rest of the samples, suggesting a small but significant amount of alkali loss as a result of post-eruptional alteration. They were not included in the calculation of the Rb and K enrichment ratio. Alteration effects are not observed for other elements. For example, Ba displays a good linear correlation with Nb. which would be destroyed if Ba was affected by alteration.

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the relative values of Dj. The method can be applied to cogenetic suites of near primary magmas formed from a chemically uniform source by variable degrees of melting. We illustrate the method on the La Grille volcanic suite. A highly incompatible element, i, is chosen as an index for the degree of partial melting. We use Nb because it is measured on all samples, behaves as a highly incompatible element, and is not easily modified by secondary processes. Element-element plots display lines or smooth curves, depending on the Dj values and the range of partial melting [ 10,151 (Fig. 1). Low and high degrees of melting (FL and F,) are represented by high and low concentrations of the index element (C NbF and C NbF,, respectively). Concentrations of other elements at F, and F, (CjF, and CjF,> are estimated based on their correlation with the index element (Fig. 1). Elements which are more compatible with the residual mineral phases are held back from the melt compared to more incompatible elements, therefore the compatible element is less enriched at F, compared with F,. This means that the ‘enrichment ratio’ (Ej = CjrL/C,rHl reflects the bulk distribution coefficient during partial melting, and for any two elements j and i, if E, > E, then Dj < Di. The relative range in the degree of partial melting for a lava suite can be estimated by the enrichment ratio of highly incompatible elements with D N 0 (i.e. R = CFL/CFH). An important aspect of this approach is that the enrichment ratios are independent of the chemical composition of the melting source, depending only on the mineralogy and the degrees of melting F, and F,. The enrichment ratio (E) is related to D in a complex way; however, general features are easily illustrated using equilibrium batch and fractional melting equations. E is a smooth function of D for both batch and fractional melting (Fig. 2). Relative D values can be evaluated in the range of D = O.OOl0.1. For more incompatible and compatible elements, E approaches constancy and relative D values cannot be estimated. Despite this limitation, the method effectively allows recognition of large differences between D values of different elements. Most element pairs in La Grille lavas show linear abundance correlations (Fig. 1). This relationship is consistent with binary mixing of distinct sources as well as with variations in the degree of partial melt-

J

0.0001

0.001

0.01

0.1

1

Distribution coefficient

Fig. 2. Enrichment ratios (E = CF, /CFH) as a function of the distribution coefficient D. Shown are curves for modal batch and fractional melting calculated for a degree of melting FL = 1% and F, = 4.5%: C Batch melting: E = $

Fractional

D+F,x(l-D) r,

=

C melting: E = 3

D+F,

X(1-D)

F, (I-(l-F,)“o) = F_ X I_ (l-(l-FL)““) FH

For highly incompatible elements with D < 0.001 the enrichment ratio approximates the value E = Fn /FL, which corresponds to the total range of partial melting and is a characteristic constant for a cogenetic lava suite. In the range of about 0.001 < 0.1 the enrichment ratios decrease smoothly as a function of the distribution coefficient and therefore can be used to estimate the sequence of incompatibility of elements during partial melting.

ing. In La Grille, the element variations are unlikely to be a consequence of binary mixing, for the following reasons. Firstly, the Th-Nb correlation passes through the origin (Fig. 11, which is a requirement for elements having similar D values if the lavas are related by partial melting but a very special and unlikely case for mixing. Secondly, the Ti-Nb variation is non-linear (Fig. 1). Ti is constant for Nb > 60 ppm and decreases regularly for lower Nb. This is inconsistent with binary mixing, rather, it indicates that Ti is buffered by a phase present in the smallest degree melts but absent in larger degree melts. Thirdly, the nearly uniform Pb, Nd, and Sr isotope ratios of La Grille lavas indicate a common source. Although linear element variations are possible as a result of mixing of melts from isotopically similar but mineralogically different sources [ 191, this would be unlikely to reproduce the large range in incompatible trace element contents of a factor greater than 4. Altogether, these considerations strongly support our conclusion that the La Grille element variations reflect different degrees of partial melting of a common mantle source.

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4. Enrichment ratio patterns and magma source mineralogy The mineralogy of the La Grille magma source can be evaluated using a trace element diagram with the elemental sequence used for oceanic basalts (Fig. 3). This sequence reflects increasing element compatibility from left to right with an anhydrous spine1 or garnet lherzolite source [ 161(01 + opx + cpx + sp f gntl. Partial melting of this assemblage using appropriate mineral-melt distribution coefficients results in enrichment ratios which plot in a smooth pattern (Fig. 3b). The enrichment ratio pattern of La Grille (Fig. 3) is not smooth but shows negative anomalies for Ba, K and, to a smaller extent, Sr, relative to neighboring elements (Fig. 3a). This is a clear indication for the presence of a residual phase or phases other than the normal anhydrous mantle assemblage in the source of the La Grille volcanics, which hold back these elements from the melts. Amphibole and phlogopite have higher mineral/melt distribution coefficients (k~n’me’t) for K, Ba, and Rb than phases of an anhydrous mineral assemblage, and are stable under certain mantle pressure-temperature (p-T) conditions. Both phases are common in alpine-type peridotite massifs and metasomatized mantle xenoliths [20]. High quality k, values of many incompatible elements are now available for pargasitic-amphibole and phlogopite [2 l-231. Both Rb and Ba behave as compatible elements with respect to phlogopite (Fig. 41, while in amphibole Rb is moderately compatible (Fig. 3). Although melting studies have indicated that amphibole breaks down at the solidus [24,25], in which case its trace element signature might not be expected in the resulting lavas, these melting experiments do not resolve phase relationships of low degree partial melts. Amphibole has been shown to be stable above the wet solidus, in equilibrium with a carbonatite melt [26], and in the form of a hydroxyl-fluoride solid solution [27]. In La Grille, the more compatible behaviors of Rb, Ba, and K, relative to the neighboring elements in the compatibility sequence for anhydrous peridotite melting (Fig. 31, combined with the observation that Rb is less compatible than Ba, are strong evidence for the presence of amphibole in the magma source. Although we see evidence for amphibole in the melting zone of La Grille, it has only been reported

a.

C. 0

I,‘#,,‘, RbBaTh

U

K

NbLaCeSrNdZrSmEuTiGdY

ErYb

Fig. 3. Magma source mineralogy of La Grille magmas. (a) Enrichment ratio patterns (E = Cr, /C rr,) for the La Grille suite. (b) Enrichment ratio patterns calculated for melts of garnet- and spinel-lherzolite and F, = 1%. Melting of normal mantle mineralogies do not reproduce the observed negative spikes in the enrichment ratio pattern of the La Grille suite. (c) Best-fit enrichment ratio pattern calculated for modal batch partial melting of an amphibole-harzburgite. For comparison, the enrichment ratio pattern of the La Grille suite is shown as a light gray curve. The excellent tit is taken as a clear indication for residual amphibole in the source of the La Grille volcanics. Data sources for (k?‘” ‘“‘e”) values: 01, opx. cpx, sp. gt [64-671 and references therein; Dr” = DNb, DRb = 0 was assumed; pargasitic amph and mica [2123.64.681. If no experimental data were available DY = DE’, and DTh = D” were assumed. Modes and F, used: garnet-Lherzohte ol:opx:cpx:gt = 60:25:9:6 (12:25:30:33); spinel-lherzolite ol:opx:cpx:sp = 58:30:10:2; amphibole-lherzolite ol:opx:cpx:gt:amph = 72:22:3: 1:2; F, = I G/F in all cases; numbers in parenthesis are melt modes. The gray curve for garnet-lherzolite illustrates the effect of non-modal versus modal melting, which is significant only for compatible elements. For spinel-lherzolite two curves calculated for different F, are shown (black: F,_ = 1%: gray: F, = 2%‘).

as a crystallizing phase in evolved OIB lavas with MgO < 6 wt%, and it has not been observed in La Grille lavas. Either the amphibole enrichment ratio pattern (Fig. 3) reflects small degree melts from the edges of the melt envelope, which then have a large effect on the trace element signature but a small effect on the major elements. or, alternatively, shal-

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Ti

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I”““‘i”l” I menite-harzburgite

Rb

Ba

Th

K

la

La

Ce

Sr

Nd

Zr

Sm

Eu

Ti

Gd

Y

Yb

Fig. 4. Magma source mineralogy of the Honolulu series magmas: (a) enrichment ratios (E = CF, /CFH) for the Honolulu Series. E was calculated for samples with Mg# > 58. Th is the index element, and ClhF,= 10 ppm and CThF, = 2 ppm. Data are from Clague and Frey [ 181. (b) Best-fit enrichment ratio pattern derived from a phlogopite-amphibole-ilmenite-harzburgite mineralogy by modal batch melting with F, = 1%. For comparison the Honolulu enrichment ratio pattern is shown as a light gray curve. The excellent tit is taken as a clear indication of residual phlogopite + amphibole in the source of the Honolulu volcanics. Data sources for (k$‘in/me’t) values are as in Fig. 3 plus ilmenite [69]. Mode used: ol:opx:cpx:gt:phlog:amph:ilm = 72:21.5:3:1:1:0.5:1.

low degassing of the magmas was sufficient to destabilize amphibole. Based on the normalized trace element pattern of La Grille lavas, the presence of amphibole in the source of the La Grille volcanics has also been suggested in an independent study [77]. The presence of amphibole in the mantle beneath La Grille is further supported by its presence in melt inclusions of La Grille mantle xenoliths [28]. We have compared a calculated enrichment ratio pattern for melting of an amphibole-bearing mantle source with the observed pattern of the La Grille volcanics (Fig. 3c) in order to constrain the modal mineralogy in the source and the degree of melting. Less than 1% garnet is needed to reproduce the low enrichment ratios for the heavy rare earth elements. The relative amounts of olivine, opx and spine1 are not constrained due to their low k, values. The range in degree of melting R = CFL/CFH, is estimated to be N 4.5 using the most incompatible element, in this case U. The absolute degree of melting depends on the amount of clinopyroxene present. For exam-

ple, in order to tit the enrichment ratios of the light rare earth elements for a harzburgite with only 3% cpx, F, is low ( - 1%). For a lherzolite with 9% cpx, F, would be higher (- 2%). With R, = 4.5 this indicates that F, = 4.5% or 9%, respectively. K is a major constituent of amphibole and the enrichment ratio depends on the phase relationships during partial melting, which are unknown. For illustrative purposes its value in the model melts is calculated, as in previous studies [21,22], as a k, based on estimated concentrations in melts in equilibrium with amphibole. The amount of amphibole in the source is estimated based on reproducing the relative enrichment ratios of Ba and K, given the absolute degrees of melting. The amount necessary is only 2% for the harzburgite and 5% for the lherzolite sources. Previous studies of oceanic island basalts have suggested that amphibole and/or phlogopite is a source phase for alkaline lavas (e.g. [17,18]). One example is the post-erosional Honolulu series of Oahu, Hawaii, where compositions include rare alkali basalts, basanites, nephelinites, and nephelinitemelilitites. We apply the trace element method to the Honolulu volcanics, which, like La Grille, represent near primary magmas from a common magma source [ 1829,301. Clague and Frey [ 181 suggested the presence of residual phlogopite, amphibole and a Ti-rich phase in the magma source, based on the small range of K-Rb-Nb-Ta-Zr-Ti-Hf-V concentrations compared to other incompatible elements. This shows up very well in the enrichment ratio pattern (Fig. 41, where the low enrichment ratios of Rb-Ba-K and Ta-Ti-Zr indicate the presence of phlogopite and a Ti-rich phase such as ilmenite in the source. There is still some debate about the k, values for Nb (and therefore Ta) in amphibole, based on the discrepancy between experimentally estimated low k,, values [21,22] and high k,, values derived from measured amphibole/clinopyroxene partition coefficients in mantle xenoliths (e.g. [31]). If amphibole serves as a major host for Nb, Ta and Ti in the source of the Honolulu volcanics [31], then there might not be a need for a Ti-rich phase in the mantle source of these rocks. However, ilmenite has been identified in rare amphibole-ilmenite-containing xenoliths from Salt Lake Crater, Oahu [32]. Irrespective of the presence of a Ti-rich phase, the element variations strongly indicate a residual K-rich phase in the magma source.

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In order to constrain the modal mineralogy of the source of the Honolulu series, we used the same approach as for La Grille (Fig. 3). The enrichment ratio pattern of melts derived from a phlogopiteamphibole-ilmenite-harzburgite source matches the pattern of the Honolulu volcanics (Fig. 4). A harzburgitic source requires lower degrees of partial melting than a lherzolitic one, as in La Grille, with F, = 1% and 2%. The amount of mica in the source is constrained based on reproducing the enrichment ratios of Rb and Ba, and given the absolute degrees of melting. Only 0.5% is necessary to reproduce the low enrichment ratios. Some amphibole might be present as well, but the amount cannot be constrained because its effect is outweighed by the more compatible behavior of Ba and Rb in phlogopite. In summary, the observations from Oahu and Grande Comore indicate that the presence of K-rich phases in the source of basanitic melts may be a general feature of the ocean basins.

5. Melting of oceanic lithosphere The identification of residual amphibole and/or phlogopite in the source of the La Grille and the Honolulu series lavas has important implications for the location of the magma sources. Fig. 5 compares the p-T stability fields of phlogopite and pargasitic amphibole with geotherms for asthenospheric upper mantle, thermal plumes, and old oceanic and continental lithosphere. The phase relationships show that amphibole is not stable in the asthenospheric upper mantle and in thermal plumes (Fig. 4). In both cases temperature is the limiting parameter rather than pressure. Both phlogopite and amphibole are stable in the old continental and oceanic lithospheric mantle (Fig. 5). In the cold and deep continental lithospheric mantle these minerals can be stable to high pressures, as indicated by high-pressure amphibole and phlogopite bearing xenoliths found in some continental lavas (e.g. [33]). The presence of fluorine or K-richterite extends amphibole stability to higher p-T conditions [27]. Only the pure F end-members are stable in astbenospheric conditions, but such a composition is improbable, because amphibole in both lherzolites and basalts has H,O/F > 1 ([34] and refs. therein). The dominance of water in mantle

Temperature

(Co)

3 15 5 E 20 3 ? 25 a 30

Fig. 5. Pressure-temperature (p-T) diagram showing stability fields of amphibole and phlogopite. Shown also are a geotherm for Archean shield (lower left comer of diagram), an asthenospheric mantle adiabat [48], a plume adiabat at about 300 K above the asthenosphere, the dry peridotite solidus [70], a water-undersaturated solidus, and amphibole-phlogopite stability fields [24.25]. In addition, solidi are shown of pargasitic amphibole with variable fluoride contents (amph(F,,OH,, ), amph(F,,,)) as well as hydroxyl- and F-K-richterite (K-r(OH,,,), K-r(F,,,)) [27]. Amphibole and phlogopite are stable in the oceanic lithosphere but not at conditions characteristic for the convecting asthenospheric mantle or thermal mantle plumes.

amphiboles suggests a low solidus temperature for ‘wet’ lithospheric mantle. Phlogopite might be stable in the asthenosphere deeper than w 150 km [35]. None of these phases are stable under thermal plume conditions. These considerations strongly suggest that an amphibole or phlogopite signature in an ocean island lava suite reflects lithospheric mantle melting. We conclude that the source of the La Grille lavas and the Honolulu series is the oceanic lithospheric mantle. For the Honolulu series this is consistent with previous suggestions that Hawaiian post-erosional lavas are formed by melting of the lithosphere, based on isotopic compositions and temporal evolution of Hawaiian lavas [3], and thermal considerations [36]. A lithospheric source is further supported by data on lithospheric mantle xenoliths from Oahu, where metasomatic veins have the same isotopic

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compositions as the host lherzolite and overlap with the Honolulu series [37]. The lower solidus temperature of ‘wet’ compared with ‘dry’ lithosphere (Fig. 5) allows melting by conductive heating from the upwelling thermal plume. This might be particularly applicable in the case of La Grille, which has been erupting contemporaneously with the shield-building basal& of neighboring Karthala volcano. Temperatures of the lithosphere-derived alkalic lavas would depend on the melting regime, and the amounts of F and CO, in the amphiboles, which increase the solidus temperature. In any case, they would be lower than parental melts of plume-associated basalts. We have not found direct temperature measurements of basanitic lavas, and temperature estimates from lava chemistry are also unavailable. Garnet-pyroxenite xenoliths from Oahu give p-T estimates of lOOO1150°C at 50-75 km [38]. Under these conditions melting would occur in the presence of hydrous phases (Fig. 5). In summary, amphibole and phlogopite are stable in the oceanic and continental lithospheric mantle but not in the asthenospheric mantle nor in thermal plumes because temperatures exceed the thermal stability limits. Therefore, the signatures of amphibole in La Grille and phlogopite in the Honolulu series strongly indicate that the magmas are derived from the oceanic lithospheric mantle. The lithosphere beneath the Indian and Pacific oceanic crust was metasomatized by volatile-bearing fluids subsequent to creation at mid-ocean ridges, which resulted in the formation of K-rich minerals, and possibly Ti-rich minerals in case of the Honolulu series.

Plume-lithosphere source compositions 6.

interactions

Koolau series in eastern Oahu. A oceanic lithosphere origin of the La Grille and Honolulu lavas raises the question of whether the lithosphere significantly contributes to melts more directly associated with the plume. In Grande Comore and Oahu, the isotopic composition of the lithospheric magma sources are nearly uniform on the scale sampled by the magmas, and can be compared with lavas more closely associated plume. The shieldwith the deep mantle-derived 0.5132

1

0.5130 B $ %

Koolau, Oahu 0.5126

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0.5126 ith. . 0.5124 0.705

0.704

0.706

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Koolau,

15.5

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Oahu

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-plume B 15.4

15.3 0.707

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,i

”

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lith.\ 0.706

% v,

0.705

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0.702

It is still a matter of debate to what extent the isotopic variations in individual ocean islands represents deep mantle plume sources themselves or contributions from other mantle sources, such as entrained lower and upper mantle or oceanic lithospheric mantle and crust. These issues can be addressed by comparing La Grille and the Honolulu series to the related main shield-building lava& represented by Karthala in Grande Comore and by the

18.5

19

“6Pb/204Pb Fig. 6. Isotope variations in lavas from Grande Comore, eastern Oahu and Heard islands. In all three cases, lavas from the volcano associated with the main constructional phase displays isotope variations indicating binary mixing between lithosphere and plume. n = La Grille; 0 = Karthala; + = Honolulu; 0 = Koolau; A = Heard; shaded field = mid-ocean ridge basalt (MORB). Data sources: Grande Comore, this study and [71]; Oahu [29,30.72.73]; Heard [60,61]; MORB, MPI data compilation.

C. Class. S.L. Goldstein/Earth

The possibility of lithospheric contributions to Hawaiian tholeiitic lavas has been discussed widely and we will highlight some main arguments (e.g. [3,40-451 and refs. therein). The depth of magma segregation of Hawaiian tholeiites is estimated from experimental data to be 50 km, in the spine1 stability heavy rare field (e.g. [45]), whereas fractionated earth element (HREE) patterns, indicating the presence of residual garnet (e.g. [46]), suggest an origin below N 78 km. A shallow origin for Hawaiian tholeiites appears to be supported by the elastically defined thickness of the Hawaiian lithosphere of N 30 km, but this corresponds only to the N 450°C isotherm [47]. The lithosphere defined as a thermal boundary layer with a bottom potential temperature of about 1280°C [48] is on the order of 100 km thick beneath Hawaii prior to plume activity (cf. [49]). About 50 km of thermal erosion of the lithosphere by the Hawaii plume would be required to enable

building lavas have higher “Sr/ “Sr and lower ld3Nd/ 144Nd ratios than the lithospheric melts (Fig. 6). Karthala and Koolau both display a range of isotope ratios trending toward those of La Grille and the Honolulu series, respectively. This implies binary mixing of the mantle plume and oceanic lithosphere components, and means that the isotopic variations of Grande Comore and Koolau lavas reflect mixing between the deep-mantle plume and the shallow lithospheric mantle. In the case of Grande Comore, this result is supported by recent U-series data, which correlate with Sr isotopic variations 1391. The shield lavas with the largest plume component are those with isotope ratios most different from the lithosphere. The isotopic compositions of both the deepmantle plume and the lithospheric components can be identified. On the scale sampled by the magmas, both the plume and the lithosphere have nearly uniform isotopic compositions.

o.‘46 7 0.142

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_1

Plume-lithosphere interaction: Mixing of plume tholeiitic melt with melts derived from metasomatized lithospheric mantle

I:

1

-

0.130

1

mamle 0.7030

J 0.7034

0.7038

0.7042

0.7046

87Sr/86Sr Fig. 7. Schematic illustration of the process of plume-lithosphere interaction for OS vs. Sr isotope variations. Percolation of plume melts through the lithosphere (not shown) and bulk peridotite incorporation would result in decoupling of OS and Sr isotopes or strongly curved mixing relationships respectively, inconsistent with the near-linear correlation in Hawaiian shield volcanics. This difference has been taken as evidence against plume-lithosphere interaction [53]. However, mixing of plume-melts with melts from the metasomatized lithospheric mantle [3,54] are consistent with the near-linear isotopic variation. A heterogeneous Hawaii plume source is not excluded by this model. Mixing curve parameters: Honolulu lithospheric mantle: *‘Sr/ s6Sr = 0.7033 [29.30]; Sr = 10 ppm; ‘“‘OS/’ ‘*‘OS = 0.124. OS = 3 ppb (peridotite values [74]); lithospheric basanitic melt Sr = 500 ppm. nephelinitic melt Sr = 1000 ppm [18]. OS content of melts OS = 0.3 ppb [53]. Koolau end-member (Fig. 1 in 15211, Sr = 300 ppm [75]. Sr and OS isotope data from Hawaii [53.54,76] and references therein: only samples with OS > 45 ppt are considered.

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shallow melting of the tholeiites. Neither a seismic estimate of the thickness of the lithospheric mantle beneath Hawaii [50] nor three-dimensional modeling of the interaction of thermal plumes with the moving lithosphere [5 11 indicate significant thermal erosion of the Hawaiian lithosphere. If garnet-pyroxenite or eclogite, instead of peridotite, is the source of high SiO, Hawaiian tholeiites (e.g. [52]), then they could form with a garnet HREE signature without a need to thermally erode the lithosphere to shallow levels. Both thermal erosion by the plume or rising of plume melts through a thick lithosphere suggest interaction between plume-derived melts and the lithospheric mantle. It has recently been suggested that OS isotopic variations in Hawaiian basalts are strong evidence against plume-lithosphere interaction [53]. This conclusion is based on the strongly compatible behavior of OS, which results in decoupling of OS from Sr, Nd and Pb during equilibrium porous flow of plume melt in peridotite. Os-Sr isotope relationships would show strong curvature if bulk peridotite mixes with plume melts, which is inconsistent with the near linear covariations (Fig. 7). However, Martin et al. 1541 have suggested that mixing of plume and depleted upper mantle melts, as suggested by Chen and Frey [3], is consistent with results from Os-Sr isotope data from Haleakala, Hawaii. Fig. 7 illustrates that the Os-Sr isotope systematics of Hawaiian lavas do not exclude plume-lithosphere interaction. Mixing of tholeiitic plume melts (Koolau end-member) and melts from metasomatized lithospheric mantle (Honolulu lithospheric end-member; basanitic or nephelinitic lithospheric melt) result in curvilinear relationships that fit the Os-Sr isotope systematics. Such a process is feasible because metasomatized lithospheric mantle mineral assemblages would melt due to conductive heating by the plume. Fast melt transport through the lithosphere [54,55] or coating of magma conduits with ‘frozen’ plume melts might explain the apparent lack of interaction between plume melts with peridotitic wall rock. A heterogeneous Hawaii plume source is not excluded by this model. It shows that the Os-Sr isotope mixing relationship does not exclude plume-lithosphere interaction but, rather, may constrain the mechanism of this interaction. The lithospheric melts represented by La Grille

and the Honolulu series have Nd-Sr isotopic compositions slightly different from Indian and Pacific MORB (Fig. 6). Either the aging oceanic lithosphere was infiltrated by small degree melts prior to plume activity, or the lithosphere was metasomatized by plume-derived fluids or melts prior to eruption of the La Grille and Honolulu lavas. Direct evidence for metasomatism of lithospheric mantle beneath Grande Comore and other intraplate oceanic and continental settings has been demonstrated based on the chemical composition, mineralogy, and high volatile contents of melt and fluid inclusions within minerals of mantle xenoliths [28]. They are not the results of in situ melting or infiltration of the host magma but, rather, are metasomatic melts from greater depths. The isotopic compositions of the lithosphere-derived melts, intermediate to MORB and the plume, are consistent with lithospheric metasomatism by plume-derived fluids. The binary mixing relationships displayed by isotopic variations of Grande Comore and Oahu show that the isotopic composition of the deep mantle plume is nearly uniform in both cases. Koolau is one of the three end-members identified in isotopic compositions of Hawaiian volcanics (e.g. [43] and refs. therein). Honolulu is the lithospheric mantle endmember. If both of the other end-members are derived from Hawaiian deep plume sources only one was sampled in Koolau. Binary mixing between a deep mantle plume and the lithosphere, both with nearly uniform isotopic compositions, was previously suggested for Heard Island [56,57], based on the isotopic evolution through time of lavas of the Ninetyeast RidgeKerguelen-Heard plume track. This interpretation (Fig. 6) is supported by helium isotopes [58]. Lavas with Pb, Sr, and Nd isotope ratios close to plume composition [56] have high 3He/4He ratios, typical of plumes, while others have 3He/ 4He ratios like MORB, typical of lithospheric mantle. In contrast to Grande Comore and Koolau, the Heard #time isotopic composition is more depleted (lower Sr/ 86Sr, higher ‘43Nd/ 144Nd) than the lithosphere. Heard is not located on normal oceanic lithosphere but, rather, on the Kerguelen Plateau, which has isotopic characteristics similar to continental crust (e.g. [59-611). Another example of binary mixing between a plume of nearly uniform composition and shallow mantle

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sources is Easter Island [62]. In this case, however, the contaminant is the convecting upper mantle due to the near-ridge position of the plume combined with the young age of the lithosphere where the island is located. In all these cases the interactions occur between shallow reservoirs and plumes. This suggests that binary isotopic variations in oceanic islands in general may result from interactions between deep plume sources having nearly uniform compositions and shallow mantle. Hawaii appears to be a special case in which the plume source itself may be heterogeneous, nevertheless both deep endmembers appear to be nearly homogeneous in composition.

lithosphere interaction. The metasomatizing agent is likely a plume-derived fluid; however, continuous metasomatism of the aging oceanic lithosphere is also possible. Shallow-level contamination of plumes with uniform compositions have been described in other ocean islands as well Grande Comore and Oahu. To the extent that these case studies are representative of OIB, they indicate that deep plume sources have distinct and nearly uniform isotopic compositions, and interaction with oceanic lithospheric mantle plays a major role in the composition of lavas erupted during the shield-building stage of plume magmatism.

7. Conclusions

Acknowledgements

Trace element systematics of the La Grille and Honolulu basanite suites indicate the presence of amphibole and phlogopite + amphibole in their melting regions. Their presence, combined with uniform isotope ratios over a large range in degree of melting is strong evidence for a lithospheric mantle source of these lavas, because the upper temperature stability limit of amphibole and phlogopite is well below temperatures for the convecting upper mantle and thermal mantle plumes. The identification of the shallow component constrains the cause of isotopic variability in some oceanic island suites. Lavas from Grande Comore and eastern Oahu form nearly binary isotopic mixing arrays. The lithospheric mantle component represented by the La Grille and Honolulu series forms one end-member. The plume-related shield-building volcanoes Karthala and Koolau show isotopic variations which extend between compositions of the shallow lithospheric end-member and the second end-member. We conclude that the isotopic diversity of Grande Comore and eastern Oahu shield-building volcanics reflects plume-lithosphere interaction. The process of plume-lithosphere interaction appears to occur through mixing of plume-derived melts with small-degree melts formed by conductive heating of the metasomatized portion of the lithospheric mantle. Mixing of melts is consistent with the Os-Sr isotope systematics of Hawaiian volcanics, previously taken as evidence against plume-

This work would not have been possible without the aid of P. Bachhlery, and Dr. D. Ben Ali, director of the CNDRS (Centre National de Documentation et de Recherche Scientifique) of the Federal Islamic Republic of the Comores. We thank H. Becker, A. Hofmann, W. White, S. Galer, and K. Haase for discussions and C. HCmond for help in the field. Reviews by F. Albarkde, C. Hawkesworth, W. McDonough, S. O’Reilly, and anonymous reviewers of a previously submitted version, helped to improve this manuscript significantly. We also thank H. Frohna-Binder and R. Gehann for conducting the XRF and INAA analyses, respectively, as well as E. Grieshaber, S. Bederke-Raczek and H. Feldmann for providing technical assistance. This is LamontDoherty Earth Observatory Contribution 5658. k’A]

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[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

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