Oxygen isotope constraints on the sources of Hawaiian volcanism

EPSL ELSEVIER

Earth and Planetary Science Letters 144 (1996) 453-468

Oxygen isotope constraints on the sources of Hawaiian volcanism John M. Eiler a7*, Kenneth A. Farley a, John W. Valley b, Albrecht W. Hofmann ‘, and Edward M. Stolper a ” Division of Geological Planetaq Sciences, California Institute of Technology, Pasadena. CA 91125. USA b Department of Geology and Geophysics. Unit1ersit.v of Wisconsin, Madison, WI 53706, USA ’ Max Plan& Institutftir Chemie. Postjkh 3060. 55020 Main:, Germany

Received 16 February 1996; revised 5 September 1996: accepted 7 September 1996

Abstract We have measured oxygen isotope ratios in 99 separates of olivine and 14 separates of plagioclase or glass from Hawaiian lavas. These data confirm that the source(s) of some Hawaiian basalts are lower in S “0 than peridotite xenoliths and the source region for mid-ocean ridge basalts (MORB). Our data document correlations between oxygen and radiogenic isotope ratios and consistent differences in 6 “0 between volcanoes. Low values of 8 “0 are associated with a ‘depleted’ component that is relatively high in 206Pb/ ‘04Pb, low in 3He/4He, and anomalously low in ‘07Pb/ ‘04Pb relative to “‘Pb/ ‘“Pb ThivL component is preferentially sampled in lavas from the so-called Kea trend volcanoes (Kilauea, Mauna Kea, Kohala and Haleakala). Low 6 “0 values in the ‘Kea’ component suggest that it is hydrothermally altered oceanic crust. The similarity of the Kea end member to Pacific MORB in terms of Sr, Nd, and Pb isotope ratios further suggests that this component is assimilated from the local Pacific plate in subcrustal magma chambers. Anomalous 206Pb/ 204Pb-207Pb/ ‘04Pb relationships indicate recent enrichment in U/Pb in this component and further support the hypothesis that this component is young ( < 10s yr) Pacific crust. The isotopic distinctions between Loa and Kea trend volcanoes implies a systematic difference in the magma supply and plumbing systems of volcanoes on these two trends. Samples from Lanai and Koolau have ‘enriched’ radiogenic isotope compositions (radiogenic Sr and non-radiogenic Nd and Pb) and higher 6’*0 than typical upper mantle values, suggesting the incorporation of recycled sediment and/or oceanic crust in their sources. Other isotopic end members to Hawaiian lavas (e.g., high ‘He/‘He and post-erosional lavas) have 6 ‘*O values within the range typical of the upper mantle. Kewvords:

Hawaii; volcanism: basalts; O-18/0-16

1. Introduction island basalts (OIBs) provide evidence of chemical and isotopic variability in the mantle [l-3]. Ocean

* Corresponding author. Fax: + 1 818 568 0935. E-mail: [email protected]

The diversity

among

OIBs in the isotope

ratios of Sr,

Nd, Pb, and the rare gases and in trace element

ratios is often attributed to the formation of the crust and atmosphere and to the recycling of crust back into the mantle [2,4,5]. Oxygen-isotopes are an excellent tracer of crustal material because the isotopic composition of oxygen is sensitive to low-temperature

0012.821)3/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SO0 12-82 1X(96)00 170-7

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J.M. Eiler et ul. / Eurth and Planetag

processes in fluid-rich systems in the crust. Variations in 6 I80 ’ due to magmatic differentiation are small (I 2%0) and related to major element chemical composition, making them relatively easy to recognize and avoid [6-81. In contrast, there is at least a 35%0 range in aI80 of silicates that have interacted with water at or near the earth’s surface [9-131. The addition of crustal material to magmatic systems, either at shallow levels or through subduction into the mantle, may thus be accompanied by large variations in S180 [8]. Unaltered OIB whole rocks and glasses have 6 “0 values of 4.6-7.5%0 [14] (excluding Icelandic lavas, which have anomalously low 6 I80 values from interaction between magmas and hydrothermally altered Icelandic crust [ 151). This range of S “0 values is wider than that of MORB glasses (5.2-6.1%0) [ 161 and has been used to infer that the mantle sources of OIBs contain large fractions of recycled crustal materials and/or substantial primary mantle oxygen isotope heterogeneity [ 14,17,18]. However, relationships between radiogenic and oxygen isotopes are weak or absent in the existing whole-rock and glass database [ 141, making interpretation of variations in oxygen isotope ratios difficult. We present here the results of a study of oxygen isotopes in phenocrysts from basaltic lavas from the Hawaiian islands. The Hawaiian islands are among the longest-lived, largest, and most extensively studied occurrences of oceanic hot spot volcanism. They include basalts carrying the most extreme known elevations in 3He/ “He [19] and are thought to be derived from at least three sources with distinct radiogenic isotope signatures [20-301. Hawaiian lavas make up over 80% of all OIB samples significantly lower in 6 “0 than typical MORB (excluding Iceland), and Hawaii is the only hot spot except Iceland dominated by such lavas [ 14,17,18,31]. The occurrence of low S “0 values in a hot spot distinctive for its strongly elevated 3He/JHe ratios has led to the hypothesis that the Hawaiian plume, and possibly the undegassed deep mantle in general, is substantially lower in S”O than the MORB reser-

I

All values of S’s0 relative to SMOW.

reported through

((‘so/

‘60)q;tandard)- 1)x 1000.

‘60)s~,D,e / ‘so/

or discussed in this paper are the relationship: 6’80 =

Science Letters 144 (19%)

453-468

voir [14,17,18]. However, previous studies of Hawaiian samples have not detected relationships between oxygen and other isotope ratios, making the origin of “0 depletions ambiguous. Resolving the origin of this distinctive geochemical characteristic is critical for understanding the sources of Hawaiian magmatism and, more generally, for understanding the significance of variations in 6 “0 values for the origin of OIB sources. This study focuses on oxygen isotope ratios in olivine and therefore differs from previous studies that have relied on analyses of whole rocks and glasses. There are two potential advantages to our approach: (I> the dominant control of 6 “0 variations in volcanic rocks is post-eruptive alteration [ 1I, 141, analyses of hand picked refractory phenocrysts can largely avoid such effects and provide more reliable records of magmatic oxygen isotope ratios; (2) oxygen isotope ratios can be fractionated during crystallization or low degrees of melting [8,14], such effects can be potentially minimized by emphasis on a single mineral such as olivine that has a restricted composition, crystallizes early in the magmatic evolution of basaltic lavas, and is present as a major constituent in the mantle.

2. Samples We analyzed oxygen isotope ratios in phenocrysts, microphenocrysts, and/or glass in 106 Hawaiian lavas (Table 1). Most samples had previously been analyzed for major and trace element abundances and radiogenic isotope ratios by other workers or were collected from previously analyzed lava flows. Olivine separates were prepared from 99 samples by crushing, sieving, and hand picking under a binocular microscope. Plagioclase separates were prepared by the same means from 11 samples (4 of which also yielded olivine separates), and glass separates were prepared from 3 samples (all of which also yielded olivine separates). Two types of mineral separates were prepared: phenocrysts (large grains, N 0.5-2.0 mm, in macroscopically phyric lavas) and microphenocrysts (150-250 pm grains, prepared from macroscopically aphyric samples). These are discriminated by typeface in Table 1. Most of the plagioclase separates were microphenocrysts.

Loa

5.27

5.33

Rl29-5.50

Rl33-6

5.04

5.18

L-92-30 05OOB.P 1

L-8-588

(1950)

AH95-6

(/,%I/)

5.44

5.3x 5.19 4.93

‘w.L+? 75.Ku/-2 76.K&h

5.16

5.12

5 I4

5 14

5.10

5 I?

KK-29-10

KK-25-4

KK-24-7

KK-16-I

KK-15-4

KK-17-2

MG-la

MG- I Kauai

posl-erosional

5.30

s.07

K-204

MC-0

KK-3l-I?dun1te

s 13

K-02X

W-II

5.23

5 71

KK-31-12

Ksuai (Nrpali)

Koo- IO

f IXOI) 5 37

5.75

5 99

I Klx-2

5.95

Koo-IYA

5.90

5.96 Ku<>-49

5.98 Koo-8A

5 39

561

5.6

Koo-3OA

KOOlaU

Koo-55

gl = 4.9x

Lan-X5-7

Lanaa# I B

AHY5-33

AH9550

Lanai

HO-9

KU/i?

C-124

HO-19

(1801 flow)

4.77

4.66

4 64

4.81

491

AHYS7(/80/I 530

AHYI-5

AHY5-4

Hualalai

AnkaramIte

Mauna Kea

Or/wr

KI-8

KI-3

MKI-8

Suhnrorine

R466-5 0

R463-7.0

1 69

J 99

4.73

4.95

J 73

4.82

4 64

HO-13

HO-20

HO-17

HO-14

5.31

W-8

40

R446-2 40

R413-I

RX%!-0.05

R347- I .O

R303.3.00

R25Y-0.8

4.78

4.76

4.76

HO-I?

HO-J

5 3s

gl -491

2

R243-8 40

R221-0.6

R?l5-7

RI, 2-O 40

5 19

Haleakala

1793.R4

w-4

5.08

DR-3

5.07

4.97

5 06

5.12

4.91

J.YX

4 83

4.76

R/f,&I..55 R ,89-8.50

1.87

R160--5.75

Mauna Kea

KK-20-4

W-l

MG-2

P,do/a

Pm

Puu 00

Avg Kohala

,995

Puu

1985

00,

AH95-22

Puu 00

Mauna Lllu 1970

5.14

3.73

1959

Mauna Ulu 1969

x0.3/

K. Ikl-81-3-

143/1959

K. 111-75-J-

AH95-15/1979

ALV-1800.RJ

5.37

I

I

sahorr,nl

SOH4-666.

SOH4-641-l

SOH4-62Y-

SOH4-490.

I

5 10

I IS- I 5.18

5.20

5.1,

521

13-I

SOH4-271-I

H,stork

I 12-I

SOH4-159-I

SOH4-

SOH4-I

SOHJ-I

SOH4-74.

MG-3

Puako Ard Rc\.

Kilauea S-OH-J drill
Loihi

AH95-2

Orhrr

5 Oh

5 14

5 13

(1926)

AHY5-9

Mauna Lad 1950

5.27

Mauna Loa 1868

Ii.10

5 16

(1868)

(1859)

AH95-3

AH95-21

S.27

AHYS-10(1)0843l

Hirror~< ruhurrul

(410 B.P.)

5 I2

B.P.)

L-81 -03 (9020

Prchtsloric

I1

5.03

R 150-0.05

5.

5 39

R142-1.60

R 153-3.05

5 20

RI X-3.50

aahaeriul

s.19

IO

521

R55-0.75

R ,03-2.6s

HSDP drill corv

Mauna

6.0X

456

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and Planeta?

Science Lettem 144 (1996) 453-468

3. Analytical techniques

Kauai PEB (Kaual)

All mineral and glass separates were analyzed by laser fluorination at the University of Wisconsin, Madison 1321. Standards were analyzed concurrently with samples on each day of analysis. A total of 31 UWG-2 (garnet) standards analyzed over a period of 7 months during this study averaged 5.75 f 0.06%0 (1 u) on 9 out of 11 days, within uncertainty of me value of 5.8 rt 0.1%0 1341. On two days, daily averages of 3-4 standards were O.l-0.2%0 lower, attributable to maintenance on the laser fluorination line immediately prior to these days. This shift was also observed in secondary silicate standards (San Carlos olivine and GPlOl pyroxene). Analyses of unknowns from these days have been corrected by O. l-0.2%0. Reproducibility on San Carlos olivine standard was rt 0.07%0, 1cr. Approximately half of the analyses were replicated, with an average deviation from the mean for replicates of _tO.06%0. Average values are given in Table 1 for replicated samples. Measured yields on olivine were 99 f 2%. Most samples yielded more than 20 pmole of CO, gas after conversion. Those that yielded less gas (between 5 and 20 kmole, mostly analyses of microphenocrysts) may be less precise, due to the greater possible effects of trace contaminants and fractionation during gas handling; they are indicated as such in Table I by reporting to only one decimal place.

4. Results Results are presented in Table 1 and Figs. 1 and 2. Table 2 lists average 6”O values from this study and radiogenic isotope ratios from the literature for each suite of samples. In those cases for which radiogenic isotope ratios were not measured on the same samples (or on samples from the same flow) as the oxygen isotopes, these suite averages have been plotted in Fig. 2. Points plotted using such averaged isotopic compositions are clearly distinguished in Fig. 2 by oversized symbols and error bars. Several observations can be made from our data (Fig. 1): (1) Many Hawaiian olivines are lower in ??I80 than olivines from well-characterized upper mantle

KO&U Lanai Hualalai Mauna

Loa Loihi

Haleakala

(Honomanu)

Haleakala Kohala

(Kula)

Kohala Mauna Kilauea

0

(Poiulu)

al

0 00

0

Kea

-00000

0 00

(prehistoric)

Kilauea

00

(other)

cxla,

Ip”“~bm’B4~ ano o

(historic)

I

o ‘,

4.6

~,

5.4

5.0

5.8

8180 Fig. 1. Values of 6 “0 for olivine from Hawaian lavas. The vertical band marks the range of typical upper mantle olivine [33] and of olivine in equilibrium with typical MORB [16]. Samples are organized into Loa trend shield and postshield lavas (distinguished with an x). Kea trend shield and post-shield lava% and post-erosional basalts (PEBs). ordered by age (oldest to youngest) within each group.

rocks such as peridotite xenoliths [33] (5.2 t_ 0.14) and the sources of MORB [16] (MORB glasses have 6 I80 values of 5.7 _t 0.17, corresponding to olivine values of approximately 5.0-5.2%0 based on a melt-olivine fractionation of 0.7-0.5%0 [6,34-361). (2) The range in aI80 values for olivine from individual volcanoes is more restricted than the total range (1.35%0); for example. 0.28-0.41%0 ranges are seen for Loihi, Mauna Loa, Koolau, and Mauna Kea. Given these narrow distributions for samples from a single volcano, differences between suites, although small, are significant. (3) Values of St80 for so-called [37] ‘Loa trend’ volcanoes (Loihi, Mauna Loa, Hualalai, Lanai, Koolau and Kauai) are clearly distinguishable from ‘Kea trend’ volcanoes (Kilauea, Mauna Kea, Kohala and Haleakala) in that the Loa trend volcanoes show no values of 6 “0 for olivine less than 5.0%0, while the Kea trend volcanoes are dominated by such low values. Values of S “0 for plagioclase (dominantly microphenocrysts) span a 1.1%0 range, comparable to that for olivine. However, plagioclase-olivine fractionations are smaller than expected and scattered - 0.4%0, compared to an expected value (averaging determined of - 0.7%0, based on experimentally fractionations [38]). Feldspar in these samples is commonly ‘cloudy’, indicating alteration. It is known

451

J.M. Eiler et al. /Earth and Planetau Science Letters 144 ( 1996) 453-468

that some samples (e.g., SOH drill core samples from Kilauea) have experienced post-eruptive hydrothermal exchange 1391. This suggests that the St80 values of plagioclases are affected by hydrothermal exchange/alteration. Alternatively, this effect may be due to late-stage shifts in magmatic 6’*0 values. Our focus will be on the olivine data set because of its relative resistance to oxygen exchange and the much larger number of samples. Major and trace element concentrations are available for most of the samples analyzed in this study. These data show 6 I80 to be uncorrelated with magma type (i.e., tholeiitic vs. alkalic) or extent of differentiation (e.g., SiO,, MgO, whole-rock and mineral Mg#, alkalinity; representative data illustrated in

h Loahi l 26k- Mauna Loa n 28k+ Mauna Loa l Lanai = Koolau q Kauai

[40]). Although the details of melt-olivine oxygen isotope fractionations are largely unknown, this suggests that S’*O values of olivines from Hawaiian lavas are not significantly influenced by melting or by equilibrium fractionations during crystallization. and we conclude that they are effective source tracers. The correlation of olivine 6 ” 0 with radiogenic isotope characteristics (below) supports this conclusion. 4.1. Radiogenic

isotope-S

“0

correlations

There is a negative correlation between 6’*0 values of olivine crystals and whole-rock *06Pb/ 20’Pb ratios (Fig. 2a). Previous studies of Pb

. K~lauea OMauna Kea 0 Kohala 0 Haleakala (Honomanu)

Fig. 2. Plot of S IgO for olivine from shield building lavas vs. (a) ‘06Pb/ ““Pb: (b) c+.,~; and (c) 87Sr/86Sr for whole rocks; and (d) ‘He/ 4He for olivine, demonstrating that these isotopes are well correlated. Small symbols are for individual samples in which both measurements have been made on the same sample or on samples from the same flow. Large symbols are group averages for other suites in which both measurements have been made, but not on the same samples or on samples from the same flow (error bars are 2 u uncertainty in the mean). Post-shield and post-erosional lavas have been excluded from this plot. Radiogenic isotope data for this plot are from literature sources, and are available on request from the corresponding author.

458

J.M. Eiler et al. /Earth

and Planetan, Science Letters 144 (19961453-466:

isotopes [24,26] have led to the identification of three isotopic end members in Hawaiian lavas; thus, although there is a strong linear correlation between oxygen and ‘06Pb/ ‘04Pb, we would not a priori expect such simple relationships in two-dimensional plots. It is thus not surprising that the relationships between 6 “0 E 87Sr/ 86Sr and 3He/4He appear more complex. &ts of 6 “0 versus Sr and Nd isotopes in shield-building lavas have overall trends of decreasing 6 “0 with increasing end and decreasing 87Sr/ 86Sr (Fig. 2b,c). There is significant scatter in the middle of both plots, although that in Fig. 2b is largely due to three lNd analyses from Loihi [22] that are substantially higher than the highest values seen in subsequent studies, making their significance unclear. When viewed as a whole, there is no apparent correlation when a”0 is plotted versus ‘He/ “He (Fig. 2d). However, when separated into Loa or Kea trend volcanoes, systematic behavior emerges. For Loa trend volcanoes, S’*O is uncorrelated with 3He/4He. In contrast, for Kea trend volcanoes, olivine 6 “0 values decrease systematically with decreasing 3He/ ‘He, from S I80 values in the ‘nor-

Table 2 Average isotopic properties Unit

for select Hawaiian s7sr/

86%

mal’ mantle range for 3He/4He = 12 R, down to 4.6%0 for 3He/‘He < 9 R,. In addition, olivine 6 “0 values correlate with the deviation of Hawaiian lavas from the northern hemisphere reference line (NHRL) in a plot of ‘07Pb/ *04Pb versus ‘06Pb/ 204Pb. This is illustrated in a plot of S’8001ivioe against A207 (a measure of deviation from the NHRL [41]; Fig. 3). Low S ‘*O samples are low in Azo7, a signature reasonably interpreted as reflecting a recent (N 1-2.108 yr) increase in U/Pb [42] (an interpretation which is required if the source was previously on or above the northern hemisphere reference line). 4.2. Phenocqsts

or xenocrysts?

Olivine crystals in Hawaiian lavas frequently display features such as undulatory extinction and kink banding that have been attributed to post-crystallization strain [43]. These grains are chemically indistinguishable from unstrained grains and commonly contain melt inclusions that cannot be distinguished from those in unstrained grains [44-471. Thus, al-

suites EN‘i _

2obPb/ *04Pb

‘He/4He

6’8O

Loa trend Loihi

0.70355 f 3

6.3 f 0.3

18.366 + 0.017

24.9 + 2.0

5.16 f 0.05

Mama Loa 28 ka + Historic Lanai Kauai Koolau

0.70371 0.70383 0.70424 0.70361 0.70420

6.2 5.4 2.2 6.6 0.8

0.4 0.2 0.6 0.2 1.0

18.184 f 0.025 18.129 f 0.016 17.83 + 0.08 18.319 f 0.080 17.833 + 0.04

16.4 + 1.4 8.3 f 0.2 - 13 19.5-2 1.o 12.8 f 0.6

5.22 f 0.08 5.16 + 0.06 5.52 f 0.10 5.30-5.43 5.89 f 0.08

f & f f f

2

I 10 6 6

+ f + f f

Kea trend Kilauea Historic SOH4 Mama Kea HSDP Submarine Kohala (pololui

0.70356 + 2 _

6.5 + 0.2 _

18.453 & 0.039 _

12.8-14.7 _

4.79 * 0.10 5.09 i- 0.06

0.70356 f 2 0.70359 + 4 0.70365 + 3

7.2 +O.l 6.6 * 0.3 6.4kO.l

18.468 f 0.038 18.353 f 0.064 18.192 &-0.060

9.3 f 1.2 12-14 8-13

4.78 f 0.05 5.05 4.97 +0.15

Haleakala Honomunu Kula

0.70377 f 3 0.70339 f 14

5.9 i 0.2 7.26

18.286 f 0.026 18.337

14.8 + 1.2 8.0 + 0.2

5.06 f 0.16 4.90

Values are means, with 2cr uncertainties in those means. Single values and ranges are for data sets with too few samples to determine a valid mean, or were reported only as ranges. Data were compiled from literature sources and are available from the corresponding author. ‘He/4He ratios are given in units of RA. He isotope ratio for Lanai are unknown and assumed to be comparable to the otherwise isotopically similar lavas from Koolau.

J.M. Eiler et al. /Earth and Planetay

459

ScienceLetters 144 (19961453-468

‘06Pb/ “‘Pb in Mauna Loa [XI], and correlations between oxygen and radiogenic isotope ratios from this study (e.g., Fig. 2). Given the size of our sample set and similar correlations among other isotopic systems, it seems implausible that these correlations are fortuitous. We conclude that olivine phenocryst populations in Hawaiian lavas are dominated by grains that crystallized from their host lavas or from closely related lavas. 4.3. A three-component model ofisotopic variations in sources of Hawaiian shield-building lar~as Fig. 3. Values of ‘a0 for olivine in shield building lavas, plotted vs. the A’“’ index of Hart [41]. Low values of this index indicate recent U/Pb enrichment. The vertical line marks the lower limit of 4”’ values in MORB. Symbols as in Fig. 2. Pb isotope data are from literature sources, and are available on request to the corresponding author.

We have constructed a three-component model for the sources of Hawaiian shield-building lavas based on covariations of isotopic ratios, including our new data on oxygen isotopes. The first step in these calculations was a factor analysis of variations in ??I80 87Sr/ 86Sr, t43Nd/ r4JNd, ?()6pb/ 2o‘tp,,, 207pb/ 20;pb, 208p,,/ 203Pb, and ‘He/ ‘He in samples for which all seven isotope ratios have been measured as well as those for which we could make reasonable estimates based on averages of related samples from the literature (Table 2). Only shieldbuilding lavas have been included in this analysis; post-shield and post-erosional lavas will be discussed in a later paper. Regression of these data using the principal components method without rotation yielded two significant eigenvectors that accounted for 89% of the variance in shield-building lavas (the next most significant eigenvector accounted for only 4% of the variance, comparable to analytical precision in several of the isotope systems). That two eigenvectors can account for so much of the variance in these seven isotopic systems signifies that a three-component mixing model will successfully describe most of the isotopic variability. Although the end members must be in the plane of the eigenvectors, the choice of end members is arbitrary.

though strained olivines appear to have crystallized from magmas closely related to those from which the unstrained grains crystallized, the strained olivines may have come from a mechanically rigid body (e.g., a cumulate pile or conduit wall) before incorporation into the host lava. This raises the possibility that olivine remobilization has confounded the oxygen isotopic record of Hawaiian olivines. Although the mechanisms of olivine accumulation thought to be active in Hawaiian volcanoes [45,46] could lead to the mixing of olivine between lava flows, correlations between the isotopic properties of olivine (e.g., He, 0 and OS) and groundmass (e.g., Sr, Nd and Pb) indicate that there has not been substantial mixing of olivine grains between isotopitally distinctive lavas. Examples of such correlations include ‘He/ “He-87Sr/ “Sr in Mauna Loa lavas in Haleakala [48], ]271, ‘He/ ‘He-” Sr/ “Sr 3He/SHe-‘4”Nd/ IJ4Nd in Mauna Kea [49], and lA4Nd and lx70s/ ‘ssOs_ 18’OS/ ‘880s_“‘Nd/

Table 3 End member isotopic compositions

for mixing model

Endmember

*‘Sr/s6Sr


206 Pb/

Koolau Loihi Kea

0.70447 0.70360 0.70340

0.0 6.1 7.7

17.75 18.30 18.64

‘04 Pb

207 pb,

15.43 15.48 15.46

204

pb

208pb, 204pb

‘He/“He

37.74 38.16 37.99

13.5 32.0 7.5

5.95 5.20 4.54

J.M. Eiler et al./ Earth and Planetary Science Letters 144 (1996) 453-468

460

Extremes in the eigenvalue scores indicate that appropriate end members would be components similar to lavas from Koolau, Loihi, and Mauna Kea. These components are essentially identical to the ‘Koolau’, ‘Loihi’, and ‘Kea’ or ‘lithospheric’ end members identified in previous studies of radiogenic isotope variability in Hawaiian shield-building lavas [2027,481, and thus the addition of oxygen isotope ratios to the database has not led to the requirement of an additional source component. To illustrate the success of the three-component model, three components (‘Loihi’, ‘Koolau’ and

‘Kea’; Table 3) were chosen that satisfy the results of the principal component analysis and correspond to reasonable end members based on extremes in the data (Fig. 2). Each sample in this study was fit as a mixture of these three components by least-squares; all isotopic variables were converted to multiples of the standard deviation from the average for that isotope so as to weight the significance of all isotopes equally. This calculation implies linear mixing, an assumption that is unlikely to be strictly valid but appears to be reasonable for this exercise, based on the linearity of isotopic arrays for Hawaiian lavas in the Sr, Nd, Pb, He and 0 isotope systems (see [23,24,48] and Fig. 2). This model successfully describes variability in 0, Sr, Nd, Pb, and He isotopes in shield-building lavas. This is shown in Fig. 4, where we have compared the measured isotopic ratios for each of the samples in this study with the value computed from the best fit proportions of the three source components. FOJ each ratio, the average difference between measured and model fit isotope ratios is a small multiple of analytical precision. Available radiogenic isotope data and our preliminary results for oxygen (Table 1) indicate that most post-shield and post-erosional samples are poorly fit by the three-component model developed for the shield-building lavas. All are lower 87S~/ 86Sr and ‘06Pb/ ‘04Pb for a given eNd than would be observed for shield-building lavas, previously noted as characteristic of post-erosional lavas [24,25]. Our preliminary oxygen data suggest that this source is within the typical upper mantle range in S”O.

5. Discussion 5.1. Identity of the low S”O end member

B

L MEBUWd

Fig. 4. Comparison of measured Sr, Nd, Pb, He and 0 isotope ratios in samples of shield building lavas from this study with those consistent with a best fit of the samples to a mixture of the three end member components in Table 3. All isotope ratios are simultaneously fit to within a small multiple of precision ( f indicates average difference between fit and measured values), indicating that the three end member model describes the data well. Averages from Table 2 used where necessary.

One of the source components defined by our data (the ‘Kea’ component) has a 6 I80 value for olivine of 5 4.6%0. This value is significantly lower than that in peridotite xenoliths and the inferred value of olivine in oxygen isotope equilibrium with the MORB source region (5.2 * 0.2%0; [16,33]). These low values are associated with: (1) high Nd and low Sr isotope ratios, characteristics usually associated with ‘depleted’ sources from which melt was previously

J.M. Eiler et al. /Earth and Planeray Science Letters 144 (19%) 453-468

extracted hundreds of millions to billions of years ago [2]; (2) the most radiogenic Pb isotope ratios known for Hawaiian lavas; (3) ‘He/ 4He ratios equal to or slightly lower than MORBs, and (4) ‘“‘Pb‘06Pb compositions falling significantly below the NHRL. These characteristics correspond to a ‘depleted’ end member, some of whose characteristics have been identified in previous isotopic studies of Hawaiian lavas, and which has been variously called the ‘Kilauea’. ‘Kea’ or ‘lithospheric’ end member ]20.21,23,25]. We now evaluate various possible sources of this low 6 “0 component. The MORB Source: The ‘Kea’ end member has been previously identified with the MORB source mantle beneath Hawaii region in the upper [20,23,26,29] on the basis of the close approach to the Sr, Nd and Pb isotope compositions of Pacific MORB (87Sr/ “Sr = 0.7028 i 0.0004, Ed,, = 9 _t 2, ‘06Pb/ 204Pb = 18.4 &-0.4 for the Pacific crust, including the Hawaiian arch [16,51] vs. 0.7034, 7.7, and 18.64. respectively for the ‘Kea’ end member defined by this study). However, the “0 depletion of this end member relative to known MORBs argues against this interpretation. No appropriately low 6 ‘a0 MORBs are known from the global sampling of ridge glasses [16]; so, if such a component is present in the MORB source regions, it has never been sampled directly. While this cannot be disproved, it is unsatisfying given the large number of oxygen isotope analyses of MORB. In addition. peridotite xenoliths from Hawaiian volcanoes, some of which are thought to be representative of the mantle lithosphere underlying Hawaii [52]. are not low in 6’*0 relative to typical upper mantle values [31]. Recycled crust: I80 depletion in silicates is associated with high-temperature water-rock interactions (such as occur in the hydrothermal cells at mid-ocean ridges), and the only known major silicate reservoir lower in 6 “0 than MORB or peridotite xenoliths is lower oceanic crust [I 11. Therefore, the low 6 “0 end member observed in Hawaiian lavas could be recycled. subducted oceanic crust, previously cited as an important component in hot spot magmas (including Hawaiian magmas) [4..5,53]. This hypothesis is difficult to test because oceanic crust undergoes a variety of geochemical changes during hydrothermal alteration at active ridges, and during subduction. Nevertheless, it would be fortuitous if

461

such a source had evolved to an isotopic composition so close to that of Pacific MORB. We also note that the one clearly non-MORB-like characteristic of ’ Kea’ radiogenic isotope compositions (i.e., low 4”‘) suggests recent, not ancient, enrichment of U relative to Pb. There is trace element evidence for recycled gabbroic crust in the mantle sources of Hawaiian lavas [53]. However, unlike IgO depletions, these characteristics are found equally in lavas spanning the full range of radiogenic isotope ratios seen at Hawaii and cannot be directly linked to the oxygen isotope variations observed in this study. The Hcrwaiim ~rolcanic edifice: Hydrothermal circulation in Kilauea is vigorous enough to have produced low 6 “0 basalts within the volcanic edifice [39]. Recent detailed study of Puu 00 lavas conducted in concert with this work finds 0.5%0 variations in S ‘a0 values of glass and coexisting olivine microphenocrysts, and significant glass-olivine isotopic disequilibrium, possibly suggesting assimilation of low aI80 material at variable times relative to the time of olivine crystallization [54]. If this assimilant is hydrothermally altered Hawaiian volcanic edifice and if this mode of assimilation is common to Hawaiian lavas. it could be a significant source of low 6 ‘“0 olivine analyzed in this study. However, the Nd and Pb isotope ratios of the volcanic edifice will not differ significantly from those of younger lavas, so it could not account for the correlations seen in Figs. 2 and 3. We therefore conclude that, while high-level assimilation may be responsible for some scatter in Fig. 2, the large number of samples demonstrating significant correlations with radiogenic isotope ratios indicates that this is a second-order effect. High-level assimilation of altered Hawaiian basalts may be a significant source of low 6”O values in glasses, which are not correlated with radiogenic isotopes [17,18]. Pacific crmr: An explanation that satisfies many of our observations is that the ‘depleted’, low 6 I80 end member in Hawaiian lavas is derived by assimilation of hydrothermally altered portions of the local Pacific crust underlying Hawaii. Oceanic crust typically contains an extensive sequence of gabbroic cumulates that have S I80 values in the range of i-5%0; that is. - l-5%0 lower than fresh MORB ([55]. and data summarized in [ 111). While the oxy-

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gen isotopic composition of altered oceanic crust is too variable to yield a precise estimate, this suggests that the N 0.5%0 lowering of a’*0 associated with the depleted Hawaiian end member could result from adding up to lo-20% of oceanic crust to primitive Hawaiian lavas. As noted above, Pacific MORBs have Sr, Nd and Pb isotope ratios similar to, but more extreme than, those we have identified for the ‘depleted’, low S’*O Hawaiian end member [11,51]. High temperature seawater alteration and radioactive ingrowth over the 110 Ma since the formation of the local Pacific crust [56] would tend to make such values approach even more closely the low 6 “0 Hawaiian end member (i.e., “Sr/ 86Sr and radiogenie Pb isotope contents would increase at nearly constant eNdI. Similar arguments, in the absence of oxygen isotope data, have been taken as evidence for a source for the ‘Kea’ end member in seawater-altered portions of the Pacific lithosphere [57]. Finally, anomalous AzoO values in the ‘Kea’ component (as well as 206Pb/ ‘04Pb at the upper end of the Pacific MORB range) can be explained by this hypothesis, because a shift in the U/Pb ratio during hydrothermal alteration at the ridge (e.g., [58]) when the 110 my local Pacific crust formed would lead to high ‘06Pb/ *04Pb and low A207 relative to recent Pacific MORB (e.g., [42]). There are several other geochemical features of Hawaiian lavas that are consistent with a significant contribution from hydrothermally altered rock (although some do not distinguish between a direct seawater contribution, assimilation of seawater-aitered rocks from the volcanic edifice, and assimilation of the underlying oceanic crust). These include: elevation of D/H ratios in Kiiauea east rift lavas [59]; correlated enrichments in B, S ‘I B and D/H in Kilauea and Loihi glasses [60]; high Cl contents in Kilauea glasses [61]; and highly radiogenic OS isotope ratios in low OS lavas from Haleakala [62]. It might be expected that there would be clear major and/or trace element manifestations if lavas rich in the Kea component had assimilated 20% lower oceanic crust. However, because of the relatively small contrasts between the concentrations of most elements in gabbros from the ocean crust and in typical Hawaiian basalts and the degree of scatter of Hawaiian lava chemistry about olivine control lines, such tests are not definitive. The effects of assimila-

Science Letters 144 (19961453-468

tion of lower crustal material on major and trace element compositions are, therefore, expected to be subtle and not readily separable from the effects of crystal fractionation, accumulation, and variations due to differences in melting conditions and mantle sources. A possible test of the hypothesis that the low 6 “0 Kea end member is a local crustal contaminant is that, in this case, one might expect a correlation between a’*0 and olivine composition (i.e., assimilation can only proceed to the degree that latent heat is released due to crystallization). No such correlation is observed among samples from the Mauna Kea suite (the only large suite of closely related samples that is low in 6 ‘*O [40]) and olivines from Mauna Kea are not systematically less forsteritic than those from Mauna Loa [47,63]. However, this may not be a definitive test, given the complex plumbing and continuous magma supply of Hawaiian volcanoes during shield building. For example, melts of the country rock that are mixed into deep Hawaiian magma chambers as assimilants are probably produced by heat released by crystallization of previous batches of magma, and therefore their availability or quantity need not be correlated with the instantaneous composition (or quantity) of olivine crystallizing in the magma chamber at the time of assimilation. Preliminary reports of OS isotope data from Mauna Kea lavas [64] indicate they are characterized by relatively non-radiogenic OS (‘870s/ ‘**OS = 0.130) that, when combined with their “0 depletions, may indicate a hydrothermally altered olivine-rich source. Such a source could be altered peridotites that are recycled by subduction (following the arguments for a recycled gabbroic component outlined above) or locally derived mixtures of mafic and ultramafic layers in the underlying Pacific crust. The OS concentrations of altered lower oceanic crust may not be high enough for assimilation of such material to have a significant impact on the OS isotope ratios of primitive lavas, and thus these low ratios may, instead, reflect a particular mixture of Loihi- and Koolau-like end members into which a low S’*O assimilant was added. We note that, similarly, nonradiogenic OS isotope ratios are seen in some of the highest 3He/ 4He Haleakala lavas [63], which are closely related to samples with more radiogenic Sr

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and less radiogenic Pb than the ‘Kea’ end member, supporting the suggestion that low OS isotope ratios may not be unique to the ‘Kea’ source. 5.2. Constraints on the addition Hawaiian lavas

of

oceanic crust into

We think that the Pacific crust underlying Hawaii is the most plausible source of the ‘*O-depleted component, based on the fact that it is known to have approximately the correct isotopic properties and is known to be present under Hawaii. We regard it as particularly significant that such a source can readily explain the anomalous. ‘young’ radiogenic Pb (i.e. low A*“) of the low S IgO component (Fig. 3). Clague [65] suggested that during the shield building and post-shield stages in the life of a Hawaiian volcano a long-lived magma chamber is present near the base of the oceanic crust. If this is the case. the high liquidus temperatures of primitive Hawaiian magmas ( > 1400°C [661) make it plausible that significant amounts of lower oceanic crust would be incorporated into the magma chamber by melting of its wall rocks. providing a mechanism for crustal contamination. Other evidence in support of this hypothesis includes: fluid inclusion barometry on olivine cumulate xenoliths [67], consistent with crystallization at the depth of the base of the oceanic crust [52]; and seismic data suggesting that the old oceanic crust beneath Oahu has been extensively underplated by a large magma chamber or sill complex [68]. However, it should be noted that this evidence, and Clague’s (1987) model [65] of magma storage at the base of the crust may be in conflict with the interpretation of patterns of seismicity beneath Kilauea [69]. If the ‘Kea’ end member reflects assimilation of oceanic crust. then the difference between the Loa and Kea trends does not reflect, as has been argued [30,50]. deep-seated properties of mantle sources but, rather, reflects volcanological phenomena, such as different geometries of magma bodies at the base of the crust, more effective armoring of Loa trend magma chambers by cumulates, and/or higher liquidus temperatures or longer residence times in lower crustal magma bodies for Kea trend lavas. However, our results do not offer insights into which, if any, of these could control or contribute to the Loa-Kea difference.

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Pre-shield and those post-erosional lavas that carry mantle xenoliths are not thought to evolve in magma chambers at the base of the crust [65], so if the ‘Kea’ component reflects assimilation in such magma chambers, we would expect there to be relatively little of this component in such lavas. The low inferred proportion of the Kea component in Loihi lavas agrees with this expectation. Our limited sampling of post-erosional lavas also agrees with this expectation, in that these lavas require a contribution from a distinctive end member [70,23] with ‘normal’ ??I80 (Table 1). The distinctive post-erosional end member can be plausibly associated with the peridotitic upper mantle (i.e., the MORB source) underlying the oceanic crust, which has previously been proposed to be a significant source component only after extensive heating by the plume or invasion by volatiles released from the plume L23.70.711. 5.3. The origin of the Koolau end member Our data on samples from Koolau and Lanai indicate that the ‘Koolau’ component, previously recognized based on its ‘primitive’ or ‘enriched’ radiogenic isotope ratios, is higher in 6 I80 than typical upper mantle sources. This IgO enrichment is unlikely to be due to fractional crystallization since these lavas are not unusually evolved. and the freshness of the olivines and the similarity of 6 IgO among 7 Koolau samples (6 of which are averages of 2-5 analyses of individual grains) makes it implausible that the IgO enrichments were caused by weathering. Distinctively non-radiogenic Pb in the Koolau end member prohibit it being derived from modern sediments or any portion of the existing Pacific crust. Analysis of olivines from other OIBs indicates that EM2 lavas also have olivines with 6”O values 5.4-6.0% [72]. Our oxygen isotope data therefore support the suggestion, made previously based on similarities in the trace element ratios between Koolau and EM2 lavas [28], that the ‘Koolau’ component represents an enriched mantle source similar to EM2 (although Pb isotope ratios in the Koolau end member more closely resemble EM I). Recycled sediment could be the source of the trace element and radiogenic isotopic characteristics of the EM2 end member [2], and such a source could also explain its high S “0 values (due to strong “0 enrich-

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ments in pelagic and terrigenous sediments). OS isotope data also suggest that the Koolau end member is derived from recycled material [64], although these data, in combination with major element variations in Hawaiian lavas, have been used to imply a recycled basaltic upper oceanic crust source rather than sediments [73]. Enrichments in “0 are common to both the basaltic upper oceanic crust and sediments, and so our data do not discriminate between these hypotheses. The range in S “0 of weathered basalt and sediments of N lo-25%0 indicates that the N 0.8%0 enrichment of the Koolau source relative to typical upper mantle could be produced by the addition of 4-20% of such material to a typical mantle peridotite of N 5.5%0.

between Sr, Nd, and Pb isotope ratios of the Kea end member and Pacific MORB lead us to prefer the hypothesis that this component is locally derived from the crust beneath Hawaiian volcanoes. The details of Pb isotope variations and other geochemical characteristics of Hawaiian lavas are also consistent with this hypothesis. Given the distinction in oxygen isotope ratio between Loa and Kea trend volcanoes, this would require a significant, long-lived difference in the magma supply and plumbing systems of the volcanoes of these two trends. (4) High 6 “0 values in Lanai and Koolau lavas suggest that the enriched, ‘Koolau’ end member contains a component of recycled “O-enriched sediments and/or upper oceanic crust at levels comparable to those seen in high 6 ‘*O EM2 lavas found elsewhere [72].

6. Conclusions ( I) Oxygen isotopes are an important tracer of the sources of Hawaiian lavas. Correlations between 6 “0 in olivine and Sr, Nd, Pb, and He isotope ratios permit the identification of 6 “0 values of distinctive source reservoirs. (2) Our data, in combination with previous study of Sr, Nd, Pb, and He isotopes, indicate that Hawaiian lavas are derived from mixtures of four sources: (i) a high 3He/4He, modestly ‘depleted’ component (the ‘Loihi’ end member) with S’xOo,iv,ne similar to the typical upper mantle value of 5.2%~ and (ii) an enriched, high 6 “0 component (S 18001ivine2 6.0%0; the ‘Koolau’ end member), both of which are reasonably attributed to the Hawaiian ‘plume’; (iii) a depleted, low S’80, low “He/‘He end member I 4.6%0) preferentially incorporated in (6 ‘*00ll”l”e lavas from the so-called Kea trend (the ‘Kea’ end member); (iv) A depleted component that is within the normal range in S “0 for upper mantle peridotite (i.e., 6 “Oolivi”e N 5.2%0). This end member is most clearly seen in post-erosional lavas. This component has previously been suggested to be derived from the lithospheric mantle, and our data support this conclusion. (3) Low 6 “0 values in the ‘Kea’ end member suggest that this source component is hydrothermally altered oceanic crust. Although we cannot rule out a role for recycled oceanic crust preferentially sampled by the mantle sources of Kea-trend lavas, similarities

Acknowledgements We thank Dave Clague, Fred Frey, Mike Garcia, and Don Thomas for graciously supplying samples for this study, John Lassiter, Mark Kurz, Erik Hauri and Mike Rhodes for sharing data prior to their publication, Mike Spicuzza and Nami Kitchen for assistance in the stable isotope laboratory at the University of Wisconsin, and Fred Frey, Emi Ito and Colin MacPherson for thorough reviews of this manuscript. This work was funded by NSF-9303975 and -9 I 17588 to EMS and 93-04372 to JWV, and DOE grants DEFG03-85ERl3445 to EMS and 93ERl4389 to JWV. [FAI

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