Volatile abundances in basaltic glasses from seamounts flanking the East Pacific Rise at 21°N and 12–14°N

Gmrh!mrca (‘I C~nmxhrmrco Copyright C 1988 Pergamon

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I

Cal&7037/88/r3.00

Arm Vol. S2. pp. 2 J-2 I9 Press pk. Printed in U.S.A.

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Volatile abundances in basaltic glasses from seamounts flanking the East Pacific Rise at 21”N and 12-14”N KWESI E. AGGREY,’ DAVID W. MUENOW’ and RODEY BATIZA~ ‘Chemistry Department and Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HI 96822, U.S.A. *Department of Geological Sciences, Northwestern University, Evanston, IL 60201, U.S.A. (Received September 29, 1987; accepted in revisedjbrm

May 25, 1988)

Abstract-Volatiles in glasses from Seamounts flanking the East Pacific Rise (EPR) in the vicinity of 21”N and 1214”N were analyzed by high-temperature mass spectrometry. Compared to mid-ocean ridge basalt (MORB) magmas erupted at the spreading axis of the EPR at 2 I “N, the seamount magmas are commonly enriched in HzO, Cl, F and S (at the same Mg#). Water and Cl abundances range from 0. I I I to I .02 I wt.% and 0.003 to 0.226 wt.%, respectively. The wide range in abundances is consistent with the diversity of magmas erupted by these seamounts as indicated previously from major and rare-earth element data. Hz0 shows a strong linear positive correlation with KzO content [H20 = 0.45 (20.04) KzO + 0.22 (20.03) with RMS error of 0.091. Based on H20/KzO-ratios and KzO abundances the glasses separate into three distinct groups: highly volatile-enriched alkalic/transitional, enriched tholeiitic, and depleted tholeiitic. Ratio-ratio plots of incompatible elements (La/Sm vs. K1O/H1O) indicate that these seamount magmas can all be related by the mixing of enriched and depleted source materials. 1. INTRODUCTION

tams two-nested pit craters with a combined depth of about 100 m. Cone C2 is much smaller, about I km wide and 200 m high with no summit crater.

RECENT ISOTOPICand trace element data from young seamounts along the East Pacific Rise (BATIZA and VANKO, 1984; ZINDLER ef al., 1984; MACDOUGALLand LUGMAIR, 1986; ALLAN etal., 1988; GRAHAMef al., 1988) are consistent with their origin from compositionally distinct mantle sources. One of these source end-members is incompatibleelement depleted, typical to that proposed for MORB sources: the other is a source which is enriched. The geochemical

1.2. Samples The samples from the PLUTO expedition (I 982) were collected using the ALVIN submersible. They are all tholeiites and were collected predominantly from pillow lavas and pahoehoe (sheet) flows. Samples range from sparsely phyric to porphyritic. Phehocrysts are sparse (~3.5 ~01%) in the fresh glasses. Plagioclase (AN,& is the dominant phenocryst and microphenocryst (ALLAN et al., 1988). Observed phenocryst t microphenocryst assemblages are PI and PI + 01. Clinopyroxene occurs only in the groundmass and groundmass textures are similar to those observed in other oceanic basal& The Cocos Plate seamounts were surveyed with Seabeam and dredged during CERES leg 3 (August 1982, R/V Thomas Washington) and during the RISE III expedition (R/V New Horizon, 1979). Samples range from tholeiitic through transitional to alkalic and are either pillow fragments or pieces of submarine pahoehoe (sheet flows). Most samples are phyric to sparsely phyric (phen. < 3 ~01%)except CDI 8-3 and 23-6 which are highly porphyritic (BATIZA and VANKO, 1984). Plagioclase (AnJ,_& is the dominant mineral phase in all the samples and the three most common phenocryst assemblages are 01 + PI + Spl, 01 + PI + Cpx. and PI + Cpx (BATIZA and O’HEARN. 1982).

diversity of lavas erupted by these seamounts suggests that the sources of MORB are more complex than would be in-

ferred, if only magmas erupted along the axis of the spreading center are examined. ZINDLER d al. (1984) have proposed that “small-scale, large-magnitude heterogeneities in the upper mantle” are emphasized in seamounts because, unlike magmas erupted along the axes of spreading centers, the sources of seamount magmas arc small and relatively isolated. These reservoirs lack the thermal and mechanical properties needed to produce large volumes of melt that is mixed and homogenized as apparently occurs below mid-oceanic ridge axes. In this paper we present volatile abundances in glasses from Pacific Plate seamounts which flank the East Pacific Rise (EPR) in the vicinity of 2 I “N (from the PLUTO expedition, 1982) and from a group between 12 and 14”N along the EPR on the Cocos Plate (the RISE III expedition, I979 and CERES Leg 3 expedition, 1982).

2. EXPERIrMENTAL

METHODS

The volatile content ofglasses were determined by high-temperature mass spectrometry on glassy fragments (16-32 mesh) handpicked with a binocular micros+ope. This technique employs a facility consisting of an effusion-vaporization source (Knudsen cell) interfaced with a computer-monitored quadrupole mass spectrometer. Details have previously been described (K~LLINGLEYand MUENOW, 1975; LIU and MUENOW,1978). Sample preparation was identical to that of BYERSet a/. (I 983). Typically, a 40-45 mg sample is heated at S”C/min to 1250°C under a vacuum of IO-’ to lop8 torr. Degassing of the sample is monitored by the mass spectrometer which rapidly scans the spectrum from 2 to I00 amu over the entire temperature range. Mass peaks and their corresponding ion-current signals are measured and stored on a magnetic tape for data reduction and analysis. Minimum signals corresponding to concentrations of I ppm for Cl, F and S and 5 ppm for HI0 can be discerned from background contributions. Mass pyrograms (computer plots of ion intensity versus temperature) are generated from the reduced data and these provide quantitative in-

I. 1 Geological background The seamounts and one small cone from which glasses were obtained are located on young oceanic crust (~6.8 my) near the East Pacific Rise between 12” and l4’N (Cocos Plate Seamounts) and on oceanic crust ~0.8 my in age near 21 “N (PLUTO samples). Sample locations with respect to the EPR are shown in Fig. I. Bathymetric charts and interpretations of the forms of the Cocos Plate seamounts are given by BATIZAand VANKO(I 983). The two seamounts and the cone which yielded the PLUTO suite (the “Larson Seamounts”) are described by ALLANet al. (I 988). These seamounts are about 6 km across and 700 m high with eccentric, 2 km wide summit calderas. Seamount B (also known as Green Seamount) con2115

2116

K. E. Aggrey,D. W. Muenow and R. Batiza Volatiles Volatile abundances for pillow rim glasses from these seamounts are listed in Table 2 and plotted versus Mg# in Fig. 2. The general enrichment in volatiles compared to those obtained from glasses along the axis of the EPR at 21”N is striking. At the same Mg# the seamount glasses commonly have higher abundances for H20, Cl, F and S than those from the EPR spreading axis at 21’N. Hz0 shows a strong positive linear correlation with PzOs (Fig. 3a) Hz0 = 1.44 (+O. 12) PzOJ + 0.04 (+0.04) with RMS error of 0.09 IO4N

and with K20 (Fig. 3b) Hz0 = 0.45 (kO.04) KzO + 0.22 (kO.03) with RMS error of 0.09.

FIG. 1. Map showing location of seamounts with respect to East Pacific Rise (EPR) (heavy line segments) (after BATIZAand VANKO, 1984).Solid circles show where samples in this study were collected. Insert (after LONSDALE et al., 1982) shows location of seamounts E,

B and C2 relative to EPR. formation on sample volatility since the area under each curve is

proportional to the total amount of that volatile released within the temperature interval. After degassing the samples are weighed for total weight loss. 3. RESULTS AND D&CUSSlON The petrography, mineralogy and whole-rock chemistry of each of the samples used in this study have been previously described (BATIZA and VANKO, 1984; ALLAN et al., 1988). Major element, selected rare-earth and isotopic data for these samples are presented for reference in Table 1.

Major,

Si0,

Ti02

*203

FeOt

1182-5 1182-2 1182-7 1180-18 1180~4A 1181-5 1181-6

49.95 49.93 49.15 49.81

2.54 1.56 1.44 2.01 1.U 1.82 1.79

13.77 15.31 16.87 15.38 16.81 15.33 15.44

12.80 9.84 9.23 10.42 8.45 10.04 10.18

5.22 7.36 7.75 6.64 8.52 6.83 6.97

20-Z Cl%-S2 06-81 CO5-16 20-l 23-6 an-6

51.01 50.61 50.48 50.83 49.90 51.20 51.12

2.25 2.41 1.75 1.09 1.34 2.04 1.17 2.01 1.70 1.30 1.79 1.78 1.69

15.54 17.58 15.19 14.90 16.65 15.20 14.52 16.70 17.04 16.94 17.80 18.02 17.80

10.11 7.85 9.91 9.48 9.01 9.76 10.71 9.21 8.93 8.90 7.78 7.55 8.18

5.23 4.44 6.86 8.05 7.74 6.04 7.72 6.75 7.71 9.06 6.96 6.96 7.69

selected

sea.uOmtglasses*

~elementandisotqicdataforEAI

1.

Table

Based on the HzO/K20 vs. I/K20 plot (Fig. 4) the glasses studied here separate into three distinct groups: (I) highly volatile-enriched, alkalic/transitional; (II) enriched tholeiitic; and (III) depleted tholeiitic. The HrO/KzO ratio is a useful parameter in evaluating similarities in the sources for mantlederived basalts because this ratio is relatively insensitive to different degrees of fractional crystahization of a source region consisting of olivines, pyroxenes, and garnet and spine1 (BYERS et al., 1985). Furthermore, ratio-ratio plots with a common denominator are useful for evaluating source homogeneity and the possibility of mixing (LANGMUIR m al.. 1978). For the seamount glasses there are three distinct linear arrays for H20/K20 vs. l/KzO, two of which are co-linear. The highly volatile-enriched group (I) contains all the alkalic and transitional basalts and they are all from the Cocos seamounts. Three of the five alkalic basalts are those from seamount N-5 and two are from seamount 6 with sample 6-B2

Na20

K20

'2'5

9.90 11.92 11.38 11.28 12.07 11.41 11.45

3.95 3.03 3.06 3.53 2.59 3.22 3.25

0.25 0.11 0.34 0.24 0.09 0.29 0.32

0.25 0.21 0.18 0.22 0.15 0.20 0.26

9.84 8.32 11.74 12.88 11.75 11.08 12.59 10.74 11.35 11.67 9.70 9.65 10.48

4.07 4.78 3.26 2.16 2.94 3.55 2.23 3.10 3.32 2.75 3.69 3.76 3.38

1.00 2.16 0.22 0.05 0.28 0.49 0.06 0.73 0.19 0.06 1.19 1.23 0.76

0.42 0.77 0.25 0.08 0.17 0.27 0.12 0.30 0.21 0.12 0.54 0.52 0.39

WC=

Total

Id

sm

8%X-/%r

PIlJm

49.38

50.21 50.44

a2

49.44

cD18-3 cD16-1 anl-5 C011-12 am-1

49.87 48.84 50.00 50.06 48.90

99.35 100.10

6.48 3.67 8.00 8.08 2.37 5.58 7.46

5.92 3.56 3.57 4.91 2.55 3.65 4.39

.70255 .70246 .70244 .70254 .70254

99.47 98.92 99.66 99.52 99.78 99.63 100.24 98.98 100.32 99.64 99.45 99.53 99.27

20.51 37.18 5.29 2.04 6.04 10.79 1.52 11.42 5.44 2.16 25.09 16.44

5.92 7.19 3.97 2.45 3.35 4.63 2.44 4.67 4.08 2.95 5.03 4.57

.70280 .70295 .70259 .70241 .70276 .70264 .70249 .70260 .70248 .70291 .70279

98.63

99.27 99.40 99.53 99.18

lFma Satiza ad Vank~ (1984),Allan et al. (1988),Graham et al. (1988)and previouslyunphlished dati. tCD = CEPE9 (1982):no UI = RISE III (1979).

Volatiles in seamount basaltic glass Table

2.

Volatile ahadmca

(wt. %) for EFR searant glasses

E

Pw#

E E E B B C2 Q

ni ni M M M M ln

46.1 61.1 63.8 57.2 67.9 58.9 58.6

.329 .307 .342 .370 .lll .356 .317

.052 .091 .045 .040 .031 .079 .061

.Oll .014 .012 .014 .007 .014 .007

.x30 .088 .090 .133 .134 .120 .107

6 6 6 6 6 7 7 7 N-2 N-2 N-5 N-5 N-5

AB AB ni M ni lR M m ln M AB AB AB

52.5 54.3 59.6 64.0 64.7 56.9 60.6 61.0 64.4 68.4 65.6 65.9 66.3

.753 1.021 .357 .148 .430 .464 .126 .606 .404 .166 .792 .881 .675

.121 .226 .021 .017 .108 .052 .046 .048 .Ol2 .003 .138 .135 .093

.016 .015 .OlO .009 .024

.122 .099 .112 .113 .087 .131 .117 .090 .108 .105 .082 .092 .082

Sauple seaiwmt

II20

cl

F

S

PLlno 1182-5 1182-2 1182-7 1180-lB 1180-4A 1181-5 1181-6

axr6 20-2 CDS-B2 CD6-Bl US-16 20-l 23-6 CDI-6 CD2 CDlS-3 ~Dl6-1 anl-5 am-12 cm2-1

.010

.004 .005 .004 .004 .023 .Oll .015

2117

the Cocos seamounts (samples l-6,5- 16 and 16- 1, from seamounts 7, 6 and N-2, respectively) and one from the Pluto seamounts (sample 1180-4A from seamount B). They span a relatively large range in H20/K20-ratios (1.23-2.96) and have the lowest Hz0 abundances of any of the seamount glasses studied (0.111 to 0.166 wt.%). The range of Hz0 abundances is similar to that observed for the 2 1“N ridge crest samples (Fig. 2). This supports the previous observation by ZINDLERet al. (1984) that some samples from seamounts 6 and 7 fall within the “range” considered “normal” for MORB. Sharing a similar H,O/K,O-ratio (0.99-2.79) with the volatile-depleted tholeiitic group there is an additional group of tholeiites (II in Fig. 4) considerably more volatile enriched. This third group is also co-linear with the highly enriched alkalic/transitional group on the H20/K20 vs. l/K20 plot, Fig. 4. Six of these samples are from the Pluto seamounts and three from the Cocos seamounts. All samples within this

rM = tholeiitic,AB = alkalic,7.R= transitional

(from seamount 6) being the most enriched. The two transitional basalts are both from seamount 7. While these seven samples collectively span a relatively large Mg# range (52.5 to 66.3) they all show a narrow H20/K20 ratio (0.5-1.0). They are the most enriched in H20 (Fig. 2) and generally have the highest halogen abundances. The most enriched sample (6-B2) has Hz0 and Cl abundances of 1.02 1 and 0.226 wt.%, respectively. The enriched nature and/or “fertile” source(s) of these seamount glasses is also shown by H20/Cl ratios. SCHILLING ef al. (1980), for example, noted that MORBS near the Iceland and Azores hot spots have higher halogen contents (also higher “Sr/“?Sr ratios and incompatible element abundances) than normal MORBs. They invoked a “fertile” source for Cl, since they could not relate these differences to partial melting, or crystal fractionation. H,O/CI ratios for these enriched alkalic and transitional seamount glasses average 7.4, while those from 2 1“N spreadingaxis glasses have the average value of 24.0. MORBs from the Mid-Atlantic Ridge show H20/Cl ratio of 34 (DELANEY et al., 1978), while basaltic glasses along the Galapagos Spreading Center at 95”W show the Galapagos “hot spot” effect with H20/Cl ratio of 4 (BYERSetal., 1986). Halogen enrichment in the seamount magmas is also apparent from fluorine values which average 0.0 11 wt.% overall compared to 0.005 wt.% found in the EPR 2 1“N spreading axis magmas (BYERS et al., 1986). The average sulfur abundance for this alkalic/ transitional group is not significantly different from those of the other two groups. However, all three samples from seamount N-5 are among the most depleted in sulfur (0.082 to 0.092%) and in this respect are similar to the 21 ON ridge crest samples (Fig. 2). In striking contrast to this highly volatile-enriched group there is a volatile-depleted group (III) containing four relatively unevolved low-K tholeiites. Three of these are from

0.005-

025,

I k'

020-

3 f = u

015-

N-5 .N-5

6.

a6

O.lO-

005-oE

0.20.

.E OC2 oc2 7m7

" 80 I

.N-5

oE

I

OE

0 15-

3 f

6.

OIO-

6.

E 005-

12I

5

o-

.6 .N-5

OS-

.6

.N-5

5

E

.N-5

0.6 -

.7

2 04d 02 00 45

0 PLUTO 0 cocos PLATE A ZION RIDGE CREST I 50

N-Z %

I 55

60 h

65

#

FIG. 2. Mg# versus H20, S, Cl and F for seamount and 2 1ON EPR ridge crest gIasses (enclosed fields). Open symbols are tholeiitic basalts; closed symbols, akalic and transitional basalts.

. . A-

K. E. Aggrey, D. W. Muenow and R. Batiza

2118 1.0

I

I

0.8 2

L 0 I”

0.6

-

I

I

.

D

0.4

00

D

08,

ps,

o’2 : Aa 0.0

0.0

I 0.4

0.2

0.6

studied span the narrow Mg# range of 65.6 to 66.3 yet HrO abundances range from 0.675 to 0.881 wt.%. Similarly, for seamount E the three glasses studied show a wide range in Mg# (46.1 to 63.8) but only span the narrow range of 0.307 to 0.342 wt.% for HrO. In the two seamount groups (i.e., PLUTO and COCOS) seamounts E and N-5 are each located farthest from the spreading axis (Fig. 1) and show the largest degree of volatile diversity among the seamounts studied. This supports the suggestion of BATIZA and VANKO (1984) that lava diversity increases with lithospheric age.

P,Os (wt%)

Magma sources

0.2 ” OOL~ 0.0

” 0.5 ”

”

”

IO

”

’

I5

K20(wt%.)

FIG. 3. Variation diagrams for PzOs , KzO and Hz0 for seamount and 2 1“N EPR ridge crest glasses.Symbols as in Fig. 2.

group are enriched in HzO, Cl, F and S at similar Mg# compared to the 21°N ridge crest samples. Water abundances range from 0.307 to 0.430 wt.%. The average water abundance in the 2 I ON ridge crest glasses is 0.120 wt.%. An additional sample with low volatile abundance but not falling within the volatiledepleted group as (defined by Fig. 4) is #1182-5 from seamount E. The sample is enriched in iron and sulfur and the most evolved (Mg# 46.1) of all the seamount glasses studied, yet is highly depleted in HzO, Cl and F (Fig. 2) but not depleted in potassium. This unusual sample from seamount E well illustrates the diversity of magmas from individual seamounts. This is especially obvious by looking at individual data points for Hz0 abundances (Fig. 2) in glasses from this seamount as well as those for seamount N-5. For seamount N-5 the three glass samples

The volatile data as plotted in Fig. 2 (vs. Mg#) clearly show and support what has been shown previously by others (using trace element data) that these seamount magmas cannot all be related by fractional crystallization or varying degrees of melting from a common parental source. ZINDLER et al. (1984) and BATIZAand VANKO (1984), however, have demonstrated that binary magma mixing of enriched and depleted sources can account for the diversity of seamount magmas. The three seamount volatile types studied here group into two distinct non-colinear arrays for H,O/KzO vs. 1/KzO (Fig. 4) that may reflect these distinct sources. This is supported by the co-linearity in s7Sr/86Sr vs. Hz0 and La/Sm vs. Hz0 (Fig. 5). In both plots the volatile depleted group (III from Fig. 4) have the lowest 87Sr/86Sr ratios (.70241-.70249 + .00004) and the lowest La/Sm ratios (cl). Similarly, the volatile enriched alkalic/transitional group (I from Fig. 4) have the highest ‘?!@Sr and La/Sm ratios (.70260 to .70295 and 2.2 to 5.2, respectively). The volatile enriched tholeiitic group II occupies a position between these two on Fig. 5. The mixing of two end-member source types is also indicated from ratio-ratio plots using highly incompatible elements.

0.7030

6

0.01 0

I

I

5

IO

I

15

I 20

I 25

l)K20

PIG. 4. HzO/K,O YS.l/K20 for seamount glasses.Fields I, II, and III are drawn around volatile-enriched alkalic/transitional, enriched tholeiitic and depleted tholeiitic basal& respectively. Symbols as in Fig. 2.

01 00

r

I

I

I

I

I

I 0.6

0.4

I 1.2

He0 (wt. %)

FIG. 5. Hz0 vs. *7Sr/86Sr and La/Sm for seamount gIasses.Fields are drawn around the aIkaIic/transitionaI and depleted tholeiitic basalts.Symbols as in Fig. 2. REE data from BATEAand VANKO(1984); isotopic data from GRAHAMet al. (1988).

Volatiles in seamount basaltic glass

2119

2 1“N: Implications concerning the mantle source composition for both seamount and adjacent EPR lavas. In Seamounts, Islands and Atolls (eds. B. KEATING, P. FRYER, R. BATIZAand G. BoEHLERT),AGU Monograph (in press). BATIZAR. and O’HEARNT. ( 1982) Major element chemistry of basalt glasses dredged from young isolated volcanoes and the East Pacific Rise, 10”-140N. Proc. Intl. Symp. on the Activity of Oceanic Volcanoes, San Miguel, Azores, Archipelago 3,20 l-230. BATIZAR. and VANKO D. (1983) Volcanic development of small oceanic central volcanoes on the flanks of the East Pacific Rise inferred from narrOw beam echo sounder surveys. Mar. Geol. 54, I

-

Oo.0

I

I

I .o

05

,

1.5

I

2.0

I 25

53-90.

FIG. 6. Variation diagram showing La/Sm VS.KzO/HzO.

BATIZAR. and VANKO D. (1984) Petrology of young Pacific Seamounts. J. Geophys. Res. 89, 11, 235-l 1, 260. BYERSC. D., MUENOWD. W. and GARCIAM. 0. (1983) Volatiles in basalts and andesites from the Galapagos Spreading Center, 85”W to 86”W. Geochim. Cosmochim. Acta 47, 1551-1558.

verSuS KzO/HzO plot (Fig. 6). It is important to note that the position of individual samples in these mixing plots is independent of their Mg# and are preserved from plot to plot. Although it is unlikely that any of the samples studied represent pure end-members we characterize samples CDS 16 and CD6-B2 as most representative of depleted and enriched end-members, respectively. Both samples are from seamount 6 which has been reported to display nearly the entire range in trace element and major element variation (ZINDLERet al., 1984). Deviation from simple binary mixing in the form of small amounts of additional components such as K and P has been indicated by BATIZA and VANKO (1984). However, postmelting processes which may have given rise to these heterogeneities observed in the seamounts apparently did not affect the water-phosphorus and water-potassium ratios, since Hz0 shows a strong linear correlation with both P,O, and K20 (Fig. 3). The ratio appears to be representative of the source(s) since data from the EPR 2 1“N spreading axis, Cocos Plate and PLUTO seamounts all lie on the same straight line.

BYERSC. D., GARCIAM. 0. and MUENOWD. W. (1985) Volatiles in pillow rim glasses from Loihi and Kilauea volcanoes, Hawaii. Geochim. Cosmochim. Acta 49, 1887- 1896. BYERSC. D., GARCIAM. 0. and MUENOWD. W. (1986) Volatiles in basaltic glasses from the East Pacific Rise 2 1“N: Implications for MORB sources and submarine lava Bow morphology. Earth Planet. Sci. Len. 79, 9-20. DELANEYJ. R., MUENOWD. W. and GRAHAMD. G. (1978) Abundance and distribution of water, carbon and sulfur in the glassy rims of submarine pillow basalts. Geochim. Cosmochim. Acta 42, 581-594. GRAHAMD. W., ZINDLERA., KURZ M. D., JENKINSW. J., BATIZA R. and STAUDIGELH. (1988) Helium, lead, strontium and neodymium isotopic constraints on magma genesis and mantle heterogeneity beneath young Pacific seamounts. Contrib. Mineral. Petrol. (submitted). KILLINGLEY J. S. and MUENOWD. W. (1975) Volatiles from Hawaiian submarine basalts determined by dynamic high temperature mass spectrometry. Geochim. Cosmochim. Acta 39, 1467-1473. LANGMUIRC. H., VOCKER. D., HANSONG. N. and HART S. R. (1978) A general mixing equation with applications to Icelandic basalts. Earth Planet. Sci. Lett. 37, 380-392.

K,O/H,O

This is shown by the La/Sm

Acknowledgements-We thank the crews of R/Vs Lulu. New Horizon, Thomas Washington and DSRV ALVIN and to Peter LonsdaIe, chief scientist during PLUTO. Thoughtful reviews and discussion on this research were provided by James Allan, David Graham, and Mike Garcia. Support for this research was in part provided by NSF grants OCE-8415270 and OCE-8508042, ONR grant NOOO14-80-C-00856 and Hawaii Institute of Geophysics. Hawaii Institute of Geophysics Contribution No. 2028. Editorial handling: D. Fisher

REFERENCES ALLAN J. F., BATIZAR. and LONSDALEP. (1988) Petrology and chemistry of lavas from seamounts flanking the East Pacific Rise,

LIU N. W. K. and MUENOWD. W. (1978) A control and data acquisition system for a high temperature/Knudsen cell quadrupole mass spectrometer. High Temp. Sci. 10, 145-154. LQNSDALEP., BATIZAR. and SIMKIN T. (1982) Metallogenesis at seamounts on the East Pacific Rise. Marine Technol. Sot. J. 16. 54-61. MACDOUGALL J. D. and LUGMAIRG. M. (1986)Sr and Nd isotopes in basalts from the East Pacific Rise: significance for mantle heterogeneity. Earth Planet. Sci. Lett. 77, 273-284. SCHILLINGJ.-G., BERGERONM. B. and EVANSR. (1980) Halogens in the mantle beneath the North Atlantic. Phil. Trans. Roy. Sot. London A297, 147-178. ZINDLERA., STAUDIGELH. and BATIZAR. (1984) Isotope and trace element geochemistry of young Pacific seamounts: Implications for the scale of upper mantle heterogeneity. Earth Planet. Sci. Lett. 70, 175-195.