Temporal helium isotopic variations within Hawaiian volcanoes: Basalts from Mauna Loa and Haleakala

0016-7037/87/53.00+ 00

Ceochrmrcaet Cosmochrmlca ANO Vol. 51, pp. 2905-2914 0 Pergamon Journals Ltd. 1987. Pnnted tn U.S.A

Temporal helium isotopic variations within Hawaiian volcanoes: Basalts from Mauna Loa and Haleakala* MARK D. KURZ’, MICHAEL 0. GARCIA~, FRED A. FREY~ and P. A. O’BRIEN’ ‘Chemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543. U.S.A. *Hawaii Institute of Geophysics. University of Hawaii, Honolulu, HI 96822, U.S.A. ‘Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology. Cambridge. MA 02139, U.S.A. (Recerved December 16, 1986: accepted in revisedfinn

Ju/y 28. 1987)

Abstract-Helium isotope ratios in basahs spanning the subaerial eruptive history of Mauna Loa and Haleakaia vary systematically with eruption age. In both volcanoes, olivine mineral separates from the oldest samples have the highest )HePHe ratios. The Haleakala samples studied range in age from roughly one million years to historic time, while the Mauna Loa samples are radiocarbon dated flows younger than 30,ooO years old. The Honomanu tholeiites are the oldest samples from Haleakala and have ‘He/‘He ratios that range from 13 to 16.8X atmospheric, while the younger Kula and Hana series alkali basalts ah have ‘HerHe close to 8X atmospheric. A similar range is observed on Mauna Loa; the oldest samples (roughly 30.000 years) have ‘HetHe ratios of I5 to 20X atmospheric, with a relatively smooth decrease to 8X atmospheric with decreasing age. The consistent trend of decreasing ‘HerHe ratio with time in both volcanoes. coherence between the helium and Sr and Nd isotopes (for Haleakala), and the similarity of ‘He/‘He in the late stage basahs to depleted mid-ocean ridge basalt (MORB) helium, argue against the decrease being the result of radiogenic ingrowth of “He. The data strongly suggest an undegassed (i.e., high ‘He/(Th + U)) mantle source for the early shield building stages of Hawaiian volcanism, and are consistent with the hotspot/mantle plume model. The data are difficult to reconcile with models for Hawaiian volcanism that require recycled oceanic crust or derivation from a MORB-related upper mantle source. We interpret the decrease in 3He/‘He with volcano evolution to result from an increasing involvement of depleted mantle and/or lithosphere during the late stages of Hawaiian volcanism. INTRODUCIION

IN RECENTYEARS,helium isotopic data for mantlederived igneous rocks have been extensively used to constrain models of mantle evolution. One obstacle to understanding the helium isotopic data in oceanic basalts is that large isotopic variability is often observed over small geographic distances. This is particularly true of Hawaii, where large ranges in 3He/4He ratios occur within and between single volcanoes (KURZ et al., 1983; RISON and CRAIG, 1983; KURZ et al.. 1985a. 198Sb). One view of this variability is that it is related to changes in the mantle source of the basalts as volcanic systems evolve (KURZ et al., 1983). Another explanation is that helium isotope ratios are altered by magma chamber processes such as degassing. contamination, and radiogenic ingrowth of 4He (CONDOMINES et al., 1983; ZINDLER and HART, 1986). The correct interpretation

of this local variability

is of general im-

portance because oceanic islands play a key role in constraininggeochemical mantle models. and because modification of ‘HerHe ratios by magmatic processes would decouple helium from the other radiogenic isotope systems. The helium isotopic variability within single Hawaiian volcanoes also has an important bearing on the origin of the Hawaiian chain, particularly with respect to the hot spot/mantle plume model (WILSON, 1963; MORGAN, 197 1). High ‘He/“He ratios (i.e., greater than * WHO1 Contribution No. 649 I.

typical MORB values) have been observed in several Hawaiian volcanoes, and these results qualitatively support an undegassed mantle origin for the Hawaiian volcanism (CRAIG and LUFTON, 1976; KANEOKAand TAKAOKA, 1980; KURZ et al., 1982, 1983; RISON and CRAIG, 1983). However, the variability within individual volcanoes must also be consistent with the hot spot model. Temporal geochemical variability has previously been observed in several Hawaiian volcanoes (e.g., CLAGUE and BEESON, 1980; FEIGENSON et al.. 1983; CHEN and FREY, 1983,1985; RODENet al., 1984: STILLE et al.. 1983), but has only been inferred for helium isotopes (KURZ et al., 1983). In order to understand the nature and origin of the inn-a-volcano helium isotopic variability, with respect to the Hawaiian hot spot, and oceanic island volcanism in general, this study focuses on determining 3He/4He variation with eruption age at two Hawaiian volcanoes. Haleakala and Mauna Loa were chosen for detailed isotopic study because in both cases the chronology of volcanic eruption is reasonably well known. The chronology and volcanic stratigraphy of the basalts are important for testing volcano evolution models (CHEN and FREY, 1983: KURZ et al., 1983) and to evaluate the possibility that radiogenic 4He contaminates ocean island magma chambers (e.g., COND~MINES el al., 1983). K-Ar studies have shown that volcanism on Haleakala has spanned a time period of at least 800,000 years (MCDOUGALL, 1964). CHEN and FREY (1983, 1985) showed that the exposed subaerial lavas display significant trace element and isotopic variability over

2905

2906

Mark D. Kurz et ol

this time period, which reflects systematic changes in the mantle source with eruption age. Subaerially exposed lavas on Mauna Loa represent a shorter time period, but recent field work (in conjunction with radiocarbon dating) provides extremely well-documented volcanic stratigraphy for the last 30,000 years (LIPMAN, 1980; LIPMAN and SWENSON,1984; LOCKWOODand LIPMAN,1987). As shown in this paper, both Haleakaia and Mauna Loa display systematic helium isotopic variability within the exposed volcanic pile. These results constrain the nature of the mantle melting beneath Hawaii. Another reason for studying Haleakala lavas is that several authors have reported extremely high 3He/4He ratios from one summit lava flow (KANEOKA and TAKAOKA,1980; RISONand CRAIG, 1983). Although these data are now known to result from cosmogenic helium, rather than mantle derived helium (e.g.. KURZ, 1986a,b; CRAIG and POREDA, 1986), it is important to document the conditions under which the effect is significant. This is particularly relevant to studies of oceanic islands, because the lavas are often exposed on the surface for extended periods of time, and in order to study mantle helium one must be able to eliminate cosmogenic effects. EXPERIMENTAL The goal of this study is to examine the evolution of a single volcano through time, which requires analysis of subaerially

Sample

1790-3

Depth (actera)

0

Mineral Analyzed

Crtahing Kc ccSTPlgr8m

gr.

0.2Ox10“ 2.38x10-’ 5.46x10-’

8.1t.l 7.9t.l 8.2t.l 7.8e.l 8.2r.2

br. CP* SANA KUU

01. 01.

‘Scl’ile (SIRa)

2 3 60 62

01. 01. 01. 01.

1.03x10“ 6.63x10-’ 9.96x10-’

7.8r.l

- 865-4 - 1185-21 - 885-14 - 885-10

164 164 228 249

01. 01. 01. 01.

- U85-8

255

CPX 01.

-

261

CPX 01.

8.53x10-’ 2.45x10-’ 1.57x10_’ 8.92x10-’ 9.77x10-’ 2.84x10-’ 2.73x.10-’ 6.04x10-

8.3z.2 7.9t.l 8.0r.l 8.k.l 8.0t.l 7.6z.2 6.6t.3 7.8e.2

-305 -415 -420 -645 -655 -665 lubrarinc

01. 01. 01. 01. 01. 01. 01.

2.92x10-’ 4.64x10-* 5.57x10-* 9.86~10-‘~ 4.41x10-9 1.60x10-’ 5.58x10-’

13.4t.3 13.k.2 16.2r.2 14.5t.6 14.72.2 16.8t.l 16.02.2

HONO -

865-14 H65-13 865-10 865-9

erupted lavas. Unfortunately, subaerially erupted lavas are largely degassed upon eruption. and the only means of measuring helium within such lavas is to analyze mineral separates (KANEOKAand TAKAOKA, 1980; KURZetol., 1982). In using this approach, one implicitly assumes that the crystals analyzed are phenocrysts rather than xenocrysts, an assumption that may not always be valid. The samples analyzed here are all olivine or clinopyroxene phyric basalts: the results are given in Tables I and 2. The quantities of helium trapped wtthin basaltic phenocrysts are generally orders of magnitude lower than within glasses. requiring either that small quantities of gas, or large sample sizes, be analyzed. For these reasons. some effort was devoted to minimizing blanks, in order to allow analysis of small mineral separates. This was particularly necessary because several of the samples were from one inch diameter drill cores, and were not available in abundance. The extraction line and cryogenic system for separation of helium from the other noble gases is conceptually similar to that described previously (w. Kuaz and JENKINS, 1981; KURZef al.. 1985a)but has been extensively modified to reduce the blanks. The modifications to the crushing apparatus and high-vacuum furnace are described by KURZ ef al. (1987). The procedural blank for crushing and melting are identical at 3 to 5 X IO-” ccSTP ‘He, with atmospheric )He/‘He ratios. The mineral separates were produced by crushing in a stainless steel mortar, followed by sieving, and handpicking the olivine and clinopyroxene phenoctysts from the I to 2 mm size fraction. The crystals were then cleaned in 2 N nitric acid, distilled water, acetone and methanol, and repicked. In all cases, the crystals were then crushed in vacuum to release helium from the inclusions. As shown by KURZ (1986a.b). cosmogenic helium is not released by this extraction method, and to eliminate the influence of cosmogenic helium, crushing in vacua was the predominant method used here. The Haleakala samples analyzed were porphyritic tholeiitic

885-7 HO-12 80-16 no-4 m-22 SO-23 C-123 gx83-0725

Meltial *lie ‘He/% ccSTP/gram WFa)

,3.56x10-’

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1.54x10-’

7.97i.11

[5.34x10-*

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2.87x10-’

10.7r.l

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16.9+.41*

l5.72xlo-*

16.2t.11

He

fhma

isotopic

variation

2907

in Hawaiian volcanoes

Loa 1.15x10-*

ML55* K1877(3)* ML8** EA82-01 KTE85-05 HA82-Ob

1.39x10-* 1.44x10-’ 8.42x104.193x10-’ 1.41x10-’

8.6z.b 8.22.1 8.0r.7 8.1z.l

KTE85-32 KTEBS-30-l-7** -3o-g-131*”

1.11x10-*

KTE85-11 KTE85-23

1.11x10-’ 6.23x10-’

9.062.2 12.2t.2 8.72.2 13.1t.l 10.95r.2 18.82.1 17.62.2 14.82.1 2o.ot.2

2.06x10-’

12.8t.2

KTE85-28 EAe.2-05B

8.45x10-’

37 110 119 910r70 2180x60 5250~100 5650t90 9020+130 11780+100 28150t900

.2464 . lb62 .1813 .0796 .3686 .1052

>31.000 >31.000

.19bl .1038

>23.000

.bOb6

Kilavea 8K82-07

*

denote8 average l * denotes averqes ALL data reported graina

of data taken from Kurz et al. (1983). of data reported by Kurz (1986b). here were obtained by crushing of l-2 m

olivine

in “mxnlm.

and alkalic basal& The Hana and Kula samples are alkalic basalt from the drill core from the Nahiku area on the north side of Haleakala originally described by STEARNS and MACDONALD (1942) and more recently by 0%~ and FREY (1985). Petrographic and geochemical descriptions of the drill cores, and the samplea analyzed here can be found in those references. The Honomanu tholeiites were collected from a section along Honomanu Gulch and on the west side of Honomanu Ray. The stratigraphic positions and geographic locations of the Haleakala samples are shown in Fig. 1. Sample locations and descriptions of the Mauna Loa and Kilauea basalts an given in the appendix. As mentioned above, in analyzing basaltic phenocrysts, one implicitly assumesthat

100-200 Ka age gap) are the alkalic to basanitic basalts of the Hana formation. The Hana lavas range in age from historic (- 1790) to roughly 200-300 Ka (no ages are published for Hana lavas).

Nahiku Drill Cores H85

H65

they arerelatedto the host magma.Examinationof the Mauna Loa samples in thin section reveals that many have minor amounts of olivine showing strain features (“kink-banding”) and resorption textures, which may indicate that these are xenocrysts. However, there is no systematic variation in the abundance of these features (or other petrographic characteristics) with age, and they are present in all the samples. Therefore, we attribute the helium isotopic variations to be characteristic of the magmas. The radiocarbon dates for the samples were taken from KELLEY etal.(1979).and RUBIN etal. (1987). RESULTS

Honomanu Bay

Haleakala

AND DISCUSSION

A. Haleakda The helium isotopic results for the Haleakala samples (Table 1) are shown with respect to depth in Fig. 2. (The data arc plotted as a function of depth because the exact age of each sample is not known.) The Honomanu lavas from the Haleakala shield constitute -99% of the subaerial portion of the volcano (MACDONALD, 1963). Conformably overlying the Honomanu lavas are the alkalic basalts to trachytes of the Kula Formation, which range in age from 400 to 800 Ka (MCDOUGALL, 1964; NAUGHTON et al., 1980). Uncomforrnably overlying the Kula lavas (roughly

FIG. I. Locations and stratigraphic positions of the Haleakala basalts studied. The depths are given in meters below the top of the section. In the case of the drill cores, this is simply depth below surface; the Nahiku area drill cores are described by STEARNS and MACDONALD(~~~~). The Honomanu Ray samples were collected from outcrop; depths were estimated based on the average dip of the section on the west side of the Ray. In the map and the three sections, Hana lavas are denoted by stippled, Kula by unshaded, and Honomanu by shaded patterns.

2908

Mark D. Km-z er al tiALEAKALA

200

Ku/a

I

FIG.2. ‘He/‘He ratio for phenocrysts m basaltic rocks from HaIeakala volcano, plotted as a function of depth. Depth between Kula and Honomanu samples is approximate. The symbols indicate the different lavas: Hana (squares) from the present to about 200-400,OOO years, Kula (diamonds) from 400,000 years to 800,000 years, and Honomanu (circles) are older than ~,~ years. The sample with attached arrow is the dredged basalt, which is inferred to be deeper in the section.

As shown in Fig. 2, Hana and Kula Iavas have relatively uniform ‘HerHe at roughly 8X atmospheric. which is indistinguishable from MORB. However, the earlier Honomanu samples have significantly higher 3HepHe ratios, rangi n g from 13 to 17X atmospheric. This progression, from older high 3He/4He ratio lavas followed by younger, low 3HePHe ratio lavas, is exactly as predicted by the volcano evolution model suggested by KURZ et al. f i 983) (based iargely on data from Loihi seamount). It is also consistent with the model of C&EN and FREY (1983) in the sense that the alkalic basalts have a larger MORB source component than the earlier tholeiitic lavas. The tholeiites are more primitive with respect to helium isotopes. There is an abrupt change in ‘He/“He between the Honomanu and Kula samples, and it may be argued the populations are in fact bimodaI. In contrast, 87Sr/s6Srdata from the HonomanuKula boundary suggest a gradational trend (CHEN et al., 1985, and in preparation). Additional Sr and He isotopic data on samples from the stratigraphic transition will be required to determine the degree of correlation between these isotope systems. One explanation for the lack of a gradational ‘He/ 4He variation at the Honomanu-Kula transition is that our samples do not necessarily inctude the boundary where a transition may occur. Ftuxgate magnetometer measurements on the samples from Honomanu Bay show that the Kula flows are reversely magnetized and the upper Honomanu flows are normally magnetized.

Using existing K-Ar ages for other Kuia and Honomanu lavas, these Kuta tavas were probably erupted in the upper part of the Matuyama (0.73 to 0.88 Ma) and the upper Honomanu flows in the Jarmillo excursion (0.88-0.94 Ma). Thus, the transition in 3He/ 4He ratio must have occurred in less than 0.2 1 Ma and probably over about 0. IO Ma. Note. also. that RISON and CRAIG ( 1983, p.426) reported a much lower ‘He/ 4He ratio of 8X atmospheric. similar to the Kula sampfes reported here. for a single sample from the Honomanu formation. The Mauna Loa data presented below suggest that the transition from high to low ‘He/ 4He ratios may take place within the tholeiites. so low 3He/4He Honomanu basalts may exist near the Honomanu-Kula boundary. The geochronology and stratigraphy indicate that Haleakala has been erupting basalt with relatively uniform 3He/4He for the last 0.9 Ma (~ginning of Kula formation to present). The 3He/dHe ofthe lavas erupted during this time is in all cases indistinguishable from MORB at roughly 8X atmospheric. This long time period of isotopically uniform lavas places important constraints on models for helium in Hawaiian voicanoes: before -0.9 Ma. the basalts erupted by Haleakala had significantly higher 3He/4He ratios. As mentioned earlier, the time required for the transition from high to lower 3He/4He ratios in Haleakala lavas is unclear because the section is not complete. The Sr and Nd isotopic compositions of the Haleakala lavas also show a clear trend with age; Honomanu lavas have higher s7Sr/s%r and lower 14-‘Ndf IaNd ratios than the overlying Kula and Hana lavas (CHEN and FREY, 1983, 1985). The relationship between the helium and strontium isotopic data is shown in Fig. 3 and demonstmte that both isotopic systems reveal systematic differences between KuIa and Honomanu lavas. Honomanu lavas have higher s7Sr/s6Sr (.7036 to .70385) and 3He/4He (13 to 17X atmospheric), while the Kula and Hana have lower “Sr/ ?Sr 1.7031 to .7035) and 3He/4He (7.8 to 8.1X atmospheric). The helium data therefore broadly support the CHEN and FREY (1983) model that the tholeiites are more primitive and are derived from a plume-type source. It is important to note that the Kuta and Hana lavas have distinct *‘Sr/%r ratios but are indistinguishable based on 3He/4He ratios. This places constraints on possible mixing relations between the different Haleakala lavas (ix.. the He/% ratios in the endmembers; see below). The observed variations in 3He/4He ratio are nor related to the presence of cosmogenic helium, because the data discussed above were obtained by crushing in vacuum. Previous studies have shown that, even in samples containing predominantly cosmogenic 3He. crushing releases only magmafic helium (KURZ, 1986a.b). because magmatic helium is predominantIy contained by inclusions. In addition, the Honomanu series samples were collected in areas that have rapid erosion rates, and are shielded from cosmic rays by

He isotopic variation in Hawaiian volcanoes

2909

higher 3He/*He ratios, up to 20 times atmospheric, while the lavas younger than 2000 years have 3He/4He ratios indistinguishable from MORB. However, in the case of Mauna Loa, the radiocarbon dates show that the transition to MORB-like helium has occurred within the last 10,000 years. In addition, the lower 3He/ 4He ratios are obtained in tholeiitic basalts. as opposed to the alkali basalts on Haleakala. As mentioned above, this variability cannot be attributed to cosmogenic helium. Recent helium data on dredged submarine basalt glasses from Mauna Loa (LUFTON and GARCIA, 1986) support this, because their 3He/4He ratios (- 15X atmospheric) are consistent with those reported here, and the submarine samples could not have been exposed to cosmic rays. %P*Sr The dramatic Mauna Loa ‘HePHe variations bring up the question of possible temporal variability within FIG. 3. ‘HerHe plotted with 87Sr/*6Srfor the Haleakala samples; see Fig. 2 for symbols indicating the different lava Kilauea. KURZ et al. (1983) showed that historical lavas series Helium isotopic data are from this study, and the from Kilauea have significantly higher ‘HepHe ratios strontium isotopic data are from CHENand FREY (1985, and than historical Mauna Loa flows (13.4 to 14.5X atunpublished). The mixing lines were calculated assuming that mospheric as compared to 8X atmospheric). Although samples Cl23 and 1790-3 were endmembers. The parameter we do not have an equivalent suite of radiocarbonR is defined as: dated Kilauea basalts, we report a single analysis of R, = (‘He/He*) X (*6Sr2/86Srl) one of the oldest lavas from the Hilina formation in and the value of R, must be between 20 and 50 if the samples Table 2 (sample HK82-07). The Hilina Formation is are related by mixing (see text). the oldest subaerially exposed sequence of lavas on Kilauea, and is older than 23,000 years (EASTONand GARCIA, 1980). The similarity in ‘He/‘He between this the overlying basalt column. It is conceivable that the sample and the historical Kilauea flows, suggests that Kilauea has not changed isotopically on the same time lava flows were exposed on the surface for long periods of time before being covered by later flows, but the scale as Mauna Loa. However, LUPTON and GARCIA constancy of the Hana and Kula series data attests to (1986) found significantly higher ‘HerHe ratios from the efficacy of the crushing in eliminating this effect dredged (i.e., possibly older) Kilauea basahs which may suggest that Kilauea has in the past undergone isotopic (see KURZ, 1986a,b). In addition, the heating experiments on some of the Haleakala samples (see Table 1) evolution consistent with that observed for Mauna Loa indicate that many of the lavas were buried soon after and Haleakala. eruption (i.e., low exposure ages). Therefore, the isotopic data obtained by crushing in vacuum reflects the characteristics inherited from the parent magma, rather than from cosmogenic effects. B. Mauna Loa and Kilauea Mauna Loa differs substantially from Haleakala in that all of its surface lavas are tholeiitic basal& while 99% of Haleakala is covered by alkali basalt. The relative youth of Mauna Loa is reflected in the fact that only 100,000 years of eruptions are exposed ahove sea level (LIPMAN, 1980), as compared to roughly 1 million years on Hale&ala (MCDOUGALL, 1964). The presence of charcoal under many Mauna Loa lavas has allowed the development of a detailed radiocarbon stratigraphy for the last 30,000 years (KELLEY el al., 1979; LIPMAN, 1980; LOCKWOODand LIPMAN, 1980; RUBIN et al., 1987). The 3He/“He ratios in some historic and radiocarbon dated flows from Mauna Loa are listed in Table 2, and plotted as a function of time in Fig. 4. As with the Haleakala basalts, the older lavas display significantly

FlG.4. ‘He/‘He ratio as a function of radiocarbonage (years before present) for Mauna Loa samples. The samples with attached arrowscan only be assigned lower age limits, because they were not dated, but directly underhe dated ash layers. Radiocarbon dates are from KELLEY etal.(1979) and RUBIN et al. (1987).

MarkD. Kurzef al.

2910

IMPLICATIONS A. Ocean island volcanism

The observed helium isotopic variability within Haleakala and Mauna Loa has several implications for studies of oceanic island volcanism. The most obvious is that without stratigraphic control on sampling, helium measurements in oceanic island lavas can be misleading. Although most of the helium data in the literature are from recently erupted basalts, relative eruption age information is clearly important in interpreting intra- and inter-volcano helium isotope variations. It is not clear how much generality may be derived from the Haleakala and Mauna Loa temporal variability. However, in a study of helium in Icelandic subglacial basalts, KURZ et al. (1985a) suggested that the isotopic variability could be related to age of rifting. Also, GRAHAM et al. (1984) showed that there is a significant range in 3He/4He ratios within single East Pacific seamounts, and inferred a relationship with eruption age. The Haleakala and Mauna Loa data also provide a useful test of the possibility that ‘HerHe ratios are modified by magma chamber processes, as suggested by COND~MWESet al. (1983). and ZINDLERand HART (1986). These authors suggest that residence within shallow magma chambers will lower 3He/4He ratios due to CO2 degassing, coupled with production of 4He (by decay of Th and U). The data presented here argue strongly ugainsf this process as an important control on 3HefHe. If production of 4He during magma chamber residence were important, one would expect ‘He/‘He ratios significantly lower that 8X atmospheric because the production ratio is between 0.1 and 0.01X atmospheric (e.g., MORRISONand PINE, 1955). It would also be difficult to explain consistent 3He/4He ratios for samples erupted over at least 800 Ka (within the Kula and Hana formations of Haleakala), because the extent of contamination would be expected to change with time and Th and U contents. In addition, both Hale&ala and Mauna Loa lavas decrease to roughly 8X atmospheric, which is the vaiue to be expected from depleted oceanic lithosphere/mantle. Finally, variability in Sr and Nd isotopes and trace elements is also observed in Haleakala (CHEN and FREY, 1985; see Fig. 3), which would be difficult to explain on the basis of magma chamber degassing. A much more plausible model to explain the helium isotopic variability is that the mantle source changes as the volcano grows, as suggested previously (KURZ et al., 1983; CHEN and FREY. 1983. 1985). The higher 3He/‘He ratios within the earlier eruptions of both Haleakala and Mauna Loa are indicative of a more primitive source, which has had a higher time-integrated ‘He/(Th + U) than MORB, and is consistent with involvement ofa deep mantle plume in the origin of the Hawaiian Islands. The decrease in 3HePHe ratio as a function of time, and convergence for both Haleakala and Mauna Loa, to a value of 8X atmospheric,

suggest the increasing involvement of a depleted MORB-type source as the volcanoes grow. The depleted mantle source can either be lithosphere or asthenosphere beneath Hawaii; these two possibilities cannot be distinguished using the present data. The mechanism for the transition in 3He/4He can quite easily be related to the movement of the Pacific plate over a fixed hotspot. As the volcano drifts away from the hotspot, supplies of both heat and high 3He/4He ratio material decrease, and both the eruption rate and 3He/4He ratio decrease. The final stages of eruption have 3He/4He (and also other geochemical characteristics) similar to normal MORB. This is essentially the model suggested previously (CHEN and FREY, 1983. 1985: KURZ et al., 1983). However. the simplest form of this model. mixing of two isotopically homogeneous (and distinct) sources, cannot explain other isotopic data for Hawaiian basalts. CHEN (1987) recently sug gested the importance of heterogeneity within the MORB-related component, while others have suggested the possibility of additional distinct components to account for the isotopic complexity (e.g., STILLEef al., 1983. 1986; FEIGENSON, 1984). The high ‘HerHe ratios in the Haleakala tholeiites. the decrease in the Mauna Loa ‘HerHe ratios over such a short time scale. and the apparent isotopic uniformity in Kilauea lavas. all suggest that the shield building phase of Hawaiian volcanism has consistently high 3He/4He ratios. Based on this hypothesis, Mauna Loa has almost completed the shield building stage. and eruption rates should decrease, but Kilauea is still within its shield building (high ‘HerHe) stage. The simple fact that Mauna Loa is the largest of all Hawaiian shields supports the notion that it may be ready to stop erupting. Although there is little other evidence that independently supports the notion that Mauna Loa has finished its shield building stage, available eruption rate and geochemical data are not inconsistent with this. LIPMAN(1980) has shown that eruption rate on Mauna Loa has varied over the last several thousand years. However, the data for eruption rates beyond 3 Ka are insufficient to evaluate whether Mauna Loa has decreased its eruption rates over the last 10 Ka (P. LIPMAN,pers. commun.). Several major and trace element studies have suggested geochemical differences between historic and prehistoric Mauna Loa lavas (BUDAHN and SCHMITT, 1985; TILLING et al., 1987). TILLING et al. (1987) suggest that Mauna Loa trace element variations are related to long-term secular variation. However, other Mauna Loa trace element studies on this timescale reveal no variations that are larger than those observed in historical flows (RHODES, 1987). Additional studies of older Mauna Loa lavas will be required to further understand the helium isotopic variations and possible relations with other geochemical tracers. An additional ramification of this hypothesis relates to the volume of material necessary to build a Hawaiian volcano. RICHTER and MACKENZIE (1985) point out

He isotopic variation in Hawaiian volcanoes that the CHEN and FREY (1983) model implies melting of unrealistic volumes of lithosphere (i.e.. depleted mantle) if even a small proportion of this component is involved in the shield building stage. The data presented here suggest that the shield building stage may involve minimal amounts of this component, and therefore may reduce the problem of volume. In constraining mixing proportions with trace element and isotope data, the assumed end-members are the most critical parameters. In the CHEN and FREY (1985) model, the mixing proportions are greatly affected by the use of “bulk earth” isotopic characteristics as one end-member. It is interesting to note that the highest 3He/4He ratios are always found in samples with “Sr/ “Sr ratios significantly lower than “bulk earth” values (KURZ et al., 1982, 1983), perhaps indicating a less enriched end-member for the Hawaiian volcanism. An enriched end-member with s7Sr/86Sr less than 0.7045 would also significantly decrease the problem of volume. The high 3He/4He ratios. and decrease in 3He/4He ratio with time, are difficult to reconcile with models for Hawaiian volcanism that invoke involvement of recycled oceanic crust (FEIGENSON, 1984) fractionation of depleted upper mantle derived melts (ANDERSON, 1985) or stress induced melting of depleted MORB-type source (SHAW and JACKSON. 1973; FEIGENSONand SPERA, 198 I). B. Melt migration Although the hotspot model can qualitatively explain the helium data, the details of melt formation, mixing and migration must also be accomodated. CHEN and FREY (1983,1985) suggest that the depleted (alkali basalt) source is generated by a small degree of partial melting (. 1 to 2 percent melt), which mixes with the undepleted source derived melts. In this model, melts from the undepleted mantle permeate into the normal (depleted) mantle and lithosphere, inducing a small degree of partial melting, which then mixes with the undepleted melts. If the decrease in ‘HerHe (and s7Sr/86Sr) near the boundary between Honomanu and Kula lavas is related to mixing of magmas or sources (e.g., CHEN and FREY, 1983). then the data in Fig. 3 can help to constrain the mixing process. In either case. the mixing lines shown in Fig. 3 indicate that the helium concentrations in the end-member liquids must be drastically different. The curvature of the mixing lines is related to the parameter: R, = (4He,/4Hez) X (86Sr2/86Sr,) where, in the present case, the subscripts 1 and 2 refer to the concentrations in the Hana and Honomanu lavas, respectively (e.g., LANGMUIRet al.. 1978). Because the strontium concentration in the Honomanu series is roughly a factor three lower than in the Hana (or Kula) series lavas (CHEN and FREY, 1983) R, values

2911

of 2(i to 50 (see Fig. 3) would require that the Hana series end-member have 60 to I50 times higher helium concentration than the Honomanu series liquids. Because the mixing parameter R, is derived from isotopic data. and we assume that shallow magma chambers have no effect on 3He/“He ratios (see above). it also seems unlikely that this concentration variation can result from shallow magma chamber degassing (i.e.. it is characteristic of the magma sources). There are several difficulties in using the mixing lines in Fig. 3 to evaluate the end-member concentrations. First, measured phenocryst helium concentrations cannot be used to independently evaluate end-member concentrations because they do not directly reflect the magmatic helium concentrations, i.e., the phenocrysts primarily contain helium within melt inclusions (KURZ et al.. 1982). In addition, the sample ages are not known, so the transition from Honomanu (high ‘He/ 4He) to Kula (lower 3HePHe) took place over an unknown time period, perhaps up to several hundred thousand years, However, if the helium isotopic variability is caused by mixing. then the helium concentration in the depleted source must be higher by a factor between 60 and 150. This is difficult to reconcile with undepleted mantle having higher time integrated 3He/ (Th + U) ratio, unless helium is somehow greatly enriched by the melting process for the depleted source. This same discrepancy is apparent in comparing the helium results from MORB and Loihi seamount: if they are related by mixing, the MORB source must have higher helium concentrations (KURZ et al., 1982). (Note that this is not an artifact of the helium data being plotted as 3He/4He; the same conclusion is reached in plotting 4He/3He against “Sr/‘%r.) Possible mechanisms of helium enrichment are closely related to the processes by which melts accumulate and migrate in the mantle. Small degrees of partial melting could produce the helium enrichment if the solid/melt helium partition coefficient is roughly 100 times lower than that for strontium, and the liquids are produced by small degrees of partial melting (as suggested by CHEN and FREY, 1983). For extremely small degrees of partial melting, the He/Sr ratio change will approach the ratio of the partition coefficients. Unfortunately, partition coefficients are not well known for helium. so it is difficult to evaluate this hypothesis. Available analyses for olivine and clinopyroxene phenocrysts are generally less than 10m8cc 4He STP/gram (e.g., Tables 1 and 2; KANEOKA and TAKAOKA, 1980; KURZ et al., 1982). Tholeiitic glasses from oceanic islands (i.e., Hawaii) and mid-ocean ridges typically contain between IO-’ and low5 cc 4He STP/ gram. As mentioned above, the phenocryst concentrations are primarily related to inclusions, and are therefore not indicative of equilibrium partitioning. Also, the glass data do not necessarily reflect data for parental liquids. We can therefore only use these numbers to place zapperlimifs of 0.00 I to 0.1 for the partition coefficients. In the absence of better partition coefficients,

Mark D. Kutz et al.

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it is impossible to rule out this explanation of the helium enrichments. Another possible explanation for the high He/% ratios implied by Fig. 3, could relate to the high diffisivity of helium in the context of disequilibrium melting processes (e.g.. RICHTER,1986; NAVONand STOLPER, 1987). For example, it is conceivable that a melt migrating through the mantle would equilibrate with respect to helium faster than strontium, leading to enrichment in helium within the melt. In this case, the concentrations depend not only on the partition coefficients, but also on the diffusion coefficients (and would be controlled by the geometry and rate of magma migration). If we view this simply as a process of “stripping out” elements from a matrix into a migrating melt, one limitation to the extent of fractionation is the ratio of the diffusion coefficients (DHe/DSr). Once again, the relevant data are not available, but it is possible to place limits on this ratio. The Dsr for clinopyroxene (at 1200 degrees) is 5 X lo-l3 (SNEERINGER et al., 1984); the only relevant helium data at this temperature are for olivine (HART, 1984), which yield a Dne of lo-“. Assuming that helium will diffuse faster in clinopyroxene than in olivine, due to a more open structure, a lower limit to the DHJDs, is therefore roughly 200. Thus, diffusive enrichment of helium with respect to strontium could also explain the implied He/Sr enrichment in the depleted Haleakala endmember. A mom rigorous treatment of these possibilities must await measurement of relevant diffusion and partition coefficients. However, a qualitative understanding of the mechanisms that control the helium concentrations (i.e., in addition to isotopic composition) in basaltic lavas is of general importance. Basaltic glasses from Loihi seamount have helium concentrations more than a factor of ten lower than MORB glasses (e.g., KURZ Edal., 1983; RISONand CRAIG, 1983), which is contrary to what would be expected based on ‘HerHe ratios, and implied He/(Th + U) ratios. In addition, as mentioned above, if the MORB-Loihi He-Sr isotopic data are related by mixing of sources, they also imply a paradoxically higher He/Sr ratio in the MORB source. Therefore, the arguments regarding helium enrichment processes may have significance not only for Hawaii, but also for understanding melt extraction beneath mid-ocean ridges. SUMMARY This study, involving stratigraphically controlled basalt suites from Haleakala and Mauna Loa, demonstrates that single Hawaiian volcanoes vary in magmatic ‘HePHe ratio as a function of age. The time span of the samples studied is quite different for the two volcanoes, more than one million years for Haleakala and roughly 30.0 years for Mauna Loa. Despite this difference, the range in ‘HerHe ratios is similar for the two volcanoes. In both cases, the earlier

(i.e., older) lavas have higher 3He/“He ratios (up to 18X atmospheric for Haleakala and 20X atmospheric for Mauna Loa), which indicates the mantle source for the shield building phase of Hawaiian volcanoes is relatively primitive and undegassed. In contrast, the younger lavas from Haieakala and Mauna Loa have much lower )HePHe ratios, indistinguishable from that of normal MORB (8X atmospheric). We interpret this transition from high to lower 3He/ 4He ratios to reflect a change in the mantle source as the volcanoes evolve. In this model, the primitive undegassed source contributes most strongly during the early shield building stages. As the volcano is pushed off the (fixed) hotspot by plate motion, the eruption rates decrease rapidly, and the magmatic 3He/4He ratios approach those of depleted mantle/lithosphere. Acknowledgements-We thank Don Thomas. Don Ehhon, and Tom Trull for their important assistance in the field, and Peter Lipman, Jack Lockwood and Reg Okamura for helping to select the Mauna Loa sites. We thank T. Trull, D. Graham, and W. Jenkins for comments on an early version manuscript, W. B. Bryan for useful discussion, 1. Kaneoka and M. Rhodes for their careful reviews, D. Lott and L. Surprenant for assic tance in the laboratory, and M. Lumping for typing the manuscript. We also thank C. Y. Chen for making available unpublished Sr data for several of the Honomanu samples. This work was supported by funds from NSF Grants GCE8315270 and GCE85-16082 to M. Kurr, GCE84-16212 to M. Garcia, and EAR84 19723 to F. Frey. Editorial handling: F. A. Podosek REFERENCES ANDERSOND. L. (1985)Hotspot magmas can form by fractionation and contamination of mid-ocean ridge basahs. Nature 318, 145-149. BUDAHNJ. R. and SCHMII-~R. A. (1985) Petrogenetic modeling of Hawaiian tholeiitic basalts: A geochemical approach. Geochim Cosmochim. Acta 49,67-88. CHEN C. Y. (1987) Lead isotope constraints on the origin of Hawaiian basahs. Nature 327,49-52. CHENC. Y. and FREY F. A. (1983) Origin of Hawaiian tholeiites and alkalic basalt. Nature 302, 785-789. CHENC. Y. and FREYF. A. (1985)Trace element and isotope geochemistry of lavas from Haleakala volcano, East Maui: rmplications for the origin of Hawaiian basahs. J. Geophys Rex 90,8743-8768. CHENC. Y., FRV F., HARTS. and GARCUM. ( 1985)Isotopic

and rare-earth element geochemistry of the transition from tholeiitic to alkalic volcanism in Hale&ala volcano. Eos 66, 1133. CLAGUED. A. and BEESONM. H. (1980) Trace element geochemistry of the east Molokai volcanic series,Hawaii.Amer .I. Sci. BOA, 820-844. CONDOMINES M., GRONVOLDK., HOOKERP. J., MUEHLENBACHSK., O’NIONSR. K., OsKARssONN. and OXBURGH E. R. (1983)Helium, oxygen. strontium and neodymium relattonships in Icelandic volcanics. Earth Planet. Ser. Lett. 66, 125-136. CRAIGH. and LUPTONJ. E. (1976)Primordial neon, helium and hydrogen in oceanic basalts. Earth Planet. Ser. Lett. 31,369-385. CRAIG H. and POREDAR. J. (1986) Cosmogenic ‘He in terrestrial rocks: the summit lavas of Maui. Proc. Natl. Acad. Sci. 88, 1970-1974. EASTONR. M. and GARCIA M. 0. (1980) Petrology of the

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He isotopic variation in Hawaiian volcanoes Hilina formation, Kilauea volcano. Bull. VolcanoL434, 657-673. FEIGEN~~N M. D. (1984) Geochemistry of Kauai volcanics and a mixing model for the origin of Hawaiian alkali basal& Contrib. Mineral. Petrol. 87, 109-l 19. FEIGEN~~NM. D. and SPERAF. J. (1981) Dynamical model for temporal variation in magma type and eruption interval at Kohala volcano, Hawaii. Geology 9, 53 l-533. FEIGENSONM. D., HOFMANNA. W. and SPERAF. J. (1983) Case studies on the origin of basalt. II. The transition from tholeiitic to aikahc volcanism on Kohala volcano, Hawaii. Contrib. Mineral. Petrol. 84, 390-405. GRAHAMD., ZINDLERA., KURZ M. D., JENKINSW. J. and BATJZAR. (1984) He, Sr, and Nd isotopes in basaltic glasses from young Pacific seamounts. Eos 65, 1078. HART S. R. (1984) Helium diffusion in olivine. Earth Planet. Sci. Lat. 70, 297-30 I. KANEOKA 1. and TAKAOKAN. (1980) Rare gas isotopes in Hawaiian ultramafic nodules and ultramatic rocks. Science 208, 1366- 1368. KELLEYM., SPIKERE., LIPMANP., LOCKWOOD J. P., HOLCOMBR. T. and RUBINM. (1979) USGS radiocarbon dates. XV: Mauna Loa and Kilauea. Radiocarbon 21,306-320. KURZ M. D. (1986a) Cosmogenic helium in a terrestrial igneous rock. Nature 320,435-439. KURZ M. D. (1986b) In situ production of cosmogenic terrestrial helium and some applications to geochronology. Geochim. Cosmochim. Acta 50.2855-2862. KURZ M. D. and JENKINSW. J. (198 I) The distribution of helium in oceanic basalt glasses. Earth Planet. Sci. Lett. 53,41-54. KURZ M. D., JENKINSW. J. and HART S. R. (1982) Helium isotopic systematics of oceanic islands: Implications for mantle heterogeneity. Nature 297,43-47. KURZ M. D., JENKINSW. J., HART S. R. and CLAGUED. (1983) Helium isotopic variations in Loihi seamount and the island of Hawaii. Earth Planet. Sci. Lett. 66, 388-406. KURZ M. D., MEYERP. and SIGURDSSON H. (1985a) Helium isotopic systematics within the neovolcanic zones of Iceland. Earth Planet. Sci. Lett. 74, 291-305. KURZ M. D., O’BRIENP., GARCIAM. and FREY F. (1985b) Isotopic evolution of Haleakala volcano: Primordial, radiogenic and cosmogenic helium. Eos 66, 1120. KURZ M. D., GURNEYJ. J., JENKINSW. J. and Lo-t-r D. E. (1987) Helium isotopic variability within single diamonds from the Orapa Kimberlite. Earth Planet. Set Left. (in Press). LANGMUIR C. H., VOCKER. D., HANSONG. H. and HART S. R. (1978) A general mixing equation with applications to Icelandic basalts. Earth Planet. Sci. Len. 37, 380-392.

LIPMANP. W. (1980) Rates of volcanic activity along the southwest rift zone of Mauna Loa volcano. Bull. Volcano/.

43-44,703-725. LIPMANP. W. and SWENSONA. ( 1984) Generalized geological map of the southwest rift of Mauna Loa. USGS Map I1323. LOCKWOODJ. P. and LIPMANP. W. (1980) Recovery of datable charcoal beneath young lavas. Bull. Volcanol.4, 609615. LOCKWODD J. P. and LIPMANP. W. (1987) Holocene eruptive history of Mauna Loa volcano. U.S. Geol. Survey ProJ:Paper 1350, 509-535. LUP~ONJ. E. and GARCIAM. 0. (1986) Helium isotopes in submarine basahs from the island of Hawaii. Eos 67,127 1. MACDONALDG. A. ( 1963) Relative abundance of intermediate members of the o&anic basalt-trachyte association-a discussion. J. Geoohvs.Res. 86. 5 100-5 102. MACDOUGAJ_L I. i 1964) Potassium-argon ages from lavas of the Hawaiian Islands. Geol. Sot. Amer. Bull. 75, 107-128.

MORGANW. J. (1972) Deep mantle convection and plate motion. Amer. Assoc. Petrol. Geol. Bull. 565, 203-2 13.

MtTRRtSON P. and PINE J. (1955) Radiogenic origin of the helium isotooes in rock. Ann. N. Y. Acad. Sci. 62. 7 l-92.

NAUGHTONJ.-J., MACDONALD G. A. and GREENBERG V. A. ( 1980) Some additional K-Ar ages of Hawaiian rocks:

The Maui complex. J. Voic. GeothermalRes. 7, 339-345. NAVON0. and E. STOLPER (1986) Geochemicai consequences of melt percolation: The upper mantle as a chromatograhic column. J. Geol. (in press). RHODES J. M. (1987) How Mauna Loa works: a geochemical perspective. Hawaii Symposium on: How VolcanoesWork (abstr. vol.), p. 208. Hawaiian Volcano Observatory, Hilo. Hawaii. RICHTERF. M. (1986) Simple models for trace element fractionation during melt segregation. Earth Planet. Sci. Left 77,333-344.

RICHTERF. M. and MCKENZIED. (1985) Dynamical models for melt segregationfrom a deformable matrix. J. Geol.92, 729-740.

RISONW. and CRAIGH. (1983) Helium isotopes and mantle volatiles in Loihi seamount and Hawaiian island basalts and xenoliths. Earth Planet. Ser. Lett. 66, 407-426.

RODEN M. F., FREY F. A. and CLAGUED. A. (1984) Geo-

chemistry of tholeiitic and alkalic lavas from the Koolau range, Gahu, Hawaii: Implications for Hawaiian volcanism. Earth Planet. Sci. Let?. 69, 141-158. RUBINM., GARGULINSKI L. K. and MCGEEHIN J. P. (1987) Radiocarbon dates of Hawaii. U.S.G.S. Prof Paper 1350, 243-260. SHAWH. and JACKSONE. D. (1973) Linear island chains in the Pacific: Result of thermal plumes or gravitational anchors. J. Geophys.Res. 78,8634-8652.

SNEERINGER M., HARTS. R. and SHIMIZUN. (1984) Sr and Sm diffusion in dionside. Geochim. Cosmochim. Acta 48. 1589-1608. STEARNSH. T. and MACDONALD G. A. (1942) Geology and

ground water resources of the island of Maui, Hawaii. HawaiiDiv. Hydrogr. Bull. 7, l-344. STILLEP., UNRUHD. M. and TATSUMOTO M. (1983) Pb, Sr, Nd and Hf isotopic evidence of multiple sources for Oahu, Hawaii basal& Nature 304.25-29. TILLING R. I., WRIGHTT. L. and MILLARD H. T. (1987) Trace element chemistry of Kilauea and Mauna Loa lava in space and time. U.S.&S. Prot Paper 1350, 64 I-690. WILSON J. T. (1963) A nossible oriain of the Hawaiian islands. Can. J. Physics41, 863-870. ZINDLERA. and HART S. R. (I 986) Helium: Problematic primordial signals. Earth Planet. Sci. Lett. 79, 1-8.

APPENDIX Mauna Loa and Kilauea sample descriptions Mauna Loa HA82-0 I

KTE85-05

HA82-04

KTESS-28

Picritic basalt containing 10-1496, 1-4 mm, subhedral to euhedral olivine crystals, and 510% vesicles. Collected from thkN.E. rift, Kipukanakekakeauadf19”29’16”. 155°23’15”). ,. 910 i 70 years (W4047). I Olivine-phyric basalt with 3-5% l-3 mm, anhedral olivine, I-2% vesicles, and rare plagioclase collected from the base of Ka’alaiki aa flow, southwest rift of Mauna Loa (19’08’15”N, 155”32’44”W).Radiocarbon age 2180 ? 60 (W4015). Puna sugar picritic basalt, containing 15-2056 (l-3 mm) olivine collected 200 feet southwest of nine and one half mile camp f 19”36’48”N). Radiocarbon age is 5250 -1-lad years (W4536). Vesicular otivine basalt with 2O-2.5%vesicles and l-2%, 1 mm, olivines collected from aa flow exposed on the north side of highway 1I near

2914

HA82-05

KTE85-32

KTE85-11

KTE85-23

Mark D. Kurz ef al. Mountain View (09’32’43”N, 155’07’17”W). The radiocarbon age of this flow is 5650 2 90 (W3862). Puu Hoakalei picritic basalt containing IO-12% (I-5mm)olivine(19”39”12”N, 155”07’2O”W). Radiocarbon age is 9020 2 130 years (W4975). Olivine phyric basalt containing 5-7% vesicles and l-2% (5-2 mm) olivines; flow sampled near the beach at Paiahaa bay (18’58’19”N, 155”37’19”W). The radiocarbon age is 11780 & 100 (W3487). Olivine-rich basalt, containing IO-126 (1-2 mm) olivines in a fine-grained vesicular groundmass collected from the base of the sea cliff south of Kahukupoko point, below the deepest exposed ash layer (19”04’2O”N, 155“33’25”W). The ash layer has been dated as 30,000 years (W3935). Olivine phyric pahoehoe flow containing l-3% olivines (5-2 mm), and sparse plagioclase and

clinopyroxene, just below Pahala Ash on the top of the sea cliffs exposed by the Kahuku fault (18”57’22”N, 155’41’42”W). Kilauea HK82-07

Olivine phyric (4-5%) basalt from the base of Hilina Formation ( 19” 17’30”. 155” 18’38”). Sampled from massive aa tloi outcrop j&t above talus pile, at base of Keana Bihopa (elevation 1200 ft.); one of the oldest subaerially exposed flows on Kilauea volcano.

All other samples listed in Table 2 are described by KURZ et al. ( 1983) and KURZ ( 1986b). The radiocarbon ages and associated uncertainties are taken from KELLEY et al. (1979) and RUBINet al. ( 1987). The four digit number preceded by W refers to USGS radiocarbon dates of the same lava flows given in those references.