Helium and lead isotope geochemistry of oceanic volcanic rocks from the East Pacific and South Atlantic

Abstract

The isotopic evolution of helium and lead in the Earth is coupled by virtue of their common radioactive parents uranium and thorium. The isotopic signatures in oceanic volcanic rocks provide constraints on the temporal evolution of mantle source regions and volcanic magmas. He and Pb isotopes were measured in glassy basalts from young seamounts in the East Pacific, and in phenocrysts and corresponding whole rocks, respectively, from the island of st. Helena. He isotopes were also measured in glassy mid-ocean ridge basalts from the South Atlantic, previously studied for Pb isotopes by Hanan et al. (1986). A precise reconstruction of He-Pb isotope relationships in volcanic source regions is complicated by post-eruptive radiogenic ingrowth of 4He in non-zero age basalts, by pre-eruptive radiogenic ingrowth of 4He in magmas with elevated (U+Th)/He, by multi-stage fractionation processes involving (U+Th)/He, U/Pb and Th/Pb and by convective mixing in the Earth's interior. Aspects of each of these problems are addressed. (U+Th)/He ages are estimated from the isotope disequilibrium of 3He/ 4He between He trapped in vesicles and that dissolved in the glass phase of young alkali basalts at seamount 6 in the East Pacific. 3He/ 4He in the glass phase of these alkali basalts is subatmospheric, while in the vesicles it ranges between 1.2-2.5 RA (RA = atmospheric ratio). 3He/ 4He in vesicles (extracted by crushing in vacuo) allows a correction to be made in the dissolved phase He (by fusion of the remaining powder) for the inherited component in order to compute the radiogenic [He]. The method is applicable to rocks containing phases with different (U+Th)/He, and the results have implications for dating lavas in the age range of 103 to 106 years, and for reconstructing the temporal evolution of young volcanic systems. Pb, Sr and Nd isotopic variability observed at a small seamount field between 9-14°N near the East Pacific Rise covers ~80% of the variability for Pacific MORB, due to small-scale heterogeneity in the underlying mantle. Tholelites at these seamounts have He, Pb, Sr and Nd isotope compositions which are indistinguishable from MORB. Associated alkali basalts show more radiogenic He, Pb and Sr signatures. The lower 3He/ 4He of He trapped in vesicles of these alkali basalts (1.2-2.6 RA ) is associated with low helium concentrations (<5x10-8 ccSTP/g). Evolved alkali basalts have lower 3He/ 4He (1.2-1.8 RA) than primitive alkali basalts (2.4-2.6 RA), suggesting some degree of magmatic control on inherited 3He/ 4He in these alkalic lavas. Collectively, the isotopic results suggest that as the lithosphere ages, material transfer from the MORB source becomes less significant because smaller degrees of melting average the chemical characteristics of heterogeneous mantle volumes less efficiently than near the ridge. Icelandites erupted at Shimada Seamount, an isolated volcano on 20 m.y. old seafloor, have Pb, Sr and Nd isotopic compositions similar to post-erosional basalts at Samoa. 3He/ 4He at Shimada ranges between 3.9-4.8 RA and helium concentrations are too large for radiogenic contamination of magma to have lowered the 3He/ 4He appreciably. These results indicate the presence of an enriched mantle component previously unidentified beneath the East Pacific. Its low 3He/ 4He may be due to the melting of domains with high (U+Th)/He which formed during accretion of the oceanic lithosphere. Alternatively, it is an inherent characteristic of the source, which contains material recycled into the mantle at subduction zones. The mid-ocean ridge between 12-46°S in the South Atlantic displays 3He/ 4He lower than typical MORB values. Local anomalies occur at the latitudes of off-axis islands to the east, apparently due to contamination of depleted mantle asthenosphere by hotspot materials (as previously shown for (La/Sm)N and Pb isotopes; Schilling et al., 1985; Hanan et al., 1986). He - Pb isotopic relationships along the 12-22°S ridge segment suggest that St. Helena has 3He/ 4He less than MORB. 3He/ 4He in two St. Helena rocks (extracted by in vacuo crushing of olivine and pyroxene) is 5.8 RA when the extracted contents are greater than 1x10- 13 ccSTP/g, consistent with the He - Pb observations along the St. Helena ridge segment. 4He/3He and radiogenic Pb isotope ratios are linearly correlated for the South Atlantic ridge segment between 2-12°S. Linear correlation of 4He/3He - 206Pb/204Pb within an oceanic rock suite reflects the temporal evolution of 204Pb/3He in the source. A linear correlation between volcanic suites derived from isotopically different sources (e.g., oceanic islands) may imply a coherent fractionation of (U+Th) from He and Pb during the evolutionary history of their respective mantle source regions.