Other
Scientific paper
Oct 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990e%26psl.100..195h&link_type=abstract
Earth and Planetary Science Letters, Volume 100, Issue 1-3, p. 195-209.
Other
34
Scientific paper
A bimodal suite of volcanic rocks collected from the Sumisu Rift by alvin provide present day examples of the first magmatic products of arc rifting during the initiation of back-arc spreading. The trace element and isotopic composition of these rocks, which are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, differ from those of arc rocks and N-MORB in their relative incorporation of both subduction-related and non-subduction-related components. Subduction-related components, i.e., those that distinguish volcanic arc basalts from N-MORB, are less pronounced in rift lavas than in arc lavas. Alkali and alkaline earth to high field strength element and REE ratios as well as 87Sr/86Sr are intermediate between those of N-MORB and Izu arc lavas and indicate that Sumisu Rift basalts are similar to BABB erupted in other, more mature back-arc basins. These results show that back-arc basins may begin their magmatic evolution with BABB rather than more arc-like lavas.
Evidence of non-subduction related components remains after the effects of subduction related components are removed or accounted for. Compared to the arc, higher HFSE and REE concentrations, contrasting REE patterns, and <= ɛNd in the rift reflect derivation of rift lavas from more enriched components. Although SR basalt resembles E-MORB in many trace element ratios, it is referred to as BABB because low concentrations of Nb are similar to those in volcanic arcs and H2O/REE and H2O/K2O exceed those of E-MORB. Differences in HREE pattern and ɛNd require that the E-MORB characteristics result from source heterogeneities and not lower degrees of melting. Enriched mantle beneath the rift may reflect enriched blobs entrained in a more depleted matrix, or injection of new, more enriched mantle. High 208Pb/204Pb and moderate 207Pb/204Pb ratios with respect to Pacific MORB also reflect ancient mantle enrichment. Trends on Pb isotopic diagrams and <= ɛNd in the rift than in the arc indicate that recent sediment recycling is not an important process in the generation of these back-arc lavas.
Gill James B.
Hochstaedter Alfred G.
Morris Julie D.
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