Isotopic and trace element constraints on mixing and melting models of marginal basin volcanism, Bransfield Strait, Antarctica

Details Isotopic and trace element constraints on mixing and melting models of marginal basin volcanism, Bransfield Strait, Antarctica Isotopic and trace element constraints on mixing and melting models of marginal basin volcanism, Bransfield Strait, Antarctica

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992e%26psl.111..287k&link_type=abstract

Earth and Planetary Science Letters, Volume 111, Issue 2-4, p. 287-303.

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12

Scientific paper

Bransfield Strait is a narrow marginal basin separating the South Shetland Islands from the northern end of the Antarctic Peninsula. Quaternary volcanism occurs in the strait as subaerial and submarine volcanoes aligned on the inferred axis of rifting, and as two subaerial, off-axis volcanoes on the northern margin of the strait. The Bransfield Strait lavas are similar to published analyses from other marginal basins, ranging from basalts and basaltic andesites to trachytes. They exhibit moderate enrichments in alkali and alkaline earth elements relative to high-field-strength elements that are typical of many back-arc basin basalts. The seamount basalts have trace element chemistry similar to enriched mid-ocean ridge basalts (E-MORB), but with variously higher alkali and alkaline earth element concentrations and, frequently, lower Nb.
Low-pressure fractional crystallization accounts for most of the compositional variation within individual volcanoes, but it does not explain intervolcano differences even though the volcanoes are closely spaced and presently or recently active. Melting of depleted mantle mixed with 0.5-2% crust or sediment explains the isotopic and trace element variations not accounted for by magma chamber processes. The off-axis volcanoes are the products of less partial melting than the on-axis volcanoes. One of the off-axis volcanoes also contains more of an enriched component in its source.
Some of the lavas have Rb/Sr too low to account for their 87Sr/86Sr, perhaps due the loss of Rb during mantle metasomatism or interaction between slab-derived fluids and the mantle source of the basalts. Strontium, neodymium, and 207Pb/204Pb isotopic signatures remained constant during the transition from Tertiary island arc volcanism to Quaternary marginal basin volcanism, but 206Pb/204Pb increased.
High CeN/SmN of basalts from some volcanoes requires residual garnet in the source. Thus the young (15-25 Ma at the trench) subducted slab is either deeper than 60 km beneath the volcanoes, or it has fractured and is no longer a coherent barrier to melts generated below it.

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