Elemental Abundances Relevant to Identification of Magma Sources
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
The search for chemical characteristics of magma sources is usually done by analysing the magmas themselves. This indirect approach has limitations : clearly the magma has only some of the source's characteristics. W hat we require are process-independent chemical characteristics, analogous to the isotopic abundance of radiogenic daughter isotopes that have been used so successfully in defining magma sources. Processindependent chemical characteristics in mid-oceanic ridge, oceanic island and
... ceanic island and islandarc basalts (m.o.r.b., o.i.b., i.a.b.) have been used to identify contrasting chemical characteristics of mantle peridotite from these three tectonically distinct regions. As an example, the abundance ratios of one group of elements (e.g. Cs, K, Rb, Ba, U, and perhaps Th) relative to another group (e.g. light r.e.e., Zr, H f) are found to be fractionation-independent during most shallow-level basalt fractionation. These ratios are presumed to reflect the chemical characteristics of the mantle source of basalt from the three tectonic environments. In particular the ratios indicate the large cationdepleted nature of all m.o.r.b. and most o.i.b. peridotite sources. In common with many other island arcs, the abundance ratios are consistently higher in mantle under the Aleutian arc than in adjacent non-arc mantle represented by oceanic ridge, oceanic island, and back-arc basalts. The contention that subduetion of sediment could result in arc mantle sources with these high ratios is substantiated by trace element analyses of Ba and Os-rich deep sea sediments of the type that are being subducted at present at the Aleutian trench. The importance of recycling of sediment into the mantle at island arcs as an im portant control on the trace element (and isotopic) evolution of the mantle is indicated.