Elemental abundances relevant to identification of magma sources

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. What 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. Process-independent chemical characteristics in mid-oceanic ridge, oceanic island and island-arc 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, Hf) 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 cation-depleted 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 subduction of sediment could result in arc mantle sources with these high ratios is substantiated by trace element analyses of Ba and Cs-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 important control on the trace element (and isotopic) evolution of the mantle is indicated. Trace element heterogeneity in the source regions of magmas as diverse as basalts and leucogranites can be established using analyses of fractionation-independent elements of the magmas. In general, crystal-melt fractionation is not an adequate explanation for much of the variability in the sources. Mantle metasomation (Bailey 1982) or analogous crustal metasomatism (Heier 1979), involving a high pressure CO$\_{2}$- or H$\_{2}$O-rich fluid phase may create some of the heterogeneity, but high concentration levels of many elements in the fluid are required. The recycling of sediment with distinctive and highly variable ratios of Cs, Rb, K, Ba, U, Th and r.e.e., which recent $^{10}$Be studies (Brown et al. 1982) have reaffirmed, creates new opportunities for quantitative mass balance calculations on crustal recycling. More emphasis on sediment chemistry (including regional variability) and on the chemistry of mantle and crustal residues (restites) will be necessary to further the at present rather generalized and speculative aspects of the recycling calculation Support from the National Science Foundation (grants 80-07751 and 82-12757) is acknowledged. I thank Bruce Coulombe and Walter Conrad for analytical assistance, and V. Von Drach for splits of the DSDP core samples. Assistance by many people at Cornell and California Institute of Technology is acknowledged, especially G. J. Wasserburg. Drafting was done by S. Fast and manuscript preparation by E. Story.