Mariana Trough lavas from 18°N: Implications for the origin of back arc basin basalts

Abstract

Geochemical data on glass separates from four dredges and two Deep Sea Drilling Project sites near 18°N in the Mariana Trough define 17 distinct chemical groups that probably represent individual magma batches. Much of the observed variation within the sample suite can be explained by low‐pressure fractionation, although effects of magma mixing and variations in melt‐generating processes are also apparent. Relative to mid‐ocean ridge basalts (MORB), Mariana Trough glasses have higher H2O and Al2O3 and lower FeO* at moderate degrees of differentiation. The high Al2O3 is a consequence of the displacement of the olivine‐plagioclase cotectic to lower MgO in the hydrous Mariana Trough magmas. Low FeO can be attributed to low T and P of melting of diapirs rising along cooler adiabats than those beneath normal mid‐ocean ridges. Extents of melting giving rise to Mariana Trough primary magmas were about 15%, within the range of melting extents for primary MORB. Relative to MORB, Mariana Trough lavas are enriched in Ba>Rb>K>light rare earth elements>Sr and depleted in Y. The degree of enrichment inversely correlates with solid/liquid distribution coefficients, suggesting control by a low‐degree melt component in the Mariana Trough source. Relative to submarine lavas of the Mariana arc, Mariana Trough volatiles are depleted in halogens. Melting of the upper part of the subducted crust could occur 125–175 km beneath the Mariana region by addition of volatiles derived from the breakdown of hydrous phases in the lower crust and upper mantle at depths of 150–210 km. Addition of such a melt component to the source region for Mariana Trough magmas will not quantitatively account for the incompatible element enrichments unless altered crust or crustal sediments are included in the slab being melted or open system hybridization reactions occur in the overlying mantle wedge.