Molybdenum isotope variations in calc-alkaline lavas from the Banda arc, Indonesia: Assessing the effect of crystal fractionation in creating isotopically heavy continental crust

Abstract

Recent studies report a large Mo isotope variability of up to 1‰ (expressed in δ98/95MoNIST3134) in convergent margin lavas. These isotopic variations have been associated with subduction zone processes and ultimately may account for heavy and variable isotope signatures in evolved continental crust. Arc lavas show both lighter and heavier Mo isotopic values when compared to the chondritic average (δ98/95MoNIST3134 −0.16 ± 0.02‰), with a concentration weighted mean isotopic composition (δ98/95MoNIST3134 +0.03‰) distinctly heavier than that of the mantle (δ98/95MoNIST3134~−0.21 to −0.16‰). The absence of isotopic fractionation during tholeiitic igneous differentiation indicates that heavy Mo isotope signatures in convergent margin rocks are intrinsic to subduction zone processes, caused either by recycling of subducted components, by Mo isotope fractionation during magmatic differentiation, or by a combination thereof. In order to gain a more detailed understanding of the Mo isotope variability in arc lavas, we have studied Mo isotopes and concentrations in calc-alkaline lavas sampled along the Banda Arc in Indonesia, an intra-oceanic subduction zone well known for variable contributions of subducting continental meterial.Banda Arc lavas exhibit Mo isotopic compositions covering a large range from −0.48 to +0.24‰ in δ98/95MoNIST3134. We find that, combined with published data from other subduction zones, the majority of arc data are isotopically heavier compared to its presumed mantle wedge source or mid-ocean-ridge basalts. Furthermore, arc lava Mo isotope signatures show an apparent covariation between indices of amphibole-clinopyroxene fractionation from the melt. Generally, heavier Mo isotopic signatures are associated with higher degrees of REE fractionation expressed as λ1, λ2, and Dy*/Dy, high Ca/Al, and higher Sc contents. These observations can be best explained with an open system magma reservoir where fractional crystallisation, recharge and eruption are tightly linked. Notable exceptions in the Banda region are four samples with isotopically light Mo. Three of these isotopically light lavas indicate the assimilation of an isotopically light Mo reservoir with heavy δ18O and radiogenic 87Sr/86Sr-206Pb/204Pb, tentatively identified as lower arc crust. Together, our data demonstrate, in line with previous findings, that magmatic processes modify the Mo isotopic composition in arc lavas, thus adding complexity to the Mo isotope systematics inherent to subduction zones.