MOLYBDENUM ISOTOPE SYSTEMATICS IN THE ALEUTIAN ARC: A SOURCE TRACER PROXY

The elemental fluxes in subduction zones control the chemical budget for the arc magmatism and allow exploration of the relative contributions of the materials involved in the crust production. Molybdenum is a fluid mobile element and can potentially shed light on the sources of fluids in subduction zones and its redox conditions. Here we explore the use of Mo isotopes as a source tracer by analyzing Mo isotope data (as δ^98/95Μο relative to NIST SRM 3134) on basalts, andesites, and dacites from the central, eastern, and western Aleutian volcanoes. The Aleutian arc is an ideal location to study the effect of source variability on Mo isotopes due to systematic changes in the sediment flux and convergence rate along the arc. We also report Mo isotope data on sediments from the north Pacific as proxies of incoming sediments, and serpentinized abyssal peridotites from South West Indian ridge as proxies of subducted serpentinized lithosphere. In order to address the effect of igneous differentiation on Mo isotopes, we specifically analyze both hydrous/amphibole dominated (Buldir) and anhydrous/amphibole free (Korovin) calc-alkaline sequences. The data shows that arc lavas from both Buldir, with low δ^98/95Mo (-0.38 to -0.13‰, avg=-0.22 ‰), and Korovin, with high δ^98/95Mo (+0.03 to +0.22‰, avg=0.11‰), show no covariation with SiO₂ and Dy/Dy* indicating absence of Mo isotope fractionation during anhydrous and hydrous differentiation. Thus, we conclude that Mo isotopes reveal source features for Aleutian magmas. All volcanoes analyzed here, except for Korovin and Seguam (+0.03 to +0.30‰), have low, MORB-like, δ^98/95Mo (-0.38 to -0.12‰). The lack of correlation between along-arc Mo isotope variance and sediment flux suggest that sediments are unlikely the source of Mo. Instead, the high δ^98/95Mo in Korovin and Seguam, where the Amlia Fracture Zone enters the trench, is consistent with the high δ^98/95Mo of serpentinized peridotites (up to 1.10‰) analyzed here, implying a role for serpentinized lithosphere in the Mo budget of these arc lavas. High Mo/Ce and MORB-like δ^98/95Mo in the western Aleutians, where slab melting under eclogitic facies has been inferred, suggest little fractionation of Mo isotopes from the slab to the overlying crust and a role for fluids in the preferential flux of Mo from the slab to the arc.