Molybdenum isotope ratios in Izu arc basalts: The control of subduction zone fluids on compositional variations in arc volcanic systems

Abstract

Molybdenum isotope variations in mafic arc lavas have mainly been attributed to the influence of slab-derived components, such as subducted sediment melts and aqueous fluids. The latter have been hypothesised to fractionate Mo isotopes through interaction with the oceanic crust and carry an isotopically heavy signal that is transferred to the source of arc magmas. Thus, understanding Mo isotope systematics in subduction zones requires characterising the Mo isotope composition of slab-derived fluids and their influence on the Mo isotope budget of arc magmas. However, Mo isotope data reported to date show a considerable influence from subducted sediments that complicate accurate constraints being placed on the fluid contribution. We present Mo isotope data for mafic lavas from the Izu arc, a highly depleted oceanic island arc whose magma compositions show a dominant control from slab-derived fluids. The lavas from the Izu volcanic front are isotopically heavier than MORB and the depleted mantle. Their d 98/95 Mo (the relative difference in measured 98 Mo/ 95 Mo to NIST 3134) systematically varies with indicators for fluid-mobile element enrichment, suggesting that slab-derived fluids in the Izu arc have heavy Mo isotope compositions. Additionally, co-variations with radiogenic 143 Nd/ 144 Nd and 176 Hf/ 177 Hf point to a relationship between the addition of aqueous fluids and compositional heterogeneity of the sub-arc mantle. We present mass balance models that show that the influence of subduction zone fluids on the trace element pattern of arc magmas is more dominant when these are added to a more depleted and refractory sub-arc mantle, which preferentially melts due to a relatively higher fluid flux. The mass balance of Mo in the Izu arc predicts a light Mo isotope composition for the residual oceanic crust as a result of the preferential removal of isotopically heavy Mo during slab dehydration, consistent with previous suggestions for the Mariana arc and isotopically light Mo previously reported for eclogites.

Molybdenum isotopes unmask slab dehydration and melting beneath the Mariana arc

“…3 ), with values both higher and lower than that of DM (−0.21 ± 0.02‰ 34 , 42 ). This δ 98/95 Mo variation is much greater than has been observed in comparable Mariana arc lavas from the Central Island province (MgO: 2.7–6.1 wt%; δ 98/95 Mo = −0.17‰ to +0.15‰ 8 ) or in lavas from the Izu volcanic front (MgO: 3.4–5.6 wt%; δ 98/95 Mo = −0.14‰ to +0.11‰ 33 ). The NW Rota-1 samples are lower in δ 98/95 Mo, from −0.20‰ to −0.02‰, and show a smaller δ 98/95 Mo range (0.18‰ ± 0.12‰) than either the Pagan samples (0.4‰ ± 0.12‰) or other IBM samples (0.32‰ ± 0.12‰).…”

“…3b, c ), consistent with an isotopically heavy Mo signature arising from slab dehydration. δ 98/95 Mo and Ce/Mo correlate inversely in the Pagan samples and show a much steeper trend than other Mariana and Izu volcanic front samples 8 , 33 , suggesting a fluid-derived δ 98/95 Mo component that is much higher than +0.2‰.

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“…Negative correlations between Mo and MgO imply that Mo behaves as a moderately incompatible element during mantle melting and magmatic differentiation, similar to the light rare earth elements. Thus, Ce/Mo ratios can be used to document Mo enrichment in volcanic rocks or their mantle sources 8 , 30 , 31 , 33 . The SW Pagan samples point to complex magmatic evolution, including magma mixing, as indicated by reverse-zoned olivine and clinopyroxene phenocrysts 7 .…”

“… Plots of δ 98/95 Mo vs. a Ce/Mo, b Ba/Nb, c Cs/Nb, and d Mo/Nb for the Asùt Tesoru serpentinite mud samples and Pagan and NW Rota-1 volcanic rock samples unaffected by amphibole fractional crystallization. The Izu arc 33 , other Mariana volcanic front volcanoes 8 , average Mariana sediments 6 , 8 , 801 C AMOC Super Composite 8 , 72 and average blueschist and eclogite data from the Raspas Complex and Cabo Ortegal Complex 32 are also plotted for comparison. The ranges for depleted mantle (DM) are estimated according to the studies of Bezard et al 34 , Gale et al 44 , Workman and Hart 73 , and Salters and Stracke 43 .…”

“…Molybdenum (Mo) is mobile in fluids, like Pb and Ba, but it can also be partially retained in rutile during slab dehydration or melting. Mo isotopic fractionation during slab devolatilization results in heavier isotopic signatures in the fluid phase, while the residual slab becomes isotopically lighter 8 , 29 – 33 . The materials of the subducting Pacific plate outboard of the Mariana arc have different Mo isotope compositions (expressed as δ 98/95 Mo, the ‰ deviation of 98 Mo/ 95 Mo from standard NIST SRM 3134).…”