Molybdenum and titanium isotopic signatures of arc-derived cumulates

Abstract

Fluid migration and/or differentiation of magmas in arc settings are important drivers of stable-isotope fractionation of elements like molybdenum and titanium, respectively. For both isotope systems, evolved magmas are heavier than average arc-basalts, which requires an isotopically light reservoir counterbalancing the heavy felsic lithologies.In an attempt to better define this isotopically light reservoir, we investigate Mo and Ti isotopic signatures of upper crustal magmatic cumulates comprising hornblendites and gabbros from the Alpine orogen, the Sierra Nevada batholith, the Sanandaj-Sirjan zone and the Kos volcano-plutonic system. The cumulates and mafic enclaves exhibit Ti isotopic compositions ranging from that of arc-basalts/andesites to significantly lighter values (δ49Ti between −0.15 and + 0.08‰), which is in agreement with a Rayleigh distillation model. The δ49Ti correlates negatively with the abundance of FeTi oxides, suggesting that in samples which have δ49Ti signatures similar to those of arc-basalts, most of the Ti is hosted in pyroxene and amphibole. This indicates that the degree to which Ti isotopes are fractionated in a melt is controlled by the fraction of Ti incorporated into silicate phases versus that incorporated into FeTi oxides.In contrast, the corresponding Mo isotopic compositions of the upper crustal magmatic cumulates and mafic enclaves are more dispersed (δ98MoNIST = −0.02 ± 0.22‰, 2 s.d.) and similar to average arc-basalts. However, Mo concentrations throughout several cumulate and mafic enclave bulk rocks measured are too enriched to be explained by pure fractional crystallization as they do not match modelled melt-cumulate fractionation trends. We distinguish between purely magmatic and predominantly fluid mediated processes leading to Mo enrichment in cumulates, and show that both can play an important role in the generation of high Mo cumulates.