THE ROLE OF SCAPOLITE IN RECORDING THE VOLATILE CYCLE OF THE LOWER ARC CRUST

Volatile cycling processes in lower-arc crust are not well understood, nor frequently studied from an igneous mineral perspective. This is because igneous volatile reservoirs are rare, and few exposures of lower-arc crust exist. Here, we use scapolite, which is considered to be a major reservoir for volatiles such as S, C, and Cl in the lower crust. We investigate igneous and metamorphic scapolite to characterize the occurrence, distribution and petrogenesis in the lower-crust of the Median Batholith, Fiordland, New Zealand. Here we report >450 major and trace-element analyses of scapolite from 15 igneous and metamorphic rocks covering an ~1800 km² area of exposed lower-arc crust.

Major element geochemical data reveal distinct differences between scapolite formed in the three rock types. Igneous and meta-igneous scapolite are S-rich with S wt% between 3.5 and 5.7%, whereas meta-sedimentary scapolite are S-poor with S wt% between 0.2 and 2.5%. All samples contain C with igneous and meta-igneous wt% between 1.4% and 2.8% and meta-sedimentary wt% between 3.3% and 5.0%. For all samples, Cl wt% is <0.05%. Trace-element geochemical data also reveal differences between the igneous and meta-igneous group and the meta-sedimentary group. Where igneous and meta-igneous are relatively enriched in LREE and depleted in HREE, the meta-sedimentary samples are depleted in LREE and are generally below detection for elements heavier than Nd. Igneous scapolite from garnet-bearing veins and pegmatites also have HREE concentrations that are below detection consistent with co-crystallization with garnet.

The presence of C and S in scapolite in lower-arc crust of the Median Batholith suggests that scapolite is a long-term volatile reservoir. Volatile abundances in scapolite are directly correlated with rock type; thus, major- and trace-element geochemical data can differentiate igneous and meta-igneous scapolite from meta-sedimentary scapolite in the case of the Median Batholith. In addition, igneous and meta-igneous scapolite are relatively high in S compared to the meta-sedimentary group suggesting that the S in igneous melts is not sediment-derived and must originate from another source.