THE ROLE OF SCAPOLITE IN RECORDING THE VOLATILE CYCLE OF THE 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.