Masters Thesis

The Role of Magmatic Scapolite in Recording the Volatile Cycle of the Lower-Arc Crust

Large-scale exchange of volatiles between Earth's surface, crust, and mantle takes place at Cordilleran arcs where volatile-bearing oceanic crust is subducted beneath continental margins. Here, we investigate scapolite, which we consider to be a major reservoir for volatiles such as S, and C in the lower crust, in order to evaluate its role in the volatile cycle by characterizing its occurrence, distribution and petrogenesis in arc crust and by calculating the estimated S and C budget contained within the mineral. We report ~550 major- and trace-element analyses of scapolite from 20 igneous and metamorphic rocks exposed over ~4600 km2 in the Median Batholith, Fiordland, New Zealand. Major and trace-element geochemical data reveal distinct differences between scapolite formed in igneous and metamorphic rocks. Igneous scapolite are S-rich with SO4 concentrations ranging between 1.6% and 5.7%, whereas metasedimentary scapolite are S-poor with S concentrations ranging between 0.2 and 2.5%. All igneous scapolites contain 1.5 to 4.1% CO2, and metasedimentary scapolite have concentrations ranging from 3.3 to 5.3%. Cl concentrations are <0.5 for all igneous and metamorphic scapolite. Trace-element data show that igneous scapolite are relatively enriched in LREE and depleted in HREE, and metasedimentary scapolite are depleted in LREE and are generally below detection (<0.01 ppm) 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. We find that volatile abundances in the Fiordland scapolites are distinctly different from each other according to the rock type and major- and trace-element concentrations; thus, geochemical data can differentiate igneous scapolite from metasedimentary scapolite in the lower crust. Igneous scapolite are relatively high in S compared to the metasedimentary group suggesting that the S in igneous melts is not sediment-derived and must originate from another source. Our calculation of the S and C budget estimates that ~14% of the carbon and ~57% of the sulfur found in the lower crust is contained within scapolite demonstrating that this mineral represents a significant and stable volatile reservoir, which formed in Fiordland during the Early Cretaceous.

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