Excess water generation during reaction-inducing intrusion of granitic melts into ultramafic rocks at crustal P-T conditions in the Sør Rondane Mountains of East Antarctica

Arc magmas are one of the main sources of aqueous geofluids in the crust, and the movement of fluids above magma chambers has been geophysically imaged. Here, we constrain the water budget (i.e., supply, consumption and release of H2O) in these areas above magma chamber by examining the hydration caused by crust-melt reactions in the Sør Rondane Mountains of East Antarctica. The study area contains a phlogopite-pargasite-peridotite unit that has been intruded by numerous granitic dikes, creating hydration reaction zones at the dike-peridotite boundary. These reactions occurred at 0.5 GPa and 700 °C, corresponding to middle crustal conditions, and generated a series of reaction zones with distance from the granitic dikes as follows: (i) granitic dike, (ii) pargasite-actinolite zone, (iii) tremolite-phlogopite zone, (iv) anthophyllite-phlogopite zone, (v) phlogopite-olivine-orthopyroxene zone, and (vi) unaltered pargasite-phlogopite peridotite. The presence of amphiboles with a preferred orientation perpendicular to the dike margins and an absence of Cr-rich magnetite indicate that the pargasite-actinolite zone [zone (ii)] grew from the dike margins as a result of the dike reacting with the host rock, with an initial melt/rock boundary located between zones (ii) and (iii). The H2O contents of reaction zones (ii)-(v) are higher than the content in the hosting pargasite-phlogopite peridotite, suggesting that the intrusion of the dike was associated with hydration reactions. Geochemical analysis along a profile through the reaction zones indicates Mg and Fe depletion, and Si enrichment in zones (iii)-(iv), and Ca depletion and K enrichment in zones (iv)-(v) relative to the hosting pargasite-phlogopite peridotite. In contrast, zone (ii) is characterized by Ca, Fe, and Mg enrichments relative to the granitic dike. These observations suggest that the reaction zone sequence was formed by the elemental transfer between granitic dike and parasite-phlogopite peridotite: Ca, Fe, and Mg were leached from the host peridotite [zones (iii)-(v)]; K, Si, and H2O were mobilized from the granitic melt; and Ca, Mg, and Fe were deposited in the overgrowth zone [(ii)]. The width of the overgrowth zone correlates with the width of the leached zone [(iii)-(v)] but does not correlate with the width of the dike. These chemical profiles and geometry of the reaction zones suggest that hydration was not rate-limited by the supply of water from granitic melt, but instead was limited by the chemical transfer of elements such as K and Ca. It is likely that 3.7 wt% of the > 5.6-5.0 wt% H2O content of the granitic melt was consumed by hydration reactions, with the other > 1.9-1.3 wt% removed from the system. These observations suggest that at least one-fourth of the water in granitic melts can be liberated as excess water during the intrusion and solidification of the granitic melts, and subsequently transported to the overriding upper crust by the upward infiltration.