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The effect of variable Na/K on CO2 solubility in slab-derived rhyolitic melts
Michelle Muth, Megan S. Duncan, Rajdeep Dasgupta
Abstract: We conducted high pressure, high temperature experiments to investigate the effect of variable alkali ratio on the CO2‐rich fluid solubility in hydrous rhyolitic melts at sub‐arc depths. Experiments were performed at 3.0 and 1.5 GPa, 1300 °C on rhyolitic compositions similar to low‐degree partial melts of subducted slab lithologies, with fixed total alkalis (Na2O+K2O ~11.5 wt.%, volatile‐free), but Na# (molar Na2O/[Na2O+K2O]) varying from 0.15 to 0.88. In the experimental glasses, total dissolved CO2 (CO2tot.) ranged from 2.14 ± 0.07 to 3.20 ± 0.07 wt.% at 3.0 GPa, and from 0.70 ± 0.02 to 1.19 ± 0.02 wt.% at 1.5 GPa. Experiments showed a general positive correlation between Na# and CO2tot., with the exception of the highest Na# experiment at 1.5 GPa. Carbon was dissolved as molecular CO2 (CO2mol.) and carbonate (CO32‐). As Na# increased, CO2mol./CO2tot. decreased from 0.94 to ~0.00 in the 1.5 GPa experiments and from 0.65 to 0.05 in the 3.0 GPa experiments. Variability in CO2 concentration is larger and more clearly correlated with Na# at 3.0 GPa, indicating that this effect is pressure dependent. Our results show that compositional variability in silicic melts must be considered to accurately place constraints on the limit of CO2 transfer in subduction zones.
Muth, M., Duncan, M. S., Dasgupta, R. (2020). The effect of variable Na/K on CO2 solubility in slab-derived rhyolitic melts. In Manning, C., Lin, A., and Mao, W. (Eds.) Carbon in Earth’s Interior, Geophysical Monograph 249, 195-208. doi:10.1002/9781119508229.ch17
Yudovskaya, M. A., Costin, G., Sluzhenikin, S. F., Kinnaird, J. A., Ueckermann, H., & Abramova, V. D.
Mafic sills of the Marginal Zone are widespread within the footwall of the Bushveld Complex with some of them being enclosed in later Bushveld intrusions. In the central sector of the northern limb of the Bushveld Complex on the Turfspruit farm, a sill of norite enclosed within the Lower Zone cumulates contains numerous semi-assimilated fragments of anorthositic and magnetite-orthopyroxenitic lithologies that are interpreted to be the partially melted xenoliths of local argillaceous, magnetite-bearing, and anhydritic shales. Almost pure anorthite with eutectic inclusions of microgranular Al-rich orthopyroxene composes the interstitial material in the Lower Zone chromite-olivine cumulates at the contact with the magnetite-bearing norite. An identical association of anorthite+Al-rich orthopyroxene occurs in the overlying Platreef pyroxenite at the contact with anhydritic hornfels suggesting that an exchange reaction has likely involved anhydrite in both cases. Strontium isotope signatures of plagioclase from the norite support its hybrid origin, whereas Sr isotopes in anorthite from harzburgite is within the range of the most primitive Bushveld rocks indicating a predominant contribution of magmatic Sr. We suggest that the observed mineral assemblages of Al-rich and Cr-poor orthopyroxene, Fe-rich anorthite with elevated Ga, and Al-rich, Cr-depleted spinel, as well as the changes in mineral chemistries towards those compositions, are indicative of assimilation of evaporites and argillaceous shales by Bushveld melts on Turfspruit and elsewhere in the northern limb.
Yudovskaya, M.A., Costin, G., Sluzhenikin, S.F. et al. (2020). Hybrid norite and the fate of argillaceous to anhydritic shales assimilated by Bushveld melts. Miner Deposita. https://doi.org/10.1007/s00126-020-00978-6
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