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Title: Identifying the granitic composition water-saturated solidus
The granitic water-saturated solidus (G-WSS) is the lower temperature limit of magmatic mineral crystallization. The accepted water-saturated solidus for granitic compositions was largely determined >60 years ago1. More recent advances in experimental petrology, improved analytical techniques, and recent observations that granitic systems can remain active or spend a significant proportion of their lives at conditions below the traditional G-WSS2–5 necessitate a careful experimental investigation of the near-solidus regions of granitic systems. Natural and synthetic starting materials were melted at 10 kbar and 900°C with 48 wt% H2O to produce hydrous glasses for subsequent experiments at lower PT conditions used to locate the G-WSS. We performed crystallization experiments and melting experiments at temperatures ranging from 575 to 800°C and 1, 6, 8, and 10 kbar on 12 granitoid compositions. First, we ran a series of isothermal crystallization experiments along each isobar at progressively lower temperatures until runs completely crystallized to identify apparent solidus temperatures. Geochemical analyses of quenched glass compositions demonstrate that progressive crystallization drives all starting compositions towards silica-rich, water-saturated rhyolitic/granitic melts (e.g., ~7578 wt% SiO2). After identifying the apparent solidus temperatures at which the various compositions crystallized, we then ran series of reversal-type melting experiments. With the goal of producing rocks with hydrous equilibrium microstructures, we crystallized compositions at temperatures ~10°C below the apparent solidus identified in crystallization experiments, and then heated isobarically to conditions that produced ~20% melt during the crystallization experiments. Importantly, crystallization experiments and heating experiments at the same PT conditions produced similar proportions of melt, crystals, and vapor. A time-series of experiments 230 days at PT conditions previously identified to produce ~10% to 20% melt did not reveal any kinetic effects on melt crystallization. Experiments at 6 to 10 kbar crystallized/melted at temperatures close to the published G-WSS. However, at lower pressures where the published G-WSS is strongly curved in PT space, all compositions investigated contained melt to temperatures ~75 to 100°C below the accepted G-WSS. The similarity of crystallization temperatures for the higher-pressure experiments to previously published results, similar phase proportions in melting and crystallization experiments, and the lack of kinetic effects on crystallization collectively suggest that our lower pressure constraints on the G-WSS are accurate. The new experimental results demonstrating that the lower-pressure G-WSS is significantly lower than unanimously accepted estimates will help us to better understand the storage conditions, evolution, and potential for eruption in mid- to upper-crustal silicic magmatic systems. (1) Tuttle, O.; Bowen, N. Origin of Granite in the Light of Experimental Studies in the System NaAlSi3O8–KAlSi3O8–SiO2–H2O; Geological Society of America Memoirs; Geological Society of America, 1958; Vol. 74. (2) Rubin, A. E.; Cooper, K. M.; Till, C. B.; Kent, A. J. R.; Costa, F.; Bose, M.; Gravley, D.; Deering, C.; Cole, J. Rapid Cooling and Cold Storage in a Silicic Magma Reservoir Recorded in Individual Crystals. Science 2017, 356 (6343), 1154–1156. (3) Andersen, N. L.; Jicha, B. R.; Singer, B. S.; Hildreth, W. Incremental Heating of Bishop Tuff Sanidine Reveals Preeruptive Radiogenic Ar and Rapid Remobilization from Cold Storage. Proceedings of the National Academy of Sciences 2017, 114 (47), 12407–12412. (4) Ackerson, M. R.; Mysen, B. O.; Tailby, N. D.; Watson, E. B. Low-Temperature Crystallization of Granites and the Implications for Crustal Magmatism. Nature 2018, 559 (7712), 94–97. (5) Glazner, A. F.; Bartley, J. M.; Coleman, D. S.; Lindgren, K. Aplite Diking and Infiltration: A Differentiation Mechanism Restricted to Plutonic Rocks. Contributions to Mineralogy and Petrology 2020, 175 (4).  more » « less
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Xth Hutton Symposium
Medium: X
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National Science Foundation
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  1. The accepted water-saturated solidus for granitic compositions (granitic water-saturated solidus, G-WSS) was largely determined >60 years ago. Significant experimental resources using modern approaches have been allocated to defining, refining, and parameterizing the solidus positions for other rock compositions, but limited work has been performed to accurately define G-WSS. Modern experimental and analytical techniques afford the opportunity to re-investigate the position of the G-WSS. Various thermobarometric applications to many granitic and rhyolitic composition rocks commonly return temperature estimates significantly lower than the widely accepted G-WSS determined largely by Tuttle and Bowen (1958). To evaluate the apparent discrepancies and help distinguish igneous from metamorphic processes recorded in granitic mineral assemblages, we performed experiments at temperatures ranging from 575 to 900°C and 0.5 to 10 kbar on 12 granitoid compositions composed of natural and synthetic starting materials. Most starting materials were melted in the presence of water at 10 kbar and 900°C in piston cylinders and quenched to room temperature in under one minute to produce water-saturated glasses for usage in subsequent crystallization experiments. The results of experiments on glass compositions were further validated in several runs using finely-ground crystalline starting materials. We ran crystallization experiments at P-T conditions spanning the accepted G-WSS. Decreasing experimental temperatures along each isobar caused systematic decreases in melt percentages until achieving complete crystallization at solidus conditions. A time-series of experiments at P-T conditions with ~20% melt did not reveal any kinetic effects on melt crystallization. All compositions investigated contained melt to temperatures ~75 to 100°C below the accepted G-WSS. Experimental results demonstrating that the G-WSS is significantly lower than unanimously accepted estimates will help us to better understand the storage conditions and evolution of silicic magmatic systems. Tuttle O, Bowen N (1958) Origin of Granite in the Light of Experimental Studies in the System NaAlSi3O8–KAlSi3O8–SiO2–H2O. Geological Society of America 
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  5. null (Ed.)
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