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Author(s): Michael R. Ackerson [sup.1] [sup.2] , B. O. Mysen [sup.1] , N. D. Tailby [sup.3] , E. B. Watson [sup.2] Author Affiliations: (1) Geophysical Laboratory, Carnegie Institution of Washington, [...], The structure and composition of granites provide clues to the nature of silicic volcanism, the formation of continents, and the rheological and thermal properties of the Earth's upper crust as far back as the Hadean eon during the nascent stages of the planet's formation.sup.1-4. The temperature of granite crystallization underpins our thinking about many of these phenomena, but evidence is emerging that this temperature may not be well constrained. The prevailing paradigm holds that granitic mineral assemblages crystallize entirely at or above about 650-700 degrees Celsius.sup.5-7. The granitoids of the Tuolumne Intrusive Suite in California tell a different story. Here we show that quartz crystals in Tuolumne samples record crystallization temperatures of 474-561 degrees Celsius. Titanium-in-quartz thermobarometry and diffusion modelling of titanium concentrations in quartz indicate that a sizeable proportion of the mineral assemblage of granitic rocks (for example, more than 80 per cent of the quartz) crystallizes about 100-200 degrees Celsius below the accepted solidus. This has widespread implications. Traditional models of magma formation require high-temperature magma bodies, but new data.sup.8,9 suggest that volcanic rocks spend most of their existence at low temperatures; because granites are the intrusive complements of volcanic rocks, our downward revision of granite crystallization temperatures supports the observations of cold magma storage. It also affects the link between volcanoes, ore deposits and granites: ore bodies are fed by the release of fluids from granites below them in the crustal column; thus, if granitic fluids are hundreds of degrees cooler than previously thought, this has implications for research on porphyry ore deposits. Geophysical interpretations of the thermal structure of the crust and the temperature of active magmatic systems will also be affected. Thermobarometry and diffusion modelling in quartz crystals show that some granites may crystallize at much lower temperatures than we had thought, possibly explaining observations of cold magma storage.

In situ Raman and Brillouin light scattering techniques were used to study thermally induced high-density amorphous (HDA) to low-density amorphous (LDA) transition in silica glass densified in hot compression (up to 8 GPa at 1100 °C). Hot-compressed silica samples are shown to retain structural and mechanical stability through 600 °C or greater, with reduced sensitivity in elastic response to temperature as compared with pristine silica glass. Given sufficient thermal energy to overcome the energy barrier, the compacted structure of the HDA silica reverts back to the LDA state. The onset temperature for the HDA to LDA transition depends on the degree of densification during hot compression, commencing at lower temperatures for samples with higher density, but all finishing within a temperature range of 250–300 °C. Our studies show that the HDA to LDA transition at high temperatures in hot-compressed samples is different from the gradual changes starting from room temperature in cold-compressed silica glass, indicating greater structural homogeneity achieved by hot compression. Furthermore, the structure and properties of hot-compressed silica glass change continuously during the thermally induced HDA to LDA transition, in contrast to the abrupt and first-order-like polyamorphic transitions in amorphous ice. Different HDA to LDA transition mechanisms in amorphous silica and amorphous ice are explained by their different energy landscapes. [ABSTRACT FROM AUTHOR]

An estimate of TiO2 activity (aTiO2s!") is necessary for the application of trace-element thermo-barometry of magmatic systems where melts are typically undersaturated with respect to rutile/anatase. Experiments were performed in the system SiO2-Na2O-TiO2 to develop two independent methods of estimating aîic!t2"sat -- one based on the commonly applied rutile-saturation technique and another utilizing a novel Ti-in-tridymite thermometer. It is demonstrated that the rutile-saturation model can lead to an overestimate of amO^ relative to TiO2 activity calculated using the solubility of Ti in tridymite (SiO2) coexisting with rutile. Overestimation via the rutile-saturation technique is due to variations in the solubility mechanisms of Ti in the melt phase as a function of Ti content. In natural systems, overestimates of aTiO^®" will lead to an underestimation of crystallization temperatures by Ti-based trace-element thermobarometers. Although this study is not directly applicable to natural systems, it lays the groundwork for future research on natural composition magmas to constrain TiO2 activity in melts. [ABSTRACT FROM AUTHOR]