Diffusion of Sr in fluorphlogopite determined by Rutherford backscattering spectrometry

International audienc

Pre-eruption recharge of the Bishop magma system

The 650 km3 rhyolitic Bishop Tuff (eastern California, USA), which is stratigraphically zoned with respect to temperatures of mineral equilibration, reflects a corresponding thermal gradient in the source magma chamber. Consistent with previous work, application of the new TitaniQ (Ti-in-quartz) thermometer to quartz phenocryst rims documents an ∼100 °C temperature increase with chamber depth at the time of eruption. Application of TitaniQ to quartz phenocryst cores, however, reveals lower temperatures and an earlier gradient that was less steep, with temperature increasing with depth by only ∼30 °C. In many late-erupted crystals, sharp boundaries that separate low-temperature cores from high-temperature rims cut internal cathodoluminescent growth zoning, indicating partial phenocryst dissolution prior to crystallization of the high-temperature rims. Rimward jumps in Ti concentration across these boundaries are too abrupt (e.g., 40 ppm across a distance of <10 µm) to have survived magmatic temperatures for more than ∼100 yr. We interpret these observations to indicate heating-induced partial dissolution of quartz, followed by growth of high-temperature rims (made possible by lowering of water activity due to addition of CO2) within 100 yr of the climactic 760 ka eruption. Hot mafic melts injected into deeper parts of the magma system were the likely source of heat and CO2, raising the possibility that eruption and caldera collapse owe their origin to a recharge event

Magmatic residence times of zoned phenocrysts: introduction and application of the binary element diffusion modelling (BEDM) technique

This paper describes a general technique, binary element diffusion modelling (BEDM), for determining single-crystal residence times in magmas that relies on modelling the diffusion of two or more elements in the crystal. BEDM has the advantage over other diffusion-based models in that it does not need a precisely defined initial compositional profile for the crystal at “zero time”, and instead requires that the concentrations of the two elements are correlated during crystallisation. Any differences subsequently observed between the two elements are caused by intracrystalline diffusion during residence in hot magma. These differences are removed by artificially ageing the slower-diffusing of the two elements, and the amount of time taken to “undo” the difference between the elements is simply related to the crystal residence time (=decoupling time) at high temperatures. The BEDM principle is demonstrated using artificial data and is then applied to literature data for Sr and Ba in a zoned sanidine crystal from the Bishop Tuff (Anderson et al., in J. Petrol 41(3):449–473, 2000). For this crystal, the method gives a residence time estimate of 114 ka at 800°C, which is then compared with estimates from other methods. In theory, the method can be further expanded for use as a geothermometer as well as geochronometer. However, this is not easily possible with the diffusivity data currently available