MAGNESIUM THERMOMETRY OF APATITE: PRELIMINARY CALIBRATION AND FUTURE PROMISE (Invited Presentation)

Apatite has several qualities that make it a compelling target for geochemical measurements: it is ubiquitous in crystalline rocks; it has an extensive pressure-temperature stability; it incorporates system volatiles during growth (F, Cl, S, C); it concentrates key refractory trace elements (P, REE, U); and it commonly exists as inclusions in more durable phases such as zircon, garnet and titanite. For these reasons, apatite in igneous and metamorphic rocks is widely exploited for geochemical information about the system of interest. To date, however, there is no proxy for the temperature of apatite crystallization in magmatic and metamorphic systems.

We conducted an exploratory experimental study of Mg incorporation in apatite as a function of temperature (875°-1100°C) at 1 GPa pressure, focusing on magmatic system analogs that contain—in addition to apatite and melt—two Ca-Mg phases (2 pyroxenes or pyroxene + amphibole) to buffer the activity of the Mg component in the system. Electron-microprobe analysis for Mg in the crystallized apatites reveals log-linear dependence on T(K)^-1, with concentrations ranging from ~2000 ppm to ~6000 ppm. A single experiment at 1100°C and 1 atm pressure produced apatites containing ~40% less Mg than the experimental counterpart at 1 GPa, suggesting a substantial positive (and very preliminary) pressure effect on Mg uptake.

Tentative application of the new Mg-in-apatite thermometer to ~200 Holocene tephra and pumice fragments from Mt. Shasta (~69-77% wt% SiO₂) yields apatite crystallization temperatures ranging from ~870°-900°C for the least silica-rich compositions (~70% SiO₂) to ~780°-810°C for the more silicic compositions (~74-77% SiO₂). This overall pattern is consistent with progressive fractionation toward high-silica magmas, and parallels the temperature trend resulting from Fe-Ti oxide thermometry (but shifted to ~50°-60°C lower temperatures). The concentration of P₂O₅ in the Shasta samples falls progressively with increasing SiO₂ content, which is consistent with monotonic cooling of an apatite-saturated system.