TEMPERATURES AND RESIDENCE TIMES OF THE AMALIA TUFF: RESULTS OF TITANIUM-IN-QUARTZ THERMOMETRY AND DIFFUSION MODELING

Cathodoluminescence (CL) images of quartz grains from the rhyolitic Amalia Tuff (NM) show a distinct concentric zoning of darker cores and bright rims. Under CL, brighter areas imply higher titanium (Ti) concentrations. As found by [1], Ti content in quartz is positively correlated to temperature of quartz crystallization (Ti-in-quartz thermometry). This suggests that these quartz grains were subjected to a reheating event prior to eruption, raising the questions: (1) What are the temperatures of long-term storage and pre-eruptive reheating? and (2) How long did the quartz spend at these higher-temperature conditions? We attempt to answer these questions with a combination of Ti-in-quartz thermometry and diffusion modeling.

We applied Ti-in-quartz thermometry to the Amalia Tuff using an equation from [2]. Using a TiO2 activity (aTiO2) of 0.5 and a pressure of 2.0 kbar, quartz rim temperatures averaged 650°C , at the lower end of the granitic solidus. Because the TiO2 activity of the Amalia Tuff is not well-constrained, we also decided to test the lowest possible aTiO2 of 0.1 which produces average quartz rim temperatures of 750°C.

We then applied diffusion modeling across the rim-core boundary, comparing results using 650°C and 750°C as the temperature input. Two equations for calculating diffusion coefficient were compared—[3] and [4]. Diffusion timescales at 650°C were 944 - 15235 a when using [3] and 2.3 to 29 Ma when using [4], which is unrealistically long. Diffusion timescales at 750°C were 29 - 470 a when using [3] and 0.035 to 0.56 Ma when using [4]. Generally, using diffusion coefficients calculated by [3] implies short timescales for rim growth, presumably related to a late-stage reheating event initiating eruption of Amalia Tuff, whereas applying [4] suggests that the rims grew early during the assembly and long-term storage of the system in the upper crust rather than being related to eruption.

[1] Wark and Watson (2006), Contrib Mineral Petrol, 152, 743–754 [2] Zhang et al. (2020), Earth and Planetary Science Letters 538, 116213 [3] Cherniak et al. (2007), Chemical Geology 236 65–74 [4] Jollands et al. (2020), Geology, 48, 654–657