A NANOSIMS STUDY: MAGMA STORAGE AS RECORDED IN QUARTZ PHENOCRYSTS FROM A LARGE SILICIC PROVINCE (PATAGONIA, ARGENTINA)

The volcanic El Quemado Complex was deposited during the breakup of Gondwana from the Middle to Late Jurassic. It is part of a large silicic igneous province, which includes the Chon Aike Province in Southern Patagonia and related rocks from the Antarctic Peninsula. We studied quartz phenocrysts to provide new insights into the timescales of volcanic processes and magma sources. Quantitative Ti-analyses were done by SIMS and several high-resolution NanoSIMS profiles were acquired across growth respectively resporption zones as identified by cathodoluminescence. All NanoSIMS transects show sharp changes in the ⁴⁸Ti/²⁹Si ratio.

We noticed distinct differences between quartz phenocrysts from (a) rhyolite flows and (b) their volcanoclastic equivalents. The former (a) show oscillatory magmatic zoning and diffusion modeling of titanium in quartz reveals a surprisingly short time-scale for quartz crystallization of 3.4 ± 1.3 yr. This suggests crystal growth during undisturbed magma ascent. The oscillatory zoning and the variation of the Ti concentration of these quartz phenocrysts do not reflect T, P but rather local a_Ti changes around the growing crystal. Hence care must be taken when using the Ti in quartz thermometry. In contrast quartz phenocrysts from (b) volcanoclastic rocks show oscillatory-zoned cores with clear resorption and overgrowth textures, rather suggesting storage in a mush, with heating or pressure cycles before eruption. Diffusion modeling shows overgrowth rims formed 0.9 to 3.5 years before eruption.

Additional information comes from oxygen isotope analyses: the high δ¹⁸O-values (11-14‰) for both types of quartz phenocrysts are compatible with a crustal magma source for the El Quemado complex. SIMS analyses further reveal the absence of δ¹⁸O-zoning and confirm that they indeed preserved their magmatic oxygen isotope signature (i.e., are not affected by late stage alteration).