NEW CLINOPYROXENE-LIQUID GEOTHERMOMETER INDICATES A BROAD CRYSTALLIZATION INTERVAL FOR LOW-AL CLINOPYROXENE IN HIGH-SILICA MAGMATIC SYSTEMS

In order to investigate the P-T-X histories of high-silica systems (HSS) and to understand the timescales leading to their eruption, it is essential to use a geothermometer that is appropriately calibrated. However, clinopyroxene (cpx) from many well-known HSS are not included in existing cpx-opx and cpx-liquid geothermometer calibration datasets, which tend to focus on cpx from mafic systems. For example, cpx from HSS such as Paektu, Pantelleria, Valles, and Yellowstone can be quite high in FeO^totwith Mg# = 0.20–66, but very low in Al₂O₃(0.13–6.2 wt%). We find that existing geothermometers do not work well for these cpx and can predict temperatures > 170°C in error of low-Al cpx-saturated experiments. This is not surprising, because geothermometers dependent on the equilibria of diopside (CaMgSi₂O₆), hedenbergite (CaFeSi₂O₆), and jadeite (NaAlSi₂O₆) are likely to be inaccurate if there is very little Al and thus very little to no Jd component.

We present a new cpx-liquid geothermometer (SEE ±20°C) calibrated with 64 experimentally-derived low-Al cpx compositions from both the literature and new CSPV experiments. This new geothermometer shows a fivefold decrease in the deviation between calculated and actual experimental temperatures as compared to the Putirka (2008, Eq. 33) cpx-liquid geothermometer (±20 vs. ±110°C). Application of this new thermometer to natural HSS cpx shows an overall lowering of cpx crystallization temperatures as compared to the Putirka geothermometer, with an average ∆T of 85°C. And although cpx is a mafic phase, our temperature calculations demonstrate that its stability is not restricted to near-liquidus temperatures; we have calculated cpx in high-silica natural systems to below 775°C for the late-erupted Bishop Tuff, Paektu comendite, and Bandelier Tuff, and indeed, experiments have also shown the mineral to crystallize at, or near, the solidus to temperatures as low as 675°C (e.g., Almeev et al. 2012; Gardner et al. 2014; Bolte et al. 2015; Iacovino et al. 2015; Befus and Gardner 2016). When present, cpx appears to be a persistent phase throughout the crystallization interval of high-silica systems, and so its presence does not necessitate the invocation of xenocrysts, mixing with a hotter magma body, nor imply a low (or high) crystallinity.