Optimizing pre-eruptive temperature estimates in thermally and chemically zoned magma chambers

• This method is suitable when multiple populations of glass and clinopyroxene occur in a single heterogeneous eruption.
• The method bases on predicted and observed components in clinopyroxene.
• The error of temperature estimate is minimized through the identification of equilibrium clinopyroxene–melt pairs.

We present a method to minimize the error of temperature estimate when multiple discrete populations of glass and clinopyroxene occur in a single heterogeneous eruptive unit. As test data we have used ~ 1100 clinopyroxene–melt pairs from isothermal and thermal gradient experiments. These latter are characterized by the crystallization of multiple modes of clinopyroxene as frequently documented for chemically and thermally zoned magma chambers. Equilibrium clinopyroxene–melt pairs are identified through the difference between predicted and measured components in clinopyroxene. The use of these equilibrium compositions as input data for one of the most recent clinopyroxene-based thermometers demonstrates that the error of temperature estimate is minimized and approaches to the calibration error of the thermometric model. To emphasize the paramount importance of this method for predicting the crystallization temperature of heterogeneous magmas, we have tested for equilibrium ~ 480 and ~ 150 clinopyroxene–melt pairs from the explosive eruptions of the Sabatini Volcanic District (Latium Region, Central Italy) and the Campi Flegrei Volcanic Field (Campanian Region, Southern Italy), respectively. These eruptions were fed by zoned magma chambers, as indicated by the occurrence of multiple modes of clinopyroxene in the eruptive units. Results from calculations demonstrate that clinopyroxene–melt pairs in equilibrium at the time of eruption are effectively captured by our method and, consequently, the error of temperature estimate is significantly reduced.