VOLATILE CONTENTS AND PRE-ERUPTIVE CONDITIONS OF RHYOLITIC MAGMAS FROM THE ORGAN CALDERA, SOUTHERN NM

The Organ caldera in southern NM formed during eruptions ~36 Ma and the volcanic deposits are now exposed in the Organ Mountains. We are studying the Organ caldera ignimbrites in order to better understand processes preceding the caldera-forming eruptions and the formation and evolution of the magma chamber system. This research project provides the first analyses of volatile contents and trace and major element concentrations of melt inclusions from the first and last erupted tuffs from the caldera, the Cueva Tuff (CT) and Squaw Mountain Tuff (SMT), respectively.

Major element compositions of melt inclusions from the CT are relatively homogenous. Melt inclusion trace elements from the CT show notably high Li, Zn, and Pb contents, the latter two likely related to the successful historical mining of base metals in the associated pluton. Conversely, variable major element compositions from the SMT suggest a heterogeneous melt with a possible mixing origin, which is supported by feldspar analyses. Feldspars from the SMT span from Ab_0-100, while majority of CT feldspars are low-K sanidines. FTIR results show volatiles from the CT magma chamber vary in H₂O contents, ranging from 2.2-6.4 wt %, and CO₂ contents up to 126 ppm. This range in values suggests varying crystallization pressures/depths. Volatile contents are much lower in the SMT (1.1-2.3 wt. % H₂O and no measurable CO₂); indicating lower pressures and depths of crystallization. Melt inclusion entrapment pressures determined using VolatileCalc 2.0 were used to estimate crystallization depths. The majority of CT inclusions were trapped at ~3-7 km, and SMT volatile contents suggest shallower crystallization depths (<2 km).

The geochemical data acquired in this study suggest that the CT erupted from a convecting magma chamber or a vertically extensive magma chamber system, accounting for the range in melt inclusion volatiles with little variation in major element chemistry. SMT volatile contents and major element data suggest an eruption from the shallow portion of a magma chamber, potentially caused by mixing or injection of a more mafic magma. Further melt inclusion trace element data will provide more information on chamber conditions including chamber stratification, melt differentiation and evolution, and pre-eruptive metal concentrations.