Effects of Glacial Loading and Unloading on Magmatic Storage Conditions Beneath Mount Waesche, Marie Byrd Land, Antarctica

Abstract Ages of lava flows between 426 and 96 ka from Mount Waesche, a stratovolcano in the Executive Committee Range of Marie Byrd Land, West Antarctica, indicate greater eruption frequency during interglacial periods, suggesting the glacial overburden influences the magmatic plumbing system. We conducted a geochemical and petrologic study on lavas that erupted in the last peak interglacial period (c. 100–130 ka) and lavas that erupted in the preceding glacial period (c. 138.7–150.5 ka) to evaluate changes to the magmatic system as a function of the ice load. These samples are alkaline (Na2O + K2O = 4.16–9.69 wt %; basalt to benmoreite) and contain plagioclase + Fe–Ti oxides ± clinopyroxene ± olivine ± apatite, with total crystallinities ranging from ~3 to 35%. Samples often contain crystal clots consisting of clinopyroxene + olivine ± plagioclase. Microprobe analyses of crystals in a subset of samples reveals clinopyroxene and olivine have discontinuous, normal zoning and range in composition from Mg#41–80 and from Fo53–79, respectively. Plagioclase also has discontinuous zoning and spans a wide range (>30 mol % An) in composition in each sample; An contents of plagioclase systematically decrease as plagioclase size decreases. We use a set of thermodynamic models to evaluate likely equilibrium mineral–melt pairs. We find that nearly all samples contain a fraction of large, An-rich (>An84) xenocrystic plagioclase. Agreement between mineral compositions within crystalline clots and equilibrium mineral compositions suggests that the clots are autoliths. We apply an iterative method of H2O-dependent clinopyroxene thermobarometry that utilizes a plagioclase hygrometer to obtain pre-eruptive conditions recorded by clinopyroxene and plagioclase. Our iterative thermobarometry + plagioclase hygrometry method reveals temperatures ranging from 1022 ± 13 (±2σ) to 1149 ± 15 °C and pressures from 747 ± 165 (±2σ) to 1039 ± 135 MPa, and H2O contents from 0.8 ± 0.2 (±2σ) to 3.4 ± 0.2 wt %. Temperatures from clinopyroxene–liquid thermobarometry closely match those from olivine–liquid thermometry (1042–1169 °C) whose temperatures are calculated based on average olivine compositions or temperatures based on forsterite-rich analyses. Patterns in whole-rock chemistry can be explained by at least two stages of crystallization, where a plagioclase-free system is required to form less evolved lavas (>3.5 wt % MgO) and a plagioclase-bearing system is required to form more evolved lavas (<3.5 wt % MgO); these two models are broadly consistent with the mineralogy of the autoliths. We find that whole-rock compositions and pre-eruptive temperatures co-vary, where the less evolved samples are hotter. We do not observe significant changes to the common lower crustal storage conditions or crystallizing assemblages at depth as a function of glacial and interglacial periods. However, interglacial periods feature a wider range of lava types, suggesting there is a more varied temperature regime in the lower crust. We suggest that there was an increase in mafic injections into the lower crust during the interglacial period, producing a varied temperature regime. During glacial periods, the temperature variations and resultant magmatic systems could still exist but are less active and less widely dispersed. The volcanic record and sample petrology at Mount Waesche suggests that reducing glacial overburden primarily affects melt generation in the underlying mantle, rather than the crustal magmatic system.