Magmatic processes and storage beneath Heard Island, southern Indian Ocean

Abstract: A young marine island called Heard Island is located in the southern Kerguelen Plateau in the Indian Ocean, a large igneous province created by the Kerguelen mantle plume. The two major geographic regions on Heard Island have two principal volcano-magmatic suites. Basanites, alkali basalts, and trachybasalts make up one group, the Big Ben Series (BBS), while basanitic to trachytic rocks make up the Laurens Peninsula Series (LPS). The most recent eruption at Big Ben volcano occurred in October 2022. To better understand magma evolution in the underlying plumbing system, clinopyroxene, feldspar, and olivine, mineral chemistry, clinopyroxene thermobarometry, and olivine thermometry were used.      The main phenocrysts from Heard Island are olivine, clinopyroxene, and feldspar. All phenocrysts share the characteristics of sieve textures and fractures. The mineral chemistry of clinopyroxene, zonation, and variation of core and rim of Mg#, Al2O3, TiO2, and Cr2O3, provides insight into the magmatic evolution of magma. Results from clinopyroxene-liquid thermobarometry suggest that clinopyroxene crystallization occurs at depths of 1 to 39 km for the cores and 1 to 47 km for the rims, with corresponding temperatures of 1098 to 1208°C and 1099 to 1254°C respectively. Comparison with olivine thermometry shows concordance in temperature estimates. The Mohorovičić discontinuity, or Moho, which marks the boundary between the Earth's crust and mantle, is believed to lie between 18 and 26 km deep at Heard Island. This study suggests that magma pockets can be found below the Moho between 51 and 18 km, and in the upper and lower crust between 18 and 2 km. Additionally, the density difference between various crustal layers determines where magma storage is located. The evidence from petrology and geochemistry points to common processes of magma mixing, recharge, and fractional crystallization during magma evolution.