Cosmic spherules from Widerøefjellet, Sør Rondane Mountains (East Antarctica). (1st February 2020) - Goderis, Steven
Abstract: A newly discovered sedimentary accumulation of micrometeorites in the Sør Rondane Mountains of East Antarctica, close to the Widerøefjellet summit at ∼2750 m above sea level, is characterized in this work. The focus here lies on 2099 melted cosmic spherules larger than 200 μm, extracted from 3.2 kg of sampled sediment. Although the Widerøefjellet deposit shares similarities to the micrometeorite traps encountered in the Transantarctic Mountains, both subtle and more distinct differences in the physicochemical properties of the retrieved extraterrestrial particles and sedimentary host deposits are discernable (e.g., types of bedrock, degree of wind exposure, abundance of metal-rich particles). Unlike the Frontier Mountain and Miller Butte sedimentary traps, the size fraction below 240 μm indicates some degree of sorting at Widerøefjellet, potentially through the redistribution by wind, preferential alteration of smaller particles, or processing biases. However, the cosmic spherules larger than 300 μm appear largely unbiased following their size distribution, frequency by textural type, and bulk chemical compositions. Based on the available bedrock exposure ages for the Sør Rondane Mountains, extraterrestrial dust is estimated to have accumulated over a time span of ∼1–3 Ma at Widerøefjellet. Consequently, the Widerøefjellet collection reflects a substantial reservoir to sample the micrometeorite influx over this time interval. Petrographic observations and 3D microscopic CT imaging are combined with chemical and triple-oxygen isotopic analyses of silicate-rich cosmic spherules larger than 325 μm. The major element composition of 49 cosmic spherules confirms their principally chondritic parentage. For 18 glassy, 15 barred olivine, and 11 cryptocrystalline cosmic spherules, trace element concentrations are also reported on. Based on comparison with evaporation experiments reported in literature and accounting for siderophile and chalcophile element losses during high-density phase segregation and ejection, the observed compositional sequence largely reflects progressive heating and evaporation during atmospheric passage accompanied by significant redox shifts, although the influence of (refractory) chondrite mineral constituents and terrestrial alteration cannot be excluded in all cases. Twenty-eight cosmic spherules larger than 325 μm analyzed for triple-oxygen isotope ratios confirm inheritance from mostly carbonaceous chondritic precursor materials (∼55% of the particles). Yet, ∼30% of the measured cosmic spherules and ∼50% of all glassy cosmic spherules are characterized by oxygen isotope ratios above the terrestrial fractionation line, implying genetic links to ordinary chondrites and parent bodies currently unsampled by meteorites. The structural, textural, chemical, and isotopic characteristics of the cosmic spherules from the Sør Rondane Mountains, and particularly the high proportion of Mg-rich glass particles contained therein, imply a well-preserved and representative new sedimentary micrometeorite collection from a previously unstudied region in East Antarctica characterized by distinct geological and exposure histories.