¹F. Hörz,²P. D. Archer Jr., ³P. B. Niles, ⁴M. E. Zolensky, ^3,4M. Evans
¹LZ Technology Inc., Houston, Texas, USA
²Jacobs, NASA Johnson Space Center, Houston, Texas, USA
³ARES, NASA Johnson Space Center, Houston, Texas, USA
⁴Texas A&M University, College Station, Texas, USA

We have investigated the carbonates in the impact melts and in a monolithic clast of highly shocked Coconino sandstone of Meteor Crater, AZ to evaluate whether melting or devolatilization is the dominant response of carbonates during high-speed meteorite impact. Both melt- and clast-carbonates are calcites that have identical crystal habits and that contain anomalously high SiO2 and Al2O3. Also, both calcite occurrences lack any meteoritic contamination, such as Fe or Ni, which is otherwise abundantly observed in all other impact melts and their crystallization products at Meteor Crater. The carbon and oxygen isotope systematics for both calcite deposits suggest a low temperature environment (100 °C) for their precipitation from an aqueous solution, consistent with caliche. We furthermore subjected bulk melt beads to thermogravimetric analysis and monitored the evolving volatiles with a quadrupole mass spectrometer. CO₂ yields were <5 wt%, with typical values in the 2 wt% range; also total CO₂ loss is positively correlated with H₂O loss, an indication that most of these volatiles derive from the secondary calcite. Also, transparent glasses, considered the most pristine impact melts, yield 100 wt% element totals by EMPA, suggesting complete loss of CO₂. The target dolomite decomposed into MgO, CaO, and CO₂; the CO₂ escaped and the CaO and MgO combined with SiO₂ from coexisting quartz and FeO from the impactor to produce the dominant impact melt at Meteor Crater. Although confined to Meteor Crater, these findings are in stark contrast to Osinski et al. (2008) who proposed that melting of carbonates, rather than devolatilization, is the dominant process during hypervelocity impact into carbonate-bearing targets, including Meteor Crater.

Reference
Hörz F, Archer Jr. PD, Niles PB, Zolensky ME, Evans M (2015) Devolatilization or melting of carbonates at Meteor Crater, AZ? Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12453]

Published by arrangement with John Wiley&Sons

¹M. Shyam Prasad, ¹N. G. Rudraswami, ¹Agnelo De Araujo, ²E. V. S. S. K. Babu, ²T. Vijaya Kumar
¹CSIR—National Institute of Oceanography, Dona Paula, Goa, India
²LAM-ICP-MS National Facility, CSIR-National Geophysical Research Institute, Hyderabad, India

Extraterrestrial particulate materials on the Earth can originate in the form of collisional debris from the asteroid belt, cometary material, or as meteoroid ablation spherules. Signatures that link them to their parent bodies become obliterated if the frictional heating is severe during atmospheric entry. We investigated 481 micrometeorites isolated from ~300 kg of deep sea sediment, out of which 15 spherules appear to have retained signatures of their provenance, based on their textures, bulk chemical compositions, and relict grain compositions. Seven of these 15 spherules contain chromite grains whose compositions help in distinguishing subgroups within the ordinary chondrite sources. There are seven other spherules which comprise either entirely of dusty olivines or contain dusty olivines as relict grains. Two of these spherules appear to be chondrules from an unequilibrated ordinary chondrite. In addition, a porphyritic olivine pyroxene (POP) chondrule-like spherule is also recovered. The bulk chemical composition of all the spherules, in combination with trace elements, the chromite composition, and presence of dusty olivines suggest an ordinary chondritic source. These micrometeorites have undergone minimal frictional heating during their passage through the atmosphere and have retained these features. These micrometeorites therefore also imply there is a significant contribution from ordinary chondritic sources to the micrometeorite flux on the Earth.

Reference
Shyam Prasad M, Rudraswami NG, De Araujo A, Babu EVSSK, Vijaya Kumar T (2015) Ordinary chondritic micrometeorites from the Indian Ocean. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12451]
Published by arrangement with John Wiley&Sons