Devolatilization or melting of carbonates at Meteor Crater, AZ?

Abstract

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 SiO 2 and Al 2 O 3 . 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. CO 2 yields were <5 wt%, with typical values in the 2 wt% range; also total CO 2 loss is positively correlated with H 2 O 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 CO 2 . The target dolomite decomposed into MgO, CaO, and CO 2 ; the CO 2 escaped and the CaO and MgO combined with SiO 2 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.

CO2 release due to impact devolatilization of carbonate: Results of shock experiments

“…However, Hörz et al. () found that calcite occurrences from Meteor Crater impactites have characteristics that are inconsistent with a melt origin and that the carbonate target was largely devolatilized. Carbonate melt was not produced in the shock experiments to 60 GPa documented here.…”

“…; Langenhorst and Deutsch ; Hörz et al. ). Early shock‐recovery experiments for calcite to quantify how much CO 2 is released at a given shock pressure yielded seemingly conflicting results: the experimental devolatilization studies of Boslough et al.…”

“…However, Hörz et al. () found that calcite occurrences from Meteor Crater impactites have characteristics that are inconsistent with a melt origin including low temperature secondary formation as indicated by stable isotope measurements and total CO 2 loss correlated with H 2 O loss from thermal analysis which is an indication that most of those volatiles derive from the secondary calcite. They interpret these and other characteristics of Meteor Crater impactites to reflect a sequence of events in which the target dolomite decomposed into MgO, CaO, and CO 2 ; the CO 2 escaped, and the CaO and MgO combined with SiO 2 from coexisting quartz and FeO from the impactor to produce the dominant impact melt at Meteor Crater.…”

Shock metamorphism of planetary silicate rocks and sediments: Proposal for an updated classification system

“…; Hörz et al. ; Bell ). Although frequently studied in shock recovery experiments, no consensus has been reached so far regarding the onset or even nature of specific shock effects in carbonates.…”

Researching the Earth—and a Few of Its Neighbors

“…More generally, the role of melting and devolatilization has been discussed and debated by Osinski et al (2008), Hörz et al (2015), and Bell (2016). Howard (2011) invoked volatiles in the shallow crust to enhance the dispersal of impact melts as tektites.…”