The reaction of carbonates in contact with laser‐generated, superheated silicate melts: Constraining impact metamorphism of carbonate‐bearing target rocks

1,2Christopher Hamann, 1,2Saskia Bläsing, 1,2Lutz Hecht, 43Sebastian Schäffer, 4Alex Deutsch, 3Jens Osterholz, 3Bernd Lexow
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13133]
1Museum für Naturkunde, Leibnitz‐Institut für Evolutions‐ und Biodiversitätsforschung, Berlin, Germany
2Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany
3Fraunhofer‐Institut für Kurzzeitdynamik, Ernst‐Mach‐Institut, Freiburg, Germany
4Institut für Planetologie, Westfälische Wilhelms‐Universität Münster, Münster, Germany
Published by arrangement with John Wiley & Sons

We simulated entrainment of carbonates (calcite, dolomite) in silicate impact melts by 1‐bar laser melting of silicate–carbonate composite targets, using sandstone, basalt, calcite marble, limestone, dolomite marble, and iron meteorite as starting materials. We demonstrate that carbonate assimilation by silicate melts of variable composition is extremely fast (seconds to minutes), resulting in contamination of silicate melts with carbonate‐derived CaO and MgO and release of CO2 at the silicate melt–carbonate interface. We identify several processes, i.e., (1) decomposition of carbonates releases CO2 and produces residual oxides (CaO, MgO); (2) incorporation of residual oxides from proximally dissociating carbonates into silicate melts; (3) rapid back‐reactions between residual CaO and CO2 produce idiomorphic calcite crystallites and porous carbonate quench products; (4) high‐temperature reactions between Ca‐contaminated silicate melts and carbonates yield typical skarn minerals and residual oxide melts; (5) mixing and mingling between Ca‐ or Ca,Mg‐contaminated and Ca‐ or Ca,Mg‐normal silicate melts; (6) precipitation of Ca‐ or Ca,Mg‐rich silicates from contaminated silicate melts upon quenching. Our experiments reproduce many textural and compositional features of typical impact melts originating from silicate–carbonate targets. They reinforce hypotheses that thermal decomposition of carbonates, rapid back‐reactions between decomposition products, and incorporation of residual oxides into silicate impact melts are prevailing processes during impact melting of mixed silicate–carbonate targets. However, by comparing our results with previous studies and thermodynamic considerations on the phase diagrams of calcite and quartz, we envisage that carbonate impact melts are readily produced during adiabatic decompression from high shock pressure, but subsequently decompose due to heat influx from coexisting silicate impact melts or hot breccia components. Under certain circumstances, postshock conditions may favor production and conservation of carbonate impact melts. We conclude that the response of mixed carbonate–silicate targets to impact might involve melting anddecomposition of carbonates, the dominant response being governed by a complex variety of factors.

Discuss

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s