Were Carbonate Impact Melts Produced from the Carbonate-rich Target Lithologies at Meteor Crater, Arizona?

Introduction: In simple impact craters like Barringer Meteorite Crater (aka Meteor Crater), very little molten material was generated. There are no detectable melt ponds within the breccia lens or any significant melt pools on the crater walls or in the ejecta blanket. There was either an insufficient volume of melt produced by the impact event and/or it was too finely disseminated, possibly because the relatively high volatile content of the target rocks dispersed the melt as ash [1]. The target sequence is composed of sandy carbonates (Kaibab Fm.) and carbonate-bearing siltstones (Moenkopi Fm.) and sandstones (Toroweap and Coconino Fms.). Previous work by Hörz and others [2] demonstrated that melting occurred dominantly in the sandy carbonate portion of the section, producing siliciate melts that were thoroughly degassed of CO2. On the other hand, two more recent, albeit preliminary, reports suggest carbonate melts were produced [3,4]. To test the concept of carbonate impact melts and to assess their abundance at Meteor Crater, we analyzed two suites of samples: (i) the ashy matrix of fall-back breccia from inside the crater and (ii) cmscale melt particles that were ejected from the crater. Methods: Standard optical microscope technieques were used to establish petrographic relationships. The fine-grained matrix of fallback breccia was examined with imaging and EDS capabilities of a JOEL 50 SEM, while ejected melt particles were examined with imaging and WDS capabilities of a CAMECA SX-100 electron microprobe. Both instruments are at NASA JSC. Interior Crater Breccia: The interior breccia lens of the crater is composed of an allogenic breccia and an overlying fall-back breccia [5]. We sampled the latter, because it contains the airborne ejecta that may have entrained any ashy carbonate melt. This unit contains all of the target lithologies, including shocked varieties like lechatellierite, and meteoritic debris. We sampled a classic outcrop exposed on the north crater wall (Fig. 15.4 of [4]). The fine-grained matrix is dominantly less than 5 μm in size (Fig. 1a and b). It is composed of angular to sub-rounded quartz grains (Fig. 1c) and minor K-feldspar and calcite. The only hints of melt are rare shards of silica that are either fractured quartz or glass (Fig. 1d), but not carbonate. Exterior Crater Ejecta: We re-examined a pair of melt particles briefly described by [4] that are 0.9 and 0.7 cm in size. Like the melts of [2], these two partiFigure 1: Grain size distribution of breccia matrix. A) Grain size distribution on 500 μm scale; B) Grain size distribution on 20 μm scale; C) SEM image of dominating sub-rounded quartz crystal; D) SEM image of irregular silica shard.