Petrography of crystalline clast breccias from the Manson M-1 core

Abstract

Drilling at the M-1 site on the flank of the central peak intersected the most diverse suite of breccias encountered at a single location within the Manson impact structure. In this chapter, we describe and categorize breccias from the lower 108 m of the drill core, which are noteworthy because of the abundant clasts from the crystalline basement. The M-1 core impact breccias roughly preserve the original stratigraphy of the target lithologies. The uppermost breccias are rich in Phanerozoic and Proterozoic sedimentary clasts, and contain less than a few volume percent of crystalline lithologies or shock-melted and shock-metamorphosed materials. The breccias that we focus on underlie a sharp but irregular contact with the sedimentary clastrich breccias. The uppermost of these crystalline clast breccias (unit 1, 106.4-146 m depth) is unsorted, matrix-rich, and predominantly consists of shock-melted and shock-metamorphosed clasts from a limited number of Proterozoic basement lithologies. Proterozoic shale clasts are rare, although the shale content of the matrix is significant. The uppermost portion of this breccia (unit 1a) has a nearly isotropic to finely crystalline matrix, whereas the lower portion (unit 1b) has a more coarsely crystalline matrix and reaction coronas around quartz. The lowermost 53 m of the M-1 core (unit 3, 161-214.3 m depth) consists of breccias with relatively coarse grained unrecrystallized matrices, higher clast abundances, more diverse clast lithologies, rare solidified melt clasts, and emplacement temperatures considerably below the solidus. Between these two breccias is a zone where the two lithologies intermingle, but do not homogeneously mix (unit 2, 146-161 m depth). Unit 3 is interpreted to be crater lining deposits that formed as the crater was excavated. Unit 1 also may have been emplaced by this mechanism, although the distinct break in lithology between the melt-rich and meltpoor breccias and the poorly sorted nature of the melt-rich breccias suggest that unit 1 may have been emplaced by a different mechanism than unit 3. This mechanism may have been a density current that originated in the ejecta curtain and flowed back into the crater. In either case, the mingling of the breccias may have been related to uplift of the central peak. The lithologies of the crystalline clast breccias are similar to those encountered at the West Clearwater impact structure, which has a similar size, but had less of a sedimentary veneer before impact. However, a clast-poor melt sheet like that of West Clearwater, has not been encountered at the Manson impact structure. In addition, the volume of melt at Manson is significantly less than for West Clearwater. This may be related to the dispersal of the melt material by explosive expansion of volatiles derived from sediments in the target area as suggested by Kieffer and Simonds (1980).

Impact melt sheet formation on Mars and its implication for hydrothermal systems and exobiology

“…For Manson, apparently ∼50% of the heat in the central uplift was derived from shock and frictional heating (McCarville and Crossey, 1996), whereas this estimate is ∼80% for PuchezhKatunki (Masaitis and Naumov, 1993). The crater fill cooled quickly at Manson and did not play a major role in hydrothermal circulation, probably because it contains little melt (e.g., Bell et al, 1996). The central uplift also appears to be the major heat source at Puchezh-Katunki, which like Manson lacks a clast-free melt sheet (Masaitis and Naumov, 1993;Naumov, 2002).…”

Experimental shock decomposition of siderite and the origin of magnetite in Martian meteorite ALH 84001

“…If ejection is a low-pressure phenomenon (Melosh 1995), how or when could the ALH material be heated to produce magnetite? Possible scenarios for prolonging thermal pulse duration after impact event(s) include insulation in a suevite layer (Bell et al 1996;Dodd 1981;Simonds 1978) after impact to bring cumulate rock (like ALH 84001) from depth to Mars's surface, or during impact events the material may have suffered before ejection from Mars (Treiman 2003). In the pressure range from 10 to 40 GPa, in experimental shock studies, localized zones of very high temperature have been observed (Lyzenga et al 1983;Boslough 1988).…”

Geochemical comparison of four cores from the Manson impact structure

“…For most elements, the strategy used in this work of analyzing a few small fragments (total mass:~200 mg) selected to avoid large clasts and areas of obvious alteration is successful in yielding analyses with less dispersion than that of Koeberl et al (this volume) in which several grams of core section ("about 5 to 15 cm in length") were powdered. in the SB (Bell et al, 1993). Although the decrease in As and Sb is consistent with a decrease in the proportion of shale component with depth in the melt breccia, this decrease also results partly from dilution by As-and Sb-poor crystalline clasts.…”

“…(1) At the top is the impact-melt breccia (IMB), the unit that reached the highest temperature (Izett et al, 1993) and was most thoroughly melted. The IMB corresponds to "unit 1" of Bell et al (1993;this volume) and the "crystalline-clast breccia with a melt matrix" ("CCB-M") of Anderson et al (1993). (2) Below the IMB is the fragmental-matrix, suevite breccia (SB), which is similar to that described as "suevitic breccia," that is, a breccia having a clastic matrix with some melt clasts (Stöffler et al, 1979), or "melt-bearing mixed breccias" (Grieve et al, 1977).…”

“…This region of the M-1 core has been divided into three major intervals or units Bell et al, 1993;this volume). (1) At the top is the impact-melt breccia (IMB), the unit that reached the highest temperature (Izett et al, 1993) and was most thoroughly melted.…”