Frontier Mountain 93001: A coarse-grained, enstatite-augite-oligoclase-rich, igneous rock from the acapulcoite-lodranite parent asteroid

Abstract

Despite some similarities, the silicate igneous component of FRO 93001 (i.e., the pyroxeneplagioclase mineral assemblage) differs in being coarser-grained and containing abundant enstatite. Melting-crystallization modeling suggests that FRO 93001 formed through high-degree partial melting (≥35 wt%; namely, ≥15 wt% silicate melting and ~20 wt% metal melting) of an acapulcoitic source rock, or its chondritic precursor, at temperatures ≥1200 °C, under reducing conditions. The resulting magnesium-rich silicate melt then underwent equilibrium crystallization; prior to complete crystallization at ~1040 °C, it incorporated lodranitic xenoliths.FRO 93001 is the highest-temperature melt from the AL parent-body so far available in laboratory. The fact that FRO 93001 could form by partial melting and crystallization under equilibrium conditions, coupled with the lack of quench-textures and evidence for shock deformation in the xenoliths, suggests that FRO 93001 is a magmatic rock produced by endogenic heating rather than impact melting.

Frontier Mountain meteorite specimens of the acapulcoite‐lodranite clan: Petrography, pairing, and parent‐rock lithology of an unusual intrusive rock

“…Geoscienze e Georisorse in Florence, following the experimental conditions described in Folco et al (2006). To discuss possible pairings with FRO 93001, a direct comparison with thin section FRO 93001,01 and literature data published in our previous work (Folco et al 2006) was carried out.…”

“…Based on the petrographic descriptions of FRO 90011, FRO 99030, and FRO 03001 given above, and that of FRO 93001 reported in Folco et al (2006), we argue that these 4 meteorite specimens from the Frontier Mountain blue ice field are paired. The apparent differences observed in thin sections are simply due to the fact that they are small fragments (cm-sized) of a relatively coarse-grained (grain size up to centimeters), heterogeneous (namely, inclusionrich) rock.…”

“…This interpretation is based on the sharp textural contrast with the igneous host crystals (see Figs. 2 and 3 in Folco et al 2006) and on the petrologic argument that the oligoclase-rich composition of the igneous mineral assemblage would require a source rock of acapulcoitic (or its chondritic precursor) composition, i.e., a chemical system which contained substantial amounts of Al, Ca, and Na, unlike the typical lodranitic one (Folco et al 2006).…”

“…According to Folco et al (2006), FRO 93001 is an unshocked (shock level S1), little weathered (weathering grade W1), unique igneous rock of the AL clan consisting of orthoenstatite (Table 3), augite (Table 4), and oligoclase (Table 6) up to >1 cm in size enclosing polycrystalline aggregates of fine-grained homeoblastic olivine (Table 2), often associated with Fe,Ni metal, troilite, chromite (Table 5), schreibersite, and phosphates. Such aggregates appear to have undergone igneous corrosion and are interpreted as lodranitic xenoliths.…”

“…They include one acapulcoite, Frontier Mountain (FRO) 95029 (Grossman 1997), 3 specimens originally classified as lodranites, FRO 90011 (McCoy et al 1993), FRO 99030 (Grossman 2000), and FRO 03001 (Russell et al 2004), and the igneous (the term "igneous" is used in this paper to strictly describe rocks or minerals with textures indicating solidification from molten material) member FRO 93001 (Folco et al 2006); the latter carries evidence for very high degree melting processes and thus it may considerably increase our understanding of differentiation mechanisms operating on the AL parent asteroid. Pairing is discussed on the basis of the direct comparison of new petrographic data from FRO 90011, FRO 95029, FRO 99030, and FRO 03001 with literature data for FRO 93001 (Folco et al 2006). In particular, we will show how meteorites that appear unrelated in thin section are actually fragments of the same fall, as in the case of the 3 lodranites FRO 90011, FRO 99030, and FRO 03001, and the igneous-textured FRO 93001.…”