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

The Frontier Mountain (FRO) 93001 meteorite is a 4.86 g fragment of an unshocked, medium- to coarse-grained rock from the acapulcoite-lodranite (AL) parent body. It consists of anhedral orthoenstatite (Fs13.3 ± 0.4Wo3.1 ± 0.2), augite (Fs6.1 ± 0.7Wo42.3 ± 0.9; Cr2O3 = 1.54 ± 0.03), and oligoclase (Ab80.5 ± 3.3Or3.1 ± 0.6) up to >1 cm in size enclosing polycrystalline aggregates of finegrained olivine (average grain size: 460 ± 210 µm) showing granoblastic textures, often associated with Fe,Ni metal, troilite, chromite (cr# = 0.91 ± 0.03; fe# = 0.62 ± 0.04), schreibersite, and phosphates. Such aggregates appear to have been corroded by a melt. They are interpreted as lodranitic xenoliths. After the igneous (the term "igneous" is used here strictly to describe rocks or minerals that solidified from molten material) lithology intruding an acapulcoite host in Lewis Cliff (LEW) 86220, FRO 93001 is the second-known silicate-rich melt from the AL parent asteroid. 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.