A 68 g portion of a stone retaining considerable fusion crust was purchased in Erfoud, Morocco by M. Farmer in October of 2004. A sample was submitted for analysis to Northern Arizona University (T. Bunch and J. Wittke), and NWA 2627 was initially determined to be an anomalous winonaite. The stone has a recrystallized texture and is composed of a heterogeneous mixture of orthopyroxene (44 vol%), olivine (41 vol%), FeS (6 vol%), metallic phases (5 vol%), and plagioclase (3 vol%), along with minor abundances of merrilite, Ca-pyroxene, and chromite.
The mineralogy and composition of this meteorite are similar to those for the lodranites, and when plotted on a diagram comparing the Δ17O-isotopic value vs. Fa mol% in olivine (Rumble, III et al, 2005), NWA 2627 clearly lies within the acapulcoite field (see Figure 2 plot in LPSC 38, #2254). It has been suggested that this meteorite represents a transitional acapulcoite (Touboul et al., 2007) according to the following classification scheme by Floss (2000) and Patzer et al. (2003) based on metamorphic stage.
primitive acapulcoites: near-chondritic (Se >12–13 ppm [degree of sulfide extraction])
typical acapulcoites: Fe–Ni–FeS melting and some loss of sulfide (Se ~5–12 ppm)
transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
lodranites: sulfide, metal, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
enriched acapulcoites (addition of feldspar-rich melt component)
The high temperatures experienced on the acapulcoite PB, manifest in the recrystallized texture and occasional partial melt phase, suggest that it accreted very early in Solar System history when radiogenic 26Al was still extant. This was a time period spanning ~1–3 m.y. after CAI formation, after the accretion of differentiated parent bodies had occurred but before the accretion of chondritic parent bodies had begun (Touboul et al., 2007). However, this formation scenario is inconsistent with some acapulcoites having ages younger than that attributed to the complete extinction of radiogenic 26Al. An impact shock heating model has been proposed by Rubin (2007), the details of which can be found on the Monument Draw page.
Cooling rate data indicate a more rapid cooling for the acapulcoite PB than that which the bulk H chondrite PB experienced (but similar to that of H4 chondrites; Kleine et al., 2007), consistent with a smaller acapulcoite PB and/or a near-surface residence for the acapulcoites. The possibility also exists for a collisional disruption early in its history, forming sub-km- to multi-km-sized fragments, which eventually succumbed to gravitational reassembly. Compared to the lodranite lithological unit, both primitive and typical acapulcoite material is thought to have originated in the outermost layer of the asteroid where it cooled earlier and faster, consistent with its older gas retention age, finer-grain size, and less intense metamorphism. The transitional acapulcoites formed at an intermediate level and exhibit accordant features, while the lodranites formed within a hotter, deeper layer, experiencing silicate partial melting and loss of a basaltic component.
This transitional acapulcoite has been weathered to grade W2/3 and has been shocked to stage S2. The 210 g NWA 4399 may be paired with NWA 2627. The photo shown above, courtesy of Jim Strope, is a 0.88 g partial slice of NWA 2627.
