Ureilite, olivine–augite type
Found 1994
28° 46.66' N., 12° 20.13' E.
A single stone weighing 136 g was found in the Libyan Sahara. This meteorite was classified at the Institut für Planetologie in Münster as a pyroxene-rich (pyroxene/pyroxene + olivine = 65 vol%), unbrecciated ureilite. It has been weathered to grade 3 and exhibits only weak shock features (S3), revealed by planar fractures and undulatory extinction in the silicates. However, the presence of shock-produced diamonds within carbon-rich areas suggests that some shock features may have been erased during annealing.
Hammadah al Hamra 064 is a member of a small group of ureilites, the olivine–augite type, which comprises less than ~10% of the known ureilites. A third type, the olivine–orthopyroxene ureilites, has been resolved in the ureilite classification scheme proposed by Goodrich et al. (2006). The olivine–augite ureilites formed as cumulates, rather than residues, in late-stage, highly refractory melt pools located at a range of depths in close association with the much more common olivine–pigeonite ureilites. The olivine–pigeonite type of ureilites, which exhibit the typical texture of residues of low degree fractional melts, constitute ~90% of the known ureilites. Two transitional members are also known, RKPA80239 and PCA 82506, which have textures that are intermediate between the typical and poikilitic groups. The small augite- and orthopyroxene-bearing subgroups (including HaH 064, Hughes 009, and FRO 90054 [and pairings]) were found to contain primary trapped melt inclusions, demonstrating a complex magmatic history for these ureilites (Goodrich et al., 2000).
Hammadah al Hamra 064 has a bimodal texture comprising ~5–10 vol% augite. One lithology has a typical ureilite texture, consisting of mm-sized augite and olivine grains with boundaries forming 120° triple junctions. These grains are separated by metal-rich veins and an interstitial, fine-grained, carbon-rich (primarily graphite) matrix. The other lithology is composed of olivine and augite poikilitically enclosed within large (≥5 mm) pigeonite (Wo ~4.5, and more accurately described as orthopyroxene; Goodrich et al., 2006) crystals constituting ~50 vol%, and with low-Ni, metal-rich veins between constituents. This poikilitic lithology is thought to have been produced when an assemblage of cumulus olivine plus augite experienced reduction during ascent of a relatively ferroan magma from deep source regions. This augite-bearing magma may eventually have been assimilated into the olivine–pigeonite residues located at shallower levels (Goodrich and Fioretti, 2007; Goodrich et al., 2009). Alternatively, the orthopyroxene may have been introduced by integration of an opx-saturated magma. Still another possible route to orthopyroxene formation is by the transformation of pigeonite as the rock experienced an increased cooling rate below the equilibration temperature of ~1200°C (Weber et al., 2003).
During ascent of the parent magma within the conduit, olivine in augite-bearing ureilites began to crystallize first, followed by augite and orthopyroxene as the magma reached shallower levels (Goodrich et al., 2009). Olivine is surrounded by reduction rims up to 0.1 mm wide and are composed of pure forsterite in the outer layers. These rims contain miniscule spherical metal grains, silica, graphite flakes, and protoenstatite grains with occasional orthopyroxene. They were probably formed through a smelting process involving silicates and graphite, which was initiated by a sudden pressure drop accompanied by rapid cooling. Spinodal decomposition of augite also attests to a relatively rapid cooling rate (15–20°C/hr) from high temperatures. A final sudden increase in the cooling rate may have been the result of a collisional disruption of the UPB.
The ureilites of the olivine–augite type define a broad range of Δ17O values, consistent with their formation at varied depths on the UPB. This segment of the ureilites comprises a small number of samples, including the following:
A cohesive model for the petrogenesis of the ureilites was presented by Goodrich et al., LPSC XXXIII, 2002, #1379, which was followed by important modifications in subsequent publications (Goodrich et al., 2007), some details of which can be found on the Kenna page. The specimen of HaH 064 shown in the photos above and below is a 4.23 g partial slice that was sectioned from the portion curated at the Museum für Naturkunde, Humboldt University, Berlin.
