HAMMADAH AL HAMRA 064


Ureilite
Monomict/Unbrecciated
Olivine–augite

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Found 1994
28° 46.66' N., 12° 20.13' E.

A single stone weighing 136 g was found in the Libyan Sahara, and 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), as reflected in planar fractures and undulatory extinction in the silicates. However, the presence of possible shock-produced diamonds within carbon-rich areas suggests that some shock features might have been erased during annealing.

Hammadah al Hamra 064 is a member of a small subgroup of ureilites, the olivine–augite type, which comprises less than ~10% of the known ureilites. A third type, the olivine–orthopyroxene ureilites, has been identified 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. They crystallized 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. Ureilites in the small augite- and orthopyroxene-bearing subgroups were found to contain primary trapped melt inclusions, demonstrating a complex magmatic history (Goodrich et al., 2000).

Hammadah al Hamra 064 has a bimodal texture and contains ~5–10 vol% augite. One lithology has a typical ureilite texture consisting of mm-sized augite and olivine grains, with grain 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%, with low-Ni metal-rich veins occurring between constituents. This poikilitic lithology is thought to have been produced when an assemblage of cumulus olivine and augite experienced reduction during the ascent of a relatively ferroan magma from deep source regions. This augite-bearing magma was ultimately 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 orthopyroxene-saturated magma. 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 the ascent stage of the parent magma within the conduit, the 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 which are composed of pure forsterite in the outer layers. These rims contain miniscule spherical metal grains, silica, graphite flakes, and protoenstatite grains along with occasional orthopyroxene. They were probably formed through a smelting process involving silicates and graphite that was initiated by a sudden pressure drop accompanied by rapid cooling. An alternative lower-temperature smelting reaction involving methane was proposed by Langendam and Tomkins (2012) to explain the observed smelting within fractures and the discontinuous smelting at grain boundaries; their scenario also supports the concept of chemical vapor deposition (CVD) of diamond. 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 parent body. This subgroup of ureilites (known as the 'Hughes cluster') comprises a small number of samples, including the following:

Hughes 009
HaH 064
ALH 82106/82130/84136
EET 96293/96314/96331
EET 87511/87523/87717
EET 87517
META78008
LEW 88774
LEW 85440/88012/88201/88281
Y-74130
FRO 90054/90228/90233/93008 (aug-bearing clasts)
DaG 999 (single aug-bearing clast)
Y-74123 (aug within melt veins)
Y-790981 (aug within melt veins)
Calama 001
NWA 3222 (37–41 vol% augite)

A cohesive model for the petrogenesis of the ureilites was presented by Goodrich et al., LPSC XXXIII, #1379 (2002), which was followed by important modifications in subsequent publications (Goodrich et al., 2007), some details of which can be found on the Kenna and Almahata Sitta pages. 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.

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click on photo for a magnified view

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Photography courtesy of Stephan Kambach


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