DHOFAR 778


Achondrite, ungrouped
Harzburgite
(Diogenite in MetBull 87)

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Found October 2000
18° 43.525' N., 54° 43.829' E.

A single lightly weathered (W1) stone, weighing 156 g, was found in the desert of Oman by a German team. It was classified by Dr. J. Otto at the Institut für Mineralogie, Universität Freiburg, as one of the most highly shocked diogenites yet discovered, with a shock stage of S4. This breccia exhibits a fragmental texture, which has been interpreted to be a characteristic of impact ejecta. For more details on the formation of diogenites visit the Johnstown page. To see an alternative classification system for the diogenites and dunites based on mineralogical and petrographical features, proposed by Beck and McSween (2010) and modified by Wittke et al. (2011), click here.

A mineralogic and petrographic analysis of Dhofar 778 and Dhofar 007 was conducted by Pang et al. (2020), and it was determined that the meteorite is composed primarily of orthopyroxene (~54 vol%) and olivine (~44 vol%) along with minor plagioclase, pigeonite, augite, chromite, troilite, FeNi-metal. The olivine abundance in Dhofar 778 exemplifies a harzburgite (40–90 vol%). The olivines contain orthopyroxene-spinel intergrowths and silicic glass inclusions of unresolved formation mechanisms. High pressure phases including ringwoodite, wadsleyite, and coesite occur adjacent to shock veins, indicative of shock pressures reaching ~13–17 GPa.

The oxygen isotopic composition of Dhofar 778 was determined by Greenwood et al. (2017), and the plot occupies a distinct space on an oxygen three-isotope diagram (see top diagram below). They suggest that this anomalous oxygen isotope composition could be attributed to contamination by an exogenous component, but they note that further analyses should be conducted. High-resolution O-isotopic data presented by Mittlefehldt et al. (2021) show that Dhofar 007 and Dhofar 778 share close similarities consistent with a genetic relationship (see bottom histogram below).

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click on image for a magnified view

Diagram credit: Greenwood et al., Chemie der Erde—Geochemistry, vol. 77, p. 25 (2017)
'Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies'
(open access: http://dx.doi.org/10.1016/j.chemer.2016.09.005)

Histogram of HED and Eucrite-type Achondrites
Deviations from HED igneous median (median absolute deviation units)
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click on photo for a magnified view

Diagram credit: Mittlefehldt et al., MAPS, Early View, (2021 open access link)
'Eucrite-type achondrites: Petrology and oxygen isotope compositions'
(https://doi.org/10.1111/maps.13730)

A Cr-isotopic analysis was subsequently conducted for both Dhofar 778 and Dhofar 007 at the University of Science and Technology of China, and the ε54Cr values of –0.51 (±0.07) and –0.46 (±0.09), respectively, support the O-isotopic evidence for a possible common parent body distinct from the HEDs and other anomalous eucrites (Pang et al., 2020; see diagram below). It was also recognized that the Δ17O value for the anomalous eucrite-like meteorite NWA 12338 (–0.18 [±0.01]‰) is very similar to that for both Dhofar 778 (–0.160 [±0.054]‰) and Dhofar 007 (–0.173 [±0.010]‰), and it was suggested that the three meteorites might be genetically related; Cr isotopes still need to be determined for NWA 12338.

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Diagram credit: Pang et al., 51st LPSC, #1947 (2020)

The photo above shows a 0.88 g half slice of Dhofar 778, which exhibits profound shock-veining. Increased magnification reveals that fragmentation is present to a very fine scale. It is thought that the melt veins were formed and quenched within a few microseconds. The top photo below is a high resolution image of this diogenite, courtesy of Stephan Kambach. This is a highly polished section revealing a greenish color similar to that of Tatahouine. The bottom photo below shows the fusion-crusted side of the main mass.

dhofar 778
click on image for a magnified view

Photo courtesy of Stephan Kambach

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standby for dhofar 778 photo
Photos of the complete stone shown courtesy of Stephan Kambach