Martian Shergottite
olivine-phyric ∗
(depleted, permafic, reduced)

standby for dhofar 019 photo
Found January 24, 2000
18° 18.97' N., 54° 08.87' E.

A stone weighing 1,056 g was found in the Dhofar region of Oman. This meteorite fell to Earth ~340 t.y. ago, making it one of the oldest recovered meteorites from a hot desert (the record is held by the lunaite, Dhofar 025, which fell 500–600 t.y. ago). Dhofar 019 is a basaltic/doleritic rock consisting primarily (~64%) of the low-Ca pyroxene, pigeonite, with rims of augite, along with a large component (25%) of maskelynite (from shock conversion of plagioclase feldspar) and a small amount (9%) of olivine. Accessory components include chromite, ulvöspinel, ilmenite, chlorapatite, merrillite, magnetite, and pyrrhotite, along with secondary weathering products including calcite, sulfates, celestite, Fe hydroxides, and smectite. While a terrestrial origin is considered reasonable for most of the secondary weathering products, the calcite has been shown through cathodoluminescence and Raman spectroscopy to have a martian origin (Nakazato et al., 2008).

In addition to maskelynized feldspar, other shock-induced effects include planar fractures, mosaicism, twinning, and melt veins and pockets, corresponding to high shock pressures of 26–29 GPa (Fritz et al., 2005). However, utilizing cathodoluminescence and micro-Raman spectral analysis of maskelynite, Kayama et al. (2008, 2009) determined a shock pressure of 30–40 GPa, consistent with that obtained for experimentally shocked plagioclase. Smectite–calcite–gypsum "orangettes" similar to those found in ALH 84001 are present within maskelynite; although these are possibly of terrestrial origin, micro-fractures present in some smectite suggests that some could have an extraterrestrial origin.

Dhofar 019 has a heterogeneous texture, with elemental compositions similar to EETA79001A, and features similar to DaG 476 and other olivine-bearing shergottites, including the presence of zoned olivine megacrysts. However, the majority of the olivine grains in Dhofar 019 are much smaller than those present in other olivine-bearing shergottites, and have lower Ni and Mg, and higher Co and Fe contents, indicative of a more evolved magma source than that of EETA79001A or DaG 476. Other differences include a lack of orthopyroxene in Dhofar 019 compared to other shergottites, and a larger compositional range for olivine, maskelynite, and spinel, indicative of a rapid cooling rate. Melt inclusions composed of pyroxene and melt glass are ubiquitous in the olivine grains.

*The establishment of a new shergottite subgroup has been proposed, one which comprises those similar shergottites with olivine–porphyritic textures. The name picritic shergottite was suggested for this new subgroup by Barrat et al., 2002, while the name olivine-phyric shergottite was suggested by Goodrich, 2002. Goodrich suggests that the term picritic shergottite implies certain petrogenetic characteristics, such as mixing of two compositionally distinct magma sources, which is not necessarily the case for all members of this new subgroup; therefore the purely descriptive term olivine-phyric is favored.

Trace element analyses suggest that Dhofar 019 and DaG 476 were derived from similar but unique incompatible element-depleted parental magma source regions, comparable to the near primary melt represented by Y-980459 but with a lower abundance of Ni; nevertheless, Dhofar 019 is texturally more similar to Zagami and Shergotty. A crystal size distribution analysis reveals a very small grain size, nearly identical to that of Zagami; the size of the pyroxene laths in Dhofar 019 (0.196 mm) is intermediate between that of Zagami (0.213 mm) and DaG 476 (0.082 mm). Calculations based on olivine zoning profiles indicate that the smaller olivine grains nucleated at depth within a late-stage, Fe-rich magma, where they underwent limited diffusion. The larger and more magnesian olivine megacrysts are an early crystallization product. After a short period, measured in days to weeks, the olivine grains were infused with a magnesian magma, which was rapidly cooled within a few meters of the surface—too rapidly to establish equilibrium with the melt. Mikouchi and Miyamoto (2002) calculated a rapid cooling rate of approximately 0.05–0.1°C/hour from 1200°C to 700°C., which corresponds to a burial depth of ~5 m, probably within a lava flow.

Probable crystallization ages of 575 (±7) and 525 (±56) m.y. were calculated by Borg et al. (2001) based on Sm–Nd and Rb–Sr, respectively, while an Ar–Ar derived age for maskelynite of 642 (±72) m.y. was obtained by Korochantseva et al. (2007), and an age of ~650 m.y. was obtained by Garrison and Bogard (2001); all of these ages are in agreement within error margins. The calculated Sm–Nd age of Tissint, the only fall representing this depleted, olivine-phyric martian meteorite group, is 596 (±23) m.y., which is consistent with that of Dhofar 019 (Brennecka et al., 2013).

Utilizing a cratering model which employs a cratering frequency per area based on a lunar reference, and a Mars/Moon cratering ratio of 1.55, Nyquist et al. (2009) reasoned that the region of Mars known as "Amazonis unit 2 north" could correspond to an age of ~0.5 b.y. This time period, known as the Hesperian epoch, is consistent with hosting the ejection of not only the older shergottites, such as Dhofar 019, but also the nakhlites and chassignites. Ejection times of the younger shergottites, such as Zagami and NWA 1460/480, are consistent with the Early Amazonian epoch. Notably, these two cratering epochs account for ~30% of the martian surface, and this fact provides a solution to the conundrum whereby too many young meteorites are delivered from what was previously thought to be a mostly old surface.

A cosmogenic 38Ar-based CRE age of maskelynite and pyroxene was calculated for Dhofar 019, which averages 15.7 (±0.7) m.y. (Korochantseva et al., 2009). When combined with the estimated terrestrial age of 0.34 (±0.04) m.y., it is concluded that ejection from Mars occurred 16.0 (±0.7) m.y. ago. This long CRE age is at the theoretical limit of the calculated delivery time of material to Earth from Mars. Cosmic ray exposure ages have now been determined for many martian meteorites, and Mahajan (2015) compiled a chart based on the reported CRE ages for 53 of them. He concluded that together these 53 meteorites represent 10 distinct impact events which occurred 0.92 m.y., 2.12 m.y., 2.77 m.y., 4.05 m.y., 7.3 m.y., 9.6 m.y., 11.07 m.y., 12.27 m.y., 15 m.y., and 16.73 m.y.—see his chart here. It was argued that Dhofar 019 was launched in a unique impact on Mars. In a subsequent review based on multiple criteria, Irving et al. (2017 [#2068]) made a new determination of the number of separate launch events associated with the known (101) martian meteorites. They speculate that the number could be as few as twenty, and concur with Mahajan (2015) that Dhofar 019 represents a unique ejection event.

The specimen of Dhofar 019 shown above is a 0.16 g interior partial slice. The photo below shows the in situ mass of Dhofar 019 as it was found in the desert.

∗ Recent geochemical research on the martian basalts has led to new petrogenetic models and classification schemes.  read more >>

standby for dhofar 019 photo