DAR AL GANI 476

Martian Shergottite
olivine–orthopyroxene-phyric ∗
(depleted, permafric, reduced)

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Found May 1, 1998
27° 21.16' N., 16° 12.04' E.

Dar al Gani 476, also known as "Lucky 13", is an olivine–orthopyroxene-phyric shergottite that was found in the Libyan Sahara Desert by an international team. The brown, loaf-shaped mass measuring ~ 15 × 10 cm and weighing 2,015 g was analyzed and classified at Germany's Max-Planck-Institut für Chemie in Mainz; this is quite appropriate since this institute also developed the APXS instrument used during the Mars Pathfinder Mission aboard the Sojourner Rover to analyze surface rock compositions.

Several other olivine-bearing shergottites were recovered in the same general area as DaG 476, including DaG 489, 670, 735, 876, 975, and 1037 (weighing 2,146 g, 1,619 g, 588 g, 6.2 g, 27.55 g, and 4,012.43 g respectively). Petrographic, mineralogic, textural, and shock features, as well as noble gas and radionuclide abundances, are indistinguishable from those of DaG 476, and all of these masses are likely paired. After many tens of thousands of years of desert exposure (40–140 t.y.), DaG 476 has completely lost its fusion crust and developed cracks and veins that are filled with carbonate and other terrestrial weathering products. Although likely falling at the same time, DaG 670 is much more highly weathered than DaG 476, while on the other hand, DaG 735 has experienced significantly less weathering than DaG 476 in exhibiting only µm-wide calcite veins. DaG 735 was actually the first of the four masses to be found, having been recovered during the winter of 1997/98.

Dar al Gani 476 contains an unusually high abundance (as high as 24 vol%) of zoned olivine megacrysts up to 5 mm in size. It also contains a low abundance of orthopyroxene megacrysts up to ~0.3 mm in size, as well as orthopyroxene cores in others, similar to the larger orthopyroxene grains present in the ol–opx shergottites NWA 1195 and NWA 2046 (Irving et al., 2004, 2005). The high-magnesian olivine megacrysts, which contain unusual chromium-pyroxene inclusions, are generally considered to be phenocrysts derived from an olivine-saturated parent magma. They are embedded in a fine-grained groundmass composed mostly of Ca-poor pigeonite and feldspathic glass, along with minor Ca-rich augite. In other studies (Koizumi et al., 2003), it was concluded that the olivine megacrysts are actually xenocrysts that crystallized within an Fe-rich melt through fractional crystallization, resulting in zoning from Fo76 to Fo58. Thereafter, these xenocrysts were entrained within an Mg-rich melt, cooling rapidly to preserve the olivine zoning, and eventually forming the groundmass.

Dar al Gani 476 shows close petrographic and mineralogical similarities to the shergottites EETA79001A, LAR 12095/240, SaU 005, NWA 1068, Dhofar 019, Y-980459, NWA 1195, NWA 2046, and others more recently found, all of which contain an abundance of inclusion-bearing olivine megacrysts, and some or all of these meteorites may be launch paired (Mikouchi and Takenouchi, 2014). In addition, the REE pattern (LREE-depleted) and Sm–Nd systematics imply that a close association once existed with the source melt of the basaltic shergottite QUE 94201, as well as to Nakhla and Chassigny. However, the less evolved, highly mafic compositions of olivine, pyroxene, chromite, and ilmenite present in DaG 476 compared to QUE 94201 suggest that the parent magma of DaG 476 experienced a late episode of fractional crystallization (Borg et al., 2002). It was also determined that the higher Rb/Sr ratio of the QUE 94201 magma could be accounted for by the presence of an amphibole component. This amphibole is chlorine-rich and water-poor, reflecting the volatile content of the source magma (Filiberto and Treiman, 2009). In a contrasting study, McCubbin et al. (2009) found that kaersutite and Ti-biotite in Chassigny melt inclusions contain higher abundances of water than previously measured, a value which correlates to a parental source magma water content of ~460–840 ppm (0.5–0.8 wt%), while lower abundances of Cl and F were observed.

The conditions under which DaG 476 crystallized were also more reducing than those of other basaltic shergottites, and it is one of the most magnesian shergottites (Fo76) of the olivine-bearing subgroup, with only Y-980459, NWA 2046, and NWA 1195 having more magnesian olivines. Overall, its mineralogy and bulk chemistry indicate that it is a distinct shergottite intermediate in composition between the basaltic and poikilitic (formerly "lherzolitic") subgroups. A xenolith-bearing basalt found on Kauai, Hawaii may be a terrestrial analog of this shergottite subgroup. The Mars Exploration Rover Spirit, which landed inside Gusev Crater, discovered several picritic basaltic rocks, subsequently named Adirondack, Humphrey, and Mazatzal. Analyses of these Adirondack-class basalts revealed they are fine-grained rocks with a plagioclase and pyroxene composition, containing dark megacrysts of ferroan olivine, very similar to DaG 476 (McSween et al., 2004). Experimental work on a synthetic analog of the Adirondack-class basalts by Filiberto et al. (2008) indicates that these rocks probably do not represent a primary mantle-derived hydrous melt, but rather, the magma likely rose and pooled into a shallow chamber within the crust where it underwent fractionation.

The isotopic systematics (i.e., Sm–Nd, Rb–Sr, Lu–Hf, Hf–W) of DaG 476 indicate that differentiation from a chondritic source occurred ~4.513 b.y. ago, and also attest to a young crystallization age of 474 (±11) m.y. Based on the Fe–Mg zoning profile of olivine, DaG 476 experienced rapid cooling calculated at 0.089°C/hr, corresponding to a burial depth during crystallization of several meters (Miyamoto et al., 2009). By some accounts, DaG 476 was formed through a high-degree of partial melting of a primitive lherzolite-like (olivine-saturated) source rock mixed with another martian rock. This was followed by the segregation of a melt containing unmelted phases of olivine, enstatite, and chromite. A residue containing a fraction of these unmelted phases was removed from this "crystal mush", leaving behind the fraction that would eventually form DaG 476. An alternative petrogenesis was outlined by Goodrich (2003) and Koizumi et al. (2004) in which a small fraction (a few percent) of zoned, xenocrystic chromite and magnesian olivine and pyroxene, which were the products of fractional crystallization, became emplaced within a pyroxene groundmass phase formed through a cumulate process, in combination with the loss of a residual fractionated melt (see also Koizumi et al., 2003, above). A crystallization history was constructed by Mikouchi and Takenouchi (2014) for LAR 12095 and its likely launch pairings such as DaG 476. They suggest that crystallization occurred initially under slow cooling conditions from temperatures of ~1400°C down to ~1250°C, followed by a rapid cooling stage of 0.03–3°C/hour which preserved the chemical zoning in silicates.

The texture of the olivine phenocrysts and pyroxene crystals are indicative of flow alignment within an extruded lava flow near the surface. Features indicative of high shock include olivine core staining, twinning of clinopyroxene, mosaicism of olivine, plagioclase converted to feldspathic glass, and abundant impact-melt veins and pockets, all which correspond to a shock stage of 44–56 GPa; it was demonstrated that these features were formed at the time of ejection. A shock-melt vein identified in the paired DaG 735 provides evidence for olivine dissociation adjacent to the vein that has resulted in a granular texture consisting of magnesiowüstite + perovskite; temperatures of at least 700°C were required to produce this phase (Miyahara et al., 2011). Both cooling models and experimental results were utilized to constrain the cooling history of melt veins and pockets in DaG 476 and other shergottites, demonstrating a rate of ~0.2°C/second over time intervals of seconds to minutes, depending on the size and shape of the melt features and their proximity to other melt features (Walton et al., 2006; Shaw and Walton, 2013). Based on these data, a preatmospheric diameter of ~19 cm was estimated for the DaG 476 meteoroid, while that estimated by Nishiizumi et al. (2011) based on cosmogenic nuclides was ~30–40 cm.

Similar to other highly shocked martian meteorites, DaG 476 contains a significant concentration of martian atmospheric Ar within the melt pockets (ave. 4.6 ppb), with a minor component present within shock veins (ave. 1.3 ppb). The favored interpretation of the existence of this trapped gas component within the melt pockets calls for the initial introduction of martian atmospheric gas into pre-existing cracks and pores. During an impact-ejection event, probably that which launched the rock to Earth, a shock wave was generated that passed through a decimeter-sized meteoroid causing sudden decompression, followed by an ensuing pressure release. Thereafter, bubbles were created within localized, in situ, mm-sized melt pockets containing superheated melt, which then cooled at rapid rates over a short time interval on the order of seconds to minutes (Walton and Herd, 2007). Thereafter, as pressures reached equilibrium, the trapped atmospheric gases migrated into the vesicles of the melt phase from the surrounding cracks and pores (Walton et al., 2007). Studies by Shaw and Walton (2013) revealed how such implanted martian atmospheric noble gases could experience diffusive loss (Xe more rapidly than Ar) in the large melt pockets with long cooling intervals, resulting in a small increase in the Ar/Xe ratio.

Comparisons with Viking inert gas measurements, as well as results from chemical, mineralogical, petrographic, and oxygen isotopic studies, clearly identify DaG 476 as martian. Combining the 21Ne-based CRE age of 1.05 (±0.1) m.y. and the calculated terrestrial age of 60 (±20) t.y., a Mars ejection 1.1 (±0.1) m.y. ago is derived. Similarly, Park et al. (2003) calculated a terrestrial age for DaG 476 of 140 t.y., with an ejection age of 1.08 m.y. As well, Nishiizumi et al. (2011) calculated a terrestrial age for DaG 476 of 60 (±20) t.y. and a CRE age of 0.95 (±0.1) m.y., resulting in an ejection age of 1.0 (±0.1) m.y. This ejection age is indistinguishable from that of at least 7 other depleted olivine-phyric shergottite falls (e.g., SaU 005 and Tissint), evidently representing a common ejection event on 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 DaG 476 was launched from Mars during the 0.92 m.y.-old impact event. 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 suggest that the DaG 476 pairing group and at least 18 other depleted (predominantly olivine-phyric) shergottites were ejected 1.1 m.y. ago in a common impact event unique from the others. Notably, the composite shergottite EETA79001 has one of the youngest ejection ages of any martian sample at 0.73 (±0.15) m.y., and while Mahajan (2015) included it with the 0.92 m.y.-old impact event, Irving et al. (2017) argue that it represents a unique ejection event.

The olivine-bearing shergottite, Sayh al Uhaymir 005, was recovered in Oman in 1999. Together with several probably paired masses recovered later, it has a total combined weight of ~9.9 kg. SaU 005 shows very close similarities to DaG 476 in bulk chemical composition and texture, but it shows mineralogical evidence of having experienced slower cooling rates, exhibiting a microgabbroic texture. Noble gas and radiometric age data place the cosmic-ray exposure age at ~0.81 m.y. (Park et al., 2003), indicating an ejection from Mars slightly later than that calculated for DaG 476. The pre-atmospheric diameter of SaU 005 was calculated to have been at least 54 cm, corresponding to a mass of ~270 kg. Compared to DaG 476, SaU 005 has experienced less weathering, with a calculated terrestrial age of only ~13 t.y. It has retained a partial fusion crust and exhibits few terrestrial alteration products. Although their respective terrestrial ages indicate they are not from the same fall, SaU 005 and Dag 476 are likely comagmatic rocks from the same igneous region on Mars.

Also in 1999, two paired stones totaling 698 g were found in a collection of meteorites in Los Angeles, California. This basaltic shergottite is one of the most evolved martian meteorites found to date, and it has been shown that similarities in element composition, petrography, and mineral chemistry to both NWA 2800 and Ksar Ghilane 002 shergottites suggest they may be launch-crater pairings to Los Angeles (Bunch et al., 2008; Llorca et al., 2013). Los Angeles crystallized in a lava flow or shallow intrusion, and experienced a slower cooling rate than most other shergottites, resulting in its very coarse-grained texture. A cosmic-ray exposure age of ~3.0 m.y. is similar to that of the basaltic shergottites KG 002, Shergotty, Zagami, QUE 94201, and others, suggesting a common ejection event for all.

Further recoveries of martian shergottites have continued, including the 1,056 g olivine-bearing shergottite found in Oman in January, 2000 named Dhofar 019. This meteorite has features quite similar to those of DaG 476, including an abundance of olivine megacrysts. Although megacrysts in Dhofar 019 are less magnesian, it may share a common parental melt with DaG 476, with similarities to QUE 94201. However, in constrast to DaG 476, Dhofar 019 represents a fractionated restite. A 21Ne-based CRE age of ~20.7 m.y. has been calculated for Dhofar 019 (Park et al., 2003). This is one-third longer than any other martian meteorite, and four times longer than any other shergottite—a CRE age at the theoretical limit of the calculated delivery time of material to Earth from Mars.

Among an already staggering number of recent martian shergottite finds, new shergottites continue to be found, e.g., NWA 480/1460, NWA 856, NWA 1195, NWA 1068 (and pairings), NWA 2046, NWA 1669, NWA 2626, NWA 3171, and others from Morocco and neighboring countries, the highly-ferroan Dhofar 378 found in Oman, and the Antarctic Y-980459, the only martian meteorite that lacks plagioclase. Northwest Africa 1068 and Y-980459 have an olivine-phyric texture, while NWA 1195, NWA 2626, and NWA 2046 join DaG 476 as a newly recognized grouping of olivine–orthopyroxene-phyric shergottites. In addition, the poikilitic (formerly "lherzolitic") shergottite NWA 1950 was discovered, as well as the unique plagioclase–olivine-clinopyroxenite, NWA 2646, related to the martian poikilitic shergottite group.

Although ALH 84001 is an orthopyroxenite, and as such was characterized by the Planetary Chemistry Laboratory at Washington University as a subgroup of the nakhlites, its parental source magma has a composition that is consistent with the same mixtures of depleted and enriched REE end-member components that are used in a geochemical classification of the shergottites (Lapen et al., 2012). It was determined that the source magma of ALH 84001 contained a higher proportion of the enriched REE component than all other shergottites studied thus far. Therefore, ALH 84001 may be most appropriately classified as a subgroup of the shergottites.

Remarkably, two separate but similar clasts of an alkaline-enriched lithology were found in the Kaidun meteorite—one that was melted in situ and the other unaffected by impact forces. Based on mineralogical and textural characteristics, they have been identified as possible basaltic shergottite material (Ivanov et al., 2001, 2003). In a further study of these two clasts (Ivanov and Zolensky, 2003), it was proposed that the circumstances of repeat encounters with the Kaidun CR-like host object of an extremely rare alkaline-rich rock, necessarily derived from a large differentiated body, was consistent with a characterization of Kaidun as the martian moon Phobos. However, Ziegler et al. (2012), studying clasts of differentiated material in Kaidun, found Δ17O values inconsistent with those of martian meteorites and which don't plot with any known meteorite material on an oxygen three-isotope diagram. Notably, numerous fragments of another alkali-rich meteorite have been found in the Sahara—NWA 7034/7533. Studies have determined that the meteorite was likely part of an extensive impact-melt-breccia lens from the near-surface crust of Mars that experienced subsequent impact gardening and mixing with aeolian-distributed, siderophile-enriched meteoritic contaminates contained in soils and dust. This martian meteorite has a texture/structure that has been likened to that of a howardite.

As of the beginning of the year 2013, approximately 65 terrestrially unpaired martian meteorites were known, all of which may represent less than half a dozen common ejection events on Mars. The specimen pictured above is a 0.7 g polished partial slice in which dark olivine xenocrysts are seen embedded throughout the greenish pyroxene matrix. The photo below shows the DaG 476 meteorite in situ.

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

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