DAR AL GANI 476

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 x 10 cm and weighing 2,015 g was analyzed and classified at Germany's Max-Planck-Institut für Chemie in Mainz; this is most 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, and 975 (weighing 2,146 g, 1,619 g, 588 g, 6.2 g, and 27.55 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; on the other hand, DaG 735 has experienced less weathering than DaG 476, 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 pyroxene cores (orthopyroxene), which are similar to the larger orthopyroxene grains present in the ol–opx shergottites NWA 1195 and NWA 2046 (Irving et al., 2004, 2005). The highly-magnesian olivine megacrysts, which contain unusual pyroxene-chromium inclusions, are generally considered to be phenocrysts derived from an olivine-saturated parent magma. They are embedded in a fine-grained groundmass composed of mostly Ca-poor pigeonite and feldspathic glass, with minor Ca-rich augite. In other studies (Koizumi et al., 2003), it was concluded that the olivine megacrysts are 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, SaU 005, NWA 1068, Dhofar 019, Y-980459, NWA 1195, and NWA 2046, all of which contain an abundance of inclusion-bearing olivine megacrysts. 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 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 became fractionated.

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 a rapid cooling rate of 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 formed by fractional crystallization, became emplaced within a pyroxene groundmass phase that was formed through a cumulate process, in combination with the loss of a residual fractionated melt (see also Koizumi et al., 2003) above.

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 including olivine core staining, twinning of clinopyroxene, mosaicism of olivine, and plagioclase converted to feldspathic glass, as well as abundant impact melt pockets, all correspond to a shock stage of 44–56 GPa, and it was demonstrated that these features formed at the time of ejection. A shock-melt vein identified in the paired DaG 735 provided evidence of adjacent olivine dissociation which formed granular textures consisting of magnesiowüstite + perovskite; temperatures of at least 700°C were required (Miyahara et al., 2011). Both cooling models and experimental results of melt pocket formation constrain the melt pocket cooling rate in DaG 476 to ~780°C/hr over a 16-minute time interval (Walton et al., 2006). These results led to an estimation of the preatmospheric meteoroid diameter for DaG 476 of ~19 cm, while an estimate of the preatmospheric diameter based on cosmogenic nuclides was reported as ~30–40 cm (Nishiizumi et al., 2011).

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, the largest in size among those studied, 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 which 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 experienced cooling at rates of 780–1560°C/hr over a short time span on the order of minutes (Walton and Herd, 2007). Finally, 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).

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 age of 1.1 (±0.1) m.y. is derived. Conversely, Park et al. (2003) calculated a terrestrial age for DaG 476 of 140 t.y., with an ejection age of 1.08 m.y. Alternatively, 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, evidently representing a common ejection event on Mars. Exposure ages of all members of the basaltic and olivine-bearing subgroups studied thus far represent only a few ejection events from Mars; these correspond to ejections at ~1.2, ~3.0, ~3.8, and ~20 m.y. The ejection age for the composite shergottite EETA79001 is the lowest of any martian sample at 0.73 (±0.15) m.y., and 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 age 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 been less weathered, 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, and given the name of, Los Angeles, California. This basaltic shergottite is the most evolved martian meteorite found to date, although NWA 2800 appears to be a "twin" of Los Angeles and may be launch-crater paired (Bunch et al., 2008). Los Angeles crystallized in a lava flow or shallow intrusion, and experienced a slower cooling rate than other shergottites, resulting in a coarser-grained texture. A cosmic-ray exposure age of ~3.0 m.y. is similar to that of the basaltic shergottites 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 have a common parental melt to DaG 476, e.g., similar 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. This extremely long CRE age is 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—NWA 480, NWA 856, NWA 1068 (and pairings), NWA 1195, NWA 2046, NWA 1669, NWA 2626, NWA 3171, and others from Morocco or 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 newly recognized olivine–orthopyroxene-phyric shergottites.

The 36 g basaltic shergottite NWA 1669 is primarily composed of the pyroxenes pigeonite and augite, the composition of which suggests that augite crystallized first. This crystallization order indicates that the magma contained a significant amount of water (Jambon et al., 2003). While distinct from other Saharan basaltic shergottites, studies of the 506 g NWA 3171 (Irving, Kuehner, Herd, Larson, Longstaffe, and Gregory) shows some close compositional similarities to this group, as well as to the basaltic shergottites Shergotty, Zagami, and Los Angeles, and future CRE age studies will help resolve the question of common ejection events among them.

Remarkably, two separate but similar clasts of an alkaline-enriched lithology were found in the Kaidun meteorite—one 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.

As of the beginning of the year 2008, a total of 49 terrestrially unpaired martian meteorites are known, which represent perhaps only eight common ejection events on Mars (Bunch et al., 2008). The specimen pictured above is a 0.7 g partial slice in which dark olivine xenocrysts are seen throughout the greenish pyroxene matrix. The photo below shows the in situ mass of DaG 476 as it was found in the desert.