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NWA 5789

Martian Shergottite
olivine-phyric ∗
(depleted, permafic, primitive mantle-derived melt)

standby for northwest africa 5789 photo
Purchased June, 2002
no coordinates recorded Three fragments constituting a single fresh stone meteorite and partially covered in a glossy, bubbly, black fusion crust were found in the Morroccan Sahara. The combined weight of the three meteorite fragments was determined to be 49 g. A 1.8 g sample of the meteorite was provided to G Hupé who sent a sample both to the University of Washington (A. Irving and S. Kuehner) for petrographic analysis, and to the Carnegie Institute (D. Rumble III) for O-isotope analysis. It was ascertained that NWA 5789 is a rare primitive martian meteorite. Thereafter, the remaining quantity of NWA 5789 was purchased by Chladni’s Heirs (S. Ralew and M. Altmann).

 

Northwest Africa 5789 is a slightly friable meteorite with a permafic composition. A terrestrial analog for this meteorite is limburgite, a rock type found in a volcanic quarry in Limburg, Germany. Limburgite is a rapidly cooled, alkaline, sometimes vesicular basaltic lava, primarily composed of olivine and augite. This rock type is often associated with hydrated minerals such as kaersutite. The reservoir from which the parent magma for NWA 5789 was derived had a composition that was depleted in incompatible elements, to which 2% olivine was added (Irving et al., 2010; Treiman and Filiberto, 2014).

 

Northwest Africa 5789 is almost identical in bulk composition, petrography, and mineral chemistry, and similar in texture to the martian meteorite Yamato 980459, an olivine-websterite, which crystallized from the most primitive martian melt known (Gross et al., 2010). However, there are textural differences between the two meteorites related to differences in their cooling rate. In Y-980459, it was shown by Greshake et al. (2004) that ascent of the magma to the surface resulted in rapid quenching that suppressed the formation of plagioclase and produced a glassy mesostasis. In contrast, NWA 5789 experienced slower cooling as it ascended from higher to lower pressures, and the mesostasis formed crystalline plagioclase with radiating sprays of clinopyroxene, spinel, ilmenite, FeS, and silica laths. Another difference between the two meteorites which is more indicative of separate parental source regions on Mars is the high-siderophile element content and Os isotopes; the HSE in Y-980459 is higher, while the 187Os/188Os ratio is lower. Furthermore, Hoffmann et al. (2010) determined that significant differences exist between their respective magnetic signatures.

 

As with Y-980459 (Fo8486), NWA 5789 is thought to represent a Mg-rich, primitive mantle melt in which olivine megacryst cores have a Mg# in equilibrium with the melt of the bulk rock composition (Fo84.4 compared to the equilibrium value of Fo85.5) (Gross et al., 2011). Olivines in both meteorites are virtually identical, present as mm-sized yellow-green megacrysts containing the highest known Mg# values among martian meteorites. In addition, smaller phenocrysts of olivine and pyroxene in equilibrium are scattered throughout a fine-grained groundmass composed of pigeonite, chromite, pyrrhotite and mesostasis. It was suggested that the two meteorites might originate from the same or similar source magmas, with NWA 5789 crystallizing more slowly in a thicker section of the melt.

 

Based on pyroxene crystallization in NWA 5789, it was shown that the cores formed at high pressure conditions, consistent with the crust or upper mantle, while the mesostasis formed at lower pressures, on or near the surface (Gross et al., 2011). This difference in pressure amounts to ~10 kbar or a difference in depth of 85 km, indicating an extended crystallization history that was completed when the magma erupted onto the surface. Temperatures of the magma at depth for both Y-980459 and NWA 5789 were calculated to have been 1300–1400°C, and were formed in reduced redox conditions (oxygen fugacity near the iron–wüstite buffer). Modeling of the composition of both Y-980459 and NWA 5789 by Filiberto and Dasgupta (2012) suggests their parent magmas separated from a primary mantle source region at a pressure of 1.2 (±0.1) GPa and temperature of 1540 (±20) °C, consistent with a relatively shallow depth of ~100 km. This temperature is at the high end of the range calculated for the average mantle temperature during the Noachian period (4.5–3.6 b.y. ago) of 1450 [±80] °C, and they attribute this to a thermal anomaly.

 

It was calculated that both NWA 5789 and Y-980459 had a pre-atmospheric diameter of <10 cm, and they exhibit similar CRE ages of 1.0 (±0.2) m.y. This CRE age is indistinguishable from that of at least 7 other depleted olivine-phyric shergottite falls, all of which evidently represent a common ejection event on Mars (Nishiizumi et al., 2011). 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 NWA 5789 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 at the time of their study) martian meteorites. They speculate that the number could be as few as twenty, and suggest that NWA 5789 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.

 

A new systematic classification method was invoked for the shergottites by Irving et al. (2010). They utilized a bulk chemical diagram comparing the CaO content with the Mg#, and established three separate categories encompassing all possible values: mafic, permafic, and ultramafic (see 41st LPSC, #1547 [2010]). In addition, they combined the resulting designation with the existing terminology pertaining to trace element abundances and isotopic systematics: enriched, intermediate, and depleted. Next, these designations are combined with the established meteorite group name: mafic or diabasic shergottite (formerly ‘basaltic’ shergottite), olivine-phyric shergottite, poikilitic shergottite (formerly ‘lherzolitic’ shergottite), nakhlite, chassignite, or orthopyroxenite. Under this scheme the primitive martian magmatic rock NWA 5789 is classified as a depleted permafic olivine-phyric shergottite.

 

The martian rover ‘Spirit’ has identified alkaline volcanic rocks at Gusev crater which are geochemically consistent with limburgite, and which may characterize highlands terrane on Mars (Kochemasov, 2006). The specimen of NWA 5789 shown above is a sub-gram fragment. The mesmerizing photos below, kindly provided by Chladni’s Heirs, show different views of this unique martian meteorite.

 

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

 

standby for northwest africa 5789 photo
Photo courtesy of Chladni’s Heirs
standby for northwest africa 5789 photo
Photo courtesy of Chladni’s Heirs
standby for northwest africa 5789 photo
Photo courtesy of Chladni’s Heirs
standby for northwest africa 5789 photo
Photo courtesy of Chladni’s Heirs
standby for northwest africa 5789 photo
Photo courtesy of Chladni’s Heirs
<!–standby for northwest africa 5789 photo
Photo courtesy of Chladni’s Heirs
–> standby for northwest africa 5789 photo
Photo courtesy of Chladni’s Heirs


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NWA 1068

Martian Shergottite
olivine-phyric ∗
(enriched, permafric, oxidized)

standby for nwa 1068 photo
Found April 2001
no coordinates recorded One large mass of 522 g along with twenty-two additional fragments, all together weighing 576.77 g, were found by a French team in the Maarir region near the border of Morocco and Algeria. This meteorite was designated Northwest Africa 1068, and a sample was submitted to École Normale Supérieure de Lyon and other institutions for analysis and classification. Unaware of the analyses being conducted by the French institutions, additional paired fragments purchased in Morocco having a combined weight of 118 g were submitted to the University of Washington (Irving and Kuehner, 2002). Since the name NWA 1110 had been previously reserved from the Nomenclature Committee for these additional fragments, this meteorite will be recognized as a pairing under both names. Subsequent to this, other individual fragments were recovered in the strewnfield, some of which were submitted to the NomCom under unique NWA numbers (e.g., NWA 1183, NWA 1775, NWA 2373, NWA 2969).

 

Northwest Africa 1068 is considered to represent a distinct olivine-phyric subgroup of shergottites, characterized by an abundance of olivine megacrysts (~22 vol%) embedded within a primarily low-Ca pyroxene groundmass (~42 vol%). This subgroup would comprise those meteorites derived from a primary magma associated with an ascending mantle plume. They are ultramafic rocks, enriched in Mg (>12%), Ti, and other incompatibles, and were formed at greater depths under higher pressures than the basaltic subgroup. Despite some important compositional differences, close petrological and geochemical similarities exist between NWA 1068 and both the olivine-phyric shergottite LAR 06319 and the olivine-basaltic shergottite NWA 4468 (Sarbadhikari et al., 2009). According to MELTS program modeling, Marks et al. (2010) found that both NWA 1068 and 4468 have compositions that are consistent with the hypothesized parental melt for LA 001; in particular, the REE patterns and initial Sr–Nd isotopic compositions are consistent with such a relationship, and the major element compositions reflect a mantle-associated fractionation. The crystallization ages for both NWA 1068 and Los Angeles are concordant at ~180 m.y., and they might be derived from a common primary magma source—with Los Angeles crystallizing after ~40% fractionation, and NWA 1068 after addition of 22% olivine (Treiman and Filiberto, 2014).

 

The occurrence of plagioclase in the form of maskelynite (~15 vol%), undulose extinction of pyroxene and olivine, impact-melt pockets, and shock veins attest to high shock metamorphism (29–55 GPa) for this meteorite. Minor amounts of Ca-phosphates, K-feldspar, FeS, ulvöspinel, and chromite are also present. Despite its lack of fusion crust, NWA 1068 is relatively fresh, with only minor calcite and clay minerals present in cracks and along grain boundaries. Using the typical increases in the Sr, Ba, and Pb abundances observed in hot desert meteorites as a barometer, NWA 1068 has not been greatly affected by weathering (Barrat et al., 2002).

 

Similar to other highly shocked martian meteorites, NWA 1068 contains a significant concentration of martian atmospheric Ar within melt pockets (ave. 8.6 ppb), with a minor component present within shock veins (ave. 0.9 ppb). The favored scenario for the existence of this trapped gas component within melt pockets is based on the argument that martian atmospheric gas was initially introduced into pre-existing cracks and pores. Following the passage of a shock wave, sudden decompression and pressure release created bubbles within sub-mm- to mm-sized localized melt pockets. Thereafter, as pressures became equilibrated, the trapped atmospheric gases migrated into the vesicles of the melt phase from the surrounding cracks and pores (Walton et al., 2007).

 

Northwest Africa 1068 is composed of abundant olivine megacrysts up to 2 mm in size that have magnesian cores (up to Fo72), and rims (as well as other smaller phenocrysts) that are more ferroan (Fo49). These olivine grains usually occur as single crystals, but many are polycrystalline and contain magmatic inclusions. They show almost identical chemical compositions to the olivines in martian poikilitic (formerly lherzolitic) shergottites, and it is considered by many investigators, based on the disparate redox conditions under which core and rim crystallized (i.e., increasing oxidation from core to rim) as well as on textural and other petrographic evidence, that the megacrysts are in fact xenocrysts that were accumulated into the magma flow from different melt reservoirs (Herd, 2006; Shearer et al., 2012). The zoning in the rims of these olivine megacrysts could be attributed to diffusion between the olivine and the magma that ensued following their incorporation. In keeping with this diffusion process is the fact that these large olivines appear to have only equilibrated with the groundmass along their rims, a feature which further supports a xenocrystic origin. Moreover, they are enriched in Co and incompatible elements compared to the groundmass, and evidence indicates that they crystallized under more oxidizing conditions.

 

In an alternative view, it has been argued that the olivine megacrysts represent phenocrysts (co-genetic), as demonstrated by the equilibrium between cumulate olivine and the olivine megacryst cores (Filiberto et al., 2010). Another observation in support of a phenocrystic origin for the olivine megacrysts is that the olivine appears to be in isotopic equilibrium with the other mineral components of the rock. The observed zoning in NWA 1068 olivine megacrysts could be interpreted as reworked phenocrysts that were subjected to a short period of gravitational settling and/or convective transport before accumulation (Shearer et al., 2008).

 

In further support of a phenocrystic origin, it was argued that the melt inclusions within the olivine megacrysts of NWA 1068, as well as in other olivine-phyric shergottites, have a similar composition to that of the bulk rock, indicating a derivation from a common parental source magma. Since the olivine megacrysts and the bulk rock are in chemical equilibrium, there is a high likelihood that the megacrysts represent phenocrysts derived from a common parental source melt. Moreover, the formation of spinel and high-Ca pyroxene in both the megacrysts and the matrix was concurrent. As such, it was undertaken by Filiberto et al. (2013) to compare the composition of the primary trapped melt in the magnesian megacrysts to that of the calculated parental melt prior to incorporation of excess olivine; their results were most consistent with a xenocrystic origin for the megacrysts.

 

Cooling rate studies place the crystallization of the low-Ti/Al pyroxene in NWA 1068 at a depth of ~85 km (~10 kbar), near the base of the crust, whereas the pyroxenes with a higher Ti/Al crystallized near the surface (<4.3 kbar), possibly upon eruption. in contrast, geochemical modeling conducted by Filiberto and Dasgupta (2012) suggest that formation occurred at a depth of ~150 km at a temperature of ~1520°C. This temperature is within the range of the calculated average mantle temperature of 1450 (±80) °C for basalt formation during the Noachian period, 4.5–3.6 b.y. ago. A scenario for the complex petrogenesis of this meteorite was constructed as follows:
Following their crystallization at depth, the cumulate olivine megacrysts were incorporated into an ascending, enriched, oxidized magma plume that originated at the upper-mantle (at depths of ~250–400 km; Kiefer, 2003), envisioned to be similar to lunar ur-KREEP that crystallized as a late-stage residual liquid of the martian magma ocean (Borg et al., 2012). The magma ponded in or near the base of the crust where olivine crystallized and accumulated from either the same (phenocrysts or antecrysts) or neighboring (xenocrysts) magma plumes. Low-Ti/Al pyroxene then crystallized and erupted onto or near the surface together with the olivine megacrysts. Cooling occurred rapidly close to the surface where shock metamorphic effects became significant. This scenario is consistent with the finding that NWA 1068 has a REE pattern that is similar to other basaltic shergottites, while other olivine-containing shergottites such as DaG 476, Dhofar 019, and SaU 005 do not. Xenocrystic olivines in EETA79001A might have a similar origin. Northwest Africa 1068 is a relatively primitive shergottite with a magnesian bulk composition, but is not as magnesian as experiments indicate it should be if it represented a primary liquid composition. Because it has incorporated a high abundance of olivine megacrysts it no longer represents a primary magma composition (Bunch et al., 2009).
Trace element data confirm that Northwest Africa 1068/1110 is unpaired with any previously found martian meteorites. In contrast to the depleted LREE evident in most all other olivine-phyric shergottites, NWA 1068 is enriched in incompatible elements similar to that which is found in the basaltic shergottites Shergotty, Zagami, and Los Angeles; incompatible element ratios are consistent with these basaltic shergottites as well. This suggests a parental magma for NWA 1068 of basaltic shergottite composition which had assimilated a late-stage, enriched, and more-oxidized cumulate component close to lherzolitic composition. Thereafter, olivine crystallized and was accumulated, perhaps as phenocrysts (Shearer et al., 2008).

 

Studies of NWA 1068 have continued in an effort to characterize the true nature of the olivine megacrysts and to better resolve the petrogenesis of the meteorite. Through advanced Fe–Mg isotope and major, minor, and trace element analyses of NWA 1068 bulk rock and olivine megacrysts, Collinet et al., (2017) determined that the meteorite is most consistent with a near-primary magma composition. In their formation model the olivine megacrysts and the groundmass of the meteorite are co-genetic. Subsequent to olivine megacryst formation, which they ascertained occurred over a time period of ~2–6 years under relatively slow cooling conditions within a deep pluton (the final span of ~100 days involved rim and groundmass crystallization at much faster cooling rates during/after magma ascent), the megacryst cores experienced simultaneous diffusion and growth of outer rims along with crystallization of the groundmass pyroxene and olivine, as evidenced by the fractionated Fe–Mg isotope and element profiles which are observed. This diffusion process reduced the Fo content by ~3.2 mol% from its original value to values as magnesian as Fo77; these current Fo values had previously been attributed to Fe–Mg equilibrium conditions attained through olivine accumulation. Therefore, the bulk rock composition of NWA 1068, and in a similar way that of LAR 06319, is likely representative of a primary magma, derived from a refractory mantle source, having an intermediate composition of at least ~Fo80 (see diagram below). standby for equilibrium diagram
Diagram credit: Collinet et al., GCA, vol. 207, p. 294 (2017)
‘Crystallization history of enriched shergottites from Fe and Mg isotope fractionation in olivine megacrysts’
(https://doi.org/10.1016/j.gca.2017.03.029)
Isotopic analyses using Sm–Nd and Rb–Sr data have determined a crystallization age for this shergottite of 185 (±11) m.y., and its CRE age has been calculated to be 2.2 (±0.2) m.y. (2.5–3.1 m.y. based on 10Be [Nishiizumi and Caffee, 2006] and 2.0 ±0.5 m.y. based on Ar systematics [Walton et al., 2007]). This CRE age is similar to several other martian meteorites, including NWA 2646, LAR 06319, and NWA 480/1460. Cosmic ray exposure ages have now been determined for many martian meteorites, and Mahajan (2015, #1166) 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 NWA 1068/1110 was launched from Mars during the 2.12 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 at the time of their study) martian meteorites. They speculate that the number could be as few as twenty, and suggest that NWA 1068/1110 might have been ejected with the large group of at least 26 enriched shergottites, or alternatively, it could represent a unique ejection event because of its disparate texture.

 

Interestingly, a determination of the Pb-isotopic composition of the original source of the olivine-phyric shergottites shows a similar plot to that of the nakhlites, and these diverse martian meteorites may have originated from the same mantle reservoir (Emil et al., 2006). The specimen of NWA 1068 pictured above is a 1.19 g partial slice with a thin black impact-shock vein along the left side and a natural edge along two sides. The photo below shows the main mass of NWA 1068.

 

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

 

standby for nwa 1068 photo
Photo courtesy of B. Fectay and C. Bidaut—Meteorite.fr


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Dho 019

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 vol%) of the low-Ca pyroxene pigeonite with rims of augite, along with a large content (25 vol%) of maskelynite (from shock conversion of plagioclase feldspar) and a small amount (9 vol%) of olivine (Borg et al., 2001). Accessory components include chromite, ulvöspinel, ilmenite, chlorapatite, merrillite, magnetite, and pyrrhotite, along with secondary weathering products including calcite, sulfates, celestine (SrSO4), barite, Fe-hydroxides, and Mg-phyllosilicates (Shukolyukov et al., 2002; Hallis et al., 2017). Rounded, zoned (smectite–calcite–gypsum) ‘orangettes’ similar to those found in ALH 84001 are present within maskelynite fractures; however, those in Dhofar 019 were determined by Hallis et al. (2017) to be of terrestrial origin through dissolution/deposition processes over its lengthy residence in the desert. Similar orange-colored features present in the shergottite SaU 094 have also been atributed to terrestrial alteration (Gnos et al., 2002). <!–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.

 

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.
*A new shergottite subgroup was proposed which comprises the shergottites having 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 at the time of their study) 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