Mineralogy and petrology of the Dar al Gani 476 martian meteorite: Implications for its cooling history and relationship to other shergottites

Abstract— Dar al Gani 476, the 13th martian meteorite, was recovered from the Sahara in 1998. It is a basaltic shergottitic rock composed of olivine megacrysts reaching 5 mm (24 vol%) set in a finegrained groundmass of pyroxene (59 vol%) and maskelynitized plagioclase (12 vol%) with minor amounts of accessory phases (spinel, merrillite, ilmenite). Dar al Gani 476 is similar to lithology A of Elephant Moraine A79001 (EETA79001) in petrography and mineralogy, but is distinct in several aspects. Low‐Ca pyroxenes in the Dar al Gani 476 groundmass are more magnesian (En₇₆Fs₂₁ Wo₃～En₅₈Fs₃₀Wo₁₂) than those in lithology A of EETA79001 (En₇₃Fs₂₂Wo₅～En₄₅Fs₄₃Wo₁₂), rather similar to pyroxenes in lherzolitic martian meteorites (En₇₆Fs₂₁ Wo₃～En₆₃Fs₂₂Wo₁₅). Dar al Gani 476 olivine is less magnesian and shows a narrower compositional range (Fo_76‐58) than EETA79001 olivine (Fo_81‐53), and is also similar to olivines in lherzolitic martian meteorites (Fo_74‐65). The orthopyroxene‐olivine‐chromite xenolith typical in the lithology A of EETA79001 is absent in Dar al Gani 476. It seems that Dar al Gani 476 crystallized from a slightly more primitive mafic magma than lithology A of EETA79001 and several phases (olivine, pyroxene, chromite, and ilmenite) in Dar al Gani 476 may have petrogenetic similarities to those of lherzolitic martian meteorites. Olivine megacrysts in Dar al Gani 476 are in disequilibrium with the bulk composition. The presence of fractured olivine grains in which the most Mg‐rich parts are in contact with the groundmass suggests that little diffusive modification of original olivine compositions occurred during cooling. This observation enabled us to estimate the cooling rates of Dar al Gani 476 and EETA79001 olivines, giving similar cooling rates of 0.03‐3 °C/h for Dar al Gani 476 and 0.05‐5 °C/h for EETA79001. This suggests that they were cooled near the surface (burial depth shallower than about 3 m at most), probably in lava flows during crystallization of groundmass. As is proposed for lithology A of EETA79001, it may be possible to consider that Dar al Gani 476 has an impact melt origin, a mixture of martian lherzolite and other martian rock (Queen Alexandra Range 94201, nakhlites?).     

Petrology and chemistry of the new shergottite Dar al Gani 476

Abstract— In 1998, Dar al Gani (DaG) 476 was found in the Libyan desert. The meteorite is classified as a basaltic shergottite and is only the 13th martian meteorite known to date. It has a porphyritic texture consisting of a fine‐grained groundmass and larger olivines. The groundmass consists of pyroxene and feldspathic glass. Minor phases are oxides and sulfides as well as phosphates. The presence of olivine, orthopyroxene, and chromite is a feature that DaG 476 has in common with lithology A of Elephant Moraine (EET) A79001. However, in DaG 476, these phases appear to be early phenocrysts rather than xenocrysts. Shock features, such as twinning, mosaicism, and impact‐melt pockets, are ubiquitous. Terrestrial weathering was severe and led to formation of carbonate veins following grain boundaries and cracks. With a molar MgO/(MgO + FeO) of 0.68, DaG 476 is the most magnesian member among the basaltic shergottites. Compositions of augite and pigeonite and some of the bulk element concentrations are intermediate between those of lherzolitic and basaltic shergottites. However, major elements, such as Fe and Ti, as well as LREE concentrations are considerably lower than in other shergottites. Noble gas concentrations are low and dominated by the mantle component previously found in Chassigny. A component, similar to that representing martian atmosphere, is virtually absent. The ejection age of 1.35 ± 0.10 Ma is older than that of EETA79001 and could possibly mark a distinct ejection. Dar al Gani 476 is classified as a basaltic shergottite based on its mineralogy. It has a fine‐grained groundmass consisting of clinopyroxene, pigeonite and augite, feldspathic glass and chromite, Ti‐chromite, ilmenite, sulfides, and whitlockite. Isolated olivine and single chromite grains occur in the groundmass. Orthopyroxene forms cores of some pigeonite grains. Shock‐features, such as shock‐twinning, mosaicism, cracks, and impact‐melt pockets, are abundant. Severe weathering in the Sahara led to significant formation of carbonate veins crosscutting the entire meteorite. Dar al Gani 476 is distinct from other known shergottites. Chemically, it is the most magnesian member among known basaltic shergottites and intermediate in composition for most trace and major elements between Iherzolitic and basaltic shergottites. Unique are the very low bulk REE element abundances. The CI‐normalized abundances of LREEs are even lower than those of Iherzolitic shergottites. The overall abundance pattern, however, is similar to that of QUE 94201. Textural evidence indicates that orthopyroxene, as well as olivine and chromite, crystallized as phenocrysts from a magma similar in composition to that of bulk DaG 476. Whether such a magma composition can be a shergottite parent melt or was formed by impact melting needs to be explored further. At this time, it cannot entirely be ruled out that these phases represent relics of disaggregated xenoliths that were incorporated and partially assimilated by a basaltic melt, although the texture does not support this possibility. Trapped noble gas concentrations are low and dominated by a Chassigny‐like mantle component. Virtually no martian atmosphere was trapped in DaG 476 whole‐rock splits. The exposure age of 1.26 ± 0.09 Ma is younger than that of most shergottites and closer to that of EETA79001. The ejection age of 1.35 ± 0.1 Ma could mark another distinct impact event.     

Petrogenesis of lunar highlands meteorites: Dhofar 025, Dhofar 081, Dar al Gani 262, and Dar al Gani 400

Abstract— The petrogenesis of four lunar highlands meteorites, Dhofar 025 (Dho 025), Dhofar 081 (Dho 081), Dar al Gani 262 (DaG 262), and Dar al Gani 400 (DaG 400) were studied. For Dho 025, measured oxygen isotopic values and Fe‐Mn ratios for mafic minerals provide corroboratory evidence that it originated on the Moon. Similarly, Fe‐Mn ratios in the mafic minerals of Dho 081 indicate lunar origin. Lithologies in Dho 025 and Dho 081 include lithic clasts, granulites, and mineral fragments. A large number of lithic clasts have plagioclase AN# and coexisting mafic mineral Mg# that plot within the “gap” separating ferroan anorthosite suite (FAN) and high‐magnesium suite (HMS) rocks. This is consistent with whole rock Ti‐Sm ratios for Dho 025, Dho 081, and DaG 262, which are also intermediate compared to FAN and HMS lithologies. Although ion microprobe analyses performed on Dho 025, Dho 081, DaG 262, and DaG 400 clasts and minerals show far stronger FAN affinities than whole rock data suggest, most clasts indicate admixture of ≤12% HMS component based on geochemical modeling. In addition, coexisting plagioclase‐pyroxene REE concentration ratios in several clasts were compared to experimentally determined plagioclase‐pyroxene REE distribution coefficient ratios. Two Dho 025 clasts have concordant plagioclase‐pyroxene profiles, indicating that equilibrium between these minerals has been sustained despite shock metamorphism. One clast has an intermediate FAN‐HMS composition. These lunar meteorites appear to represent a type of highland terrain that differs substantially from the KREEP‐signatured impact breccias that dominate the lunar database. From remote sensing data, it is inferred that the lunar far side appears to have appropriate geochemical signatures and lithologies to be the source regions for these rocks; although, the near side cannot be completely excluded as a possibility. If these rocks are, indeed, from the far side, their geochemical characteristics may have far‐reaching implications for our current scientific understanding of the Moon.     

A new martian meteorite from the Sahara: The shergottite Dar al Gani 489

Abstract— Dar al Gani 489 (DaG 489) is a meteorite fragment of 2146 g found in the Libyan Sahara by a meteorite finder during one of his search campaigns in 1997–98. It is a porphyritic rock with millimetersized olivine crystals (Fo_79–59) set in a fine‐grained groundmass (average grain size 0.1 mm) consisting of pigeonite (En_75–57 Wo_5–15) crystals and interstitial feldspathic glass (An_67–56 Or_0–1). Minor phases include enstatite (En_82–71 Wo_2–4), augite (En_48–52 Wo_29–32), chromite, Ti‐chromite, ilmenite, pyrrhotite, merrillite, and secondary calcite and iron oxides. On the basis of mineralogical, petrographic, bulk chemical, O‐isotopic, and noble gas data, DaG 489 can be classified as a highly shocked martian meteorite (e.g., Fe/Mn_(bulk) = 42.1, Ni/Mg_(bulk) = 0.002; δ¹⁷O = 2.89, δ¹⁸O = 4.98, and Δ¹⁷O = 0.305), belonging to the basaltic shergottite subgroup. The texture and modal composition of DaG 489 are indeed those of basalts; nonetheless, the bulk chemistry, the abundance of large olivine and chromite crystals, and enstatitic pyroxene suggest some relationship with lherzolitic shergottites. As such, DaG 489 is similar to the hybrid shergottite Elephant Moraine (EET) A79001 lithology A; however, there are some relevant differences including a higher olivine content (20 vol%), the lack of orthopyroxene megacrysts, a higher molar Mg/(Mg + Fe)_(molar) = 0.68, and a lower rare earth element content in the bulk sample. Therefore, DaG 489 has the potential of providing us with a further petrogenetic link between the basaltic and lherzolitic shergottites. Noble gases data show that DaG 489 has an ejection age of ～1.3 Ma. This young age lends support to the requirement of several ejection events to produce the current population of shergottites, nakhlites, and chassignites (SNC) meteorites. In terms of texture, mineral and bulk compositions, shock level, and weathering features, DaG 489 is essentially identical to DaG 476, another basaltic shergottite independently found ～25 km due northnortheast of DaG 489. Because DaG 489 also has the same exposure history as DaG 476, it is very likely that both meteorites are fragments of the same fall. In addition to the existing hypotheses on the petrogenesis of the similar EETA79001 lithology A and the identical DaG 476, we propose that DaG 489 could have formed through high‐degree partial melting of a lherzolite‐like material.     

Defining the mechanisms that disturb the Sm‐Nd isotopic systematics of the Martian meteorites: Examples from Dar al Gani 476 and Allan Hills 77005

Abstract— Microbeam studies of Martian meteorites Dar al Gani (DaG) 476 and Allan Hills (ALH) 77005 have been conducted to identify potential causes of disequilibrium exhibited in their Sm‐Nd isotopic systematics. Olivine and maskelynite mineral fractions on the DaG 476 isochron are displaced relative to their positions as dictated by measured mineral compositions. The olivine mineral fractions from ALH 77005 not only have a relatively low Sm/Nd ratio, but appear to contain an unradiogenic component that shifts the olivine mineral fraction off the isochron defined by the pyroxene and maskelynite mineral fractions. Trace components such as melt inclusions, impact melt, high‐Si mesostasis, and altered olivine were analyzed using scanning electron microscopy, quantitative electron microscopy, and secondary ion mass spectrometry to determine their potential for disturbing the isotopic systematics of the mineral fractions, assuming that the mineral fractions were not completely pure. Mixing models indicate that the presence of melt inclusions in the DaG 476 olivine mineral fraction lowered its Sm/Nd ratio. The maskelynite mineral fraction contains a related but more evolved mesostasis component that raised the Sm/Nd ratio of the fraction. The position of two olivine mineral fractions below the ALH 77005 isochron is interpreted to reflect small additions of impact melt with a light rare earth element enriched pattern and a non‐indigenous, unradiogenic Nd component. Furthermore, the presence of rare earth elements in olivine and maskelynite from both igneous and non‐igneous components such as melt inclusions, mesostasis, and impact melt is observed on a fine (<30 μm) scale. Despite the addition of this material, the Sm‐Nd ages are not affected. This study demonstrates that detailed mineral separation procedures as employed by modern geochronology laboratories permit reliable ages to be derived from shocked and altered samples.     

The Dar al Gani meteorite field (Libyan Sahara): Geological setting,pairing of meteorites,and recovery density

Abstract— As of July 2001, 1238 Libyan meteorites have been reported. Most were found in two areas called Dar al Gani and Hamadah al Hamra. Dar al Gani is located on a plateau of marine carbonate rocks with marly components. Eight‐hundred and sixty‐nine meteorites between 6 g and 95 kg totalling 687 kg have been found here but the calculated mean recovery density is comparatively low with one meteorite on 6.5 km². Dar al Gani is a perfect site for the recognition and preservation of meteorites. The existence of meteorites is the result of a combination of specific geological and geomorphological conditions: there is a bright‐colored, old limestone plateau (<2 Ma), under arid weather conditions over long periods of time, with rapid elimination of surface water if present and low erosion rates. The preservation of meteorites is guaranteed through the absence of quartz sand on the plateau, strongly reducing wind erosion and a basic environment emerging from the carbonate ground retards rusting of metallic meteorite components. A supposed soil cover during pluvial times has probably protected older meteorites and led to a concentration of meteorites of different periods. An evaluation of Dar al Gani meteorites suggests the existence of at least 26 strewnfields and 26 meteorite pairs reducing the number of falls to, at most, 534. Shock and weathering grades as a tool for the recognition of pairings turned out to be problematic, as several strewnfields showed paired meteorites which had been classified to different shock and weathering grades.     

Effects of liquid immiscibility on trace element fractionation in magmatic iron meteorites: A case study of group IIIAB

Abstract— Magmatic iron meteorites are generally agreed to represent metal that crystallized in asteroidal cores from a large pool of liquid. Estimates suggest that the metallic liquid contained significant amounts of S and P, both of which are incompatible and exert a strong effect on trace element partitioning. In tandem, S and P are also prone to cause immiscibility between sulfide liquid and P-rich metal liquid. The liquid immiscibility field occupies ～70% of the portion of the Fe-Ni-S-P system in which Fe is the first phase to crystallize. In spite of this, previous fractional crystallization models have taken into account only one liquid phase and have encountered significant discrepancies between the meteorite data and model values for the key elements Ni, Ir, Ga, Ge and Au at even moderate degrees of fractionation. For the first time, a model for trace element partitioning between immiscible liquids in the Fe-Ni-S-P system is presented in order to assess the effects on fractionation in magmatic iron meteorite groups. The onset of liquid immiscibility causes a significant change in the enrichment patterns of S and P in both liquids; so elements with contrasting partitioning behavior will show trends deviating clearly from one-liquid trends. A trend recorded in the solid metal will either be a smooth curve as long as equilibrium is maintained between the two liquids or the trend may diverge into a field limited by two extreme curves depending on the degree of disequilibrium. Bulk initial liquids for most magmatic groups have S/P (wt%) ratios well below 25. In these cases and due to the constitution of the Fe-Ni-S-P system, most of the metal will crystallize from the rapidly decreasing volume of metal liquid and only a subordinate amount from the sulfide liquid. Because of the strong extraction of P into the metal liquid, P will have a much larger influence on trace element partitioning than a low initial P content might suggest. My model calculations suggest that liquid immiscibility played a significant role during the solidification of the IIIAB parent body's core. The two-liquid model reproduces the IIIAB trends more closely than previous one-liquid models and can account for: (a) the general widening of the IIIAB trend with increasing Ni and decreasing Ir contents, (b) the occurrence of high-Ni members that are not strongly depleted in Ir, Ga and Ge; and (c) an upper limit at ～11 wt% Ni where the metal liquid was almost consumed.     

Petrogenesis of basaltic shergottite Northwest Africa 8657: Implications for fO2 correlations and element redistribution during shock melting in shergottites

Northwest Africa (NWA) 8657 is an incompatible trace element-enriched, low-Al basaltic shergottite, similar in texture and chemistry to Shergotty, Zagami, and NWA 5298. It is composed of zoned pyroxene, maskelynite, merrillite, and Ti-oxide minerals with minor apatite, silica, and pyrrhotite. Pyroxene grains are characterized by patchy zoning, with pigeonite or augite cores zoned to Fe-rich pigeonite mantles. The cores have rounded morphologies and irregular margins. Combined with the low Ti/Al of the cores, the morphology and chemistry of the pyroxene grains are consistent with initial crystallization at depth (30–70 km) followed by partial resorption en route to the surface. Enriched rare earth element (REE) equilibrium melt compositions and calculated oxygen fugacities (fO₂) conditions for pigeonite cores indicate that the original parent melts were enriched shergottite magmas that staged in chambers at depth within the Martian crust. NWA 8657 does not represent a liquid but rather entrained a proportion of pyroxene crystals from magma chambers where fractional crystallization was occurring at depth. Variation between fO₂ and bulk-rock (La/Yb)_N of the enriched and intermediate shergottites suggests that oxidation conditions and degree of incompatible element enrichment in the source may not be correlated, as thought previously. Shock melt pockets are characterized by an absence of phosphates and oxide minerals. It is likely that these phases were melted during shock. REEs were redistributed during this process into maskelynite and to a lesser extent the shock melt; however, the overall normalized REE profile of the shock melt is like that of the bulk-rock, but at lower absolute concentrations. Overall, shock melting has had a significant effect on the mineralogy of NWA 8657, especially the distribution of phosphates, which may be significant for geochronological applications of this meteorite and other Martian meteorites with extensive shock melt.     

A petrologic study of the IAB iron meteorites: Constraints on the formation of the IAB‐Winonaite parent body

Abstract— We studied 26 IAB iron meteorites containing silicate‐bearing inclusions to better constrain the many diverse hypotheses for the formation of this complex group. These meteorites contain inclusions that fall broadly into five types: (1) sulfide‐rich, composed primarily of troilite and containing abundant embedded silicates; (2) nonchondritic, silicate‐rich, comprised of basaltic, troctolitic, and peridotitic mineralogies; (3) angular, chondritic silicate‐rich, the most common type, with approximately chondritic mineralogy and most closely resembling the winonaites in composition and texture; (4) rounded, often graphite‐rich assemblages that sometimes contain silicates; and (5) phosphate‐bearing inclusions with phosphates generally found in contact with the metallic host. Similarities in mineralogy and mineral and O‐isotopic compositions suggest that IAB iron and winonaite meteorites are from the same parent body. We propose a hypothesis for the origin of IAB iron meteorites that combines some aspects of previous formation models for these meteorites. We suggest that the precursor parent body was chondritic, although unlike any known chondrite group. Metamorphism, partial melting, and incomplete differentiation (i.e., incomplete separation of melt from residue) produced metallic, sulfide‐rich and silicate partial melts (portions of which may have crystallized prior to the mixing event), as well as metamorphosed chondritic materials and residues. Catastrophic impact breakup and reassembly of the debris while near the peak temperature mixed materials from various depths into the re‐accreted parent body. Thus, molten metal from depth was mixed with near‐surface silicate rock, resulting in the formation of silicate‐rich IAB iron and winonaite meteorites. Results of smoothed particle hydrodynamic model calculations support the feasibility of such a mixing mechanism. Not all of the metal melt bodies were mixed with silicate materials during this impact and reaccretion event, and these are now represented by silicate‐free IAB iron meteorites. Ages of silicate inclusions and winonaites of 4.40‐4.54 Ga indicate this entire process occurred early in solar system history.     

Expanding the application of the Eu‐oxybarometer to the lherzolitic shergottites and nakhlites: Implications for the oxidation state heterogeneity of the Martian interior

Abstract— Experimentally rehomogenized melt inclusions from the nakhlite Miller Range 03346 (MIL 03346) and the lherzolitic shergottite Allan Hills 77005 (ALH 77005) have been analyzed for their rare earth element (REE) concentrations in order to characterize the early melt compositions of these Martian meteorites and to calculate the oxygen fugacity conditions they crystallized under. D(Eu/Sm)_{pyroxene/melt} values were measured at 0.77 and 1.05 for ALH 77005 and MIL 03346, respectively. These melts and their associated whole rock compositions have similar REE patterns, suggesting that whole rock REE values are representative of those of the early melts and can be used as input into the pyroxene Eu‐oxybarometer for the nakhlites and lherzolitic shergottites. Crystallization fO₂ values of IW + 1.1 (ALH 77005) and IW + 3.2 (MIL 03346) were calculated. Whole rock data from other nakhlites and lherzolitic shergottites was input into the Eu‐oxybarometer to determine their crystallization fO₂ values. The lherzolitic shergottites and nakhlites have fO₂ values that range from IW + 0.4 to 1.6 and from IW + 1.1 to 3.2, respectively. These values are consistent with some previously determined fO₂ estimates and expand the known range of fO₂ values of the Martian interior to four orders of magnitude. The origins of this range are not well constrained. Possible mechanisms for producing this spread in fO₂ values include mineral/melt fractionation, assimilation, shock effects, and magma ocean crystallization processes. Mineral/melt partitioning can result in changes in fO₂ from the start to the finish of crystallization of 2 orders of magnitude. In addition, crystallization of a Martian magma ocean with reasonable initial water content results in oxidized, water‐rich, late‐stage cumulates. Sampling of these oxidized cumulates or interactions between reduced melts and the oxidized material can potentially account for the range of fO₂ values observed in the Martian meteorites.     

Roosevelt County 075: A petrologic,chemical and isotopic study of the most unequilibrated known H chondrite

Abstract— Roosevelt County (RC) 075 was recovered in 1990 as a single 258-gram stone. Classification of this meteorite is complicated by its highly unequilibrated nature and its severe terrestrial weathering, but we favor H classification. This is supported by O isotopes and estimates of the original Fe, Ni metal content. The O isotopic composition is similar to that of a number of reduced ordinary chondrites (e.g., Cerro los Calvos, Willaroy), although RC 075 exhibits no evidence of reduced mineral compositions. Chondrule diameters are consistent with classification as an L chondrite, but large uncertainties in chondrule diameters of RC 075 and poorly constrained means of H, L and LL chondrites prevent use of this parameter for reliable classification. Other parameters are compromised by severe weathering (e.g., siderophile element abundances) or unsuitable for discrimination between unequilibrated H, L and LL chondrites (e.g., Co in kamacite, δ¹³C). Petrologic subtype 3.2± 0.1 is suggested by the degree of olivine heterogeneity, the compositions of chondrule olivines, the thermoluminescence sensitivity, the abundances and types of chondrules mapped on cathodoluminescence mosaics, and the amount of presolar SiC. The meteorite is very weakly shocked (S2), with some chondrules essentially unshocked and, thus, is classified as an H3.2(S2) chondrite. Weathering is evident by a LREE enrichment due to clay contamination, reduced levels of many siderophile elements, the almost total loss of Fe, Ni metal and troilite, and the reduced concentrations of noble gases. Some components of the meteorite (e.g., type IA chondrules, SiC) appear to preserve their nebular states, with little modification from thermal metamorphism. We conclude that RC 075 is the most unequilibrated H chondrite yet recovered and may provide additional insights into the origin of primitive materials in the solar nebula.     

Formation of a martian pyroxenite: A comparative study of the nakhlite meteorites and Theo's Flow

Abstract— The unusual composition of the nakhlites, a group of pyroxenitic martian meteorites with young ages, presents an opportunity to learn about nonbasaltic magmatic activity on another planet. However, the limited number of these meteorites makes unraveling their history difficult. A promising terrestrial analog for the formation of the nakhlites is Theo's Flow in Ontario, Canada. This atypical, 120 m-thick flow differentiated in place, forming distinct layered lithologies of peridotite, pyroxenite, and gabbro. Theo's pyroxenite and the nakhlites share strikingly similar petrographies, with concentrated euhedral to subhedral augite grains set in a plagioclase-rich matrix. These two suites of rocks also share specific petrologic features, mineral and whole-rock compositional features, and size and spatial distributions of cumulus grains. The numerous similarities suggest that the nakhlites formed by a similar mechanism in a surface lava flow or shallow intrusion. Their formation could have involved settling of crystals in a phenocryst-laden flow or in situ nucleation and growth of pyroxenes in an ultramafic lava flow. The latter case is more likely and requires steady-state nucleation and growth of clusters of pyroxene grains (and olivine in the nakhlites), circulating in a strongly convecting melt pool, followed by settling and continued growth in a thickening cumulate pile. Trapped pockets of intercumulus liquid in the pile gradually evolved, finally growing Fe-enriched rims on cumulus grains. With sufficient evolution, the melt reached plagioclase supersaturation, causing rapid growth of plagioclase sprays and late-stage mesostasis growth.     

Major and trace element compositions of georgiaites: Clues to the source of North American tektites

Abstract— Electron microprobe and laser ablation, inductively‐coupled plasma mass spectrometer analyses of 24 georgiaites show that these tektites are all Si‐rich (79–83 wt% SiO₂) glasses with variable major and trace element abundances (e.g., FeO varies from 2.1 to 3.7 wt%). Glass compositions are similar to but not identical with average upper continental crust. For example, georgiaites are light rare earth element enriched with small negative Eu anomalies (Eu/Eu*=0.73‐0.86) and La‐Th‐Sc systematics are intermediate between that of Archean and post‐Archean continental crust. When the georgiaite data are placed in the context of data for all North American tektites, triangular arrays appear on some oxide‐oxide plots (e.g., FeO‐MgO). Large variations in refractory element abundances and ratios compared to the variation in SiO₂ favors mixing over volatilization as a cause of the compositional variation. If all the tektites formed as a result of a single impact, then triangular arrays in oxide‐oxide variation diagrams require at least three source components. These components include a Si‐rich material, probably a quartz‐rich sand that was predominant in the formation of georgiaites. Two relatively silica‐poor and Fe‐rich components have compositional characteristics similar to shales and greywackes. The La‐Th‐Sc systematics of the georgiaites and most other North American tektites are distinctive and could potentially be used to link the tektites to Eocene sediments at the Chesapeake Bay impact structure.     

A new Martian meteorite from Oman: Mineralogy,petrology, and shock metamorphism of olivine‐phyric basaltic shergottite Sayh al Uhaymir 150

Abstract— The Sayh al Uhaymir (SaU) 150 meteorite was found on a gravel plateau, 43.3 km south of Ghaba, Oman, on October 8, 2002. Oxygen isotope (δ¹⁷O 2.78; δ¹⁸O 4.74), CRE age (?1.3 Ma), and noble gas studies confirm its Martian origin. SaU 150 is classified as an olivine‐phyric basalt, having a porphyritic texture with olivine macrocrysts set in a finer‐grained matrix of pigeonite and interstitial maskelynite, with minor augite, spinel, ilmenite, merrillite, pyrrhotite, pentlandite, and secondary (terrestrial) calcite and iron oxides. The bulk rock composition, in particular mg (68) [molar Mg/(Mg + Fe) x 100], Fe/Mn (37.9), and Na/Al (0.22), are characteristic of Martian meteorites. Based on mineral compositions, cooling rates determined from crystal morphology, and crystal size distribution, it is deduced that the parent magma formed in a steady‐state growth regime (magma chamber) that cooled at <°C/hr. Subsequent eruption as a thick lava flow or hypabyssal intrusion entrained a small fraction of xenocrystic olivine and gave rise to a magmatic foliation, with slow cooling allowing for near homogenization of igneous minerals. SaU 150 experienced an equilibration shock pressure of 33–45 GPa in a single impact event. Post‐shock heat gave rise to localized melting (?11 vol%). Larger volume melts remained fluid after pressure release and crystallized dendritic olivine and pyroxene with fractal dimensions of 1.80–1.89 and 1.89–1.95, respectively, at ‐ΔT >70–365 °C. SaU 150 is essentially identical to SaU 005/094, all representing samples of the same fall that are similar to, but distinct from, the DaG shergottites.     

An experimental study of silver and palladium partitioning between solid and liquid metal,with applications to iron meteorites

Abstract— Solid metal/liquid metal partition coefficients for Ag and Pd were determined experimentally as a function of the S concentration of the metallic liquid. Silver is incompatible in solid metal and strongly sensitive to the S content of the metallic liquid; partition coefficients for Ag decrease more than an order of magnitude with increasing S content of the metallic liquid and can be expressed as: where k(Ag) is the molar solid metal/liquid metal partition coefficient and X_S is the molar S content of the metallic liquid. The partition coefficient of Pd is less variable but changes from modestly incompatible to modestly compatible in solid metal with increasing S content of the metallic liquid: With these new partition coefficients for Pd and a fractional crystallization model, Pd abundance trends recorded in iron meteorite groups are modeled successfully. Measured Ag distribution between troilite-rich nodules and adjacent metal in iron meteorites also agree well with experimental solid metal/liquid metal equilibrium values. However, observed Pd metal/nodule distributions do not agree with experimentally determined partition coefficients, which suggests a more complex history than simple solid metal/liquid metal equilibrium.     

An overview of the new Moroccan regulation on collection and export of meteorites: A geoheritage to promote and preserve

Morocco is known for the high number of meteorites collected in its territory, including finds and falls. This is explained by the large size of the Moroccan Sahara, the guarantee of security in this desert, and the large community of well-trained Moroccan hunters and nomads who roam through it. Despite this richness, most meteorites collected in Morocco are sold abroad and exported. The exportation of meteorites as well as other geoheritage samples such as fossils and minerals was not completely legal or illegal as there was no dedicated regulation. Since 2000, the APPGM (Association pour la Protection du Patrimoine Géologique du Maroc) a Non-Governmental Organization (NGO) dedicated to the preservation of the Moroccan geoheritage began working with the Moroccan Geological Survey, on a draft of a specific law dedicated to geoheritage. It was fundamental to benefit from the experience of other countries with a high number of meteorites where exportation is not allowed and that are losing their meteorites to illegal exportation. The author recommended a win-win regulation that would allow the legal collection and exportation of meteorites under clear rules benefiting both hunters and scientists but also the country. In 2014, Morocco updated its law regarding mines. One article cited geoheritage as including fossils, minerals, and meteorites and mentioned that their collection and exportation would be regulated by decree. In 2019, the Moroccan Geological Survey and APPGM prepared the application decree of this article that has been discussed and approved by the Moroccan government and implemented in February 2020. This situation is unique in the region as well as compared to the other countries with a high potential of meteorites collection. Meteorite researchers and collectors all over the world should be aware of this regulation in Morocco to make their acquisitions legal. They should request a copy of the “End of the work” from local traders, the receipt from the Geological survey, and the certificate of export from customs. It is an important ethical and scientific responsibility of our community.     

Searching for nonlocal lithologies in the Apollo 12 regolith: A geochemical and petrological study of basaltic coarse fines from the Apollo lunar soil sample 12023,155

New data from a petrological and geochemical examination of 12 coarse basaltic fines from the Apollo 12 soil sample 12023,155 provide evidence of additional geochemical diversity at the landing site. In addition to the bulk chemical composition, major, minor, and trace element analyses of mineral phases are employed to ascertain how these samples relate to the Apollo 12 lithological basalt groups, thereby overcoming the problems of representativeness of small samples. All of the samples studied are low‐Ti basalts (0.9–5.7 wt% TiO₂), and many fall into the established olivine, pigeonite, and ilmenite classification of Apollo 12 basaltic suites. There are five exceptions: sample 12023,155_1A is mineralogically and compositionally distinct from other Apollo 12 basalt types, with low pigeonite REE concentrations and low Ni (41–55 ppm) and Mn (2400–2556 ppm) concentrations in olivine. Sample 12023,155_11A is also unique, with Fe‐rich mineral compositions and low bulk Mg# (=100 × atomic Mg/[Mg+Fe]) of 21.6. Sample 12023,155_7A has different plagioclase chemistry and crystallization trends as well as a wider range of olivine Mg# (34–55) compared with other Apollo 12 basalts, and shows greater similarities to Apollo 14 high‐Al basalts. Two other samples (12023,155_4A, and _5A) are similar to the Apollo 12 feldspathic basalt 12038, providing additional evidence that feldspathic basalts represent a lava flow proximal to the Apollo 12 site rather than material introduced by impacts. We suggest that at least one parent magma, and possibly as many as four separate parent magmas, are required in addition to the previously identified olivine, pigeonite, and ilmenite basaltic suites to account for the observed chemical diversity of basalts found in this study.     

A nuclear microprobe study of the distribution and concentration of carbon and nitrogen in Murchison and Tagish Lake meteorites,Antarctic micrometeorites,and IDPs: Implications for astrobiology

Abstract— Using a nuclear microprobe, we measured the carbon and nitrogen concentrations and distributions in several interplanetary dust particles (IDPs) and Antarctic micrometeorites (MMs), and compared them to 2 carbonaceous chondrites: Tagish Lake and Murchison. We observed that IDPs are richest in both elements. All the MMs studied contain carbon, and all but the coarse‐grained and 1 melted MM contained nitrogen. We also observed a correlation in the distribution of carbon and nitrogen, suggesting that they may be held in an organic material. The implications for astrobiology of these results are discussed, as small extraterrestrial particles could have contributed to the origin of life on Earth by delivering important quantities of these 2 bio‐elements to the Earth's surface and their gas counterparts, CO₂ and N₂, to the early atmosphere.     

A stability study of asteroid families near the 3:1 and 5:2 resonance with Jupiter

By using theD-criterion Lindblad (1992) has identified 14 asteroid families from a sample of 4100 numbered asteroids with proper elements from Milani and Kneevi (1990). Taxonomic types and other physical properties for a significant number of objects in five of the families show strong homogeneity within each family, further strengthening their internal relationship.To test the hypothesis of a common origin in, e.g., a catastrophic collision event, we have set out to integrate the orbits of the members of the Maria, Dora and Oppavia-Gefion families over some 10⁶ years. The mean distance for the Maria family is close to the 3:1 mean-motion resonance with Jupiter, while the other two families lie close to the 5:2 resonance.We used a simplified solar system model which included the perturbations by Jupiter and Saturn only and implemented Everhart's variable stepsize integrator RA15. All close encounters between the family members (within 0.1 AU) were recorded as well. Preliminary results from integrations over 4×10⁵ years are presented here.The statistics of close encounters show pronounced peaks for several members within each family, while for others no significant levels above the background of random encounters or even very low frequencies were found. This indicates a subclustering within the families. Quite a lot of very close (<0.005 AU) mutual encounters are found, which suggest that, at least for the larger members in a family, the mutual gravitational interactions could be of some importance for the real orbital evolutions.The encounter statistics between the Dora and Oppavia family members suggest a possible interrelationship between this two groups.     

A multidisciplinary study of silica sinter deposits with applications to silica identification and detection of fossil life on Mars

Surface features observed on Mars and evidence from martian meteorites both suggest that hydrothermal systems have operated in the crust of the planet. Hydrothermal systems are a potential habitat for living organisms and identifying these on Mars is, therefore, important in the search for life beyond the Earth. One of the surface expressions of hydrothermal systems on Earth are silica sinters, deposited during the cooling of hydrothermal solutions. In this paper we present analyses of the mineralogy, textures, chemistry and organic chemistry of silica sinters from two very different geothermal provinces, Waiotapu, New Zealand and Haukadalur, Iceland, in order to determine common features by which silica sinters can be identified. Infrared reflectance spectroscopy was utilised in combination with textural studies to evaluate the mineralogy of sinter deposits in terms of the abundances of different polymorphs of SiO₂. Concentrations of organic molecules, principally lipids, within regions of the sinters in which there is textural evidence for micro-organisms were identified in the infrared spectral data and their presence was confirmed using gas chromatography mass spectroscopy. The results of this study indicate that reflectance spectra in the wavelength region from 2.5 to 14 μm, when calibrated against natural terrestrial analogues, can be used to identify silica sinters, as well as the possible presence of recent microbial communities on Mars.