Search

The Northwest Africa (NWA) 773 clan of meteorites is a group of paired and/or petrogenetically related stones that contain at least six different lithologies: magnesian gabbro, ferroan gabbro, anorthositic gabbro, olivine phyric basalt, regolith breccia, and polymict breccia. Uranium-lead dates of baddeleyite in the magnesian gabbro, ferroan gabbro, and components within breccia lithologies of paired lunar meteorites NWA 773, NWA 3170, NWA 6950, and NWA 7007 indicate a chronologic link among the meteorites and their components. A total of 50 baddeleyite grains were analyzed and yielded weighted average 207Pb-206Pb dates of 3119.4 ± 9.4 (n = 27), 3108 ± 20 (n = 13), and 3113 ± 15 (n = 10) Ma for the magnesian gabbro, ferroan gabbro, and polymict breccia lithologies, respectively. A weighted average date of 3115.6 ± 6.8 Ma (n = 47/50) was calculated from the baddeleyite dates for all lithologies. A single large zircon grain found in a lithic clast in the polymict breccia of NWA 773 yielded a U-Pb concordia date of 3953 ± 18 Ma, indicating a much more ancient source for some of the components within the breccia. A U-Pb concordia date of apatite and merrillite grains from the magnesian gabbro and polymict breccia lithologies in NWA 773 is 3112 ± 33 Ma, identical to the baddeleyite dates. Magnesian and ferroan gabbros, as well as the dated baddeleyite and Ca-phosphate-bearing detritus in the breccia lithologies, formed during the same igneous event at about 3115 Ma. These data also strengthen proposed petrogenetic connections between magnesian and ferroan gabbro lithologies, which represent some of the youngest igneous rocks known from the Moon.

Mars Reconnaissance Orbiter HiRISE images and Opportunity rover observations of the ~22 km wide Noachian age Endeavour Crater on Mars show that the rim and surrounding terrains were densely fractured during the impact crater-forming event. Fractures have also propagated upward into the overlying Burns formation sandstones. Opportunity’s observations show that the western crater rim segment, called Murray Ridge, is composed of impact breccias with basaltic compositions, as well as occasional fracture-filling calcium sulfate veins. Cook Haven, a gentle depression on Murray Ridge, and the site where Opportunity spent its sixth winter, exposes highly fractured, recessive outcrops that have relatively high concentrations of S and Cl, consistent with modest aqueous alteration. Opportunity’s rover wheels serendipitously excavated and overturned several small rocks from a Cook Haven fracture zone. Extensive measurement campaigns were conducted on two of them: Pinnacle Island and Stuart Island. These rocks have the highest concentrations of Mn and S measured to date by Opportunity and occur as a relatively bright sulfate-rich coating on basaltic rock, capped by a thin deposit of one or more dark Mn oxide phases intermixed with sulfate minerals. We infer from these unique Pinnacle Island and Stuart Island rock measurements that subsurface precipitation of sulfate-dominated coatings was followed by an interval of partial dissolution and reaction with one or more strong oxidants (e.g., O_2) to produce the Mn oxide mineral(s) intermixed with sulfate-rich salt coatings. In contrast to arid regions on Earth, where Mn oxides are widely incorporated into coatings on surface rocks, our results demonstrate that on Mars the most likely place to deposit and preserve Mn oxides was in fracture zones where migrating fluids intersected surface oxidants, forming precipitates shielded from subsequent physical erosion.

We have analyzed by thermal ionization mass spectrometry (TIMS) the isotopic composition of Cr in five progressive etches of size-sorted plagioclase grains separated from lunar soils 60601 and 62281. Aliquots of the etch solutions were spiked for isotopic dilution (ID) analysis of Cr and Ca. The Ca ID data indicate that the initial etch steps represent dissolution of an average 0.1 to 0.2 m depth from the grain surfaces, the approximate depth expected for implanted solar wind. The Cr/Ca ratio in the initial etches is several fold higher than that expected for bulk plagioclase composition, but in subsequent etches decreases to approach the bulk value. This indicates a source of Cr extrinsic to the plagioclase grains, surface-correlated and resident in the outermost fraction of a m, which we provisionally identify as solar wind Cr. The surface-correlated Cr is isotopically anomalous and by conventional TIMS data reduction has approximately 1 permil excess 54 Cr and half as great excess 53 Cr. In successive etches, as the Cr/Ca ratio decreases and approaches the bulk plagioclase value, the magnitude of the apparent anomalies decreases approaching normal composition. If these results do indeed characterize the solar wind, then either the solar wind is enriched in Cr due to spallation in the solar atmosphere, or the Earth and the various parent bodies of the meteorites are isotopically distinct from the Sun and must have formed from slightly different mixes of presolar materials. Alternative interpretations include the possibility that the anomalous Cr is meteoritic rather than solar or that the observed (solar) Cr is normal except for a small admixture of spallation Cr generated on the Moon. We consider these latter possibilities less likely than the solar wind interpretation. However, they cannot be eliminated and remain working hypotheses. Copyright © 2003 Elsevier Ltd

This study reports the use of Raman spectral features to characterize the structural and compositional characteristics of different types of pyroxene from rocks as might be carried out using a portable field spectrometer or by planetary on-surface exploration. Samples studied include lunar rocks, martian meteorites, and terrestrial rocks. The major structural types of quadrilateral pyroxene can be identified using their Raman spectral pattern and peak positions. Values of Mg/(Mg + Fe + Ca) of pyroxene in the (Mg, Fe, Ca) quadrilateral can be determined within an accuracy of ± 0.1. The precision for Ca/(Mg + Fe + Ca) values derived from Raman data is about the same, except that corrections must be made for very low-Ca and very high-Ca samples. Pyroxenes from basalts can be distinguished from those in plutonic equivalents from the distribution of their Mg ′ [Mg/(Mg + Fe)] and Wo values, and this can be readily done using point-counting Raman measurements on unprepared rock samples. The correlation of Raman peak positions and spectral pattern provides criteria to distinguish pyroxenes with high proportions of non-quadrilateral components from (Mg, Fe, Ca) quadrilateral pyroxenes.

Correspondence analysis is a dual Q and R mode factor analysis that allows simultaneous visualization of relationships between normalized variables and samples. The usefulness of the technique is first illustrated with an analysis of a simulated feldspar compositional data set and then applied to analysis of patterns among variables and samples for compositions obtained for Viking Lander 1 soils and Pathfinder rocks and soils. For the Martian data the first two factors capture 90% of the variance. Data projected onto the plane defined by these two factors fall within a mixing triangle defined by three geologically plausible end-members: basaltic shergottite, andesite, and a halide-kieserite evaporite. However, examination of data projected onto the second and third factors shows that the compositions are systematically displaced from the mixing triangle plane in the same general direction as the Fe vector. Inclusion of a Fe oxide (e.g., hematite or maghemite) as a reasonable fourth end-member forms a mixing tetrahedron that encapsulates the data. Least squares mixing analysis of the data with these four end-members and the elemental suite measured at both landing sites (Si, Al, Ti, Fe, Mg, Ca, S, Cl) implies that aeolian dust on rocks is a physical mixture of basalt and andesite-derived materials in approximately equal proportions, soil components at the two landing sites are similar to each other and less basalt-like than the aeolian dust, and Viking 1 soils are more andesite-rich than the Pathfinder soils.

Additional co-authors: Thanasis ECONOMOU, Steven P. GOREVAN, Brian C. HAHN, Gostar KLINGELHOFER, Timothy J. McCOY, Harry Y. McSWEEN Jr, Douglas W. MING, Richard V. MORRIS, Daniel S. RODIONOV, Steven W. SQUYRES, Heinrich WANKE, Shawn P. WRIGHT, Michael B. WYATT, Albert S. YEN

Merrillite, Ca18(Ca,o)Mg2(PO4)14-Ca18 Na2Mg2(PO4)14-Ca16REE2(Mg,Fe)2(PO4)14 occurs as a primary phosphate along with apatite, in lunar and martian rocks, and in meteorites. It is nomi- nally anhydrous, but attempts to directly measure H in this mineral have not previously been re- ported. Because of the occurrence on Earth of whitlockite, Ca18(Mg,Fe 2+ )2(PO4)12(HPO4)2, and the apparent incorporation in whitlockite of a merrillite component, the lack of a whitlockite com- ponent in extraterrestrial merrillite could be taken as an indicator of low hydrogen fugacity, and this implication has been applied to lunar merrillite. On the other hand, for martian rocks, where magmat- ic OH or H2O contents were likely higher, apatite accordingly contains higher OH contents, yet coex- ists with anhydrous, Na-rich merrillite. With direct measurements by SIMS, McCubbin et al. (2014), which is in the July issue of American Mineralo- gist (p. 1347-1354), show that Shergotty merrillite is anhydrous and infer that the high T of crystal- lization of Shergotty precluded incorporation of a whitlockite component. The mineral pair apatite- merrillite in extraterrestrial rocks constitutes a powerful pair for recording magmatic conditions; however, as McCubbin et al. show, the implications of these minerals and their compositions must be interpreted in light of careful and complete analy- ses and crystal chemical constraints. Keywords: Merrillite, apatite, shergotty, planetary materials

Northwest Africa (NWA) 2727 and NWA 3170, two different stones of the same lunar meteorite, are mineralogically and chemically similar [1,2]. Both are breccias that contain an apparently related suite of lithic and mineral clasts. To investigate petrogenetic relationships, we have determined mineral compositional zoning characteristics with Electron Probe Microanalysis (EPMA). The mineralogy includes alkali feldspar (K, Ba, Na), plagioclase, pyroxene (zoned Mg-Fe-Ca), olivine (Fe-rich), and minor amounts of Fe-Ti oxides, phosphates, and troilite. Alteration (Ca-carbonate fracture fillings) occurs in both meteorites, resulting from hot-desert weathering. In the lithic clasts, mineral-chemical zoning trends, e.g., in pyroxene, are especially useful for comparing lithic clasts and constraining their origins. We used quantitative compositional mapping to study compositional variations, especially for pyroxenes as diagnostic of petrogenetic relationships. We find that quantitative compositional maps extend the characterization of mineral-chemical trends beyond that obtained by EPMA spot analyses.

[1] We have used visible and near-infrared Panoramic Camera (Pancam) spectral data acquired by the Opportunity rover to analyze 15 rock fragments at the Meridiani Planum landing site. These spectral results were then compared to geochemistry measurements made by the in situ instruments Mossbauer (MB) and Alpha Particle X-ray Spectrometer (APXS) to determine the feasibility of mineralogic characterization from Pancam data. Our results suggest that dust and alteration rinds coat many rock fragments, which limits our ability to adequately measure the mineralogy of some rocks from Pancam spectra relative to the different field of view and penetration depths of MB and APXS. Viewing and lighting geometry, along with sampling size, also complicate the spectral characterization of the rocks. Rock fragments with the same geochemistry of sulfate-rich outcrops have similar spectra, although the sulfate-rich composition cannot be ascertained based upon Pancam spectra alone. FeNi meteorites have spectral characteristics, particularly ferric oxide coatings, that generally differentiate them from other rocks at the landing site. Stony meteorites and impact fragments with unknown compositions have a diverse range of spectral properties and are not well constrained nor diagnostic in Pancam data. Bounce Rock, with its unique basalt composition, is easily differentiated in the Pancam data from all other rock types at Meridiani Planum. Our Pancam analyses of small pebbles adjacent to these 15 rock fragments suggests that other rock types may exist at the landing site but have not yet been geochemically measured.

Multispectral imaging from the Panoramic Camera (Pancam) instruments on the Mars Exploration Rovers Spirit and Opportunity has provided important new insights about the geology and geologic history of the rover landing sites and traverse locations in Gusev crater and Meridiani Planum. Pancam observations from near-UV to near-IR wavelengths provide limited compositional and mineralogic constraints on the presence abundance, and physical properties of ferric- and ferrous-iron bearing minerals in rocks, soils, and dust at both sites. High resolution and stereo morphologic observations have also helped to infer some aspects of the composition of these materials at both sites. Perhaps most importantly, Pancam observations were often efficiently and effectively used to discover and select the relatively small number of places where in situ measurements were performed by the rover instruments, thus supporting and enabling the much more quantitative mineralogic discoveries made using elemental chemistry and mineralogy data. This chapter summarizes the major compositionally- and mineralogically-relevant results at Gusev and Meridiani derived from Pancam observations. Classes of materials encountered in Gusev crater include outcrop rocks, float rocks, cobbles, clasts, soils, dust, rock grindings, rock coatings, windblown drift deposits, and exhumed whitish/yellowish salty soils. Materials studied in Meridiani Planum include sedimentary outcrop rocks, rock rinds, fracture fills, hematite spherules, cobbles, rock fragments, meteorites, soils, and windblown drift deposits. This chapter also previews the results of a number of coordinated observations between Pancam and other rover-based and Mars-orbital instruments that were designed to provide complementary new information and constraints on the mineralogy and physical properties of martian surface materials.

Veneers and thicker rinds that coat outcrop surfaces and partially cemented fracture fills formed perpendicular to bedding document relatively late stage alteration of ancient sedimentary rocks at Meridiani Planum, Mars. The chemistry of submillimeter thick, buff-colored veneers reflects multiple processes at work since the establishment of the current plains surface. Veneer composition is dominated by the mixing of silicate-rich dust and sulfate-rich outcrop surface, but it has also been influenced by mineral precipitation, including NaCl, and possibly by limited physical or chemical weathering of sulfate minerals. Competing processes of chemical alteration (perhaps mediated by thin films of water or water vapor beneath blanketing soils) and sandblasting of exposed outcrop surfaces determine the current distribution of veneers. Dark-toned rinds several millimeters thick reflect more extensive surface alteration but also indicate combined dust admixture, halite precipitation, and possible minor sulfate removal. Cemented fracture fills that are differentially resistant to erosion occur along the margins of linear fracture systems possibly related to impact. These appear to reflect limited groundwater activity along the margins of fractures, cementing mechanically introduced fill derived principally from outcrop rocks. The limited thickness and spatial distribution of these three features suggest that aqueous activity has been rare and transient or has operated at exceedingly low rates during the protracted interval since outcropping Meridiani strata were exposed on the plains surface.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

The soils at the Opportunity site are fine-grained basaltic sands mixed with dust and sulfate-rich outcrop debris. Hematite is concentrated in spherules eroded from the strata. Ongoing saltation exhumes the spherules and their fragments, concentrating them at the surface. Spherules emerge from soils coated, perhaps from subsurface cementation, by salts. Two types of vesicular clasts may represent basaltic sand sources. Eolian ripples, armored by well-sorted hematite-rich grains, pervade Meridiani Planum. The thickness of the soil on the plain is estimated to be about a meter. The flatness and thin cover suggest that the plain may represent the original sedimentary surface.

The scientific exploration of the lunar surface addresses important questions for understanding the origin of the solar system, the relationship of the Moon to the Earth, fundamental processes of planetary formation and evolution such as planetary melting, volcanism and impact, and the history of ancient and modern impact events and radiation fluxes in the inner solar system. Gaining this understanding will require much more intensive surface exploration than has been possible in the past, with a very large number of interesting sites scattered over the whole lunar surface. Attaining global access for the Moon will be expensive. A purely robotic strategy probably cannot be sustained with the current priorities associated with lunar science. A robotic outpost strategy could be more cost-effective if it contains a long-range surface mobility component. Humans would add to the outpost capability by extending the life of exploration systems and by selecting samples for intensive study on Earth. A human outpost program could significantly lower costs of lunar scientific exploration by reducing transportation costs, providing scientific capability at marginal costs, and by improving performance of scientific instrumentation and systems through on-site operations.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Arising from: T. M. McCollom & B. M. Hynek Nature 438, 1129–1131 (2005); McCollom & Hynek reply. The Mars Exploration Rover Opportunity discovered sulphate-rich sedimentary rocks at Meridiani Planum on Mars, which are interpreted by McCollom and Hynek1 as altered volcanic rocks. However, their conclusions are derived from an incorrect representation of our depositional model2, 3, which is upheld by more recent Rover data4, 5, 6, 7. We contend that all the available data still support an aeolian and aqueous sedimentary origin for Meridiani bedrock.

[1] In the May 2000 Field Integrated Design and Operations (FIDO) Mars Rover Field Test, an off-site science team was charged with the goal of characterizing the geology of a previously unknown field location by commanding and collecting observations from a robotic rover, similar to those planned for future Mars missions. One unique feature of this test was the degree to which the science team was able to make use of previously acquired overhead remote-sensing data together with ground-based observations by the rover. In particular, an Airborne Visible/Infrared Imaging Spectrometer scene covering the region of the field test was used in concert with observations from the rover's near-infrared point spectrometer to develop a coherent and essentially correct geologic story for the test site. Using the overhead data, the science team was able to form hypotheses that were testable using rover observations on the ground. The team was able to establish the provenance of exotic samples within reach of the rover by making a spectroscopic link to units observed from above. The science team was also able to “extend” the spectral range of the rover's onboard spectrometer via inferred connections to units mapped from above with greater spectral range. The net result of the success of this rover trial is an increase in experience and confidence for NASA's Mars mission strategy, which features strong ties between early orbital reconnaissance missions for geologic mapping and subsequent rover missions to the surface.