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At least 253 records · Page 14
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NASA NTRS1991

Iddingsite in the Nakhla meteorite: TEM study of mineralogy and texture of pre-terrestrial (Martian?) alterations

Rusty-colored veinlets and patches in the Nakhla meteorite, identified as iddingsite, are pre-terrestrial. The rusty material is iddingsite (smectites + hematite + ferrihydrite); like terrestrial iddingsites, it probably formed during low-temperature interaction of olivine and water. Fragments of rusty material with host olivine were removed from thin sections of Nakhla with a tungsten needle. Fragments were embedded in epoxy, microtomed to 100 nanometers thickness, and mounted on Cu grids. Phase identifications were by Analytical Electron Microscopy/Energy Dispersive X-ray Analysis (EM/EDX) standardless chemical analyses (for silicates), electron diffraction (hematite and ferrihydrite), and lattice fringe imaging. This iddingsite in Nakhla is nearly identical to some formed on Earth, suggesting similar conditions of formation on the Shergottites-Nakhlites-Chassigny (SNC) meteorite parent planet. A more detailed account of the results is presented.

Treiman, Allan H.↗
NASA NTRS1992

The stable isotopic compositions of indigenous carbon-bearing components in EETA 79001

It is now widely accepted that the most likely source of SNC meteorites is Mars. An oblique impact on Mars, or vaporization of permafrost, by an impactor seem to be the most likely ejection mechanisms capable of accelerating material to the 5 km/s velocity needed to overcome the gravitational field of Mars. These ejection mechanisms involve a large shock event in the SNC class, the shergottites EETA 79001 and ALHA 77055 are the most likely shocked samples, in which whole rock pressures of 35 to 45 GPa have been estimated. Martian weathering products have also been identified in EETA 79001. Here, the author started a series of analyses of EETA 79001 using a high-sensitivity static mass spectrometer capable of measuring sub-nanogram quantities of carbon. Recent measurements of lithology C confirm that the shock-implanted atmospheric CO2 is released during the 1100 to 1200 C step.

Hartmetz, C. P.↗
NASA NTRS1992

Recent Elysium volcanism: Effects on the Martian atmosphere

A vast area of smooth plains in southeastern Elysium near 5 deg N, 195 deg was identified as young volcanic plains. These plains represent flood eruptions of very low viscosity lavas at a point in time much later than the volcanism that produced the major volcanoes of Elysium. Since volcanism is a principal mechanism by which gases are supplied to the atmosphere, and since these gases can have important climatic effects, it is important to consider the amount of H2O, CO2, and other gases that were possibly released. We present estimates of input and release rates from this volcanic episode. The age is defined by the density of superposed craters; there are 89 +/- 15 craters greater than or equal to 1 km per 10(exp 6) sq km corresponding to the Upper Amazonian making this the youngest major volcanic episode. Lavas, covering 1.5 x 10(exp 6) sq km and presumably erupted through fissures, filled a topographic low and then flowed through knobby terrain into westernmost Amazonis. The western plains are characterized by scattered low shields having radial lava flows. Unit thickness is uncertain, individual flows are about 10 m thick; the entire deposit buries older knobby terrain having local relief of 200-400 m (estimated by Earth-based radar data); hence the volcanics must be thick enough to bury that relief. On the basis of unit morphology, length of flows, and the extent of the deposit, it can be inferred that the lava was of low viscosity and mafic to ultramafic composition. The age and petrology of the shergottites suggest they may have come from this unit.

Plescia, J. B.↗
NASA NTRS1992

Mid-infrared spectra of Martian komantiite

Synthetic mid-infrared emissive spectra of basalts of possible Martian komatiitic compositions were generated to show what might be seen in data returned from the Thermal Emission Spectrometer (TES). TES is flying on the Mars Observer arriving at Mars in late 1993. The synthetic Martian komatiite spectra were generated using mineralogy based on Shergottite, Nakhlite, and Chassigny (SNC) meteorites and Viking X-Ray Fluorescence (XRF) data. These data sets provide strong evidence for the existence of ultramafic lavas on Mars, particularly lavas of komatiitic composition.

Reyes, David P.↗
NASA NTRS1992

Aqueous-alteration products in S-N-C meteorites and implications for volatile/regolith interactions on Mars

The shergottite, nakhlite, and Chassigny (SNC) meteorites are inferred to be samples from the Martian surface, and so provide a wealth of information on Martian petrology and geochemistry. Because the SNCs are igneous rocks, it has not been obvious that they could reveal much about low-temperature geochemical processes and atmosphere surface interactions on Mars. However, five of the eight SNCs are known to contain low-temperature hydrous minerals and related phase of likely martian origin. Here, we review these martian weathering and alteration products in SNCs and outline possible implications for volatile/regolith interactions and regolith sinks for volatiles on Mars.

Treiman, A. H.↗
NASA NTRS1992

Zagami - Product of a two-stage magmatic history

Results of petrologic studies of new large samples of the Zagami shergottite are presented. Pyroxene crystals have homogeneous Mg-rich pigeonite and augite cores, overgrown by Fe-rich zoned pyroxene rims. Amphibole-bearing magmatic inclusions occur exclusively in the cores. It is concluded that Zagami experienced a two-stage crystallization history. The first stage occurred in a deep-seated, slowly cooling magma chamber. There, the homogeneous Mg-rich cores of the pyroxenes crystallized during relatively slow cooling. During the second stage, the Mg-rich pyroxenes were entrained into a magma that either intruded to the near-surface and cooled in a relatively thin dike or sill, or extruded to the surface and crystallized in a lava flow greater than 10 m thick, again without indications of crystal settling. The estimated depth of the magma chamber for Zagami of greater than 7.5 km and thickness of the putative lava flow of greater than 10 m are consistent with calculations and observations of volcanic constructs and flows in the Tharsis region of Mars.

Mccoy, Timothy J.↗
NASA NTRS1992

Workshop on chemical weathering on Mars, part 2

The third Mars Surface and Atmosphere Through Time (MSATT) Workshop, which was held 10-12 Sep. 1992, at Cocoa Beach/Cape Kennedy, focused on chemical weathering of the surface of Mars. The 30 papers presented at the workshop described studies of Martian weathering processes based on results from the Viking mission experiments, remote sensing spectroscopic measurements, studies of the shergottite, nakhlite, and chassignite (SNC) meteorites, laboratory measurements of surface analog materials, and modeling of reaction pathways. A summary of the technical sessions is presented and a list of workshop participants is included.

Burns, Roger↗
NASA NTRS1992

Aqueous geochemistry on Mars: Possible clues from salts and clays in SNC meteorites

All subgroups of the shergottite, nakhlite, and chassignite (SNC) meteorites contain traces of water precipitated minerals that include various combinations of carbonates, sulfates, halides, ferric oxides, and aluminosilicate clays of preterrestrial origin. Oxygen three-isotope analysis of thermally extracted bulk water has confirmed that at least some of the water in SNC's is, indeed, extraterrestrial. A mixture of aqueous precipitates found in the SNC's, comprising smectite, illite, and gypsum (with minor halite +/- calcite and hematite), provides a self-consistent, though not unique, model for the bulk elemental composition of surface sediments at the Viking Lander sites. Therefore, if the salts and clays in SNC's are truly linked to aqueous alteration and soil formation on Mars, then the suite of SNC secondary minerals might provide the best currently available insight into near-surface martian chemistry.

Gooding, James L.↗
NASA NTRS1992

Soil mineralogy and chemistry on Mars - Possible clues from salts and clays in SNC meteorites

If the shergottite, nakhlite, and chassignite (SNC) meteorites' parent planet is Mars, then the aqueous precipitates found in them imply that oxidizing, water-based solutions may have been chemically active on that planet over the past 200-1300 million yrs. It is suggested that the mixture of aqueous precipitates found in the SNCs furnish a self-consistent model for the bulk elemental composition of surface sediments at the Viking Lander sites. Further mineralogical and stable-isotope studies of the secondary minerals may establish the limits for biological activity over the last 1300 million years of Mars' water-based chemistry.

Gooding, James L.↗
NASA NTRS1993

Outgassed water on Mars - Constraints from melt inclusions in SNC meteorites

The SNC (shergottite-nakhlite-chassignite) meteorites, thought to be igneous rocks from Mars, contain melt inclusions trapped at depth in early-formed crystals. Determination of the pre-eruptive water contents of SNC parental magmas from calculations of the solidification histories of these amphibole-bearing inclusions indicates that Martian magmas commonly contained 1.4 percent water by weight. When combined with an estimate of the volume of igneous materials on Mars, this information suggests that the total amount of water outgassed since 3.9 billion years ago corresponds to global depths on the order of 200 meters. This value is significantly higher than previous geochemical estimates but lower than estimates based on erosion by floods. These results imply a wetter Mars interior than has been previously thought and support suggestions of significant outgassing before formation of a stable crust or heterogeneous accretion of a veneer of cometary matter.

Mcsween, Harry Y., Jr.↗
NASA NTRS1993

Moessbauer spectroscopy of the SNC meteorite Zagami

We have performed Mossbauer spectroscopy on two different pieces of the meteorite Zagami belonging to the group of SNC meteorites. In one of the samples we found a substantial amount of olivine inter grown with one kind of pyroxene, and also another kind of pyroxene very similar to the pyroxene in the other sample we examined. Both samples showed less than 1 percent of Fe(3+) in the silicate phase. The group of SNC meteorites called shergottites, to which Zagami belongs, are achondrites whose texture, mineralogy and composition resembles those of terrestrial diabases. The results from the investigation are presented.

Agerkvist, D. P.↗
NASA NTRS1993

Noble gas evidence of an aqueous reservoir near the surface of Mars more recently than 1.3 Ga

Considerable evidence points to a Martian origin of the SNC meteorites. One of these meteorites, Nakhla, contains a leachable component which has an elevated Xe-129/Xe-132 ratio relative to its Kr-84/Xe-132 ratio when compared to the approximately linear array defined by Chassigny, most shergottites, and lithology C of EETA 79001. This array is thought to be a mixing line between Martian mantle and Martian atmosphere. The leachable component probably consists in part of iddingsite, an alteration product produced by interaction of olivine with aqueous fluid at temperatures lower than 150 C. The radiogenic Xe component may represent a distinct reservoir in the Martian crust or mantle. More plausibly, it is Martian atmosphere, fractionated by solution in liquid water and by interaction with sediment. The crystallization age of Nakhla is 1.3 Ga. Its low shock state suggests that it was ejected from near the surface of Mars. Liquid water is required for the formation of iddingsite. These observations provide further evidence for the near surface existence of aqueous fluids more recently than 1.3 Ga.

Drake, Michael J.↗
NASA NTRS1993

Nitrates in SNCs: Implications for the nitrogen cycle on Mars

Nitrogen is the second most abundant constituent of the Martian atmosphere, after CO2, present at a level of ca. 2.7 percent. Several authors have hypothesized that earlier in the planet's history, nitrogen was more abundant, but has been removed by processes such as exospheric loss from the atmosphere. However, an alternative sink for atmospheric nitrogen is the regolith; model calculations have predicted that, via the formation of NOx, HNO2 and HNO3 in the lower layers of the Martian atmosphere, the regolith might trap nitrite and nitrate anions, leading to the build-up of involatile nitrates. Integrated over 4.5 x 10(exp 9) yr, such a mechanism would contribute the equivalent of a layer of nitrates up to 0.3 cm thick distributed across the Martian surface. Features in thermal emission spectra of the surface of Mars have been interpreted tentatively as emanating from various anions (carbonates, bicarbonates, sulphates, etc.), and the presence of nitrates has also been addressed as a possibility. The identification of carbonates in SCN meteorites has allowed inferences to be drawn concerning the composition and evolution of the Martian atmosphere in terms of its carbon isotope systematics; if nitrites, nitrates, or other nitrogen-bearing salts could be isolated from SNC's, similar conclusions might be possible for an analogous nitrogen cycle. Nitrates are unstable, being readily soluble in water, and decomposed at temperatures between 50 C and 600 C, depending on composition. Any nitrates present in SNC's might be removed during ejection from the planet's surface, passage to Earth, or during the sample's terrestrial history, by weathering etc. The same might have been said for carbonates, but pockets of shock-produced glass (lithology C) from within the EET A79001 shergottite and bulk samples of other SNC contain this mineral, which did apparently survive. Nitrates occurring within the glassy melt pockets of lithology C in EET A79001 might likewise be protected. Lithology C glass was therefore selected for nitrate analysis, first by non-destructive infra red spectroscopy, and then by stepped combustion.

Grady, Monica M.↗
NASA NTRS1993

High D/H ratios of water in magmatic amphiboles in Chassigny: Possible constraints on the isotopic composition of magmatic water on Mars

The D/H ratios of kaersutitic amphiboles contained in magmatic inclusions in the Shergottites Nakhlites Chassignites (SNC) meteorite Chassigny using the ion microprobe were measured. A lower limit on the delta(D(sub SMOW)) of the amphiboles is +1420 +/- 47 percent. Assuming Chassigny comes from Mars and the amphiboles have not been subject to alteration after their crystallization, this result implies either that recycling of D-enriched Martian atmosphere-derived waters into the planetary interior has taken place, or that the primordial hydrogen isotopic composition of the interior of Mars differs significantly from that of the Earth (delta(D(sub SMOW)) approximately 0 percent). In addition, the measurements indicate that the amphiboles contain less than 0.3 wt. percent water. This is much lower than published estimates, and indicates a less-hydrous Chassigny parent magma than previously suggested.

Watson, L. L.↗
NASA NTRS1993

Further carbon isotope measurements of LEW 88516

Douglas et al. have previously analyzed the carbon content and isotopic composition of a crushed sample (sub-sample 13) of the shergottite, LEW 88516. The powder, which was from a relatively large portion of the meteorite in order to obtain a representative sample, was distributed amongst the scientific community. However, it is probable that the preparation procedure was not optimized for the purposes of carbon measurements. Indeed, it was found that LEW 88516,13 contained over 1200 ppm carbon, a concentration which is greater than that present in any other SNC meteorite. That a close relative, ALH A77005, contains only 141 ppm carbon seems to implicate the preparation procedure as being responsible for the apparently high carbon content of LEW 88516. However, it may also be the nature of the fine powder which has resulted in contamination. The observation of high carbon content in LEW 88516,13 highlights the extreme difficulty of trying to obtain representative samples of whole meteorites for this kind of investigation. Presented herein are some further measurements of LEW 88516 which should serve to clarify some of the issues raised by the previous investigation.

Wright, I. P.↗
NASA NTRS1993

Magmatic volatiles and the weathering of Mars

The sources for volatiles on Mars have been the subject of many hypotheses for exogenous influences including late accretion of volatile-enriched material, impact devolatilization to create massive early atmospheres, and even major bombardment by comets. However, the inventory of chemically active volatiles observable at the contemporary surface of Mars is consistent with domination by endogenous, subsequent planetary processes, viz., persistent magmatic outgassing. Volcanism on Mars has been widespread in both space and time. Notwithstanding important specific differences between the mantles of Earth and Mars, the geochemical similarities are such that the suite of gases emitted from Martian volcanic activity should include H2O, CO2, S-containing gases (e.g. H2S and/or SO2), and Cl-containing gases (e.g., Cl2 and/or HCl). H2O and CO2 exist in the atmosphere of Mars. Both are also present as surface condensates. However, spectroscopic observations of the Martian atmosphere clearly show that the S- and Cl-containing gases are severely depleted, with upper limits of less than or equal to 10(exp -7) the abundance of CO2. Likewise, there is no evidence of polar condensates of compounds of these elements as there is for CO2 and H2O. Within the soil, on the other hand, there has been direct measurement of incorporated H2O and abundant compounds containing S and Cl. Barring some as yet implausible geochemical sequestering process, the S/Cl ratio of about 6:1 in Martian soils implies a limit of 5% on the contribution of matter of solarlike composition (e.g., carbonaceous chondrite or cometary material) to these volatiles. Hence, exogenous sources are minor or not yet observed. From analysis of elemental trends in Martian soils, it has been recently shown that a simple two-component model can satisfy the Viking in situ measurements. Component A includes Si and most or all the Al, Ca, Ti, and Fe. Component B, taken as 16 +/- 3% by weight of the total, contains S and most or all the Cl and Mg. These results constrain several models of Martian soil mineralogy but are consistent with a mixture of silicates (such as Fe-rich clays and accessory minerals and soluble salts). The overall element profile is notably like shergottites, with significant incorporation of chemically reactive atmospheric gases from magmatic degassing.

Clark, B. C.↗
NASA NTRS1994

ALH84001, a cumulate orthopyroxenite member of the Martian meteorite clan

ALH84001, originally classified as a diogenite, is a coarse-grained, cataclastic, orthopyroxenite meteorite related to the martian (SNC) meteorites. The orthopyroxene is relatively uniform in composition, with a mean composition of Wo3.3En69.4Fs27.3. Minor phases are euhedral to subhedral chromite and interstitial maskelynite, An31.1Ab63.2Or5.7, with accessory augite, Wo42.2En45.1Fs12.7, apatite, pyrite and carbonates, Cc11.5Mg58.0Sd29.4Rd1.1. The pyroxenes and chromites in ALH84001 are similar in composition to these phases in EETA79001 lithology a megacrysts but are more homogeneous. Maskelynite is similar in composition to feldspars in the nakhlites and Chassigny. Two generations of carbonates are present, early (pre-shock) strongly zoned carbonates and late (post-shock) carbonates. The high Ca content of both types of carbonates indicates that they were formed at moderately high temperature, possibly approximately 700 C. ALH84001 has a slightly LREE-depleted pattern with La 0.67x and Lu 1.85x CI abundances and with a negative Eu anomaly (Eu/Sm 0.56x CI). The uniform pyroxene composition is unusual for martian meteorites, and suggests that ALH84001 cooled more slowly than did the shergottites, nakhlites of Chassigny. The nearly monomineralic composition, coarse-grain size, homogeneous orthopyroxene and chromite compositions, the interstitial maskelynite and apatite, and the REE pattern suggest that ALH84001 is a cumulate orthopyroxenite containing minor trapped, intercumulus material.

Mittlefehldt, David W.↗
NASA NTRS1994

Comparison of the LEW88516 and ALHA77005 martian meteorites: Similar but distinct

By mineral and bulk compositions, the Lewis Cliff (LEW) 88516 meteorite is quite similar to the ALHA77005 martian meteorite. These two meteorites are not paired because their mineral compositions are distinct, they were found 500 km apart in ice fields with different sources for meteorites, and their terrestrial residence ages are different. Minerals in LEW88516 include: olivine, pyroxenes (low- and high-Ca), and maskelynite (ater plagioclase); and the minor minerals chromite, whitlockite, ilmenite, and pyrrhotite. Mineral grains in LEW88516 range up to a few mm. Texturally, the meteorite is complex, with regions of olivine and chromite poikilitically enclosed in pyroxene, regions of interstitial basaltic texture, and glass-rich (shock) veinlets. Olivine compositions range from Fo(sub 64) to Fo(sub 70), (avg. Fo(sub 67)), more ferroan and with more variation than in ALHA77005 (Fo(sub 69) to Fo(sub 73)). Pyroxene compositions fall between En(sub 77)Wo(sub 4) and En(sub 65)Wo(sub 15) and in clusters near En(sub 63)Wo(sub 9) and En(sub 53)Wo(sub 33), on average more magnesian and with more variation than in ALHA77005. Shock features in LEW88516 range from weak deformation through complete melting. Bulk chemical analyses by modal recombination of electron microprobe analyses, instrumental neutron activation, and radiochemical neutron activation confirm that LEW88516 is more closely related to ALHA77005 than to other known martian meteorites. Key element abundance ratios are typical of martian meteorites, as is it nonchondritic rare earth pattern. Differences between the chemical compositions of LEW88516 and ALHA77005 are consistent with slight differences in the proportions of their constituent minerals and not from fundamental petrogenetic differences. Noble gas abundances in LEW88516, like those in ALHA77005, show modest excesses of Ar-40 and Xe-129 from trapped (shock-implanted) gas. As with other ALHA77005 and the shergottite martian meteorites (except EETA79001), noble gas isotope abundances in LEW88516 are consistent with exposure to cosmic rays for 2.5-3 Ma. The absence of substantial effects of shielding from cosmic rays suggest LEW88516 spent this time as an object no larger than a few cm in diameter.

Treiman, A. H.↗