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NASA NTRS2007

Ar-39-Ar-40 Ages of Two Nakhlites, MIL03346 and Y000593: A Detailed Analysis

Radiometric dating of martian nakhlites by several techniques have given similar ages of approx.1.2-1.4 Ga [e.g. 1, 2]. Unlike the case with shergottites, where the presence of martian atmosphere and inherited radiogenic Ar-40 produce apparent Ar-39-Ar-40 ages older than other radiometric ages, Ar-Ar ages of nakhlites are similar to ages derived by other techniques. However, even in some nakhlites the presence of trapped martian Ar produces some uncertainty in the Ar-Ar age. We present here an analysis of such Ar-Ar ages from the MIL03346 and Y000593 nakhlites.

Park, Jisun↗
NASA NTRS2007

Comparison of Synthetic and Natural Nakhlite Pyroxenes: Complexity of Minor Elements

Zoning in pyroxenes in martian meteorites contains a rich record of the petrogenesis of these samples. In the clinopyroxene cumulate nakhlite group, major element zoning is generally limited to the outer rims of the pyroxenes. However, minor element zoning, especially of Al, Ti, and Cr, is extensive, complex, and difficult to interpret [e.g., 1-3]. To help mine the rich information about petrogenetic processes from these samples, we have been comparing minor element zoning in synthetic pyroxenes grown under known conditions with zoning observed in natural nakhlite pyroxenes. We have focused on two nakhlites, MIL03346 (MIL), which is one of the most rapidly cooled nakhlites, and Y000593 (Y593), which cooled at a more moderate rate [e.g., 4].

McKay, G.↗
NASA NTRS2007

Al, Ti, and Cr: Complex Zoning in Synthetic and Natural Nakhlite Pyroxenes

Nakhlites are olivine-bearing clinopyroxene cumulates. The cumulus pyroxenes have cores that are relatively homogeneous in Fe, Mg, and Ca, but show complex zoning of minor elements, especially Al, Ti, and Cr. Zoning patterns contain information about crystallization history parent magma compositions. But it has proven difficult to decipher this information and translate the zoning patterns into petrogenetic processes. This abstract reports results of high-precision Electron Probe MicroAnalysis (EPMA) analysis of synthetic nakhlite pyroxenes run at fO2 from IW to QFM. It compares these with concurrent analyses of natural nakhlite MIL03346 (MIL), and with standardprecision analyses of Y000593 (Y593) collected earlier. Results suggest that (1) different processes are responsible for the zoning of MIL and other more slowly-cooled nakhlites such as Y593, and (2) changes in oxidation conditions during MIL crystallization are not responsible for the unusual Cr zoning pattern

McKay, G.↗
NASA NTRS2008

Duration of a Magma Ocean and Subsequent Mantle Overturn in Mars: Evidence from Nakhlites

It is now generally accepted that the heat produced by accretion, short-lived radioactive elements such as Al-26, and gravitational energy from core formation was sufficient to at least partially melt the silicate portions of terrestrial planets resulting in a global-scale magma ocean. More particularly, in Mars, the geochemical signatures displayed by shergottites, are likely inherited from the crystallization of this magma ocean. Using the short-lived chronometer Sm-146 - Nd-142 (t(sup 1/2) = 103 Myr), the duration of the Martian magma ocean (MMO) has been evaluated to being less than 40 Myr, while recent and more precise ND-142/ND-144 data were used to evaluate the longevity of the MMO to approximately 100 Myr after the solar system formation. In addition, it has been proposed that the end of the crystallization of the MMO may have triggered a mantle overturn, as a result of a density gradient in the cumulate layers crystallized at different levels. Dating the mantle overturn could hence provide additional constraint on the duration of the MMO. Among SNC meteorites, nakhlites are characterized by high epsilon W-182 of approximately +3 and an epsilon Nd-142 similar to depleted shergottites of +0.6-0.9. It has hence been proposed that the source of nakhlites was established very early in Mars history (approximately 8-10 Myr). However, the times recorded in HF-182-W-182 isotope system, i.e. when 182Hf became effectively extinct (approximately 50 Myr after solar system formation) are less than closure times recorded in the Sm-146-Nd-142 isotope system (with a full coverage of approximately 500 Myr after solar system formation). This could result in decoupling between the present-day measured epsilon W-182 and epsilon Nd-142 as the SM-146 may have recorded later differentiation events in epsilon ND-142 not observed in epsilon W-182 values. With these potential complexities in short-lived chronological data for SNC's in mind, new Hf-176/Hf-177, Nd-143/Nd-144 and Nd-142/Nd-144 were obtained for three nakhlites (Nakhla, MIL03346 and Yamato000593). These new data are combined with previous epsilon W-182 data, to investigate potential discrepancies between the Hf-182-W-182 and Sm-146-Nd-142 systematics, and the relationship between the source of nakhlites and a crystallizing magma ocean

Debaille, V.↗
NASA NTRS2010

Sm-Nd Isotopic Studies of Two Nakhlites, NWA 5790 and Nakhla

NWA 5790 is a Martian meteorite recently found in the Mauritania part of the Saharan desert and is classified as a nakhlite, containing a small amount of interstitial plagioclase. Unlike other Martian meteorites ( e.g., shergottites), nakhlites have been only moderately shocked and their original igneous textures are still well-preserved. In this report, we present Sm-Nd isotopic data for NWA 5790 and Nakhla, a rare "fall" nakhlite, correlate their ages with those of other nakhlites and discuss their petrogenesis.

C.-Y. Shih↗
NASA NTRS2017

Multiple Igneous Bodies for Nakhlites and Chassignites as Inferred from Olivine Cooling Rates using Calcium Zoning

Nakhlites and chassignites are ultramafic cumulate rocks of clinopyroxene and olivine, respec-tively, considered to have been formed in a thick lava flow or shallow intrusion near the Martian surface [e.g., 1,2]. Although more than 100 Martian meteorites have been found so far, most of them are shergottites and only nine nakhlites and three chassignites are known (considering paired samples) [3]. In contrast to shergottites which show large variations in both mineralogy and ages, nakhlites and chassignites are suggested to have been petrogenetically related, crystallized at about the same time and been ejected by the same impact event because of their identical crystallization (approximately 1.3 Ga) and cosmic-ray exposure (10-11 My) ages [e.g., 1]. In this study we discuss the possibility of a common igneous body for all samples belonging to these two groups as suggested by previous studies [e.g., 4]. To do this we estimated cooling rates of olivine using Ca zoning profiles, especially by paying attention to the newest samples of each group (NWA 10720 nakhlite and NWA 8694 chassignite).

Mikouchi, T.↗
NASA NTRS1980

Comparative petrology and origin of Governador Valadares and other nakhlites

The Governador Valadares is a cumulate augite achondrite showing many petrographic and geochemical similarities to the other known nakhlites, Nakhla and Lafayette. However, subtle differences in minor-element composition and textures suggest that each nakhlite is unique and probably represents a separate meteorite fall. Cumulus clinopyroxene compositions (En39FS23WO38) suggest crystallization from a common silica-saturated (tholeiite-like) liquid. Intercumulus mesostasis material shows evidence for a two-stage cooling history (slow, followed by quenching) and may represent highly fractionated, foreign pore liquid filtered into present nakhlites by compaction of lower-level cumulates.

Berkley, J. L.↗
NASA NTRS2003

Crystallization and Alteration Ages of the Antarctic Nakhlite Yamato 000593

Nakhlites are unbrecciated, olivine-bearing clinopyroxenites that probably came from Mars. A total of eight nakhlites have been identified, including five recent finds: two samples (104 g and 456 g) from the hot desert of Morocco (NWA 817 and NWA 998), and three samples, a total weight of approx.15 kg, from the Yamato Mountains of Antarctica (Y000593, Y000749 and Y000802). Preliminary isotopic results for Y000593 have been given by Shih et al.. In this report, we present Rb-Sr and Sm-Nd isochron data for Y000593 and discuss the age correlation with other nakhlites and the timing of aqueous alteration on Mars.

Musawa, K.↗
NASA NTRS2006

Ar-Ar Dating of Martian Chassignites, NWA2737 and Chassigny, and Nakhlite MIL03346

Until recently only three nakhlites and one chassignite had been identified among martian meteorites. These four exhibit very similar radiometric ages and cosmic ray exposure (CRE) ages, indicating that they may have derived from a common location on Mars and were ejected into space by a single impact. This situation is quite different from that of martian shergottites, which exhibit a range of radiometric ages and CRE ages (1). Recently, several new nakhlites and a new martian dunite (NWA2737) have been recognized. Here we report our results of Ar-39-Ar-40 dating for the MIL03346 nakhlite and the NWA2737 "chassignite", along with new results on Chassigny.

Bogard, D. D.↗
NASA NTRS2011

The Miller Range Nakhlites: A Summary of the Curatorial Subdivision of the Main Mass in Light of Newly Found Paired Masses

The 2003-2004 ANSMET team re-covered a 715.2 g nakhlite from the Miller Range (MIL) region of the Transantarctic Mountains (MIL 03346). This was the first nakhlite for the US Antarctic meteorite program, and after the announcement in 2004 [1], JSC received over 50 requests for this sample for the Fall 2004 Meteorite Working Group meeting. Since then it has been subdivided into >200 splits, and distributed to approx.70 scientists around the world for study. The 2009-2010 ANSMET team recovered three additional masses of this nakhlite [2], making the total amount of mass 1.871 kg (Table 1). Given that the original find (MIL 03346) has been heavily studied and these new masses are available, we will present a comprehensive overview of the subdivision of the original mass as well as the scientific findings to date.

Righter, Kevin↗
NASA NTRS2017

Isotopic Composition of Carbonates in Antarctic Ordinary Chondrites and Miller Range Nakhlites: Insights into Martian Amazonian Aqueous Alteration

The martian surface contains features of ancient fluvial systems. Stable isotope analysis of carbonates that form in aqueous systems can reveal their formation conditions. The Nakhlite meteorites originally formed on Mars 1.3 Ga and were later exposed to aqueous fluids that left behind carbonate minerals [1], thus analysis of these carbonates can provide data to understand Amazonian climate conditions on Mars. Carbonates found in the Nakhlite meteorites contain a range of delta(sup 13)C values, which may be either martian carbonates or terrestrial contamination. To better under-stand terrestrial weathering products and martian carbonate formation processes, we conducted a set of carbonate isotope analyses on Antarctic meteorites focusing on Miller Range (MIL) Nakhlites as well as Ordinary Chondrites (OCs) (Figure 1)[1-11] [12]. OCs of petrology type H, L, and LL 3-6 were selected since they are not expected to contain preterrestrial carbonates, yet they have visible evaporite minerals on the fusion crust indicating terrestrial alteration. These cryogenically formed terrestrial carbonates may also provide an analog for cryogenic carbonate formation on Mars.

Evans, M. E.↗
NASA NTRS2017

Shergottite Lead Isotope Signature in Chassigny and the Nakhlites

The nakhlites/chassignites and the shergottites represent two differing suites of basaltic martian meteorites. The shergottites have ages less than or equal to 0.6 Ga and a large range of initial Sr-/Sr-86 and epsilon (Nd-143) ratios. Conversely, the nakhlites and chassignites cluster at 1.3-1.4 Ga and have a limited range of initial Sr-87/Sr-86 and epsilon (Nd-143). More importantly, the shergottites have epsilon (W-182) less than 1, whereas the nakhlites and chassignites have epsilon (W-182) approximately 3. This latter observation precludes the extraction of both meteorite groups from a single source region. However, recent Pb isotopic analyses indicate that there may have been interaction between shergottite and nakhlite/chassignite Pb reservoirs.Pb Analyses of Chassigny: Two different studies haveinvestigated 207Pb/204Pb vs. 206Pb/204Pb in Chassigny: (i)TIMS bulk-rock analyses of successive leaches and theirresidue [3]; and (ii) SIMS analysis of individual minerals[4]. The bulk-rock analyses fall along a regression of SIMSplagioclase analyses that define an errorchron that is olderthan the Solar System (4.61±0.1 Ga); i.e., these define amixing line between Chassigny’s principal Pb isotopic components(Fig. 1). Augites and olivines in Chassingy (notshown) also fall along or near the plagioclase regression [4].This agreement indicates that the whole-rock leachateslikely measure indigenous, martian Pb, not terrestrial contamination[5]. SIMS analyses of K-spars and sulfides definea separate, sub-parallel trend having higher 207Pb/206Pbvalues ([4]; Fig. 1). The good agreement between the bulkrockanalyses and the SIMS analyses of plagioclases alsoindicates that the Pb in the K-spars and sulfides cannot be amajor component of Chassigny.The depleted reservoir sampled by Chassigny plagioclaseis not the same as the solar system initial (PAT) andrequires a multi-stage origin. Here we show a two-stagemodel (Fig. 1) with a 238U/204Pb (μ) of 0.5 for 4.5-2.4 Gaand a μ of 7 for 2.4-1.4 Ga. This is not a unique model butdoes produce a Pb composition that falls on the plagioclaseregression at 1.4 Ga, the approximate igneous age of Chassigny [1]. It should be noted that low-μ single-stage modelsare not capable of producing sufficiently radiogenic 206Pb/204Pb at 1.4 Ga.Relation to Shergottites: The Chassigny K-spars and sulfides fall along a second mixing line defined by leachesand residues of depleted and intermediate shergottites [6]. This mixing line falls above the plagioclase regression.Therefore, we also interpret the radiogenic component of this mixing line to represent indigenous martian Pb. It ispossible that the depleted and intermediate shergottites and the Chassigny plagioclases sample radiogenic Pb from thethe same source, i.e., the mixing lines may intersect at high 206Pb/204Pb.Both K-spar and sulfide are late-stage phases. At the time of their crystallization, the Chassigny system appearsto have remained open to a depleted shergottite Pb reservoir. The depleted component of the shergottite mixing linecan be generated by a single-stage evolution from PAT (4.5 to 1.4 Ga) in a reservoir having a μ ~2. A similar modelfor the most depleted shergottites is also possible: μ = 1.5 for 4.5 to 0.3 Ga.Nakhlites: Nakhlite analyses plot between the shergottite and Chassigny plagioclase regressions [3]. So again,members of the nakhlite/chassignite suite show affinities to shergottite Pb.

Jones, J. H.↗
NASA NTRS2018

The Contrast in Outgassing of Germanium Between Shergottites and Nakhlites

Final Paper and not the abstract is attached. Introduction: Germanium is generally thought to follow Si in its geochemical behavior, but little data has existed to rigorously understand the behavior of Ge in the Martian mantle. Germanium is known to be more siderophile than Si, and its partitioning into the martian core has been studied by. Typical abundances in igneous martian meteorites range from 0.5-3 ppm, a larger range than what is observed in terrestrial basalts (1.5 +/- 0.1 ppm). In situ measurements by the MER and MSL rovers have revealed a surprisingly large range in Ge abundances in surface rocks (30-650 ppm), but many igneous rocks and soils are shown to have greater than 30 ppm Ge (the detection limit of the APXS). Recently, reported that shergottite minerals showed a depletion of Ge with increasing fractionation, while nakhlites and chassignites exhibited Ge behavior compatible with closed system igneous differentiation. They interpreted their observations as tentatively indicating volcanic outgassing of Ge from shergottites (but not from nakhlite-chassignites) with recondensation of the Ge vapor into soils and breccias. Recent experimental studies show that Ge is significantly volatile from magmas. In this study, we followed up on those results by analyzing minerals in five nakhlites and five shergottites by laser ablation ICP-MS (LA-ICP-MS). See Attached

Yang, S.↗
NASA NTRS1975

The Nakhlites. I - Petrography and mineral chemistry. II - Where, when, and how

The two Nakhlite meteorites (Nakhla and Lafayette) are described, and their possible origin is discussed. It is shown that both objects are cumulates of clinopyroxene with augite as the major mineral and with lesser abundances of olivine, plagioclase, K-feldspar, Fe-Ti oxides, FeS, pyrite, chalcopyrite, and possibly iddingsite. Also, both meteorites are unbrecciated, and their texture is dominated by elongate subhedral to euhedral prisms of clinopyroxene. The data are shown to be compatible with the Nakhlites' being cumulative rocks from a common basaltic parent liquid. It is established that these objects are not of lunar origin, and it is concluded that the position of the parent body relative to other planetary bodies remains to be determined.

Bunch, T. E.↗
NASA NTRS1982

A note against a small-body origin for shergottites, nakhlites, and chassignites

With the possible exception of Brachina, shergottites, nakhlites, and chassignites all crystallized about 1.3 b.y. ago on the same planet, and they were probably ejected from it together about 180 m.y. ago. No breccias from the parent planet have yet been identified, thus a dominantly impact-processed surface or a small body internal melt scenario is virtually untenable. These meteorites must have formed in a magmatic complex, and on a planet from which it was difficult to remove material: as far as we know, it only happened once. Everything known about shergottites, nakhlites, and Chassigny is consistent with an igneous origin on Mars.

Ryder, G.↗
NASA NTRS1987

Meteoritic basalts: The nakhlites, their parental magmas, cooling rates, and equivalents on Earth

Field study in northern Ontario was planned to compare cumulate rocks reported in the literature with the nakhlites in order to study the crystallization rates of the nakhlites and their possible geological settings. Experimental studies have progressed slowly because of the demands of the field work and teaching. The furnace is fully functional, and its thermocouple and oxygen sensor cells are functional and calibrated.

Treiman, Allan H.↗
NASA NTRS1987

Meteoritic basalts: The nakhlites, their parental magmas, cooling rates, and equivalents on Earth

Proposed one-bar phase equilibrium experiments, designed to determine the compositions of the nakhlites' parental magmas, are in progress. Proposed field studies on Earth, designed to find occurrences of rocks like the nakhlites, were extraordinarily successful. Other work supported in the past year included: attendance at the 1986 national meeting of the Geological Society of America; attendance at the 18th Lunar and Planetary Science Conference; completion and publication of a study of core formation in the SNC parent body; initiation of a study of the flux of SNC meteorites onto the Earth; and initiation of petrologic study of the Angra dos Reis achondrite.

Treiman, Allan H.↗
NASA NTRS1992

Petrogenesis of the nakhlite meteorites - Evidence from cumulate mineral zoning

Attention is given to a simple igneous petrogenesis for the meteorite Nakhla, which was previously called into question because Mg/Fe ratios in olivine indicate substantial disequilibrium between the predominant cumulus minerals (olivine and augite). Comparative analyses of simulated diffusive zoning and the observed cumulus mineral zoning for all three nakhlites (Nakhla, Governador Valadares, and Lafayette) show that their current compositions do not necessarily reflect parental magma compositions. Diffusion has altered primary cumulus compositions to varying degrees, Nakhla being the least affected, and Lafayette being almost completely reequilibrated. Since mineral zoning in each meteorite is strongly concentric around mesostasis areas, it is inferred that reaction with intercumulus liquid has controlled the observed zoning. It is argued that the nakhlites appear to be a series of relatively simple cumulate rocks which have undergone various amounts of late-magmatic and subsolidus diffusion, possibly reflecting their relative positions in a cooling cumulate pile.

Harvey, Ralph P.↗