Report

Tissint Martian Meteorite: A Fresh Look at the Interior, Surface, and Atmosphere of Mars

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Science  09 Nov 2012:
Vol. 338, Issue 6108, pp. 785-788
DOI: 10.1126/science.1224514

Abstract

Tissint (Morocco) is the fifth martian meteorite collected after it was witnessed falling to Earth. Our integrated mineralogical, petrological, and geochemical study shows that it is a depleted picritic shergottite similar to EETA79001A. Highly magnesian olivine and abundant glass containing martian atmosphere are present in Tissint. Refractory trace element, sulfur, and fluorine data for the matrix and glass veins in the meteorite indicate the presence of a martian surface component. Thus, the influence of in situ martian weathering can be unambiguously distinguished from terrestrial contamination in this meteorite. Martian weathering features in Tissint are compatible with the results of spacecraft observations of Mars. Tissint has a cosmic-ray exposure age of 0.7 ± 0.3 million years, consistent with those of many other shergottites, notably EETA79001, suggesting that they were ejected from Mars during the same event.

Demonstration in the early 1980s that an important group of meteorites was of martian origin represented a breakthrough in attempts to understand the geological evolution of Mars (13). Unfortunately, most of the samples were collected long after their arrival on Earth and thus have experienced variable degrees of terrestrial weathering (4). Even the few martian meteorites that were collected shortly after their observed fall to Earth have been exposed to organic and other potential contaminants during storage. Here, we report on the Tissint martian meteorite, which fell on 18 July 2011 in Morocco (figs. S1 and S2). This is only the fifth witnessed fall of a meteorite from Mars and therefore provides an opportunity to improve our understanding of processes that operated on that planet at the time the meteorite was ejected from its surface.

The largest recovered stones from the Tissint fall are almost fully covered with a shiny black fusion crust (Fig. 1). Internally the meteorite consists of olivine macrocrysts set in a fine-grained matrix of pyroxene and feldspathic glass (maskelynite) (5) (figs. S3 to S6 and tables S1 to S6). The matrix is highly fractured and penetrated by numerous dark shock veins and patches filled with black glassy material enclosing bubbles (fig. S7). The petrology of Tissint shows similarities to that of other picritic shergottites (an important group of olivine-rich martian basaltic rocks), in particular, lithologies A and C of EETA79001 (2). The grain density and magnetic properties of Tissint (fig. S8) also match previous results from basaltic and picritic shergottites (6).

Fig. 1

The Natural History Museum (London) stone. This 1.1-kg stone (BM.2012,M1) exhibits a black fusion crust with glossy olivines. The olivine macrocrysts (pale green) and the numerous black glass pockets and veins are characteristics of this shergottite. The scale is in centimeters.

Tissint is an Al-poor ferroan basaltic rock, rich in MgO and other compatible elements (Ni, Cr, Co). Its major element abundances are similar to those of the other picritic shergottites, especially EETA79001. Furthermore, key element weight (wt) ratios (wt %/wt %) such as FeO/MnO (39.7), Al/Ti (7.2), Na/Ti (1.41), Ga/Al (3.9 10−3), Na/Al (0.20) (3, 4, 7), and Δ17O [+0.301 per mil (‰)] (fig. S9) (8) are also typical of martian meteorites. The average composition of the black glass (tables S7 and S8) is identical to a mixture of the major phases of the rock (augite, maskelynite, and olivine: 50:20:30) with compositional variations reflecting incomplete dissolution of one phase or another (fig. S7). Among minor elements in the black glass, chlorine is always below the detection level of electron microprobe analysis [100 parts per million (ppm)], whereas fluorine and sulfur exhibit variations in the range 0 to 4000 ppm and 0 to 6000 ppm, respectively (5).

Like most other picritic shergottites, bulk Tissint displays a marked depletion in light rare earth elements (LREEs) and other highly incompatible elements, such as Rb, Li, Be, Nb, Ta, Th, and U (Fig. 2). Its Lu/Hf ratio (0.2) is in the range of values measured for EETA79001 and other basaltic shergottites [0.1 to 0.2; e.g., (9)], and lower than those of the picritic shergottites DaG 476/489, SaU 005, and Dhofar 019 [about 0.3 (1012)]. Although the sizes of the two samples analyzed here are somewhat limited (0.49 and 1.25 g), their trace-element abundances are very similar and so are likely to be representative of the whole rock composition, despite the irregular distribution of olivine megacrysts.

Fig. 2

REE patterns. (Top) Tissint in comparison with other depleted picritic shergottites. (Bottom) Black glass and groundmass-rich fraction in comparison with enriched shergottite Zagami. Data from (1012). CI chondrite normalization values are from (24).

To evaluate the possible heterogeneity of this stone, we analyzed two additional samples: a groundmass-rich sample (devoid of large olivine crystals and weighing 181 mg) and a fragment of the same glassy pocket selected for volatile analysis (40 mg). Both samples display markedly higher LREE abundances, with REE patterns generally similar to those of the enriched shergottites, as exemplified by Zagami. However, there is a minor, but analytically valid, positive Ce anomaly (Ce/Ce* = 1.1) (Fig. 2), and the La/Nd, La/Nb, and Th/La ratios are higher than those of other enriched shergottites (fig. S10). These two samples indicate that an LREE-enriched component, different from those previously recorded in other shergottites, is heterogeneously dispersed throughout the matrix of Tissint.

The presence of short-lived 48V (half-life = 16 days), among other cosmogenic isotopes, demonstrates that the stones we analyzed are from the fall of 18 July (table S10). We measured stable cosmogenic isotopes of He, Ne, and Ar in three aliquots, consisting of matrix-rich, glass-matrix mixed, and glass-rich separates (table S11). The cosmic-ray exposure (CRE) ages computed for 3Hec, 21Nec, and 38Arc are 1.2 ± 0.4, 0.6 ± 0.2, and 0.9 ± 0.4 million years (My), respectively, resulting in an average CRE age of 0.7 ± 0.3 My for Tissint. This age is in the range of CRE ages of other shergottites, notably that of EETA79001 [0.73 ± 0.15 My (2)], suggesting that Tissint and other shergottites were ejected during a single event. Nitrogen isotopes were analyzed together with the noble gases. The glass aliquot displayed a well-defined excess of 15N, which persisted after correction for contribution of cosmogenic 15Nc (assuming a production rate of 6.7 ± 2.6 × 10−13 mol 15N/gMa) (13). This excess 15N is best explained by trapping of a martian atmospheric component (2). Using a δ15N versus 40Ar/N correlation and taking a martian atmospheric value from the Viking measurements of 0.33 ± 0.03 (14), we obtain a δ15N value of 634 ± 60‰ (1σ), which agrees well with the Viking measurement of 620 ± 160‰ (7) (Fig. 3).

Fig. 3

Gas analyses of the black glass. Both bulk analyses and step-heating analyses plot on a single mixing line between terrestrial atmospheric gas (at left) and Mars atmospheric gas (14). Zagami data from (25).

Simultaneous measurement of carbon and nitrogen was carried out by stepped combustion–mass spectrometry on a small chip (21 mg) from the same sample that we used for oxygen isotopic analysis (5). The sample had a total carbon abundance of 173 ppm and δ13C of –26.6 ‰ and contained 12.7 ppm nitrogen with total δ15N of –4.5‰. At temperatures above 600°C, both carbon and nitrogen were distributed among three discrete martian components (fig. S11 and table S12). Below 600°C, readily resolvable components of organic material combusted; although these may have been introduced during postfall collection and sample storage and are an unavoidable consequence of sample handling procedures, we cannot yet rule out the presence of small quantities of indigenous martian organic matter (5). At the highest temperatures of the extraction, there was a clear indication of the presence of trapped martian atmosphere, with elevated δ13C and δ15N (even allowing for a cosmogenic component, blank-corrected δ13C reaches +16‰ and δ15N reaches +298 ± 25‰). At intermediate temperatures (600° to 800°C), there were maxima in both δ13C (–14‰) and δ15N (+110‰), suggesting that the component bears some relationship to the martian atmosphere. In addition, there was clear analytical evidence for a simultaneous release of sulfur (~19 ppm), presumably from either sulfide or sulfate decomposition. This intermediate component probably corresponds to a surface-derived weathering component, as identified in Tissint glass on the basis of REE, S, and F data (see below). The third martian component represents magmatic carbon, which is present in low abundance (1.4 ppm with δ13C of –26.3‰) and is associated with isotopically light nitrogen (δ15N < +10‰).

Our study demonstrates that Tissint is a picritic shergottite comparable in many respects to EETA79001. The black shock glass resembles lithology C of EETA79001, as well as shock melt pockets commonly found in other shergottites (15). Major elements and oxygen isotope data indicate that this glass represents a melted mixture of the surrounding bulk rock, composed of olivine, maskelynite, and clinopyroxene (5). However, this glass is substantially different from the bulk meteorite and igneous groundmass in that it has a variable but generally high S and F content; a distinct LREE-enriched composition; and a high δ15N value indicative of trapped martian atmosphere.

The LREE-enriched composition of the glass is somewhat enigmatic. Phosphates are often invoked as a carrier of REE. However, the P content of the black glass relative to bulk rock is not consistent with enrichment in phosphates. One possibility to explain the LREE composition of the glass might be selective crustal contamination before final emplacement of the Tissint magma. Although LREE-enriched magmatic rocks have been generated on Mars, as exemplified by the Nakhlites and Chassignites, these do not exhibit anomalous Ce abundances (16, 17). In addition, crustal contamination of magma is unlikely to result in REE ratio variations at the subcentimeter scale, as observed here. Decoupling of Ce from the other REE indicates partial oxidation to Ce4+, a process that requires oxidizing conditions, such as those that prevail in the near-surface environment of Mars. Surface weathering caused by leaching of phosphates by acid aqueous fluids, the process that is responsible for terrestrial alteration of eucritic meteorites in Antarctica (18), would also explain the LREE-enriched composition of the Tissint glass. The high δ15N value of the Tissint glass, as well as its enrichment in S and F, demonstrates that it has been contaminated by martian surface components. In view of this evidence, the most likely explanation for the relatively LREE-enriched composition of the glass, and the origin of the Ce anomaly, is that these features also reflect the presence of a near-surface martian component in Tissint. A martian soil component was previously suggested for EETA79001 lithology C, which also contains martian atmospheric gases (19). However, because this meteorite is a find, rather than a fresh fall like Tissint, there’s the possibility of terrestrial contamination, which complicates the interpretation (20).

We propose the following scenario to explain the composite nature of Tissint. A picritic basalt was emplaced at or near the surface of Mars. After some period, the rock was weathered by fluids, which had leached elements from the martian regolith. Subsequently, these fluids deposited mineral phases within fissures and cracks. The martian weathering products are the most likely source of the required LREE, incompatible, and volatile elements. Upon impact, preferential, shock-induced melting occurred in the target rock along fractures where weathering products were concentrated. This melting produced the black glass and retained in it chemical signatures characteristic of the martian surface. Shock melting also trapped a component derived from the martian atmosphere, as revealed by stepped combustion–mass spectrometry. About 0.7 My ago, the sample was ejected from Mars and eventually landed on Earth in July 2011. The martian weathering features in Tissint described here are compatible with spacecraft observations on Mars, including those made by the NASA Viking landers, MER Spirit rover, and ESA’s Mars Express orbiter (5, 2123).

Supplementary Materials

www.sciencemag.org/cgi/content/full/science.1224514/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S11

Tables S1 to S13

References (2635)

References and Notes

  1. Supplementary materials are available on Science Online.
  2. Acknowledgments: We acknowledge D. N. Menegas and family for their generous donation enabling the acquisition of Tissint (BM.2012,M1), M. Aoudjehane for fieldwork, A. Aaranson for field information, J. Gibson for assistance with oxygen isotope analysis, L. Labenne for loan of a sample, and A. Irving for 400 mg of powdered sample. This study was funded at Hassan II University Casablanca, Faculté des Sciences Ain Chock, by Centre de Recherches Scientifiques et Techniques Morocco and CNRS France (Projet International de Coopération Scientifique, Sciences de l’Univers 01/10), and by Comité Mixte Inter Universitaire Franco-Marocain Volubilis (MA/11/252); CRPG Nancy, France, by the CNES, CNRS, and European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 no. 267255); Université de Bretagne Occidentale–Institut Universitaire Européen de la Mer, Plouzané, France, by the Programme National de Planétologie, Institut National des Sciences de l’Univers; Open University, by Science and Technology Facilities Council grant to the Planetary and Space Sciences Discipline; and University of Alberta, by the Natural Sciences and Engineering Research Council of Canada (grant 261740-03).
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