Petrology and trace element geochemistry of Tissint, the newest shergottite fall

Abstract

The fall and recovery of the Tissint meteorite in 2011 created a rare opportunity to examine a Martian sample with a known, short residence time on Earth. Tissint is an olivine‐phyric shergottite that accumulated olivine antecrysts within a single magmatic system. Coarse olivine grains with nearly homogeneous cores of Mg# >80 suggest slow re‐equilibration. Many macroscopic features of this sample resemble those of LAR 06319, including the olivine crystal size distribution and the presence of evolved oxide and olivine compositions. Unlike LAR 06319, however, no magmatic hydrous phases were found in the analyzed samples of Tissint. Minor and trace element compositions indicate that the meteorite is the product of closed‐system crystallization from a parent melt derived from a depleted source, with no obvious addition of a LREE‐rich (crustal?) component prior to or during crystallization. The whole‐rock REE pattern is similar to that of intermediate olivine‐phyric shergottite EETA 79001 lithology A, and could also be approximated by a more olivine‐rich version of depleted basaltic shergottite QUE 94201. Magmatic oxygen fugacities are at the low end of the shergottite range, with log fO2 of QFM‐3.5 to ‐4.0 estimated based on early‐crystallized minerals and QFM‐2.4 estimated based on the Eu in pyroxene oxybarometer. These values are similarly comparable to other depleted shergottites, including SaU 005 and QUE 94201. Tissint occupies a previously unsampled niche in shergottite chemistry: containing olivines with Mg# >80, resembling the enriched olivine‐phyric shergottite LAR 06319 in its crystallization path, and comparable to intermediate olivine‐phyric shergottite EETA 79001A, depleted olivine‐phyric shergottite DaG 476, and depleted basaltic shergottite QUE 94201 in its trace element abundances and oxygen fugacity. The apparent absence of evidence for terrestrial alteration in Tissint (particularly in trace element abundances in the whole‐rock and individual minerals) confirms that exposure to the arid desert environment results in only minimal weathering of samples, provided the exposure times are brief.

Martian low-temperature alteration materials in shock-melt pockets in Tissint: Constraints on their preservation in shergottite meteorites

“…Our results for shock melt REE (Fig. 8, Table S6) are comparable to those previously reported (Chennaoui Aoudjehane et al, 2012;Barrat et al, 2014;Balta et al, 2015;Liu et al, 2016). There is no discernable difference in the REE concentrations of shock melt, whether texturally homogeneous and glassy, or crystallite-rich with interstitial glass (Fig.…”

“…Pyroxene, olivine, and maskelynite are depleted in LREE, which is typical for depleted shergottites. Although only one merrillite analysis was possible, results are generally consistent with REE reported for Tissint merrillite (Balta et al, 2015;Liu et al 2016). Our results for shock melt REE (Fig.…”

“…The abundances of REE in the major minerals in Tissint (Fig. 8, Table S5) are consistent with what is expected based on known partitioning behavior, and with previous studies (e.g., Balta et al, 2015, Liu et al, 2016: abundances are lowest for olivine; pyroxene and maskelynite have similar abundances, although a positive europium anomaly is present for maskelynite; and abundances are greatest in merrillite. Pyroxene, olivine, and maskelynite are depleted in LREE, which is typical for depleted shergottites.…”

Oxygen Isotope Thermometry of DaG 476 and SaU 008 Martian Meteorites: Implications for Their Origin

“…Furthermore, the range in SaU 005 olivine Mg# (68 at core) [86] compared to that of other shergottites is restricted (est. 60-72; see Figure 3c in [87]). It is evident that olivine Mg# is directly proportional to its MgO content that, in turn, is correlated with olivine crystallization temperatures (1050-1525 • C) observed in basalts (see Figure 1 in [88]).…”

“…It is believed that shergottites-the most abundant of the Martian meteorites found to date-have been generated by the partial melting of the Martian mantle [8,19]; therefore, their geochemical signatures can provide insights into the Martian mantle chemistry. Furthermore, the depleted shergottites-for example, having low chondrite-normalized La/Y (~0.1), low initial 87 Sr/ 86 Sr (~0.7013), low magmatic oxygen fugacity (i.e., quartz-fayalite-magnetite, QFM −3.5), and large positive ε 143 Nd values [5,7,8,20,21]-originated from a depleted mantle source region that has not seen significant alteration or melting since planetary differentiation at 4504 ± 6 Ma [9,17]. Compared to the nakhlites and chassignites, shergottites, with the exception of Northwest Africa (NWA) 7635 (2405 Ma) [17], represent more than three-quarters of the known Martian meteorites (i.e., SNCs) with relatively young crystallization ages (150-574 Ma) [7,13,22].…”