Mineral chemistry of the Tissint meteorite: Indications of two‐stage crystallization in a closed system

Abstract

Abstract-The Tissint meteorite is a geochemically depleted, olivine-phyric shergottite. Olivine megacrysts contain 300-600 lm cores with uniform Mg# (~80 AE 1) followed by concentric zones of Fe-enrichment toward the rims. We applied a number of tests to distinguish the relationship of these megacrysts to the host rock. Major and trace element compositions of the Mg-rich core in olivine are in equilibrium with the bulk rock, within uncertainty, and rare earth element abundances of melt inclusions in Mg-rich olivines reported in the literature are similar to those of the bulk rock. Moreover, the P Ka intensity maps of two large olivine grains show no resorption between the uniform core and the rim. Taken together, these lines of evidence suggest the olivine megacrysts are phenocrysts. Among depleted olivine-phyric shergottites, Tissint is the first one that acts mostly as a closed system with olivine megacrysts being the phenocrysts. The texture and mineral chemistry of Tissint indicate a crystallization sequence of: olivine (Mg# 80 AE 1) ? olivine (Mg# 76) + chromite ? olivine (Mg# 74) + Ti-chromite ? olivine (Mg# 74-63) + pyroxene (Mg# 76-65) + Cr-ulv€ ospinel ? olivine (Mg# 63-35) + pyroxene (Mg# 65-60) + plagioclase, followed by late-stage ilmenite and phosphate. The crystallization of the Tissint meteorite likely occurred in two stages: uniform olivine cores likely crystallized under equilibrium conditions; and a fractional crystallization sequence that formed the rest of the rock. The two-stage crystallization without crystal settling is simulated using MELTS and the Tissint bulk composition, and can broadly reproduce the crystallization sequence and mineral chemistry measured in the Tissint samples. The transition between equilibrium and fractional crystallization is associated with a dramatic increase in cooling rate and might have been driven by an acceleration in the ascent rate or by encounter with a steep thermal gradient in the Martian crust.

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

“…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.…”

“…Shock melt in Tissint is observed as shock veins and shock melt pockets (Figure 1; Chen et al 2015;Liu et al 2016). Shock veins typically cut across and displace igneous minerals but also follow grain boundaries.…”

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

“…However, oxygen isotopic compositions of bulk materials and minerals of both meteorites are well in isotopic equilibrium (Figure 1), suggesting that estimated temperatures are robust. Higher olivine modal abundance in SaU 005 (22-31 vol %; [6], Ni = 310 ppm; [85]) may support the idea that SaU 005 in comparison to DaG 476 (10-24 vol %; [6], Ni = 240 ppm; [85]) had crystallized at deeper levels in the Martian interior. Furthermore, the range in SaU 005 olivine Mg# (68 at core) [86] compared to that of other shergottites is restricted (est.…”

“…They represent rocks from Martian volcanoes and magmatic intrusions into the crust of Mars [1][2][3][4]. These mafic to ultramafic rocks provide unique samples that can be used to decipher and understand the magmatic processes and the crystallization history of Mars [5,6]. Based on the crystallization ages of SNCs (150-4500 Ma) obtained from various geochronometers such as Sm-Nd, Rb-Sr, Pb-Pb, and Ar-Ar [7][8][9][10][11][12][13][14][15][16][17], it appears that the Martian magmatism extended over most of the solar system history that agrees well with the time span of crater retention ages [18].…”