Ancient geologic events on Mars revealed by zircons and apatites from the Martian regolith breccia NWA 7034

Abstract

Zircons and apatites in clasts and matrix from the Martian breccia NWA 7034 are well documented, timing ancient geologic events on Mars. Furthermore, in this study, zircon trace elemental content, apatite volatile content, and apatite volatile isotopic compositions measured in situ could constrain the evolution of those geologic events. The U‐Pb dates of zircons in basalt, basaltic andesite, trachyandesite igneous clasts, and the matrix are similar (4.4 Ga) suggesting intense volcanism on ancient Mars. However, two metamict zircon grains found in the matrix have an upper intercept date of ~4465 Ma in crystalline, whereas amorphous areas have a lower intercept date of 1634 ± 93 Ma. The younger date is consistent with the date of apatites (1530 ± 65 Ma), suggesting a metamorphic event that completely reset the U‐Pb system in both the amorphous areas of zircon and all apatites. δD values in all apatites negatively correlate with water content in a two‐endmember mixing trend. The D (δD up to 2459‰) and 37Cl heavy core (3.8‰) of a large apatite grain suggest a D‐, 37Cl‐rich fluid during the metamorphic event ~1.6 Ga ago, consistent with the trace elements Y, Hf and Ti and P in zircons. The fluid was also therefore P‐rich. The D‐, 37Cl‐poor H2O‐rich rim (<313‰) suggests the degassing of water from the Martian Cl‐poor interior at a later time. This D‐, 37Cl‐poor Martian mantle reservoir could have derived from volcanic intrusions postdating the younger metamorphic event recorded in NWA 7034.

Hydrogen Isotopic Variations in the Shergottites

“…Almost all phases in the shergottites, e.g., apatite, merrillite, melt inclusions, maskelynite, and impact melt glasses, have δD values up to~4000-6830% [35,[51][52][53][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73]. A summary of water content and hydrogen isotopic compositions of the shergottites is listed in Table 1.…”

“…The positive correlation between the water and Cl content measured in impact melt glasses from Tissint [60] and melt inclusion glasses from NWA 6162 [68] also indicate that these Martian rocks retain some Cl sourced from the Martian subsurface water reservoir. It is also suggestable that the Martian crustal reservoir had Cl contributions from various sources [35,[79][80][81]102]. Additional studies estimated the concentration of water in the Martian crust to vary from 350 ppm to 1.3 wt.…”

“…We interpret these observations using an evidence-based approach, considering two particular scenarios: (1) water-rock crustal interactions versus (2) magmatic-based processes. We consider the implications of these measurements and the scope they have for future studies, paying particular attention to future works on H, S, and Cl isotopes in situ, shedding light on the nature of volatiles in the hydrosphere and lithosphere of Mars.The crystallization ages of Martian meteorites vary from: (1)~4.4 Ga for various igneous clasts from the regolith breccia NWA 7034 and its pairings [4,35,36] based on U-Pb dating of zircons, (2)~4.1 Ga for the orthopyroxenite ALH 84001 [37], (3)~1.3 Ga for clinopyroxene nakhlites, and dunitic chassignites [6], and (4)~600-150 Ma for most shergottites based on bulk Rb-Sr and Sm-Nd analyses [6], Pb-Pb isochrons measured in situ [38], and U-Pb dating of baddeleyite and phosphates [39][40][41][42], except for the two recent identified shergottites NWA 8159 and NWA 7635 which are dated at~2.4 Ga [19,20]. Around 80% of Martian meteorites are shergottites (Meteoritical Bulletin).…”

“…The current hydrogen isotopic composition of the Martian atmosphere is thought to have equilibrated with surficial/underground water via exchange between the Martian hydrosphere and lithosphere [55][56][57].A meteorite should equilibrate with its parent body in hydrogen isotopic compositions during crystallization (~0% , [50][51][52][53]). However, most of the shergottites display δD values higher than that presumed upon basaltic crystallization (up to~4000-6830% ) [35,[51][52][53][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75]. Such high values are inconsistent with parent magma degassing on Mars [76].…”

“…have been observed in the shergottites, although they are probably terrestrial in origin [83-88] (see Section 3). Although the shergottites lack any unambiguous signs of Martian mineral alteration, their highest δD values among the SNCs in both bulk (e.g.,~2100% for Shergotty vs~800% for ALH 84001) [89] and in situ (up to~4000-6830% for shergottites vs <3000% for non-shergottites) [35,[51][52][53][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75], is consistent with the progressive loss of Martian water over time [5,90].In this paper, we mainly focus on the progresses made on the hydroxyl, water, and hydrogen isotopic content of different phases in the shergottities, namely: apatite, merrillite, melt inclusions, maskelynite, impact melt glass, groundmass glass, and nominal anhydrous minerals. We aim to collate various measurements and studies in order to explore water-rock verses magmatic H isotopic processes.…”