Heterogeneous distribution of H₂O in the Martian interior: Implications for the abundance of H₂O in depleted and enriched mantle sources

Abstract

Abstract-We conducted a petrologic study of apatite within 12 Martian meteorites, including 11 shergottites and one basaltic regolith breccia. These data were combined with previously published data to gain a better understanding of the abundance and distribution of volatiles in the Martian interior. Apatites in individual Martian meteorites span a wide range of compositions, indicating they did not form by equilibrium crystallization. In fact, the intrasample variation in apatite is best described by either fractional crystallization or crustal contamination with a Cl-rich crustal component. We determined that most Martian meteorites investigated here have been affected by crustal contamination and hence cannot be used to estimate volatile abundances of the Martian mantle. Using the subset of samples that did not exhibit crustal contamination, we determined that the enriched shergottite source has 36-73 ppm H 2 O and the depleted source has 14-23 ppm H 2 O. This result is consistent with other observed geochemical differences between enriched and depleted shergottites and supports the idea that there are at least two geochemically distinct reservoirs in the Martian mantle. We also estimated the H 2 O, Cl, and F content of the Martian crust using known crust-mantle distributions for incompatible lithophile elements. We determined that the bulk Martian crust has~1410 ppm H 2 O, 450 ppm Cl, and 106 ppm F, and Cl and H 2 O are preferentially distributed toward the Martian surface. The estimate of crustal H 2 O results in a global equivalent surface layer (GEL) of~229 m, which can account for at least some of the surface features on Mars attributed to flowing water and may be sufficient to support the past presence of a shallow sea on Mars' surface.

Validity of the Apatite/Merrillite Relationship in Evaluating the Water Content in the Martian Mantle: Implications from Shergottite Northwest Africa (NWA) 2975

“…The magma source for Martian basalts is the Martian mantle; thus, the basaltic phosphates provide information on the Martian mantle composition as well as its evolution in the course of melting progression [11,[18][19][20][21]. Volatile-bearing apatite (Ca 10 (PO 4 ) 6 (Cl, F, OH) 2 ), volatile-free merrillite (Ca 18 Na 2 Mg 2 (PO 4 ) 14 ) and the high-pressure phase tuite (γ-Ca 3 (PO 4 ) 2 ) are frequently observed in extra-terrestrial bodies ( [1,3,5,6,12,[22][23][24][25] and references therein).…”

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

“…Furthermore, NWA 6234 recorded that merrillite reacted with an OH-Cl-F-rich melt to form apatite [55]. Shergottites contain trace quantities of apatite that is relatively more abundant in the enriched ones compared to their depleted counterparts [46]. It has been observed that all phosphate grains present in SaU 094 and SaU 150 meteorites, paired with SaU 005/008, represent merrillite, and no apatite has been found [69,101].…”

“…Furthermore, it is envisaged that water in the Martian mantle is heterogeneously distributed based on the estimates from the SNCs (i.e., 14-250 ppm; [46]. However, depleted shergottite mantle source is relatively water-poor (i.e., 14-23 ppm; [46]) compared to the enriched ones (i.e., 36-73 ppm; [46]). Mantle derived water that exhumed on the Martian surface in large fluxes during the earlier history of Mars (Noachian to early Hesperian periods; [23]), has been followed by further water contributions from shergottite-like magmatism during the Amazonian era [23].…”

“…Experiments suggest that the parental magma of Y980459 could have been hydrous [100]. Furthermore, it is envisaged that water in the Martian mantle is heterogeneously distributed based on the estimates from the SNCs (i.e., 14-250 ppm; [46]. However, depleted shergottite mantle source is relatively water-poor (i.e., 14-23 ppm; [46]) compared to the enriched ones (i.e., 36-73 ppm; [46]).…”

“…The volatile budget in Martian magmas is also important to understand the eruption history on Mars [37] and the chemistry of the SNCs [43][44][45][46]. Water content can constrain the extent of oxidation [47] that may possibly affect the oxygen isotope compositions of the mantle minerals through isotopic exchange.…”