Exploring fractionation models for Martian magmas

Abstract

[1] Martian primary compositions, i.e., magmas that did not experience fractionation and/or contamination after extraction from the mantle, occur as a subset of Martian meteorites and a few lavas analyzed on the planet's surface by rovers. Eruptions of primary magmas are rare on Earth and presumably on Mars. Previous studies of fractional crystallization of Martian primary magmas have been conducted under isobaric conditions, simulating idealized crystallization in magma chambers. Polybaric fractionation, which occurs during magma ascent, has not been investigated in detail for Martian magmas. Using the MELTS algorithm and the pMELTS revision, we present comprehensive isobaric and polybaric thermodynamic calculations of the fractional crystallization of four primary or parental Martian magmas (Humphrey, Fastball, Y-980459 shergottite, and nakhlite parental melts) using various pressure-temperature paths, oxygen fugacities, and water contents to constrain how these magmas might evolve. We then examine whether known Martian alkaline rock compositions could have formed through fractional crystallization of these magmas under the simulated conditions. We find that isobaric and polybaric crystallization paths produce similar residual melt compositions, but given sufficient details, we may be able to distinguish between them. We calculate that Backstay (Gusev Crater) likely formed by fractionation of a primary magma under polybaric conditions, while Jake_M (Gale Crater) may have formed through melting of a metasomatized mantle, crustal assimilation, or fractional crystallization of an unknown primary magma. The best fits for the Backstay composition indicate that consideration of polybaric crystallization paths can help improve the quality of fit when simulating liquid lines of descent.

Identification and Description of a Silicic Volcaniclastic Layer in Gale Crater, Mars, Using Active Neutron Interrogation

“…Hydrated SiO has been identified in many locations on Mars in addition to Gale crater (e.g., Glotch et al, ; Milliken et al, ; Mustard et al, ; Squyres et al, ; Sun & Milliken, ; Weitz et al, ). Multiple pathways for SiO enrichment in Martian environments have been proposed, including active enrichment through SiO sintering (Frydenvang et al, ; Ruff et al, ; Skok et al, ), passive enrichment through acid leaching (McAdam et al, ; Squyres et al, ; Weitz et al, ; Yen et al, ), and fractional crystallization in an evolved magmatic system (Carter & Poulet, ; Christensen et al, ; Edwards et al, ; McCubbin et al, ; Udry et al, , ). The crystalline mineralogy of the Buckskin sample is consistent with a silicic volcanic deposit from an evolved source (Morris et al, ; Rampe et al, ).…”

Dusty Rocks in Gale Crater: Assessing Areal Coverage and Separating Dust and Rock Contributions in APXS Analyses

“…This is especially true for rocks that are very different from average Mars or for studies that are particularly sensitive to alkali or sulfur concentrations. Such studies include those that use APXS data to interpret indices of chemical weathering (e.g., Chemical Index of Alteration; Hurowitz et al, ), igneous classification (e.g., Mangold et al, ), or petrological modeling (e.g., Stolper et al, ; Udry et al, ). The probable impact of surface dust coverage on interpretation may be determined by recalculating or replotting using estimated dust‐free rock compositions, such as those in Table .…”

Constraints on the depth and thermal vigor of melting in the Martian mantle

“…They yield an average condition of melt-mantle equilibration of~2 GPa and 1490°C. Wishstone-class basalts:The Wishstone-class basalts, unlike the Adirondack-class basalts and Home Plate basalts, are fractionated alkalic basalts [e.g., Usui et al, 2008a;Udry et al, 2014a]. The pressures and temperatures of formation calculated needed to produce the parental magma to these basalts are just above the mantle solidus, suggesting that their parental magmas may represent low-degree partial melts (≪ 5 wt %) from deep Martian mantle.…”