Current Projects in Mars Petrology
What is the likely emplacement mechanism of magma erupted on Mars as Yamato 980459?
First et al. (in press) perform dynamic crystallization experiments on a liquid having the bulk composition of olivine-phyric shergottite Yamato 980459, to constrain the igneous thermal history of this meteorite. Key characteristics of the meteorite’s mineralogy and texture, including several morphologically distinct olivine and pyroxene crystal populations and a glassy mesostasis devoid of plagioclase, were replicated upon cooling from 1435 to 909 deg C at 1 atmosphere under reducing conditions. Three sequential cooling ramps, <1, ~10, and ~100 deg C/h, are required to produce synthetic samples with textures and compositions matching Yamato 980459. Although Y-980459 is unique among martian meteorites (i.e. preserving a primary glassy mesostasis), its emplacement did not require unique physical conditions. Rather, the second and third cooling stages may reflect cooling within the core of a pahoehoe-like flow and subsequent breakout on the surface of Mars.
First, E and Hammer, J (in press, 2016) Igneous cooling history of olivine-phyric shergottite Yamato-980459 constrained by dynamic crystallization experiments Meteoritics and Planetary Science.
Funding: NASA MFR NNX11AM29G
What conditions are necessary to produce a high magnetic remanence carrier on Mars?
Brachfeld et al. (2015) evaluate the relationship between the intensity of remanent magnetization and oxygen fugacity (fO2) in natural and synthetic Mars meteorites. The olivine-phyric shergottite meteorite Yamato 980459 (Y-980459) and a sulfur-free synthetic analog (Y-98*) of identical major element composition were analyzed to explore the rock magnetic and remanence properties of a basalt crystallized from a primitive melt, and to explore the role of magmatic and alteration environment fO2 on Mars crustal anomalies.
The reducing conditions under which Y-980459 is estimated to have formed (QFM-2.5; Shearer et al. 2006) were replicated during the synthesis of Y-98*. The remanence-carrying capacity of Y-980459 is comparable to other shergottites that formed in the fO2 range of QFM-3 to QFM-1. The remanence carrying capacity of these low fO2 basalts is 1–2 orders of magnitude too weak to account for the intense crustal anomalies observed in Mars’s southern cratered highlands. Moderately oxidizing conditions of >QFM-1, which are more commonly observed in nakhlites and Noachian breccias, are key to generating either a primary igneous assemblage or secondary alteration assemblage capable of acquiring an intense remanent magnetization, regardless of the basalt character or thermal history. This suggests that if igneous rocks are responsible for the intensely magnetized crust, these oxidizing conditions must have existed in the magmatic plumbing systems of early Mars or must have existed in the crust during secondary processes that led to acquisition of a chemical remanent magnetization.
Brachfeld, S., Shah, D., First, E., Hammer, JE, and Bowles, J. (2015) Influence of redox conditions on the intensity of Mars crustal magnetic anomalies. Meteoritics. Planet. Sci. 50, 1703-1717.
Funding: NNX11AM29G and NSF MRI 0619402 and 0948262
Brachfeld et al. (2014) synthesized two basalts with compositions relevant to the crusts of Mars and Earthwere at igneous temperatures and held at 650°C for 21 to 257 days under quartz-fayalite-magnetite fO2 buffer conditions. Both basalts acquired intense thermoremanent magnetizations and intense but easily demagnetized anhysteretic remanentmagnetizations carried by homogeneous multidomain titanomagnetite. Hypothetical intrusions on Mars composed of these materials would be capable of acquiring intense remanences sufficient to generate the observed anomalies. However, the remanence would be easily demagnetized by impact events after the cessation of the Mars geodynamo. Coercivity enhancement by pressure or formation of single domain regions via exsolution within the multidomain grains is necessary for long-term retention of a remanence carried exclusively by multidomain titanomagnetite grains.
Brachfeld, S, Cuomo, D, Tatsumi-Petrochilos, L, Bowles, J, Shah, D, and Hammer, J. (2014) Contribution of Multidomain Titanomagentite to the Intensity and Stability of Mars Crustal Magnetic Anomalies. Geophys. Res. Lett. 41, 7997–8005, doi:10.1002/ 2014GL062032.
Funding: NAG5-12486, MFR04-0000-0021, and NNX11AM29G, and NSF-EAR/IF 0948262
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