Observational and Experimental Results for Tissint Magma Formation:
The Story Thus Far
Nicholas Castle, Christopher Herd
ATLAS symposium
Short Version
Tissint Shergottite/Fugacity and Petrology/Open System Control
Abstract
Tissint is a recent Martian meteorite fall, one of only five ever observed,
providing a fresh look at part of Mars’ igneous history. As an olivine-phyric
shergottite with an intermediate Fe/Mg ratio and a near-liquidus composition,
Tissint promises to provide new insights into olivine-phyric shergottites and their
relationship to other Martian meteorites. The oxygen fugacity (ƒO2) of the magma
has been calculated using two geothermometer/oxybarometers, one based on
olivine-spinel-pyroxene (Ol-Sp-Px) equilibrium, the other based on titanomagnetiteilmenite (Tmt-Ilm) equilibrium. The early-formed Ol-Sp-Px assemblage yielded an
ƒO2 of QFM-3.6±0.1 (n=36), while the late formed Tmt-Ilm yielded QFM-1.35±0.01
(n=11), showing that the system was not buffered. A calibrated petrologic model is
required in order to calculate whether the magma evolved as a closed system (autooxidation) or if an external control is required (open system oxidation).
Petrologic experiments were run on the Tissint parental composition and
compared to MELTS model results to determine if MELTS is a viable model for
Tissint. The Tissint parental composition was calculated from the Tissint bulk
composition subtracting 5 wt% olivine to account for crystal accumulation.
Experimental results from constant ƒ02 equilibrium experiments yielded similar
results to MELTS model outputs, both in crystallization curves and compositions,
with some overestimation of temperature in the MELTS model.
The MELTS prediction for closed system evolution starting between QFM-3
and QFM-4 is 1.1 to 1.6 log units of oxidation, significantly smaller than the 2.3 log
units of oxidation observed in Tissint, even accounting for a potential ±0.5 log unit
correction between the two oxybarometers. The greater than closed system model
oxidation demonstrates that Tissint must have experienced an open system
oxidation event, although the nature of this event is open for speculation.