The parental melt of lherzolitic shergottite ALH 77005: A study of rehomogenized melt inclusions

Abstract

Lherzolitic shergottite ALH 77005 is one of the most primitive martian meteorites. To characterize the parental melt of this primitive meteorite, olivine and chromite-hosted melt inclusions have been experimentally rehomogenized. The rehomogenization was performed with hydrostatic pressures (800-1000 bars) of CO 2 + CO gas along with finely powdered graphite at temperatures of 1150-1185 °C. Equilibrium between the host and inclusion melt was determined based on the lack of zonation in the host surrounding the melt inclusion, equilibrium K D values of host and melt inclusions, and textures of the melt inclusion. Chromite-hosted melt inclusions, where chromite is poikilitically enclosed by olivine, contain ~7.5 wt% MgO. This composition most closely reflects the parental melt of ALH 77005. The melts trapped in Fo 75 olivine contain ~7.1 wt% MgO when brought to equilibrium with the host. This olivine-hosted melt inclusion composition has lower SiO 2 (~50 vs. 53.9 wt%) and higher Cr 2 O 3 (~1.2 vs. 0.2 wt%) and P 2 O 5 (~1.2 vs. 0.5 wt%) than previous estimates for ALH 77005. In addition, compared with the chromite-hosted inclusions, the olivine-hosted ones have higher Al 2 O 3 and lower CaO than can be explained through crystallization of phases known to be on the liquidus. This finding suggests that magma mixing occurred between chromite and olivine crystallization or olivine-hosted inclusions were contaminated by secondary minerals such as phosphate. Both olivineand chromite-hosted melt inclusions in ALH 77005 have slightly higher Al 2 O 3 than olivine inclusions in Chassigny but significantly higher Al 2 O 3 than nakhlites such as MIL 03346 and Nakhla at similar values of MgO.

Expanding the application of the Eu‐oxybarometer to the lherzolitic shergottites and nakhlites: Implications for the oxidation state heterogeneity of the Martian interior

“…Experiments were run on approximately 1 mm 3 chips of each meteorite which were rehomogenized in TZM (titanium-zirconium-molybdenum) pressure vessels in sealed Pt-tubes Calvin and Rutherford 2008). Samples were enclosed in graphite powder that generates a G-CO mixture through C reaction with the oxidized sample, thereby fixing the fO 2 at low values between iron wustite (IW) and quartz-fayalitemagnetite (QFM) depending on the pressure of the experiment.…”

“…Fe-Mg reequilibration in ALH 77005 olivine would potentially result in all olivine phenocrysts throughout the sample sharing the same composition (similar to that seen in high metamorphic grade chondrites). Previous studies of ALH 77005 have observed grain to grain major element compositional variations in olivine (Ikeda 1994;Calvin and Rutherford 2008) implying that major element diffusive reequilibration was not complete. As Fe-Mg diffusion is faster than REE diffusion in olivine, these results suggest that post-entrapment reequilibration of the studied melt inclusions is not significant for the REEs.…”

“…1D; Table 3), as well as some melting of the matrix between grains. Criteria including experimental length (run time between 48 and 72 hours) and melt-olivine K D (experimental average = 0.27; K D for melts in equilibrium with olivinẽ 0.30 [Roeder 1974]) were used to determine if the experiment approached equilibrium (Calvin and Rutherford 2008). REE measurements were made on totally rehomogenized olivine-hosted melt inclusions (diameter >50 µm) and groundmass melts (Calvin and Rutherford 2008).…”

“…Criteria including experimental length (run time between 48 and 72 hours) and melt-olivine K D (experimental average = 0.27; K D for melts in equilibrium with olivinẽ 0.30 [Roeder 1974]) were used to determine if the experiment approached equilibrium (Calvin and Rutherford 2008). REE measurements were made on totally rehomogenized olivine-hosted melt inclusions (diameter >50 µm) and groundmass melts (Calvin and Rutherford 2008). Although the melt contained in olivine-hosted inclusions does not represent the most primitive melts measured in ALH 77005 (Goodrich and Harvey 2002), they do represent magmas that were trapped during olivine crystallization early in the crystallization sequence and therefore can be studied to understand the REE concentrations in ALH 77005 melts.…”

“…Therefore, to measure the REE composition of the melt, the inclusions must first be rehomogenized (e.g., Gaetani and Watson 2000;Varela et al 2001;Danyushevsky et al 2002;Stockstill et al 2005). The research presented here focuses on nakhlite MIL 03346 and lherzolitic shergottite ALH 77005, both of which have had melt inclusions experimentally rehomogenized Calvin and Rutherford 2008).…”