Alkalic parental magmas for chassignites?

Abstract

Abstract-Detailed analysis of cumulate and melt inclusion assemblages in the chassignites provide important constraints on the nature of the melt trapped as inclusions in cumulus olivine (and, by extension, parental magma compositions), the pressures of crystallization, and magmatic volatile contents. These mineral assemblages show strong similarities to the experimental fractionation assemblages that produce the sodic silica-saturated alkalic lavas on Earth (e.g., Ascension Island, Azores, the Nandewar volcano of Australia). The experimental assemblages were produced from silica-saturated hawaiite at pressures above 4.3 kbar with dissolved water contents above 0.5 wt%. Such pressures are consistent with Ti:Al ratios of the melt-inclusion pyroxenes in the Chassigny meteorite. Pyroxene compositions suggest early high crystallization temperatures and thus relatively low initial water and F contents. Feldspars indicate that melt evolution proceeded to rhyolite compositions both within the interstices of the cumulate olivine and within the melt inclusions, even though rhyolitic glass is only found within olivine-hosted polyphase melt inclusions. The observed rhyolite glass is compositionally similar to the alkali-rich rhyolite of Ascension Island which is produced experimentally by crystallization of hawaiite. It is proposed that the melt trapped in cumulus olivine of the Chassigny dunite was similar to a terrestrial silica-saturated hawaiite, while that trapped in olivine of the Northwest Africa (NWA) 2727 dunite was less evolved, perhaps mildly alkalic basalt. Melts similar to terrestrial intra-plate tholeiite could be parental to the cumulus minerals and evolve upon crystallization at pressures above 4.3 kbar and water contents abovẽ 0.4 wt% to mildly alkalic basalt, silica-saturated hawaiite, and alkali-rich rhyolite. The melt inclusion assemblages are inconsistent with either crystallization of a low-Al, high-Fe basalt, or lowpressure crystallization of a terrestrial-like tholeiite.

Linking the Chassigny meteorite and the Martian surface rock Backstay: Insights into igneous crustal differentiation processes on Mars

“…Based on the presence of hy in the CIPW norm it can be further classified as a silica-saturated hawaiite. Backstay bears strong compositional similarities to the evolved hawaiitic liquid proposed by Nekvasil et al (2007) to have been trapped by the olivine growing from the magma parental to the Chassigny dunite at a pressure close to that of the base of the Martian crust. As shown in Fig.…”

“…Table 6 shows the composition of the derivative liquid from Backstay saturated with Fo 68 and chromite at 1150 °C that simulates the liquid trapped by growing olivine. Table 6 also shows the derivative liquid from a terrestrial hawaiite proposed by Nekvasil et al (2007) to simulate the trapped liquid of the Chassigny melt inclusion. For the most part, the differences between the starting compositions (Backstay and Nandewar hawaiite 49000) are retained in the evolving liquids, although the CaO contents of the evolved liquids become more similar to each other and the MgO more dissimilar ( Fig.…”

“…Any lithologies that represent residual liquids must be saturated at the pressure of formation with the assemblages with which they were in equilibrium before separation and produce, upon cooling, the later-formed minerals of the melt inclusions within the cumulus grains. If, as suggested by Nekvasil et al (2007), the type of melt trapped in the cumulus olivine grains was a silica-saturated hawaiite, then surface rocks with this characteristic would represent a reasonable first focus for the investigation of potential parental liquids and high pressure derivative liquids.…”

“…The olivine hosts large polyphase inclusions that have been interpreted as "melt" inclusions that contain pyroxenes, kaersutite, rare Ti-biotite, maskelynite, alkali maskelynite, apatite, chromite, and additional minor phases (e.g., Floran et al 1978;Johnson et al 1991;Wadhwa and Crozaz 1995;Nekvasil et al 2007). Nekvasil et al (2007) concluded, based on the mineralogy of the cumulus and melt inclusion phases, that the melt trapped by the growing olivine was alkalic in composition and specifically a hawaiite of the silica-saturated alkalic variety (i.e., hy normative) that follows the Coombs trend of Miyashiro (1978). Furthermore, they inferred that this melt was trapped at pressures above 4.3 kbar for several reasons.…”

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

“…However, experimental studies suggest that 2 wt. % water may be sought to reproduce the mineralogy and temperatures of crystallization of shergottites [57,[91][92][93]. Furthermore, D/H isotope compositions of DaG 476 show negative correlation between δD (i.e., 352 ± 18 to 2347 ± 85‰) and water abundance (relatively higher) in their feldspathic glasses (shocked plagioclase; e.g., maskelynite).…”