Petrology: This specimen is very coarse grained with an ophitic to mostly subophitic texture and preferred orientation of elongate grains (some up to 7 mm long see Figure 2). It is composed mainly of maskelynite (An55.5-72.3Or0.6-3.8; non-stoichiometric) and clinopyroxene (augite Fs26.3-69.1Wo35.3-23.3, pigeonite Fs38.760.1Wo10.2-19.1, FeO/MnO = 33 41). Fine symplectitic intergrowths (Figures 2-4) of fayalite + silica + ferroaugite (after pyroxferroite [1]) are associated with larger grains of chlorapatite, merrillite, ulvospinel, ilmenite with included pyrrhotite, silica needles and silicarich glasses.
Crystalline pargasite-rich spinel peridotite mylonite from St. Paul's Rocks containing 5.7% water bound in hydrous minerals was reacted in sealed platinum capsules in a piston-cylinder apparatus from 10 - 30 kb. At 10 kb the subsolidus assemblage is amphibole, olivine, orthopyroxene, clinopyroxene and spinel, an amphibole lherzolite; with increasing pressure garnet appears at 18 kb, spinel and amphibole disappear at about 25 kb; the resulting high pressure assemblage is that of a garnet lherzolite. The solidus was located in the presence of water-rich vapor, but vapor dissolves completely in the liquid at higher temperatures, and the liquid becomes water-undersaturated. Stability limits in the melting interval were determined for amphibole, garnet, spinel, clinopyroxene, orthopyroxene, and olivine (the liquidus mineral). The results are consistent with a published conclusion that St. Paul's Rocks is a diapiric, solid-state mantle intrusion initially mobilized at a depth between 45 km and 70 km near 1,000 - 1,050°C. An estimate of the solidus of peridotite with 0.2% water is presented and compared with other studies. At intermediate pressures this solidus is determined by the breakdown of amphibole. Discrepancies among results of the various studies probably arise at least in part from experimental problems involved in the complex systems.
Abstract– Northwest Africa (NWA) 4797 is an ultramafic Martian meteorite composed of olivine (40.3 vol%), pigeonite (22.2%), augite (11.9%), plagioclase (9.1%), vesicles (1.6%), and a shock vein (10.3%). Minor phases include chromite (3.4%), merrillite (0.8%), and magmatic inclusions (0.4%). Olivine and pyroxene compositions range from Fo 66–72 ,En 58–74 Fs 19–28 Wo 6–15 , and En 46–60 Fs 14–22 Wo 34–40 , respectively. The rock is texturally similar to “lherzolitic” shergottites. The oxygen fugacity was QFM−2.9 near the liquidus, increasing to QFM−1.7 as crystallization proceeded. Shock effects in olivine and pyroxene include strong mosaicism, grain boundary melting, local recrystallization, and pervasive fracturing. Shock heating has completely melted and vesiculated igneous plagioclase, which upon cooling has quench‐crystallized plagioclase microlites in glass. A mm‐size shock melt vein transects the rock, containing phosphoran olivine (Fo 69–79 ), pyroxene (En 44–51 Fs 14–18 Wo 30–42 ), and chromite in a groundmass of alkali‐rich glass containing iron sulfide spheres. Trace element analysis reveals that (1) REE in plagioclase and the shock melt vein mimics the whole rock pattern; and (2) the reconstructed NWA 4797 whole rock is slightly enriched in LREE relative to other intermediate ultramafic shergottites, attributable to local mobilization of melt by shock. The shock melt vein represents bulk melting of NWA 4797 injected during pressure release. Calculated oxygen fugacity for NWA 4797 indicates that oxygen fugacity is decoupled from incompatible element concentrations. This is attributed to subsolidus re‐equilibration. We propose an alternative nomenclature for “lherzolitic” shergottites that removes genetic connotations. NWA 4797 is classified as an ultramafic poikilitic shergottite with intermediate trace element characteristics.
Subsolidus phase relationships have been determined to pressures of 15–27 kb for a garnet clinopyroxenite, a garnet-plagioclase clinopyroxenite, a spinel-garnet websterite, and a two-pyroxene granulite occurring as xenoliths in the Delegate basaltic breccia pipes. Assuming all the garnet pyroxenite suite xenoliths formed together or last equilibrated together, the experimental data constrain the P-T conditions of their formation to 13–17 kb and 1050–1100 °C; for the pyroxene granulites, pressures of formation of 6–10 kb at temperatures around 1100 °C are indicated. In the case of the spinel-garnet websterite, the texturally implied exsolution of garnet and orthopyroxene from clinopyroxene, and reaction of spinel with clinopyroxene to yield garnet, are shown to be explicable in terms of approximately isobaric cooling of a pre-existing aluminous clinopyroxene+spinel aggregate. The garnet of the garnet and garnet—plagioclase clinopyroxenites cannot, however, have been derived wholly by exsolution processes. New chemical data are presented for the xenoliths studied experimentally and for several similar examples from Delegate and other eastern Australian localities. Consideration of available major and trace element and isotopic data for garnet pyroxenite suite xenoliths from Delegate and elsewhere in the world strongly suggests genetic relationships with their host basaltic rocks. The Delegate examples are interpreted as a series of accumulates from local pockets of alkaline basaltic magma within the Earth's upper mantle, and which have subsequently undergone exsolution and/or recrystallization in response to subsolidus cooling. A similar origin is suggested for the analogous garnet pyroxenites found as layers within western Mediterranean peridotite massifs. The Delegate two-pyroxene granulite xenoliths are considered to be accidental fragments of metamorphic rocks from the deep crust beneath eastern Australia.
