Primary trapped melt inclusions in olivine in the olivine-augite-orthopyroxene ureilite Hughes 009

We describe the first known occurrence of primary melt inclusions in a urei lite. The ureilite is Hughes 009, one of a small number of ureilites whose primary mineralogy is olivine-augite-orthopyroxene, rather than olivine-pig eonite. Hughes 009 has a coarse-grained, equilibrated texture typical of ur eilites, and homogeneous primary mineral compositions: olivine - mg 87.3; a ugite - mg 89.2, Wo 37.0, Al2O3 = 1.6 wt.%; orthopyroxene - mg 88.3, Wo 4.9 . It shows only limited secondary reduction effects and no petrographically recognizable carbon phases, which indicates that its original carbon conte nt was lower than in most ureilites. The melt inclusions occur in olivine crystals. They are concentrated in the central regions of their hosts, showing elongate (mostly 20-60 mum in maxi mum dimension), negative olivine crystal shapes and parallel alignment. The se and other features indicate that they were trapped during initial growth of their hosts from a liquid, and are likely to be representative samples of that liquid. They consist of glass and single, subhedral crystals of hig h-Ca pyroxene, with minor Cr-rich spinel and metal-phosphide-sulfide spheru les. They are surrounded by halos of olivine with rounded outlines defined by tiny bits of metal and thin arcs of glass. Pyroxenes within each inclusi on show zonation patterns indicating that they nucleated at the olivine/liq uid interface with compositions close to that of the primary augite, and th en,grew inward with dramatically increasing Al2O3 (to 10.8 wt.%), Wo (to ap proximate to 50), TiO2 and Cr2O3 contents. Glasses within each inclusion ar e relatively homogeneous. Glasses from all inclusions show well-defined tre nds of CaO, TiO2, Cr2O3, Na2O and SiO2 vs. Al2O3, (16-23 wt.%) that can be modelled as resulting principally from crystallization of various amounts o f the pyroxene. The halos, which represent olivine that grew from the trapp ed melts, are zoned in Cr and Ca with concentrations decreasing inward, ref lecting cocrystallization of pyroxene; they have homogeneous Fe/Mg identica l to that of the primary olivine, indicating reequilibration with the host. We develop a petrologic model for the postentrapment history (crystallizati on, reaction and reequilibration) of the inclusions, based on which we reco nstruct the composition of the primary trapped liquid (PTL). The PTL was sa turated only with olivine. This result implies that Hughes 009 is a cumulat e (consistent with the high Mn/Mg ratio of its olivine and a low abundance of graphite) and that the composition of the PTL is close to that of its pa rent magma. The low-pressure equilibrium crystallization sequence predicted by MAGPOX calculations for the PTL (olivine --> augite --> plagioclase --> pigeonite) is not, however, consistent with the primary mineralogy of Hugh es 009. If the conditions of these calculations are, indeed, appropriate, t hen complex processes such as magma mixing must have been involved in the p etrogenesis of this ureilite. This conclusion is consistent with other evid ence that the olivine-augite-orthopyroxene ureilites record a more complex magmatic evolution than is evident in the olivine-pigeonite ureilites. TEM investigations of microtextural features in all phases and XRD determin ation of Fe2+-Mg site distribution in orthopyroxene have elucidated the coo ling and shock history of this ureilite. Hughes 009 experienced an extremel y high cooling rate (7 +/- 5 degreesC/h at the closure T of 630 degreesC) l ate in its evolution, and two distinguishable shock events-the first at pea k pressures of 5 to 10 GPa, resulting in mechanical polysynthetic twinning in augite and orthopyroxene and mild undulatory extinction in olivine; and the second at lower pressures, resulting only in brecciation and redistribu tion of metal. Its late history is similar to that of most ureilites, and p robably reflects impact excavation. Copyright (C) 2001 Elsevier Science Ltd .