John L. Berkley

The nature and origin of ureilites

Abstract Ureilites are carbon-bearing olivine-pigeonite achondrites which constitute a unique achondrite type. We performed a comprehensive mineralogical and petrological study on eight ureilites (Kenna, Novo Urei, Goalpara, Havero, Dingo Pup Donga, Dyalpur, North Haig, and ALHA77257) the results of which were used to construct a hypothesis for the origin of ureilites. This hypothesis suggests that ureilites are primarily olivine-pigeonite cumulates which crystallized from a silicate liquid that also contained suspended solid carbon phases, mainly graphite. This carbon caused reduction of the melt and influenced ureilite mineral compositions. Carbonaceous material was trapped by settling cumulus mafic silicates along with other intercumulus material to form the carbon-rich ‘veins’ common to ureilites. Petrofabric analyses show that mafic silicates are oriented in lineated and foliated patterns characteristic of cumulate rocks, specifically adcumulates. Strain state of silicates suggests that ureilites were deformed subsequent to lithification by mild tectonic stress as well as by moderate to severe shock. The latter event caused the formation of micron-sized diamonds and lonsdaleite from graphite and carbon-induced reduction of silicate grain margins during temporal shock-heating.

Origin and evolution of the ureilite parent magmas: multi-stage igneous activity on a large parent body

Analyses of olivine and pigeonite cores (interpreted as cumulate crystals) in eight low-shock ureilites show well-defined correlations between Fe/X (where X is one of the minor elements Mn, Cr, Ca, Al, Ti, P, or Ni) and Fe/Mg ratios. For the lithophile minor elements these trends are linear, with positive slope, and pass through or near the origin, indicating various degrees of FeO reduction of the parent magmas. The trends shown by P and Ni are consistent with this interpretation, and, additionally, require equilibrium crystallization of 20–27 mole % metal. Graphite was most likely the reducing agent, which implies that the ureilites equilibrated at different pressures, because the graphite fO2 buffer is pressure dependent. There is no evidence for fractionation of olivine or pryroxene in any of these trends. The Ca, Ti and Al trends show significant scatter, indicating that there must have been multiple parent magmas, with very similar Mg/Mn and MgCr ratios, but different MgAl, MgCa and MgTi ratios. CaO/Al2O3 ratios vary by a factor of ~3 among the eight magmas, and are superchondritic (~ 14–32, molar), similar to lunar mare basalts. We propose the following model for generation and crystallization of ureilite parent magmas (UPM). Chondritic material undergoes 10–25% partial melting, leaving a plagioclase-depleted residue. The magma or magmas form a differentiated crust on the ureilite parent body (UPB), with mafic cumulates and plagioclase cumulates spatially separated. Later remelting (<10%) of the mafic cumulates produced the UPM. Siderophile elements indicate that the source region of the UPM had not experienced segregation of metal such as would occur during core formation. In the source region, the pressure must have been at least 0.5–1 kb, because only at pressures this high does the graphite buffer allow iron oxide to be the stable phase relative to metallic iron. The graphite-bearing UPM were emplaced at various levels in the crust, where they were reduced to various extents, dependent on depth. Ureilites are early olivine-pigeonite cumulates from these magmas. Compositions of interstitial, low-Ca pyroxenes indicate that the trapped interstitial liquids from which they crystallized had lower MnMg, CrMg, CaMg, AlMg and NaMg ratios than the UPM, and indicate an abrupt change in liquid composition coincident with impact excavation of ureilites. This interstitial material may be derived by mixing a shock melt of nearby olivine-pigeonite cumulates with residual primary liquid. Our model predicts that the UPB had a differentiated crust, did not have a core, and was at least 235 km in radius. The lack of a ureilite-like body among spectrally analyzed asteroids suggests that either the spectral signature of the UPB is not that of ureilites themselves (perhaps of carbon-free feldspathic material instead), that the UPB is not intact, or that ureilites did not originate in the asteroid belt.

The Kenna ureilite - An ultramafic rock with evidence for igneous, metamorphic, and shock origin

Abstract The Kenna ureilite was found in February, 1972 near the town of Kenna, Roosevelt County, New Mexico U.S.A., weighed 10.9 kg, and measured 26.7 × 14.7 × 14.2 cm; it is the seventh known ureilite. The meteorite is composed of xenoblastic olivine (Fo79.2), commonly rimmed by forsterite (Fo99), and pigeonite (En73Wo9Fs18), in a volumetric ratio of 3:1, set in a matrix of three carbon polymorphs (graphite, lonsdaleite, and diamond) plus nickel-iron metal and troilite. Some thin metalliferous veins penetrating silicate grains contain secondary inclusions of melt with high-calcium clinopyroxene (high-Ca, Mg-rich augite to augite), andesine, K-feldspar, chromite, and siliceous CaO- and alkali-rich glasses of variable compositions. Textural, mineralogical and fabric information suggest a complex history for Kenna, involving igneous, metamorphic and shock processes. The rock appears to have originated as an ultramafic cumulate whose texture and structure was modified by adcumulus processes and by solution and redeposition in a weak deviatoric stress field. A strong mineral elongation lineation was produced during this high-temperature phase accompanied by mild plastic deformation of olivine on the system 0kl[100]. Superimposed on this original texture and fabric are processes resulting from light to moderate (50–250 kbar) shock deformation, as manifested by fracturing of the silicates, slip parallel to (001) in olivine, and twin and translation gliding parallel to (100) in the clinopyroxene. Lonsdaleite and diamond probably formed during this shock phase, which may be associated with the break-up of the parent body, but the relative time of introduction of the carbon-rich matrix is still unresolved.

Studies of Brazilian meteorites: III. Origin and history of the Angra dos Reis achondrite

The Angra dos Reis meteorite fell in 1869 and is a unique achondrite. It is an ultramafic igneous rock, pyroxenite, with 93% fassaite pyroxene which has 15.7% Ca-Tschermaks molecule, plus calcic olivine (Fo53.1; 1.3% CaO), green hercynitic spinel, whitlockite (merrillite), metallic Ni-Fe, troilite, as well as magnesian kirschsteinite (Ks62.3Mo37.7), within olivine grains, and celsian (Cs90.2An7.7Ab1.7Or0.4) which are phases reported in a meteorite for the first time, and plagioclase (An86.0), baddeleyite, titanian magnetite (TiO2, 21.9%), and terrestrial hydrous iron oxide which are phases reported for the first time in this meteorite. Petrofabric analysis shows that fassaite has a preferred orientation and lineation which is interpreted as being due to cumulus processes, possibly the effect of post-depositional magmatic current flow or laminar flow of a crystalline mush. The mineral chemistry indicates crystallization from a highly silica-undersaturated melt at low pressure. Since the meteorite formed as a cumulate, pyroxene crystals may have gravitationally settled from a melt which crystallized melilite first. Plagioclase would be unstable in such a highly undersaturated melt, and feldspathoids would be rare or absent due to the very low alkali contents of the melt. The presence of rare grains of plagioclase and celsian may be the result of late-stage crystallization of residual liquids in local segregations. Thus, the Eu anomaly in Angra dos Reis may be the result of pyroxene separation from a melt which crystallized melilite earlier, rather than plagioclase as previously suggested.

The Shaw meteorite: History of a chondrite consisting of impact-melted and metamorphic lithologies

Abstract The Shaw L-group chondrite consists of three intermingled lithologies. One is light-colored and has a poikilitic texture, consisting of olivine (many skeletal and euhedral) and augite crystals surrounded by larger (up to 1 mm) orthopyroxene grains; plagioclase occurs between orthopyroxene crystals and rare, small ( c -axes of the olivines are aligned. The light-colored lithology also contains numerous vugs and vesicles: SEM studies reveal euhedral, possibly vapor-deposited, crystals of olivine and pyroxene in the vugs. A second lithologic type is dark-colored, contains remnant chondrules. and has a microgranular texture. Poikilitic orthopyroxene crystals, where present, are smaller (0.1–0.2mm) than they are in the light-colored lithology. Microgranular olivine crystals contain 2 slab shows, contrary to published opinions, that Shaw contains normal L-group chondrite abundances of metal and troilite. However, these phases are distributed irregularly throughout the meteorite. The light colored lithology is nearly devoid of metal and troilite and centimeter-sized metal-troilite globules occur between the three silicate lithologies. Wherever the metal occurs, it consists of nearly homogeneous martensite (13.9 wt% Ni) rimmed by kamacite (7.1 wt% Ni). These data indicate that Shaw is a partly-melted shock-breccia. The light-colored lithology must have been totally melted, as shown by the presence of aligned. CaO-rich, skeletal olivines; Si-K-rich residual material: and vugs and vesicles lined with euhedral crystals of mafic silicates. The dark areas appear to be unmelted target rock of L-group composition. Analysis of the growth of kamacite at the taenite (now martensite) borders indicates a cooling rate of ~ 3 C/10 3 yr. or one thousand times faster than most ordinary chondntes. The Shaw impact event probably formed a crater several kilometers in diameter on its meteorite parent body.

Primary magmatic carbon in ureilites: Evidence from cohenite-bearing metallic spherules

Metallic spherules containing cohenite, Fe-Ni metal, sulfides, and rare phosphorus-bearing minerals occur as inclusions within olivine and pigeonite grains in five low-shock ureilites. The bulk compositions, mineralogy, and textures of these spherules indicate that they represent Fe-Ni-C-S alloys that existed as droplets of immiscible metallic liquid in the magma(s) from which ureilites crystallized. At magmatic temperatures these alloys were carbon-saturated and probably coexisted with graphite in the magma. This is consistent with the occurrence of graphite crystals as inclusions in olivine in some ureilites and supports the view that graphite in ureilites was a primary magmatic component. Graphite could have been a residue from the solid source materials that melted to produce the ureilite parent magma(s) or could have crystallized from the magma(s). The metallic liquids separated into immiscible Fe-C liquids and Fe-S liquids after they were trapped within silicates. The Fe-C liquids contained 4–5 wt.% C and 3–7 wt.% Ni. Cohenite was the liquidus phase and started to crystallize at 1200–1225°C, forming euhedral and subhedral crystals. At the eutectic (

Primary igneous carbon in ureilites: Petrological implications

1175°C) γ-iron containing 7–9% Ni and 1.8% C crystallized along with cohenite. At lower temperatures the γ-iron exsolved cohenite, and possibly α-iron.