Olivines and olivine coronas in mesosiderites

Olivines and their surrounding coronas in mesosiderites have been studied texturally and compositionally by optical and microprobe methods. Most olivine is compositionally homogeneous but some is irregularly zoned. It ranges from Fo_58–92 and shows no consistent pattern of distribution within and between mesosiderites. Olivine occurs as large single crystals or as partially recrystallized mineral clasts, except for two lithic clasts. One is in Emery, the other in Vaca Muerta, and they are both shock-modified olivine orthopyroxenites. $\text{FeO}\text{MnO}$ ratios in olivine exhibit a variety of differing trends and range from 22–46, most commonly 35–40. These values are lower than those in olivine from diogenites sensu stricto (45–50) and have therefore experienced a different history. Some of the olivine clasts could have coexisted with some of the large orthopyroxene clasts as equilibrium assemblages, but some could not. Much of the olivine may be derived from mesosiderite olivine orthopyroxenites, which differ from diogenites sensu stricto. More magnesian olivine may be a residue from one or more source rocks, with varying degrees of melting. These events probably occurred in a highly evolved and differentiated parent body.Fine-grained coronas surround olivine, except for those in impact-melt group mesosiderites (Simondium, Hainholz, Pinnaroo) and those without tridymite in their matrices (Bondoc, Veramin). Coronas consist largely of orthopyroxene, plagioclase, clinopyroxene, chromite, merrillite and ilmenite and are similar to the matrix, but lack metal and tridymite. Coronas contain abundant orthopyroxene but are unusually rich in chromite (up to 7%) and merrillite (up to 20%). The outer parts of the corona grade into the matrix, but have little or no metal and tridymite. Texturally the innermost part of the corona can be divided into three stages of development: I Radiating acicular; II Intermediate; III Granular. Stage I is the result of the greatest disequilibrium between olivine and matrix orthopyroxene and Stage III has the least disequilibrium. Coronas are the result of the reaction olivine + tridymite = orthopyroxene, probably because FeO (and MgO) diffuse from olivine to tridymite in the matrix. Absence of metal and concentration of chromite in the corona are probably the result of an FeO potential gradient away from the olivine. Merrillite concentrations are a result of P₂O₅ migration into the corona but are controlled by the availability of calcic pyroxene, or possibly plagioclase. Although the coronas are texturally similar to terrestrial and lunar counterparts, they are unique and represent different kinds of reactions marked by a large degree of intra-corona diffusion under dry conditions. Opaque oxide-silicate-metal buffer assemblages yield apparent equilibration conditions of about 840°C and fO₂ near 10^?20. Poikiloblastic pyroxene textures in some coronas suggest a closing of reaction systems between 900 and 1000°C and such systems may record a higher temperature stage of development.     

The nature and origin of ureilites

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, Haverö, 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.