Alastair W. Tait^a, Andrew G. Tomkins^a, Bélinda M. Godel^b, Siobhan A. Wilson^a and Pavlina Hasalova^a,c

^aSchool of Geosciences, Monash University, Melbourne, Victoria 3800, Australia
^bCSIRO Earth Science and Resource Engineering, Australian Resources Research Centre, 26 Dick Perry Ave., Kensington, Western Australia 6151, Australia
^cCurrent Address: Česká Geologická Služba, 118 21 Praha 1, Czech Republic

Despite the fact that the number of officially classified meteorites is now over 45,000, we lack a clearly defined sequence of samples from a single parent body that records the entire range in metamorphic temperatures from pristine primitive meteorites up to the temperatures required for extensive silicate partial melting. Here, we conduct a detailed analysis of Watson 012, an H7 ordinary chondrite, to generate some clarity on the textural and chemical changes associated with equilibrium-based silicate partial melting in chondritic meteorites. To do this we compare the textures in the meteorite with those preserved in metamorphic contact aureoles on Earth. The most distinctive texture generated by the partial melting that affected Watson 012 is an extensively interconnected plagioclase network, which is clearly observable with a petrographic microscope. Enlarged metal-troilite grains are encapsulated at widenings in this plagioclase network, and this is clearly visible in reflected light. Together with these features, we define a series of other characteristics that can be used to more clearly classify chondritic meteorites as being of petrologic Type 7. To provide comprehensive evidence of silicate partial melting and strengthen the case for using simple petrographic observations to classify similar meteorites, we use high-resolution X-ray computed tomography to demonstrate that the plagioclase network has a high degree of interconnectedness and crystallised as large (cm-scale) skeletal crystals within an olivine-orthopyroxene-clinopyroxene framework, essentially pseudomorphing a melt network. Back-scattered electron imaging and element mapping are used to show that some of the clino- and orthopyroxene in Watson 012 also crystallised from silicate melt, and the order of crystallisation was orthopyroxene clinopyroxene plagioclase. X-ray diffraction data, supported by bulk geochemistry, are used to show that plagioclase and ortho- and clinopyroxene were added to the Watson 012 sample by through-flowing basaltic melt. Along with the absence of glass and granophyre, this interconnected network of coarse-grained skeletal plagioclase indicates that the sample cooled slowly at depth within the parent body. The evidence of melt flux indicates that Watson 012 formed in the presence of a gravitational gradient, and thus at significant distance from the centre of the H chondrite parent body (the gravitational gradient at the centre would be zero). Our interpretation is that incipient silicate partial melting in Watson 012 occurred when a region of radiogenically heated H6 material located at considerable depth (possibly at ~15-20 km from surface) was heated by an additional ca. 200-300°C in association with a large shock event. Due to insulation at depth within an already hot parent body, the post-shock temperature equilibrated and remained above the solidus long enough for widespread equilibrium-based silicate partial melting, and for melt to migrate. Although the observed melting may have been facilitated by additional heating from an impact event, this is not an example of instantaneous shock melting, which produces thermal disequilibrium at short length scales and distinctly different textures. A small number of H, L and LL chondrites have been previously classified as being of petrologic Type 7; with our new criteria to support that classification, these represent our best opportunity to explore the transition from high temperature sub-solidus metamorphism through the onset of silicate partial melting in three different parent bodies.

Reference
Tait AW, Tomkins AG, Godel BM, Wilson SA and Hasalova P (in press) Investigation of the H7 ordinary chondrite, Watson 012: Implications for recognition and classification of Type 7 meteorites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.039]
Copyright Elsevier

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Gwendolyn D. Bart

Univ. of Idaho, Dept. of Physics, 875 Perimeter Drive MS 0903, Moscow, ID, 83844, USA

Small impact craters (~10-300 m) that encounter a strength transition in the target (like a regolith over bedrock) have unique morphologies. Previous studies have used these morphologies as indicators of regolith depth. This paper reports on several new analyses that expand our understanding of the quantitative relationship between small crater morphology and target layering. I describe three practical situations where the application of the updated method is ambiguous because the specific relationship between the target layering and the crater morphology has never been analyzed. In order to resolve the ambiguity, I report on new analyses of computer models and lunar data that demonstrate how the dimensions of the crater shape relate to layer depth. I also analyze the boundary conditions under which the crater-layering relationship will enable determination of layering depth. Finally, in light of the greater understanding of the crater-layering relationship, I discuss the possible application of this method to Mars.

Reference
Bart GD (in press) The Quantitative Relationship Between Small Impact Crater Morphology and Regolith Depth. Icarus
[doi:10.1016/j.icarus.2014.03.020]
Copyright Elsevier

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J.-H. Wang and R. Brasser

Institute for Astronomy and Astrophysics, Academia Sinica; 11F AS/NTU building, 1 Roosevelt Rd., Sec. 4, 10617 Taipei, Taiwan

The origin of the Halley-type comets (HTCs) is one of the last mysteries of the dynamical evolution of the solar system. Prior investigation into their origin has focused on two source regions: the Oort Cloud and the scattered disc. From the former it has been difficult to reproduce the non-isotropic, prograde skew in the inclination distribution of the observed HTCs without invoking a multi-component Oort Cloud model and specific fading of the comets. The scattered disc origin fares better, but needs an order of magnitude more mass than is consistent with theory and observations. Here we revisit the Oort Cloud origin and include cometary fading. Our observational sample stems from the JPL catalogue. We only keep comets discovered and observed after 1950, but place no a priori restriction on the maximum perihelion distance of observational completeness. We then numerically evolve half a million comets from the Oort Cloud through the realm of the giant planets and keep track of their number of perihelion passages with perihelion distance q < 2.5 AU, below which the activity is supposed to increase considerably. We can simultaneously fit the HTC inclination and semi-major axis distribution very well with a power-law fading function of the form m^-k, where m is the number of perihelion passages with q < 2.5 AU and k is the fading index. We match both the inclination and semi-major axis distributions when k ~ 1 and the maximum imposed perihelion distance of the observed sample is q_m ~ 1.8 AU. The value of k is higher than the one obtained for the long-period comets (LPCs), for which typically k ~ 0.7. This increase in k is most likely the result of cometary surface processes. We argue the HTC sample is now most likely complete for q_m < 1.8 AU. We calculate that the steady-state number of active HTCs with diameter D > 2.3 km and q < 1.8 AU is on the order of 100.

Reference
Wang J-H and Brasser R (2014) An Oort Cloud origin of the Halley-type comets. Astronomy & Astrophysics 563:A122.
[doi:10.1051/0004-6361/201322508]
Reproduced with permission © ESO

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