The Gove relict iron meteorite from Arnhem Land, Northern Territory, Australia

1,2Alex W. R. Bevan,1Peter J. Downes,3Dermot A. Henry,4Michael Verrall,5Peter W. Haines
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13307]
1Department of Earth and Planetary Sciences, Western Australian Museum, Locked Bag 49, Welshpool DC, Western Australia, 6986 Australia
2Department of Imaging and Applied Physics, Curtin University, GPO Box U1987, Western Australia, 6845 Australia
3Geosciences Department, Museum Victoria, GPO Box 666, Melbourne, Victoria, 3001 Australia
4CSIRO Mineral Resources, Australian Resources Research Centre, 26 Dick Perry Avenue, Technology Park, Kensington, Western Australia, 6151 Australia
5Geological Survey of Western Australia, Department of Mines, Industry Regulation and Safety, 100 Plain Street, East Perth, Western Australia, 6004 Australia
Published by arrangement with John Wiley & Sons

On February 24, 1979, a deeply oxidized mass of iron meteorite was excavated from bauxite at an open cut mine on the Gove Peninsula, Northern Territory, Australia. The meteorite, measuring 0.75–1 m in diameter and of unknown total weight, was found at coordinates 12°15.8′S, 136°50.3′E. On removal from the ground, the meteorite is reported to have disintegrated rapidly. A preliminary analysis at the mine laboratory reportedly gave 8.5 wt% Ni. A modern analysis of oxidized material gave Ni = 32.9, Co = 3.67 (both mg g−1), Cr = 168, Cu = 195, Ga = 22.5, Ge = <70, As = 4.16, W = 1.35, Ir = 10.5, Pt = 21.2, Au = 0.672 (all μg g−1), Sb = <150, and Re = 844 (both ng g−1). Competent fragments of oxidized material retain a fine to medium Widmanstätten pattern with an apparent average bandwidth of 0.5 mm (range 0.2–0.9 mm in plane section). Primary mineralogy includes rare γ–taenite and daubréelite, and secondary minerals produced by weathering include awaruite (with up to 78.5 wt% Ni) and an, as yet, unnamed Cu‐Cr‐bearing sulfide with the ideal formula CuCrS2 that is hitherto unknown in nature. Deep weathering has masked many of the features of the meteorite; however, the analysis normalized to the analyses of fresh iron meteorites favors chemical group IIIAB. The terrestrial age of the meteorite is unknown, although it is likely to be in the Neogene (2.5–23 Ma), which is widely accepted as the major period of bauxite formation in the Northern Territory of Australia. Gove is the second authenticated relict meteorite found in Australia.

Understanding the emplacement of Martian volcanic rocks using petrofabrics of the nakhlite meteorites

1,2,3Luke Daly et al. (>10)
Earth and Planetary Science Letters 220-230 Link to Article [https://doi.org/10.1016/j.epsl.2019.05.050]
1School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, UK
2Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
3Australian Centre for Microscopy and Microanalysis, University of Sydney, Sydney 2006, NSW, Australia
Copyright Elsevier

In order to validate calculated ages of the Martian crust we require precise radiometric dates from igneous rocks where their provenance on the Martian surface is known. Martian meteorites have been dated precisely and quantitatively, but the launch sites are currently unknown. Inferring the formation environment of a correlated suite of Martian meteorites can constrain the nature and complexity of the volcanic system they formed from. The nakhlite meteorites are such a suite of augite-rich rocks that sample the basaltic crust of Mars, and as such can provide unique insights into its volcanic processes. Using electron backscatter diffraction we have determined the shape-preferred and crystallographic-preferred orientation petrofabrics of four nakhlites (Governador Valadares, Lafayette, Miller Range 03346 and Nakhla) in order to understand the conditions under which their parent rocks formed. In all samples, there is a clear link between the shape-preferred orientation (SPO) and crystallographic-preferred orientation (CPO) of augite phenocrysts. This relationship reveals the three-dimensional shape of the augite crystals using CPO as a proxy for 3D SPO, and also enables a quantitative 3-dimensional petrofabric analysis. All four nakhlites exhibit a foliation defined by the CPO of the augite <c> axis in a plane, although individual meteorites show subtle textural variations. Nakhla and Governador Valadares display a weak CPO lineation within their <c> axis foliation that is interpreted to have developed in a combined pure shear/simple shear flow regime, indicative of emplacement of their parent rock as a subaerial hyperbolic lava flow. By contrast, the foliation dominated CPO petrofabrics of Lafayette and Miller Range 03346 suggest formation in a pure shear dominated regime with little influence of hyperbolic flow. These CPO petrofabrics are indicative of crystal settling in the stagnant portion of cooling magma bodies, or the flattening area of spreading lava flows. The CPO foliation of Lafayette’s is substantially weaker than Miller Range 03346, probably due to its higher phenocryst density causing grain-grain interactions that hindered fabric development. The CPO petrofabrics identified can also be used to determine the approximate plane of the Martian surface and the line of magma flow to within ∼20°. Our results suggest that the nakhlite launch crater sampled a complex volcanic edifice that was supplied by at least three distinct magmatic systems limiting the possible locations these rocks could have originated from on Mars.

The Milton pallasite and South Byron Trio Irons: Evidence for oxidation and core crystallization

1T.J.McCoy, 1C.M.Corrigan,1,2K.Nagashim, 1,3V.S.Reynolds, 4R.D.Ash, 4W.F.McDonough,5,6,7J.Yang, 5J.I.Goldstein, 4C.D.Hilton
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.005]
1Department of Mineral Sciences, Smithsonian Institution, Washington, D.C. 20560-0119, USA
2Hawai’i Institute of Geophysics and Planetology, Univ. of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
3Dept. of Geography and Earth Sciences, UNC-Charlotte, Charlotte, NC 28223 USA
4Department of Geology, University of Maryland, College Park, Maryland 20742 USA
5Dept. of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, Massachusetts 01003 USA
6Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China 100029
7Institutions of Earth Science, University of Chinese Academy of Sciences, Beijing, China 100029
Copyright Elsevier

The link between the Milton pallasite and the South Byron Trio irons is examined through metallography and metallogaphic cooling rates; major, minor, and trace element compositions of metal; inclusion mineralogy and mineral compositions; and oxygen isotopic compositions. The metallic hosts of these Ni-rich meteorites (18.2-20.3 wt.% Ni) are dominated by plessite with spindles of kamacite and schreibersite. The presence of ∼50 nm wide tetrataenite and absence of high-Ni particles in the cloudy zone in Milton suggest cooling of ∼2,000 K/Myr or >10,000 K/Myr. Compositionally, the metallic host in all four meteorites exhibits modest (1-2 orders of magnitude compared to CI chondrites) depletions of volatile elements relative to refractory elements, and marked depletions in the redox sensitive elements W, Mo, Fe, and P. Oxygen isotopic compositions (Δ17O) are, within uncertainty, the same for the Milton and the South Byron Trio and for IVB irons. Similarities in metallography, metal composition, inclusion mineralogy, and oxygen (Δ17O), molybdenum and ruthenium isotopic composition suggest that the Milton pallasite and South Byron Trio irons could have originated on a common parent body as chemically distinct melt, or on separate parent bodies that experience similar cosmochemical and geochemical processes. The Milton pallasite and South Byron Trio irons share a number of properties with IVB irons, including metallography, enrichment in highly siderophile elements and nickel, inclusion mineralogy and oxygen isotopic composition, suggesting they formed in a similar nebular region through common processes, although Milton and the South Byron Trio did not experience the dramatic volatile loss of the IVB irons. Depletions in W, Mo, Fe, and P relative to elements of similar volatility likely result from oxidation, either in the nebula prior to accretion or on the parent body during melting. Oxidation ∼73 wt.% of Fe is indicated, with a correspondingly FeO-rich mantle and smaller core. If Milton and the South Byron Trio sample a common core, Milton formed near the surface of the core after stripping of the silicate shell and may have experienced rapid solidification and contamination by an impactor. The molten core, from which the South Byron Trio irons crystallized, solidified from the outside in.

Constraints on asteroid magnetic field evolution and the radii of meteorite parent bodies from thermal modelling

1James F.J.Bryson,2,3,4Jerome A.Neufeld,5Francis Nimmo
Earth and Planetary Science Letters 521, 68-78 Link to Article [https://doi.org/10.1016/j.epsl.2019.05.046]
1Department of Earth Sciences, University of Cambridge, Cambridge, UK
2BP Institute, University of Cambridge, Cambridge, UK
3Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Cambridge, UK
4Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, UK
5Department of Earth and Planetary Sciences, University of California, Santa Cruz, CA, USA
Copyright Elsevier

Paleomagnetic measurements of ancient terrestrial and extraterrestrial samples indicate that numerous planetary bodies generated magnetic fields through core dynamo activity during the early solar system. The existence, timing, intensity and stability of these fields are governed by the internal transfer of heat throughout their parent bodies. Thus, paleomagnetic records preserved in natural samples can contain key information regarding the accretion and thermochemical history of the rocky bodies in our solar system. However, models capable of predicting these field properties across the entire active lifetime of a planetary core that could relate the processes occurring within these bodies to features in these records and provide such information are limited. Here, we perform asteroid thermal evolution models across suites of radii, accretion times and thermal diffusivities with the aim of predicting when fully and partially differentiated asteroids generated magnetic fields. We find that dynamo activity in both types of asteroid is delayed until ∼4.5-5.5 Myr after calcium-aluminium-rich inclusion formation due to the partitioning of 26Al into the silicate portion of the body during differentiation and large early surface heat fluxes, followed by a brief period (<12.5 Myr for bodies with radii <500 km) of thermally-driven dynamo activity as heat is convected from the core across a partially-molten magma ocean. We also expect that gradual core solidification produced compositionally-driven dynamo activity in these bodies, the timing of which could vary by tens to hundreds of millions of years depending on the S concentration of the core and the radius of the body. There was likely a pause in core cooling and dynamo activity following the cessation of convection in the magma ocean. Our predicted periods of magnetic field generation and quiescence match eras of high and low paleointensities in the asteroid magnetic field record compiled from paleomagnetic measurements of multiple meteorites, providing the possible origins of the remanent magnetisations carried by these samples. We also compare our predictions to paleomagnetic results from different meteorite groups to constrain the radii of the angrite, CV chondrite, H chondrite, IIE iron meteorite and Bjürbole (L/LL chondrite) parent bodies and identify a likely nebula origin for the remanent magnetisation carried by the CM chondrites.

Unusual neon isotopic composition in Neoproterozoic sedimentary rocks: Fluorine bearing mineral contribution or trace of an impact event?

1Chavrit, D.,1Moreira, M.A.,2Fike, D.A.,1,3Moynier, F.
Chemical Geology 520, 52-59 Link to Article [DOI: 10.1016/j.chemgeo.2019.04.025]
1Université de Paris, Institut de physique du globe de Paris, CNRS, Paris, F-75005, France
2Department of Earth and Planetary Sciences, Washington University, St Louis, MO 63130, United States
3Institut Universitaire de France, Paris, France

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Mafic minerals in the South Pole‐Aitken basin

1Xunyu Zhang,2Meng‐Hua Zhu,1Roberto Bugiolacchi
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005870]
1Space Science Institute, Macau University of Science and Technology, Macau, China
2Chinese Academy of Sciences Center for Excellence in Comparative Planetology, Hefei, China
Published by arrangement with John Wiley & Sons

The formation of the South Pole‐Aitken (SPA) basin is thought to excavate the deep crust or mantle because of its large size. The pervasive orthopyroxene‐dominated materials found across the basin suggest that they either represent the SPA impact melt or the excavated materials from the lower crust and/or upper mantle. This study analyzes the relative content and distribution of mafic minerals in the SPA area based on the spectra from small fresh craters. The orthopyroxene‐dominated materials in the non‐mare regions are classified into two types based on their distribution and different composition. One is distributed from the center to the edge across the SPA basin and interpreted as the SPA impact melt. The other is Mg‐richer and generally located in some plagioclase‐rich regions (e.g., some large impact craters/basins and the SPA edge), thought to represent materials from the lower crust and/or upper mantle. For the maria in the SPA area, the basaltic materials in the northwest are found to be richer in olivine and/or clinopyroxene than the southern ones.

The Pressure and Temperature Limits of Likely Rocky Exoplanets

1C.T. Unterborn,2W.R. Panero
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005844]
1School of Earth and Space Exploration, Arizona State University
2School of Earth Sciences, The Ohio State University
Published by arrangement with John Wiley & Sons

The interior composition of exoplanets is not observable, limiting our direct knowledge of their structure, composition, and dynamics. Recently described observational trends suggest that rocky exoplanets, that is, planets without significant volatile envelopes, are likely limited to <1.5 Earth radii. We show that given this likely upper limit in the radii of purely‐rocky super‐Earth exoplanets, the maximum expected core‐mantle boundary pressure and adiabatic temperature is relatively moderate, 630 GPa and 5000 K, while the maximum central core pressure varies between 1.5 and 2.5 TPa. We further find that for planets with radii less than 1.5 Earth radii, core‐mantle boundary pressure and adiabatic temperature are mostly a function of planet radius and insensitive to planet structure. The pressures and temperatures of rocky exoplanet interiors, then, are less than those explored in recent shock‐compression experiments, ab‐initio calculations, and planetary dynamical studies. We further show that the extrapolation of relevant equations of state does not introduce significant uncertainties in the structural models of these planets. Mass‐radius models are more sensitive to bulk composition than any uncertainty in the equation of state, even when extrapolated to TPa pressures.