¹Paula Lindgren et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.08.021]
¹Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden
Copyright Elsevier

The CM carbonaceous chondrites have all been aqueously altered, and some of them were subsequently heated in a parent body environment. Here we have sought to understand the impact of short duration heating on a highly aqueously altered CM through laboratory experiments on Allan Hills (ALH) 83100. Unheated ALH 83100 contains 83 volume per cent serpentine within the fine-grained matrix and altered chondrules. The matrix also hosts grains of calcite and dolomite, which are often intergrown with tochilinite, Fe(Ni) sulphides (pyrrhotite, pentlandite), magnetite and organic matter. Some of the magnetite formed by replacement of Fe(Ni) sulphides that were accreted from the nebula. Laboratory heating to 400 °C has caused partial dehydroxylation of serpentine and loss of isotopically light oxygen leading to an increase in bulk δ¹⁸O and fall in Δ¹⁷O. Tochilinite has decomposed to magnetite, whereas carbonates have remained unaltered. With regards to infrared spectroscopy (4000–400 cm^-1; 2.5 – 25 µm), heating to 400 °C has resulted in decreased emissivity (increased reflectance), a sharper and more symmetric OH band at 3684 cm^-1 (2.71 µm), a broadening of the Si-O stretching band together with movement of its minimum to longer wavenumbers, and a decreasing depth of the Mg-OH band (625 cm^-1; 16 µm). The Si-O bending band is unmodified by mild heating. With heating to 800 °C the serpentine has fully dehydroxylated and recrystallized to ∼Fo_60/70 olivine. Bulk δ¹⁸O has further increased and Δ¹⁷O decreased. Troilite and pyrrhotite have formed, and recrystallization of pentlandite has produced Fe,Ni metal. Calcite and dolomite were calcined at ∼700 °C and in their place is an un-named Ca-Fe oxysulphide. Heating changes the structural order of organic matter so that Raman spectroscopy of carbon in the 800 °C sample shows an increased (D1 + D4) proportional area parameter. The infrared spectrum of the 800 °C sample confirms the abundance of Fe-bearing olivine and is very similar to the spectrum of naturally heated stage IV CM Pecora Escarpment 02010. The temperature-related mineralogical, chemical, isotopic and spectroscopic signatures defined in ALH 83100 will help to track the post-hydration thermal histories of carbonaceous chondrite meteorites, and samples returned from the primitive asteroids Ryugu and Bennu.

^1,2Z.Ren,²P.M.Vasconcelos
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.08.019]
¹State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, 430074, China
²School of Earth and Environmental Sciences, The University of Queensland, Brisbane, Qld 4072, Australia
Copyright Elsevier

Jarosite [KFe₃(SO₄)₂(OH)₆] occurs both as a hydrothermal mineral or as the product of weathering and chemical sedimentation. It has been used in ⁴⁰Ar/³⁹Ar geochronology to date water-rock interaction and weathering processes on the surfaces of Earth and Mars, but the lack of information about Ar diffusivity parameters relevant to specific types of jarosites makes the interpretation of geochronological results tentative. We have filled this gap by investigating Ar diffusion parameters in representative supergene and hypogene jarosites. Detailed diffusion studies were carried out on a hypogene jarosite sample from Gilbert, Nevada, and two supergene jarosite samples from Baiyin, China. The diffusion studies were accompanied by in-situ heating investigations in a transmission electron microscope to directly determine the thermal stability and the phase transformations that jarosite undergoes under progressive heating under ultra-high vacuum. The TEM results suggest that jarosite is stable under vacuum up to ∼ 400 °C, when it undergoes phase transition to yavapaiite [KFe(SO₄)₂] and hematite (Fe₂O₃). Incremental-heating experiments reveal average diffusion parameters of E_a = 138.6 ± 4.2 kJ/mol and ln(D_o/a²) = 9.9 ± 0.9 ln(s^-1) for hypogene jarosite; E_a = 110.3 ± 3.2 kJ/mol and ln(D_o/a²) = 5.7 ± 0.7 ln(s^-1) for one supergene jarosites; and E_a = 141.2 ± 7.9 kJ/mol and ln(D_o/a²) = 11.3 ± 1.7 ln(s^-1) for the other supergene jarosite sample from the same weathering profile. Jarosite closure temperatures depend strongly on sieve size. For samples between 500-200 µm (grain size usually used for samples in Ar geochronology), at a cooling rate 100 °C·Ma^-1, the closure temperatures are 143 ± 18 °C for hypogene and 105 ± 8 and 113 ± 14 °C, respectively, for the two supergene jarosites. Forward modelling of incremental-heating results predicts that coarse-grained hypogene jarosite is retentive of Ar below 50 °C for 100 Ma and below 25 °C for 4 Ga. Densely packed supergene jarosite grains larger than 200 µm are suitable for ⁴⁰Ar/³⁹Ar geochronology at the timescales suitable for investigating water-rock interaction at the surface of Earth and Mars. Fine-grained, porous jarosites require detailed diffusion analyses prior to geochronology due to possible high Ar losses over Ma timescales. The absence of jarosites older than ∼40 Ma on Earth suggests that jarosite may require continuous exposure to acid oxidizing conditions, and that it does not survive burial and exhumation. Therefore, the occurrence of jarosite on Earth and Mars may identify segments of the planets’ surfaces continuously exposure to acid-oxidizing conditions since jarosite precipitation.