¹Lang Zhang,^1,2Ai-Cheng Zhang,¹Shu-Zhou Wang
American Mineralogist 108, 1597-1611 Link to Article [http://www.minsocam.org/msa/ammin/toc/2023/Abstracts/AM108P1597.pdf]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China 2
²CAS Center for Excellence in Comparative Planetology, China
Copyright: The Mineralogical Society of America

Apatite is an important petrogenetic indicator in extraterrestrial materials. Here, we report the
mineralogical features of apatite and associated phases in three brachinites Northwest Africa (NWA)
4969, NWA 10637, and NWA 11756. Two types of apatite are observed: intergranular apatite and apatite
inclusion within chromite and silicate minerals. The intergranular chlorapatite is enclosed by or penetrated by irregular porous merrillite, indicating chlorapatite replacement by merrillite. The intergranular
chlorapatite is closely associated with a fine-grained pyroxene-troilite intergrowth along olivine grain
boundaries, which is a sulfidization product of olivine. High-Ca pyroxene is observed as a constituent
phase in the intergrowth for the first time. The apatite inclusions are either monomineralic or closely
associated with subhedral-euhedral pore-free merrillite. In NWA 4969, the apatite inclusions show a
large compositional variation from chlorapatite to fluorapatite and are systematically more F-rich than
intergranular apatite; while the apatite inclusions in NWA 10637 and NWA 11756 are chlorapatite. Most
of the two apatite types in brachinites contain oriented tiny or acicular chromite grains, suggesting the
exsolution of chromite from apatite. We propose that apatite replacement by merrillite, formation of
pyroxene-troilite intergrowth, and exsolution of chromite in apatite were caused by a shock-induced,
transient heating event (~930–1000 °C) on the brachinite parent body. This heating event resulted in
halogen devolatilization during replacement of the intergranular apatite by merrillite, which probably
disturbed the Mn-Cr isotopic system in brachinites as well. We also propose that the apatite inclusions
could be a residual precursor material of the brachinites.

¹Ian Szumila,¹Dustin Trail,²Timmons Erickson,³Justin I. Simon,⁴Matthew M. Wielicki,⁵Tom Lapen,¹Miki Nakajima,³Marc Fries,⁶Elizabeth A. Bell
American Mineralogist 108, 1516-1529 Link to Article [http://www.minsocam.org/msa/ammin/toc/2023/Abstracts/AM108P1516.pdf]
¹Univesity of Rochester, Earth and Environmental Science, Rochester, New York 14611, U.S.A.
²Jacobs – JETS, NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
³Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
⁴University of Alabama, Department of Geological Sciences, Tuscaloosa, Alabama 35487, U.S.A.
⁵University of Houston, Department of Earth and Atmospheric Sciences, Houston, Texas 77004, U.S.A.
⁶University of California Los Angeles, Department of Earth, Planetary and Space Sciences, Los Angeles, California 90095, U.S.A.
Copyright: The Mineralogical Society of America

Impact events modify and leave behind a complex history of rock metamorphism on terrestrial
planets. Evidence for an impact event may be recorded in physical changes to minerals, such as mineral
deformation and formation of high P-T polymorphs, but also in the form of chemical fingerprints,
such as enhanced elemental diffusion and isotopic mixing. Here we explore laboratory shock-induced
physical and chemical changes to zircon and feldspar, the former of which is of interest because its trace
elements abundances and isotope ratios are used extensively in geochemistry and geochronology. To
this end, a granular mixture of Bishop Tuff sanidine and Kuehl Lake zircon, both with well characterized Pb isotope compositions, was prepared and then shocked via a flat plate accelerator. The peak
pressure of the experiment, as calculated by the impedance matching method, was ~24 GPa although a
broader range of P-T conditions is anticipated due to starting sample porosity. Unshocked and shocked
materials were characterized via scanning electron microscopy (SEM), electron backscatter diffraction
(EBSD), and Raman spectroscopy. These methods show that the starting zircon material had abundant
metamict regions, and the conversion of the feldspar to glass in the post-shock material. Analyses of
the shocked product also yielded multiple occurrences of the high-pressure ZrSiO4 polymorph reidite,
with some domains up to 300 μm across. The possibility of U-Pb system disturbance was evaluated
via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and secondary ion
mass spectrometry (SIMS). The isotopic data reveal that disturbance of the U-Pb geochronometer
in the reidite was minimal (<2% for the main U-Pb geochronometers). To better constrain the P-T conditions during the shock experiment, we complement impedance matching pressure calculations with iSALE2D impact simulations. The simulated results yield a range of P-T conditions experienced during the experiment and show that much of the sample may have reached >30 GPa, which
is consistent with formation of reidite. In the recovered shocked material, we identified lamellae of
reidite, some of which interlock with zircon lamellae. Reidite {112} twins were identified, which we
interpret to have formed to reduce stress between the crystal structure of the host zircon and reidite.
These two findings support the interpretation that shear transformation enabled the transition of zircon
to reidite. The size and presence of reidite found here indicate that this phase is probably common in
impact-shocked crustal rocks that experienced ~25 to ~35 GPa, especially when the target material
has porosity. Additionally, shock loading of the zircon and transformation to reidite at these pressures
in porous materials is unlikely to significantly disturb the U-Pb system in zircon and that the reidite
inherits the primary U and Pb elemental and isotopic ratios from the zircon.