Wittmann and two co-authors with IODP-ICDP Expedition 364 Scientists reported in 2019 on the Allanite from the Chicxulub impact is a REE-rich epidote group mineral with complex compositional variability (e.g., [1‒2]), which makes it a challenging target for electron microprobe analysis. To add to the challenge, we chose to characterize the composition of allanite crystals in granitoid rocks that experienced shock pressures of 12‒17 GPa in the peak ring of the 66 Ma old, 200 km-Ø K-Pg Chicxulub impact structure [2]. Our drill core samples were recovered by International Ocean Discovery Program (IODP) Expedition 364 [4‒5].
Using Arizona State’s JEOL JXA-8530F electron microprobe, the authors surveyed petrographic thin sections of 15 samples from the 584.7 m thick Lower Peak Ring section [5]. They characterized allanite grains and their petrographic settings with back-scattered electron images and energy-dispersive spectrometry. Utilizing wave-length-dispersive spectrometry, 20 kV accelerating voltage, 20 nA beam current and beam diameters of 1 to 5 μm, and peak-overlap corrections for X-ray lines of V, Sm, Gd, and Eu with Ti, Ce, Ce and Pr, respectively, they also determined the concentrations of up to 29 elements (Ti, Si, Zr, Th, U, Al, V, La, Ce, Nd, Pr, Sm, Gd, Dy, Er, Eu, Y, P, Nb, Ba, Mg, Fe, Ca, Mn, Sr, Zn, Na,Cl, and F) in selected allanite grains. They evaluated the stoichiometry based on 12.5 oxygen-atoms, including an estimate for Fe2+ and Fe3+ and H2O content, following the scheme devised by [2].
Allanite occurs in granitoid rocks throughout the 584.7 m thick Lower Peak Ring section.
However, they identified large, ~50 to 1000 μm grains only between ~779 and ~1221 mbsf (meters below sea floor). These allanite-(Ce) grains are typically associated with epidote and some of them are transected by veins of epidote. Alteration features include cellular voids, the crystallization of thorite (ThSiO4), and replacement with spherulitic to acicular REE hydroxyl/halogen carbonates, likely bastnäsite [(Ce,La)(CO3)F], parisite [Ca(Ce,La)2(CO3)3F2], and/or synchysite [Ca(Ce,La)(CO3)2F],) that are only a few μm in size; the fine-grained, spherulitic shapes of these REE carbonates may indicate growth due to rapid heating to relatively high temperatures >200⁰C under hydrothermal conditions because slower approach to supersaturation produces euhedral crystal shapes in Nd-hydroxyl carbonate [6].
Well-preserved allanite-(Ce) grains are euhedral and composed of domains with modest ThO2 concentrations <~1 wt% that are stoichiometric allanite (Table 1). However, most allanite-(Ce) grains are compositionally zoned and domains with ThO2 > ~1.30 wt% tend to be non-stoichiometric and metamict (Figure 1). MgO shows a moderately negative correlation with ThO2 (R2=0.46), while FeO and Fe2O3 show weak negative correlations with ThO2 (R2=0.29 and 0.25, respectively). While their alteration is variable and apparently increased if they are located close to impact melt-rich sections in the drill core, we found that overall, the major and minor element concentrations of large, well-preserved allanite-(Ce) grains in the lower peak ring have grossly similar compositions
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