¹Tahnee Burke,¹Andrew G. Tomkins,²Zsanett Pinter,³Andrew D. Langendam,⁴Laura A. Miller
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70016]
¹School of Earth, Atmosphere and Environment, Monash University, Melbourne, Victoria, Australia
²CSIRO Mineral Resources, Microbeam Laboratory, Clayton, Victoria, Australia
³ANSTO-Australian Synchrotron, Clayton, Victoria, Australia
⁴Research School of Earth Sciences, Australian National University, Canberra, Australian Capital Territory, Australia
Published by arrangement with John Wiley & Sons

The phosphates, apatite and merrillite, are accessory phases in all martian meteorites. Although apatite is commonly used to assess volatile content and speciation in martian meteorites, merrillite is at least twice as abundant in most samples, but poorly understood. Given that shergottites are divided into enriched, intermediate, and depleted subgroups based on bulk differences in light rare earth element (LREE) abundance and isotopic compositions, an understanding of phosphate mineral behavior is essential to deciphering the petrogenetic differences between these groups because they are the main REE-bearing phases. This study examines 10 enriched shergottites, six intermediate shergottites, and four depleted shergottites to investigate systematic variations in phosphate mineralogy and geochemistry. Two nakhlites, a chassignite, ALH 84001, and two pairs of NWA 7034 were also examined to cover all martian meteorite types known to date. Fourteen of the shergottites were previously classified into enriched, intermediate, and depleted subgroups based on bulk rock REE trends and La/Yb ratios. The remaining six shergottites had not been subgrouped during classification. All samples were elementally mapped using the XFM beamline at the Australian Synchrotron, which provided the relative abundance of merrillite, apatite, K-feldspar, and maskelynite within each sample (the same can be achieved with electron microprobe or SEM). We show that it is possible to classify shergottites from a single representative thin section using apatite to merrillite ratios (A10/M, where A10 is apatite abundance × 10) and K-feldspar to phosphate ratios (K10/P, where K10 is K-feldspar abundance × 10). Enriched shergottites typically have A10/M of 1.08 to 8.72 and K10/P of 1.85 to 13.34; intermediate shergottites have A10/M ranging from 0.5 to 0.96 and K10/P of 0.36 to 0.94; and depleted shergottites have A10/M ranging from 0.26 to 0.42 and K10/P of 0.09 to 0.39. Calculating these ratios thus provides a quick and straightforward method of chemically classifying shergottites that avoids the need to destroy samples for bulk rock REE analysis.

^1,2Malika Singhal,³Himela Moitra,⁴Souvik Mitra,⁵Aurovinda Panda,⁵Jayant Kumar Yadav,⁵D. Srinivasa Sarma,⁵Devender Kumar,¹Naveen Chauhan,³Saibal Gupta,¹Ashok Kumar Singhvi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70021]
¹Atomic and Molecular Physics Division, Physical Research Laboratory, Ahmedabad, India
²Indian Institute of Technology, Gandhinagar, Palaj, India
³Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, India
⁴Department of Geology, Presidency University, Kolkata, India
⁵CSIR-National Geophysical Research Institute, Hyderabad, India
Published by arrangement with John Wiley & Sons

In this study, naturally occurring jarosite samples from Kachchh, India (considered to be Martian analogue) were characterized using Fourier Transform Infrared Spectroscopy (FTIR), Cathodoluminescence–Energy Dispersive X-ray Spectroscopy (CL-EDXS), and Luminescence (thermoluminescence [TL], blue and infrared stimulated luminescence [BSL and IRSL]) methods. FTIR and CL-EDXS studies suggested that jarosite preserves its luminescence characteristics even after annealing the samples to 450°C. This facilitated luminescence studies (TL/BSL/IRSL) to assess the potential use of luminescence-dating methods to establish the chronology of jarosite formation or its transport. Jarosite exhibited TL, BSL, and IRSL signals with varied sensitivities. The TL glow curve of jarosite comprised glow peaks at 100, 150, 300, and 350°C, reproducible over multiple readout cycles. The least bleachable TL glow peak at 350°C is reduced to (1/e)th of its glow peak intensity (i.e., 36%) with ~100 min of light exposure under a sunlamp. BSL and IRSL optical decay signals comprised three components. These signals exhibited athermal fading of g ~ 6%/decade, but pIRIR signal at 225°C showed a near zero fading. The saturation doses (2D0) ranged from 700 Gy to 2600 Gy for different signals, which suggests a dating range of ~25 ka using a reported Martian total dose rate of 65 Gy/ka, primarily due to cosmic rays. Multiple TL glow peaks and their widely differing stability also offer promise to discern changes in cosmic ray fluxes over a century to millennia time scale through inverse modeling and laboratory experiments.