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Physical, chemical and mineral properties of the Polonnaruwa Stones

Wallis, Jamie, Wickramasinghe, N. C., Wallis, Daryl H., Miyake, Nori, Wallis, M. K., Hoover, Richard B., Samaranayake, Anil, Wickramarathne, Keerthi and Oldroyd, Anthony 2013. Physical, chemical and mineral properties of the Polonnaruwa Stones. Proceedings of SPIE 8865 , 886508. 10.1117/12.2029607

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We report on the physical, chemical and mineral properties of a series of stone fragments recovered from the North Central Province of Sri Lanka following a witnessed fireball event on 29 December 2012. The stones exhibit highly porous poikilitic textures comprising of isotropic silica-rich/plagioclase-like hosts. Inclusions range in size and shape from mm-sized to smaller subangular grains frequently more fractured than the surrounding host and include ilmenite, olivine (fayalitic), quartz and accessory zircon. Bulk mineral compositions include accessory cristobalite, hercynite, anorthite, wuestite, albite, anorthoclase and the high pressure olivine polymorph wadsleyite, suggesting previous endurance of a shock pressure of ~20 GPa. Further evidence of shock is confirmed by the conversion of all plagioclase to maskelynite. Here the infrared absorption spectra in the region 580 cm-1 to 380 cm-1 due to the Si-O-Si or Si-O-Al absorption band shows a partial shift in the peak at 380 cm-1 towards 480 cm-1 indicating an intermediate position between crystalline and amorphous phase. Host matrix chemical compositions vary between samples, but all are rich in SiO2. Silica-rich melts display a heterogeneous K-enrichment comparable to that reported in a range of non-terrestrial material from rare iron meteorites to LL chondritic breccias and Lunar granites. Bulk chemical compositions of plagioclase-like samples are comparable to reported data e.g. Miller Ranger 05035 (Lunar), while Si-rich samples accord well with mafic and felsic glasses reported in NWA 1664 (Howardite) as well as data for fusion crust present in a variety of meteoritic samples. Triple oxygen isotope results show Δ17O = -0.335 with δ18O (‰ rel. SMOW) values of 17.816 ± 0.100 and compare well with those of known CI chondrites and are within the range of CI-like (Meta-C) chondrites. Rare earth elemental abundances show a profound Europium anomaly of between 0.7 and 0.9 ppm while CI normalized REE patterns accord well with those of high potassium and high aluminium glasses found in lunar and 4 Vesta samples. Twoelement discrimination maps of FeO vs SiO2, FeO vs TiO2, FeO vs Al2O3 and FeO vs Na2O similarly match those of impact glasses present in lunar samples and remain within relatively close proximity of the KREEP component. Iridium levels of between 1-7ppm, approximately 104 times that of terrestrial crustal rocks, were detected in all samples.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Publisher: Society of Photo-optical Instrumentation Engineers
ISSN: 0277-786X
Last Modified: 04 Jun 2017 08:02

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