Abstract

Discovery and first characterization: On 2011, senior ge- ologists at the Chilean National Survey, found exposed deposits of green molten glasses in the Central Depression of northern Atacama Desert, Chile. According to them [1], the glass layers are distributed in an area of ca. 65 km long, in isolated and frag- mented tabular bodies, with ellipsoidal shapes of 2-4 m of diame- ter and 15-25 cm thick. The rocks correspond to green, highly vesiculate glasses, with irregular forms, that present different de- grees of fusion of the underlying sediments, mainly composed by sands and clays from a paleowetland deposit dated by 14C in 12400 ± 70y. Some samples, studied under the microscope, showed neoformed crystals and relict quartz with planar defor- mation features (PDF’s). They also found baked clay layers. First paleomagnetic characterization done in oriented samples by our team [2], found highly stable remanent magnetization and magnet- ic evidences that indicates the soil was heated in situ above 700oC (>10 cm depth). Fulgurite, metallurgic scoria and volcanic origin where ruled out by different criteria [1, 2]. Methodology: New thin and polished sections were prepared for optical and electronic microscopy. We obtained mineral and glass chemistry using microprobe and measured magnetic suscep- tibility in centimeter fragmented samples to select the most mag- netic ones in order to characterize their magnetic minerals doing hysteresis loops. Results: The samples are characterized by high heterogeneity ranging from almost completely molten ones, where green, trans- parent glass is preserved showing fluid textures, to others with many relict clast, some with reaction borders, and dirty glass. Glass recristallization to melilite and/or clinopyroxenes is com- mon, and the opaques phases are dominated by iron sulphides (~FeS), not obiquitously present, and phosphides in a less amount. Studied samples didn’t have PDF’s in quartz grains, but planar fractures were found. Magnetic susceptibility is low in most samples, and the stronger values are associated with eolian magnetite particles trapped in vesicles. Future work: Parallel studies will attempt Ar-Ar datation of the glass and its chemistry. New field trips are programmed to constraint glass and mineral phase’s distribution and study a pos- sible impact structure proposed by the discoverers. Acknowledgments: FONDECYT project No3140562. References: [1] Blanco et al. 2012 Servicio Nacional de Geología y Minería, Registered inform No IR-12-50, Santiago, Chile; [2] Roperch et al. 2015. Abstract for the PMag Colloqui- um, Aix en Provence, France.