Aguas Zarcas        10°23’29.03"N, 84°20’28.58"W

Alajuela, Costa Rica

Confirmed fall: 2019 Apr 23

Classification: Carbonaceous chondrite (CM2)

History: (G.J. Soto; P. Madrigal, O. Lücke, Escuela Centroamericana de Geología, Universidad de Costa Rica; M. Farmer, Arizona) At 21:07 local time on 23 April 2019, a meteorite fall was reported in Aguas Zarcas, San Carlos county, Alajuela province, Costa Rica. The fireball traveled WNW to ESE and was caught on cameras of the National Seismological Network (RSN) at the summit of Poás and Turrialba volcanoes, and from the Volcanological and Seismological Observatory of Costa Rica (OVSICORI). Sightings were reported from Quepos (Central Pacific) in the south and north to La Palmera in San Carlos. The first piece recovered, 1152 g, crashed through a house at 10°23’29.03"N, 84°20’28.58"W. A dog house was hit by a 280 g piece at 10°24’9.35"N 84°21’51.26"W. The BRAMON (Brazilian Meteors Observation Network), UNESP (São Paulo State University) and USP (University of São Paulo) teams determined the atmospheric trajectory of the bolide from four security videos and dashcam cameras. The bolide had an entry angle of 73° relative to the ground, traveling 20.7 km in the 4-s analyzed interval, with an atmospheric velocity of 14 km/s. Analysis shows a projected elliptical strewn-field with major axis length of 6.3 and minor axis of 3.3 km. Hundreds of stones were recovered within the projected strewn-field. The bolide orbit was determined, with the follow preliminary elements: semi-major axis 2.7 AU, eccentricity 0.63, inclination 3.09°, pericenter longitude of 185.3° and ascending node 33.4°. Current total recovered mass is around 27 kg, of which ~11 kg was collected before rain fell over the fall site.

Physical characteristics: (L. Garvie, ASU) Hundreds of fusion-crusted stones ranging from 0.1 to 1868 g. Stones under ~50 g are typically angular to blocky and lack regmaglypts. Also found were several plate-like, oriented stones. For example, one oriented plate is 13 cm wide and 1 cm thick. Many stones are oriented with a domed leading edge and well-developed roll-over lip along the rim of the trailing edge. Larger stones, especially those near 1 kg, show broad regmaglypts, some well developed. The fusion crust on several of the trailing edge surfaces is iridescent. Pre-rain material crushed in a few ml of water emits a powerful "Murchison-like" odor, though with a more prominent compost-like scent.

Petrography: (L. Garvie, ASU) The interior of the stones is uniformly dark gray and studded with small light-colored speckles. Stones are brecciated and dominated by two lithologies: chondrule poor, constituting ~80 areal% of the broken surfaces examined (~2 kg of material), and chondrule rich. Chondrules constitute ~10 areal% of the chondrule-poor lithology and ~40 areal% in the chondrule-rich lithology. Some broken surfaces show clasts of the chondrule-rich lithology in the chondrule-poor type. Clasts with lower chondrule to matrix ratio also present; for example, one 3 g half stone shows <1 areal% chondrules. A probe mount of the dominant chondrule-poor lithology (sample area 1.5 × 2.2 cm) shows scattered small chondrules (mean 275 μm, n=40), with prominent fine-grained rims. Rare chondrules to 2 mm. Fe-Ni metal, which is Cr-bearing, was only found as rounded grains in one 100-μm type I chondrule. The section contains several Al-rich CAIs to 300 μm. The matrix is dominated by two components of roughly equal proportions: anhedral clumps (typically 50 microns) that are Fe and S rich (e.g., 37.5 wt% Fe and 5.8 wt% S, with microprobe total of 93%), surrounded by fine-grained material poorer in Fe and S (e.g., 12.8 wt% Fe, 13.9 wt% Mg, and 0.5 wt% S, with microprobe total of 80.0%). Powder XRD from five stones (one chondrule rich) shows patterns dominated by serpentine. In two stones, the serpentine 001 reflection sits on a broad peak centered at 0.843 nm. Reflections for tochilinite and calcite occur in all patterns. The intensities of olivine and pyroxene reflections were generally low, but most prominent in the chondrules-rich stone.

Geochemistry: Oxygen isotopes (K. Ziegler, UNM): 7 pre-rain fragments analyzed by laser fluorination gave δ¹⁸O= 10.342, 10.807, 11.591, 12.314, 12.249, 12.669, 9.748; δ¹⁷O= 2.381, 2.932, 3.360, 3.843, 3.995, 3.959, 2.143; Δ¹⁷O=-3.080, -2.775, -2.759, -2.659, -2.472, -2.730, -3.004 (linearized, all per mil, TFL slope=0.528). Microprobe (L. Garvie, ASU): Fa_0.4-41.0, n=10.

Classification: Geochemistry and oxygen isotopic data consistent with CM2.