(Eucrite in MetBul 106)
Fell June 29, 2017
5° 57.135' S., 49° 39.238' W.
The approach of a bright fireball at 10:35 A.M. was seen and heard by people in several towns along its path from NE to SW. In the Serra Pelada Village, State of Pará, northern Brazil, students and staff members on the playground at Rita Lima Souza Municipal School heard multiple detonations and saw an ~6 kg stone fall and break apart; some pieces from this stone were shared among themselves and with local residents. In addition, the nearby fall of a separate 5.4 kg stone was witnessed by an electrician who recovered the stone and sold it to an anonymous buyer.
Analyses of a sample from the broken mass were conducted at the National Museum of Brazil, the Federal University of Rio de Janeiro, and the Federal University of Bahia (M. E. Zucolotto, A. Tosi, C. Villaça, D. Rios), and Serra Pelada was classified as a monomict basaltic eucrite breccia. It is composed primarily of clinopyroxene (pigeonite) and calcic plagioclase (bytownite, anorthite) with accessory quartz, phosphates (fluorapatite, merrillite), low-Ni iron, troilite, ilmenite, chromite, zircon, and baddeleyite, (Zucolotto et al., 2018; J. Roszjar et al., #6311). The meteorite contains very fine- to coarse-grained basaltic and gabbroic clasts embedded in a fine- to medium-grained clastic matrix. Shock features present include mosaicism in pyroxene, undulatory extinction in plagioclase, impact melt veins, and recrystallized melt pockets; however, no maskelynite or planar deformation features were observed consistent with a shock stage of S3.
In a manner similar to that employed for the chondrite groups, eucrites have been petrologically divided into a metamorphic sequence comprising seven types (after Takeda and Graham, 1991; Yamaguchi et al., 1996). Serra Pelada has features consistent with a high thermal equilibration to type 5:
Type 5homogenous host composition with readily resolvable exsolved pigeonite lamellae; pigeonites extensively clouded by reheating; mesostasis glass recrystallized or absent (e.g., Juvinas, Sioux County, Lakangaon)
In addition, a combination of thermal- and shock-metamorphism and metasomatism has produced a variety of secondary alteration features in the meteorite which were studied by J. Roszjar (2019). Such features that have been observed include augite exsolution in pigeonite, pigeonite replacement by augite, phosphate replacement, intergrown chromite and ilmenite, zircon exsolution in ilmenite, zircon rims on silicate clasts, and Fe-rich olivine veinlets in pyroxene grains.
A plot of the Fe/Mn ratio in silicates has proven to be a useful tool in distinguishing the source parent bodies among different meteorites. It was established by Zucolotto et al. (2018) that Serra Pelada has a slope value (0.03115) which is similar to that for the HEDs (0.0336). In addition, utilizing a coupled Fe/Mn vs. Fe/Mg diagram they demonstrated that Serra Pelada plots in the field for basaltic eucrites, in particular overlapping the low-Ca pyroxenes of the anomalous eucrite Emmaville (see diagrams below). Zucolotto et al. (2018) also noted that Serra Pelada has a lower TiO2 wt% than the other noncumulate eucrites.
Fe vs. Mn Diagram (atoms per formula unit)
click on image for a magnified view
Fe/Mn vs. Fe/Mg Diagram
Diagrams credit: Zucolotto et al., Anais da Academia Brasileira de Ciências, vol. 90, #1, (2018) 'Serra Pelada: the first Amazonian Meteorite fall is a Eucrite (basalt) from Asteroid 4-Vesta' (http://dx.doi.org/10.1590/0001-3765201820170854)
The PbPb data (weighted average of 18 analyses) obtained for two zircon grains in Serra Pelada by Yin et al. (2019) corresponds to an age of 4.5560 (±0.0061) b.y., which likely represents the timing of zircon crystallization on the parent body. This is an ancient age that falls within the range of protracted magmatism (~50 m.y.) as determined for zircons in other basaltic eucrites (e.g., Zhou et al., 2013; Iizuka et al., 2015, see diagram below).
Diagram credit: Yin et al., 82nd MetSoc, #6051 (2019)
Based on O- and Cr-isotopic composition data (unpublished), Yin et al. (2019) assert that Serra Pelada is an anomalous eucrite similar to NWA 8671 (Irving et al., 2018 #2247, see diagram below). These anomalous basaltic eucrites represent a separate parent body distinct from that of typical basaltic eucritesthe latter generally considered to derive from asteroid (4) Vesta.
click on image for a magnified view
Diagram credit: Irving et al., 49th LPSC, #2247 (2018)
Because there are now a number of eucrite-like meteorites that are not grouped with normal eucrites for various reasons, it was proposed that the term 'eucrite' be used as a description of a rock type rather than to imply an origin on the presumed HED parent body (4) Vesta. The photo of Serra Pelada shown above is a 3.0 g fragment. The top photos below show a large ~750 g fragment of Serra Pelada with glassy fusion crust and flow lines, while the bottom photo shows Paulo Anselmo Matioli beside the impact pit which measures 30 x 20 cm wide and 15 cm deep.