Angrite

Type of evolved achondrite meteorite that represent some of the earliest stages of asteroidal differentiation and magmatism in our solar system. Angrites are named for the Angra dos Reis meteorite, which fell in Rio de Janeiro, Brazil, in early 1869. They are basaltic (mafic) rocks, often containing porous areas, and vesicles that can sometimes reach 2.5 cm in diameter. They are thought to have formed on one of the earliest differentiated asteroids, and generally fall into one of the following two categories: coarse-grained plutonic angrites or fine-grained volcanic (quenched) angrites. Due to the wide distribution of cosmic ray exposure ages, the angrite parent body (or bodies) may still exist. By comparing the reflectance spectra of angrites to that of several main belt asteroids, three potential parent bodies have been identified: 5261 Eureka (~1.3 km across), 289 Nenetta and 3819 Robinson. Siderophile element depletion and remnant magnetization suggest that the original parent body was >70 km in diameter before it was broken apart.1

Angrites are notable for their depletion in silica and volatile alkali (Na and K), and enrichment in calcium. Their mineralogy generally consists of almost equal proportions of Ca-rich plagioclase (An99.6), high Al-Ti diopside pyroxene (fassaite), and a distinctive Ca‐rich olivine (kirschsteinite). Surprisingly, Angra dos Reis, differs from the others and is composed almost entirely (93 vol%) of pyroxene.1

Angrites are the most ancient igneous rocks known in our solar system. Various chronometers based on isotopes of Mn-Cr, Hf-W and U-Pb have been used to successfully validate the starting crystallization age of angrites to 4563.2 ± 0.3 Ma, i.e., 4.1 Myr after CAIs formation2 (condensation from the solar nebula). Crystallization continued for another 8 Myr after that. As would be expected, the volcanic angrites crystallized first and quickly over the first ~2 Myr, while the plutonic angrites cooled second and over a much longer period of ~6 My.


Some or all content above used with permission from J. H. Wittke.


 

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