Angrites
The oldest volcanic rocks known, from a vanished early planetesimal.
- Classification
- Stony meteorites › Achondrites › Asteroidal achondrites › Angrites
- Share of witnessed falls
- about 0.1%
- In our catalogue
- 21 meteorites, 1 seen to fall
- Heaviest
- D'Orbigny, 16.55 kg
- Earliest recorded fall
- Angra dos Reis (stone), 1869
- Type specimen
- Angra dos Reis, fell in Brazil in 1869
Named after Angra dos Reis in Brazil, which fell in 1869, angrites are basalts rich in calcium and titanium, with unusual minerals such as calcium-rich olivine and aluminium-titanium-rich pyroxene. They are very poor in volatile elements.
Some angrites crystallised just 4 to 5 million years after the first solids in the Solar System, about 4,564 million years ago, making them the oldest volcanic rocks known. They are used as precise time-markers for early Solar System history. Their parent body is unknown.
Mineralogy and chemistry
Angrites are basaltic rocks made of calcium-rich plagioclase (anorthite), calcium-rich olivine, sometimes as kirschsteinite, and a distinctive aluminium- and titanium-rich pyroxene, with spinel, troilite and phosphates. Orthopyroxene is absent. They are extraordinarily depleted in volatile elements such as sodium and potassium, more than any other basalts, and enriched in refractory calcium and titanium. Their oxygen isotopes lie just below the terrestrial line, with a Δ17O of about −0.07 per mil, distinct from every other group.
There are two textural kinds. Quenched or volcanic angrites, such as D'Orbigny, Sahara 99555 and Asuka 881371, cooled quickly, some with gas bubbles, like lava flows. Plutonic or metamorphosed angrites, such as Angra dos Reis, Lewis Cliff 86010 and Northwest Africa 2999, cooled slowly and are coarse-grained.
Parent body and formation
The quenched angrites crystallised about 4,563 to 4,564 million years ago, only about 4 million years after the first solids. The plutonic angrites are a few million years younger. Some angrites preserve remanent magnetism, evidence that their parent body had a molten, churning metal core that generated a magnetic field. The parent is unknown; it was a melted planetesimal, perhaps a few hundred kilometres across, that formed in the inner Solar System and has since been destroyed or lies unrecognised among the asteroids.
Angrites are of special value as clocks. Because their minerals formed quickly and stayed undisturbed, ages from uranium-lead, aluminium-magnesium, manganese-chromium and hafnium-tungsten can all be compared in the same rocks. This cross-checking lets scientists turn the decay of short-lived isotopes into absolute ages for other early meteorites, including chondrules and the oldest achondrites.
Abundance, type specimen and famous examples
Angra dos Reis, the type specimen, fell in January 1869 into the bay at Angra dos Reis, near Rio de Janeiro, and about 1.5 kg was recovered. For over a century it was almost the only one known, and it was recognised as a group of its own only in the 1970s and 1980s, as Antarctic finds such as Lewis Cliff 86010 appeared. D'Orbigny, found in Argentina in 1979 (about 16.5 kg), is the largest. Some three dozen are known.
How to recognise one
Angrites are dark grey to greenish-black, fine- to medium-grained igneous rocks; volcanic ones may have round gas holes. They contain little metal and are weakly magnetic. Their fusion crust is black and glossy. They resemble terrestrial basalt so closely that only chemical and isotopic analysis can identify them.
Famous examples
- Angra dos Reis (stone) Angrite, Brazil, 1869 · 1.5 kg
- D'Orbigny Angrite, Argentina, 1979 · 16.55 kg
- Sahara 99555 Angrite, 1999 · 2.71 kg
- Lewis Cliff 86010 Angrite, Antarctica, 1986 · 6.9 g
- Asuka 881371 Angrite, Antarctica, 1988 · 11 g
- Northwest Africa 2999 Angrite, 2004 · 392 g
Related groups
- Eucrites Volcanic basalt from the crust of Vesta.
- Ungrouped achondrites Melted rocks that match no known group: each may be the only sample of its own world.
- Asteroidal achondrites Fully melted, differentiated rocks from the crusts and mantles of asteroids, Vesta above all.
Photos shared under Creative Commons licences (Wikimedia Commons).