Lunar basalts and gabbros
Dark volcanic rock from the lunar maria, the Moon's great lava plains.
- Classification
- Stony meteorites › Achondrites › Planetary achondrites › Lunar meteorites › Lunar basalts and gabbros
- In our catalogue
- 24 meteorites
- Heaviest
- Kalahari 009, 13.5 kg
Lunar basalts are pieces of the dark lava plains, the maria, that fill the Moon's great impact basins. Gabbros are the same melt cooled slowly underground into coarser crystals. Some are single lumps of lava; others are breccias of basalt fragments.
Mineralogy and chemistry
Mare basalts are made mainly of pyroxene, often pigeonite and augite, with calcium-rich plagioclase, olivine in some, and ilmenite, a titanium-iron oxide. They are much richer in iron than highland rocks, typically 18% to 22% iron oxide. Most mare basalt meteorites are low- or very-low-titanium types; the high-titanium basalts found at some Apollo sites are rare among meteorites, perhaps because they come from fewer, more limited regions.
Like all lunar rocks, they contain no water-bearing minerals, have scarce nickel-poor metal, and match Earth's oxygen isotopes. They are distinguished from basaltic eucrites by their pyroxene iron-to-manganese ratio, about 60 rather than 30, and by their minerals.
Origin and age
The maria formed when basalt melted from the lunar mantle and flooded the great impact basins, mainly between about 3.1 and 3.9 billion years ago. Lunar meteorites widen this range. Kalahari 009, a basaltic breccia from Botswana, has fragments about 4.35 billion years old, evidence of very early volcanism; Northwest Africa 032 and LaPaz Icefield 02205 crystallised about 2.8 to 3.0 billion years ago. Gabbros such as Asuka 881757 and Yamato 793169 cooled slowly underground, probably in thick lava flows or intrusions, and are about 3.8 to 3.9 billion years old.
Basalt returned by the Chang'e 5 mission is only about 2 billion years old, showing that some mare volcanism continued later than any lunar meteorite yet records. Together these rocks trace how the Moon's mantle cooled and how its volcanism slowly died away.
Abundance and famous examples
Basalts and gabbros make up a minority of lunar meteorites, as the maria cover only about 17% of the Moon's surface. Kalahari 009, about 13.5 kg, is one of the largest. Asuka 881757, Yamato 793169, Elephant Moraine 87521 and LaPaz Icefield 02205 from Antarctica, and Northwest Africa 032, are among the best studied.
How to recognise one
Lunar basalts are dark grey to brownish rocks with visible pyroxene and feldspar crystals, or fine-grained if they cooled quickly. Breccias show basalt fragments in a darker matrix. They are only weakly magnetic and, without a fusion crust, are hard to tell from Earth basalt by eye; analysis of the pyroxene and of trace elements is needed.
Famous examples
- Kalahari 009 Lunar (basalt), Botswana, 1999 · 13.5 kg
- Asuka 881757 Lunar (gabbro), Antarctica, 1988 · 442 g
- Yamato 793169 Lunar (gabbro), Antarctica, 1979 · 6.07 g
- Northwest Africa 032 Lunar (basalt), Morocco, 1999 · 300 g
- LaPaz Icefield 02205 Lunar (basalt), Antarctica, 2002 · 1.23 kg
Related groups
- Feldspathic lunar meteorites Pale breccias from the ancient lunar highlands, the most common kind of Moon rock to reach Earth.
- Mingled lunar breccias Breccias that mix highland and mare rock, from the borders between the two.
- Eucrites Volcanic basalt from the crust of Vesta.
- Lunar meteorites Rocks from the Moon, found mainly in deserts and Antarctica.
Photos shared under Creative Commons licences (Wikimedia Commons).