Geology is full of wonders. Even non-geologists who happen to visit a mountain, mine, cave, or coastal sands marvel at the Earth's features. But some aspects of geology are more mysterious, even to us geologists. This edition of Core Elements shares two new stories on the “rare geology” of two igneous rock types.
Rasoul Sorkhabi
Editor, Core Elements
Distribution and Generation of Carbonatites
James St. John/WikiMedia Commons
Carbonatites are rare igneous rocks. They host rich concentrations of rare-earth elements (REEs) and are economically valued for that.
However, their scarce distribution on Earth and their little-known generation mechanism within the subsurface place them at the forefront of geoscience research.
What are carbonatites?
Carbonatites are often confused with marble because both contain more than 50 percent carbonate minerals, but carbonatites are igneous rocks, whereas marbles are metamorphic.
Carbonatites may be plutonic (intrusive) or volcanic (extrusive) rocks—a double mode of formation that adds to their mystery.
Go deeper: For a detailed mineralogical classification of carbonatites see this article by Peter Kresten.
Tectonic settings: Carbonatites occur in diverse tectonic settings, including large igneous provinces, ocean islands, continental rifts, and continental collisional mountains.
Most geologists believe that carbonatites are generated in small volumes inside the mantle.
Carbonatite mines: Three major REE mines from carbonatite are:
Mountain Pass in California, which produced the bulk of REEs globally from the 1960s to the 1990s, before production from China dominated the world market
Bayan Obo in China, which produces nearly half of the global REEs
Mount Weld in Australia—a 2-billion-year-old volcanic plug
Distribution: A new study by Gibson and colleagues compiled a global map of reported carbonatite occurrences and compared their distribution with lithospheric thickness derived from seismic tomography. They found:
Most carbonatites occur on the rift margins of cratons with thick (greater than 160 km) lithospheric roots.
The world’s oldest carbonatites are Archaean age (about 3 billion years old) found in Australia, Canada, and Greenland.
The youngest known carbonatite is from the active Ol Doinyo Lengai volcano in Tanzania.
Generation mechanism: The new study used thermal modeling to investigate the formation of carbonatite melts. The model results suggest:
Heat conduction on rifted continental margins or rapidly stretched continental crust can mobilize carbonated peridotite at temperatures of 950–1250 C and pressures of 2–3 gigapascal to form carbonatites.
Heat conduction from upwelling mantle plumes on rift margins in particular facilitate melting of carbonatite peridotites.
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Komatiite is another rare igneous rock. A recent study published in Geology reports the discovery of Permian age komatiites in northeastern Iran.
What are komatiites? Let’s first refresh our igneous petrology.
Igneous magma type has plutonic (intrusive) and volcanic (extrusive) varieties. For felsic (feldspar and silica rich) rock, granite is plutonic, and rhyolite is volcanic.
For ultramafic (extremely rich in magnesium and iron continent), peridotite is plutonic, and komatiite is volcanic.
Ultramafic rocks are made up of less than 45 percent silica and more than 18 percent magnesium oxide.
Komatiites also display spinifex texture, formed by a dendritic pattern of olivine plates.
Komatiite is named after the type locality in the Komati River valley in South Africa.
Go deeper: For more information on the geochemistry of komatiites see this article by J. Dostal.
Age distribution: Komatiites were discovered in 1969 in the Barberton Greenstone Belt in South Africa.
Almost all known komatiites are Archean-age (from 4.0 to 2.5 billion years ago) and related to the cooling of Earth’s extremely hot mantle.
Proterozoic-age komatiites (2.5 billion years to 540 million years old) occur on Gilmour Island, Canada.
The youngest known komatiite is from Gorgona Island in Colombia and dates to 87 Ma.
There are other young komatiites reported from Costa Rica and Vietnam, but these lack the spinifex textures. They may be called picrite basalt.
New discovery: Mobasheri and colleagues report komatiites from northeastern Iran.
These rocks occur in mountain belts formed when the Paleo-Tethys Ocean floor between the Eurasian plate and the Cimmerian microcontinent subducted and closed.
It's possible the ascent of a mantle plume into the Permian subduction zone at the margin of the Paleo-Tethys Ocean generated the komatiites.
Rock description: Komatiite sections in these locations range from 5 to 100 meters.
Each sections include an olivine cumulate in the lower part, and gabbro, pyroxene spinifex, and olivine spinifex in the upper part.
There is a chilled volcanic flow at the base and on top of each section.
MgO contents range from 31 to 38 percent in olivine cumulates, 7 to 10 percent in gabbros, 30 to 35 percent in olivine spinifex, and 35 to 36 in chilled margins.
Thank you to all who sent answers to last week’s quiz! The question was: Why does the far side of the Moon remain invisible to us on Earth, even though the Moon is rotating?
I have selected the following comprehensive response from Robert Gunn from Calgary:
“From Earth, we never see 41 percent of the far side of our Moon, because the Moon rotates one full rotation on its axis at the same time as it makes one orbit around Earth [that is, 27.3 Earth days]. The gravity-caused bulge position of the Moon is always aligned and facing Earth’s gravitational pull - hence Tidal Locking.”
Go deeper: See this videoclip from NASA for even more information.
Now, to this week’s question: The terms geology, geoscience and Earth Science are sometimes used interchangeably. But they have subtle differences. How do you distinguish the terms geology, geoscience, and Earth science?
Please send your response by November 28 to editorial@aapg.org (subject line: Core Elements Quiz).
Happy Thanksgiving to our readers in the United States!
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