Ultramafic rocks are igneous or metamorphosized igneous rocks that are very low in silica (usually less than 45 wt.%) and with more than 18% magnesium oxide.
These rocks are high in iron, low in potassium, and have more than 90% mafic minerals. Mafic minerals are iron and calcium-rich, dark-colored rock-forming minerals or silicates.
Also, ultramafic rocks have high magnesium-to-iron (Mg: Fe) ratios and contain mostly olivine, pyroxene, and less to no feldspar.
Most of these rocks are often ultrabasic or ultrapotassic. However, that is not always the case.
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Ultrabasic are not always ultramafic
Most people use ultramafic and ultrabasic synonymously. However, they are not always the same.
Ultrabasic refers to rocks with less than 45 wt.% silica (SiO2). These rocks are feldspar-poor, and most are ultramafic too.
However, some, like eclogite, are ultramafic since they have mostly red garnet and green clinopyroxene. However, they are not ultrabasic since they have more than 45 wt.% silica.
Also, rocks like ultramafic pyroxenites mostly have pyroxenes. However, they are not ultrabasic since some can have as high as 55 wt.% silica.
On the other hand, not all ultrabasic rocks are ultramafic. Some, like carbonatite, are not since they are not enriched with magnesium or iron minerals.
Ultramafic vs. ultrapotassic
Ultrapotassic rocks are silica-poor igneous rocks with a high potassium oxide to magnesium oxide ratio, usually more than 3. Examples include kimberlites, melilite rocks, lamprophyres, leucitites, etc.
Those with less than 45% silica, more than 18% magnesium oxide, and more than 90% mafic minerals are ultramafic. Examples are lamprophyre, lamproite, and kimberlite.
Note: Ultramafic isn't a formal term for igneous or metamorphic rock classification recognized by IUGS. Also, it doesn't refer to a certain texture like phaneritic, aphanitic, or igneous rock type, i.e., intrusive or extrusive.
Examples
Examples of ultramafic rocks are peridotites, lamprophyres, and komatiites. Peridotites include dunite, kimberlite, harzburgite, lherzolite, among others. More are on the diagram below.
Colors and color index
Ultramafic rocks are dark-colored. They are usually greenish to dark greenish. However, others are pale yellow-green, reddish, dark gray, brownish-black, and nearly black.
The greenish to dark-green color is from olivine and orthopyroxene. Clinopyroxenes are gray, pale green, or brownish. Other colors are from plagioclase, opaques, or accessory minerals.
Lastly, the color index of ultramafic rock M is > 90. It indicates the amount or percentage of mafic minerals by composition.
Density
These rocks are dense because they have mostly dense iron and magnesium-rich minerals. Their densities are 3.1-3.4 g/cm3.
Composition
Ultramafic rocks have mainly olivine, pyroxene (clinopyroxene or orthopyroxenes), and sometimes plagioclase. However, some may have smaller amounts of biotite or phlogopite, hornblende, spinel, garnet, and opaque oxides.
Usually, it is not easy to find fresh ultramafic rocks on the surface. This is because they form at relatively higher temperatures. Therefore, most are serpentinized or altered. In such rocks, most or all mafic minerals have been replaced.
The altered ultramafic rocks will have secondary minerals like serpentine, brucite, calcite, chlorite, and talc.
Where are ultramafic rocks found?
Ultramafic rocks occur in the Earth’s upper mantle. Also, some exist as intrusive rocks, as part of ophiolite, and a few as volcanic rocks.
Exposed ones occur in orogenic belts such as ophiolites and Archean or Proterozoic terranes.
1. Upper mantle
The upper mantle is made of mainly peridotites. The enriched mantle has mostly lherzolite and depleted harzburgites.
These rocks have mainly olivine and pyroxene with smaller amounts of garnet, plagioclase, or spinel.
2. Ophiolites
Ophiolites are obducted and exposed to sections of the earth’s crust (oceanic or continental) and upper mantle.
These ophiolites will have various ultramafic rock sequences in the lower area. These rocks include peridotites like dunite, harzburgite, lherzolite and pyroxenites.
3. Intrusive
Intrusive ultramafic rocks occur in large layered igneous intrusions. Examples are peridotites and pyroxenites.
Usually, these rocks don’t crystallize from magma but form as cumulates. Cumulates form when crystallized minerals in magma settle.
In layered intrusions, these occur with mafic rocks troctolite, gabbro, and norite, with upper parts having intermediate rocks like diorite or felsic like granophyre.
4. Xenoliths
Mantle-derived magmas like basalt, nephelite, or nephelinites may carry xenoliths of ultramafic peridotites. Xenoliths are foreign pieces of rock found in igneous rocks.
5. Volcanic
Most volcanic ultramafic rocks are of Archean Eon, with few in the Neoproterozoic Era or earlier. Archean eon began about 4 billion years ago when the Earth’s crust formed and ended about 2.5 billion years ago.
On the other hand, the Neoproterozoic Era began about 1 billion to 538.8 million years ago.
Examples of these rocks are komatiite and picritic basalt.
Ultramafic tuff is another example. It has olivine or serpentine (altered olivine) with little or no quartz and feldspar. Such occurs as kimberlite maars in various diamond fields in the South African region.
Besides Earth, these rocks probably occur in Jupiter I (Io), Jupiter, and Mercury.
Metamorphism
Ultramafic rocks undergo metamorphism in the presence of water, carbon dioxide, or both to form talc carbonate and serpentinite.
Talc carbonate occurs in metamorphism involving higher fluids with a CO2 molar proportion of more than 10%. In contrast, serpentinite forms when fluids involved have less than 10% carbon dioxide.