Ultramafic rock

2025-10-22T15:06:36Z

2003:DF:7730:B22B:8D58:1576:FE74:902B:

{{Short description|Type of igneous and meta-igneous rock}}
[[File:Peridotite 5773.jpg|thumb|upright=1.35|[[Peridotite]], a type of ultramafic rock]]
[[File:Melilithit Hohenstoffeln.jpg|thumb|upright=1.35|Melilitite from [[Hohenstoffeln]] ([[Hegau]])]]
'''Ultramafic rocks''' (also referred to as '''ultrabasic rocks''', although the terms are not wholly equivalent) are [[igneous rock|igneous]] and [[Metamorphic rock#Classification|meta-igneous]] rocks with a very low [[silica]] content (less than 45%), generally >18% [[magnesium oxide|MgO]], high [[iron(II) oxide|FeO]], low [[potassium]], and are usually composed of greater than 90% [[mafic]] [[mineral]]s (dark colored, high [[magnesium]] and [[iron]] content). [[Earth's mantle]] is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as [[carbonatite]]s and [[ultrapotassic igneous rocks]].

==Intrusive ultramafic rocks==

[[Image:Peridotite Olivine-Orthopyroxene-Clinopyroxene - Common Peridotites highlighted.png|thumb|400px|IUGS Classification diagram for intrusive ultramafic rocks based on modal percentages of mafic minerals. Green area represents typical [[Earth's mantle|mantle]] [[peridotite]].]]
Intrusive ultramafic rocks are often found in large, layered [[Layered intrusion|ultramafic intrusions]] where [[Igneous differentiation|differentiated]] rock types often occur in layers.<ref>Ballhaus, C.G. & Glikson, A.Y., 1995, [[Petrology]] of layered mafic-ultramafic intrusions of the Giles Complex, western [[Musgrave Block]], [[central Australia]]. AGSO Journal, 16/1&2: 69–90.</ref> Such [[Cumulate rocks|cumulate]] rock types do not represent the chemistry of the magma from which they crystallized. The ultramafic intrusives include the [[dunite]]s, [[peridotite]]s and [[pyroxenite]]s. Other rare varieties include [[troctolite]] which has a greater percentage of calcic plagioclase. These grade into the [[anorthosite]]s. [[Gabbro]] and [[norite]] often occur in the upper portions of the layered ultramafic sequences. [[Hornblendite]] and, rarely [[phlogopite]], are also found.

==Volcanic ultramafic rocks on Earth==
[[Volcanic]] ultramafic rocks are rare outside of the [[Archean|Archaean]] and are essentially restricted to the [[Neoproterozoic]] or earlier. [[Subvolcanic rock|Subvolcanic]] ultramafic rocks and [[Dyke (geology)|dyke]]s persist longer, but are also rare. There is evidence of ultramafic rocks elsewhere in the [[Solar System]].

Examples include [[komatiite]]<ref>Hill R.E.T, [[Sarah-Jane Barnes|Barnes S.J.]], Gole M.J., and Dowling S.E., 1990. Physical [[volcanology]] of komatiites; A field guide to the komatiites of the Norseman-Wiluna [[Greenstone Belt]], Eastern [[Goldfields-Esperance|Goldfields Province]], [[Yilgarn Block]], [[Western Australia]]., [[Geological Society of Australia]]. {{ISBN|0-909869-55-3}}</ref> and [[Picrite|picritic basalt]]. Komatiites can be host to [[ore]] deposits of [[nickel]].<ref>Lesher, C.M., Arndt, N.T., and Groves, D.I., 1984, Genesis of komatiite-associated nickel [[sulfide]] deposits at [[Kambalda]], Western Australia: A distal volcanic model, in Buchanan, D.L., and Jones, M.J. (Editors), Sulphide Deposits in Mafic and Ultramafic Rocks, [[Institution of Mining and Metallurgy]], [[London]], p. 70–80.</ref>

=== Ultramafic tuff ===
Ultramafic [[tuff]] is extremely rare. It has a characteristic abundance of [[olivine]] or [[Serpentine subgroup|serpentine]] and a scarcity or absence of [[feldspar]] and [[quartz]]. Rare occurrences may include unusual surface deposits of [[maar]]s of [[kimberlite]]s in the [[diamond]] fields of southern Africa and other regions.

==Ultrapotassic ultramafic rocks==
Technically [[Ultrapotassic igneous rocks|ultrapotassic rocks]] and [[Melilite|melilitic]] rocks are considered a separate group, based on melting model criteria, but there are ultrapotassic and highly silica-under-saturated rocks with >18% MgO which can be considered "ultramafic".

Ultrapotassic, ultramafic igneous rocks such as [[lamprophyre]], [[lamproite]] and [[kimberlite]] are known to have reached the surface of the Earth. Although no modern eruptions have been observed, analogues are preserved.

Most of these rocks occur as [[Dike (geology)|dikes]], [[diatreme]]s, [[lopolith]]s or [[laccolith]]s, and very rarely, intrusions. Most kimberlite and lamproite occurrences occur as [[volcanic]] and subvolcanic diatremes and [[maar]]s; lavas are virtually unknown.

Vents of [[Proterozoic]] lamproite ([[Argyle diamond mine]]), and [[Cenozoic]] lamproite ([[Gaussberg]], [[Antarctica]]) are known, as are vents of [[Devonian]] lamprophyre ([[Scotland]]). Kimberlite pipes in Canada, Russia and South Africa have incompletely preserved [[tephra]] and [[agglomerate]] [[facies]].

These are generally [[diatreme]] events and as such are not lava flows although tephra and [[Volcanic ash|ash]] deposits are partially preserved. These represent low-[[volume]] volatile melts and attain their ultramafic [[chemistry]] via a different process than typical ultramafic rocks.

==Metamorphic ultramafic rocks==
[[Metamorphism]] of ultramafic rocks in the presence of [[water]] and/or [[carbon dioxide]] results in two main classes of metamorphic ultramafic rock; [[talc carbonate]] and [[serpentinite]].

Talc carbonation reactions occur in ultramafic rocks at lower [[greenschist]] through to [[granulite]] facies metamorphism when the rock in question is subjected to metamorphism and the metamorphic fluid has more than 10% molar proportion of CO<sub>2</sub> ([[carbon dioxide]]).

When such metamorphic fluids have less than 10% molar proportion of CO<sub>2</sub>, reactions favor serpentinisation, resulting in [[chlorite group|chlorite]]-[[Serpentine group|serpentine]]-[[amphibole]] type assemblages.

==Distribution in space and time==
The majority of ultramafic rocks are exposed in [[orogenic]] belts, and predominate in [[Archean|Archaean]] and [[Proterozoic]] terranes. Ultramafic magmas in the [[Phanerozoic]] are rarer, and there are very few recognised true ultramafic lavas in the Phanerozoic.{{cn|date=April 2021}}

Many surface exposures of ultramafic rocks occur in [[ophiolite]] complexes where deep mantle-derived rocks have been [[obduction|obducted]] onto [[continental crust]] along and above [[subduction]] zones.

==Soil, regolith, and biology==

[[Serpentine soil]] is a magnesium rich, calcium, potassium and phosphorus poor soil that develops on the [[regolith]] derived from ultramafic rocks. Ultramafic rocks also contain elevated amounts of chromium and nickel which may be toxic to plants. As a result, a distinctive type of [[vegetation]] develops on these soils. Examples are the ultramafic [[woodland]]s and barrens of the [[Appalachian Mountains]] and [[Piedmont (United States)|piedmont]], the "wet [[maquis shrubland|maquis]]" of the [[New Caledonia rain forests]], and the ultramafic [[forest]]s of [[Mount Kinabalu]] and other peaks in [[Sabah]], [[Malaysia]]. Vegetation is typically stunted, and sometimes includes [[endemic (ecology)|endemic]] species adapted to the soils.

Often thick, [[magnesite]]-[[calcrete]] [[caprock]], [[laterite]] and [[duricrust]] forms over ultramafic rocks in [[tropical]] and [[subtropical]] environments. Particular [[floral]] assemblages associated with highly nickeliferous ultramafic rocks are indicative tools for [[mineral exploration]].

Weathered ultramafic rocks may form [[lateritic nickel ore deposits]].<ref>Golightly, J.P. (1981): Nickeliferous Laterite Deposits. [[Economic Geology (journal)|Economic Geology]] 75, pp. 710–735</ref><ref>Schellmann, W. (1983): [[Geochemical]] principles of lateritic nickel ore formation. Proceedings of the 2. International Seminar on Lateritisation Processes, [[São Paulo]], pp. 119–135</ref>

[[Lichen]] communities on ultramafic rocks show distinctive characteristics, including the unusual co-presence of species that typically grow on either [[acidic rock|acidic]] or calcium-rich rocks, due to the rocks' unique chemical composition. While some lichen species appear to be characteristic of ultramafic environments within specific geographical regions, very few species are found exclusively on these rocks. Studies have shown that lichen communities on ultramafic rocks can be more diverse than those on adjacent mafic rocks, with some localities showing notably higher species counts on serpentinites compared to other rock types. These communities often display [[xerophyte|xerophytic]] characteristics and may include species with [[disjunct distribution]] patterns. The weathering action of lichens on ultramafic rocks can promote [[biogeochemistry|biogeochemical]] processes, including the complete depletion of magnesium from serpentine minerals beneath lichen [[thallus|thalli]] and the formation of [[secondary mineral]]s common in serpentine soils.<ref>{{cite journal |last1=Favero-Longo |first1=Sergio E. |last2=Isocrono |first2=Deborah |last3=Piervittori |first3=Rosanna |title=Lichens and ultramafic rocks: a review |journal=The Lichenologist |volume=36 |issue=6 |year=2004 |doi=10.1017/S0024282904014215 |pages=391–404}}</ref>

==Other celestial bodies==

=== Io ===
Ultramafic lava may have been detected on [[Io (moon)|Io]], a moon of [[Jupiter]], because [[Thermography|heat-mapping]] of Io's surface found ultra-hot areas with temperatures in excess of {{convert|1200|C|F}}. The [[magma]] immediately below these hot spots is probably about {{convert|200|C-change|0}} hotter, based on surface-to-subsurface temperature differences observed for lava on Earth. A temperature of {{convert|1400|C|F}} is thought to indicate the presence of ultramafic magma.<ref>{{cite news |title=Space Volcanoes |url=https://www.bbc.co.uk/programmes/b08ry9l9 |access-date=6 March 2019 |work=Horizon |issue=Series 54, Episode 6 |publisher=BBC |date=7 July 2018}}</ref><ref>{{Citation |last=Lopes |first=Rosaly M. C. |title=Chapter 43 - Volcanism on Io |date=2015-01-01 |work=The Encyclopedia of Volcanoes (Second Edition) |pages=747–762 |editor-last=Sigurdsson |editor-first=Haraldur |url=https://www.sciencedirect.com/science/article/abs/pii/B9780123859389000432 |access-date=2025-05-17 |place=Amsterdam |publisher=Academic Press |isbn=978-0-12-385938-9 |last2=Williams |first2=David A.}}</ref>

=== Mercury ===
[[Mercury (planet)|Mercury]] appears to have ultramafic volcanic rock.<ref name="Charlier_etal_2013">{{cite journal | title=Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy | first1=B. | last1=Charlier | first2=T.L. | last2=Grove | first3=M.T. | last3=Zuber | journal=Earth and Planetary Science Letters | year=2013 | volume=363 | pages=50–60 | doi=10.1016/j.epsl.2012.12.021| bibcode=2013E&PSL.363...50C | url=https://orbi.uliege.be/bitstream/2268/173794/1/Charlier%20et%20al%202013%20Mercury.pdf }}</ref>

=== Mars ===
The undifferentiated crust of [[Mars]] is largely composed of [[mafic]] and ultramafic rocks.<ref>{{Cite journal |last=Müntener |first=Othmar |date=2010-10-01 |title=Serpentine and serpentinization: A link between planet formation and life |url=https://pubs.geoscienceworld.org/gsa/geology/article/38/10/959/130149/Serpentine-and-serpentinization-A-link-between |journal=Geology |volume=38 |issue=10 |pages=959–960 |doi=10.1130/focus102010.1 |issn=0091-7613|url-access=subscription }}</ref> Dark lobate flows of upper [[Hesperian]] and early [[Amazonian Age|Amazonian]] age, probably extruded from a regional network of extension faults, can be traced in the Ladon Basin. Spectral analysis data confirm the ultramafic character of these flows and the underlying rocks.<ref>{{Cite journal |last=Mège |first=Daniel |last2=Gurgurewicz |first2=Joanna |last3=Massironi |first3=Matteo |last4=Pozzobon |first4=Riccardo |last5=Tognon |first5=Gloria |last6=Pajola |first6=Maurizio |last7=Tornabene |first7=Livio L. |last8=Lucchetti |first8=Alice |last9=Baschetti |first9=Beatrice |last10=Davis |first10=Joel M. |last11=Hauber |first11=Ernst |last12=De Toffoli |first12=Barbara |last13=Douté |first13=Sylvain |last14=Keszthelyi |first14=Laszlo |last15=Marinangeli |first15=Lucia |date=2023 |title=Hydrothermal Alteration of Ultramafic Rocks in Ladon Basin, Mars—Insights From CaSSIS, HiRISE, CRISM, and CTX |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022JE007223 |journal=Journal of Geophysical Research: Planets |language=en |volume=128 |issue=1 |article-number=e2022JE007223 |doi=10.1029/2022JE007223 |issn=2169-9100|hdl=10150/672993 |hdl-access=free }}</ref>

=== Exoplanets ===
[[Mid-infrared]] (12.8 μm) observations have shown that the measured [[flux]] of [[TRAPPIST-1b]] (an [[exoplanet]]) is consistent with a surface model of ultramafic rocks.<ref>{{Cite web |last=Shrestha |first=Sabnam |date=2025-03-26 |title=The enigma of Trappist-1 b: a thick atmosphere or airless rock? |url=https://physicsworld.com/a/the-enigma-of-trappist-1-b-a-thick-atmosphere-or-airless-rock/ |access-date=2025-05-17 |website=Physics World |language=en-GB}}</ref>

== See also ==
* [[Chromitite]]
* [[Magnetite]]
* [[Kambalda type komatiitic nickel ore deposits]]
* [[Fractional crystallization (geology)]]
* [[Volcanism on Io]], a moon of Jupiter

==References==
{{Reflist}}

{{Igneous rocks}}
{{Authority control}}

[[Category:Ultramafic rocks| ]]
[[Category:Metamorphic rocks]]

Vero - Wikipedia - User contributions [en]

Ultramafic rock