Eclogite

Last updated
Eclogite piece from Norway with a garnet (red) and omphacite (greyish-green) groundmass. The sky-blue crystals are kyanite. Minor white quartz is present, presumably from the recrystallization of coesite. A few gold-white phengite patches can be seen at the top. A 23 millimetres (0.91 in) coin added for scale. Eclogite Norway.jpg
Eclogite piece from Norway with a garnet (red) and omphacite (greyish-green) groundmass. The sky-blue crystals are kyanite. Minor white quartz is present, presumably from the recrystallization of coesite. A few gold-white phengite patches can be seen at the top. A 23 millimetres (0.91 in) coin added for scale.

Eclogite ( /ˈɛklət/ ) is a bi-mineralic metamorphic rock containing the primary phases of garnet (almandine-pyrope) hosted in a matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisite, dolomite, corundum, and, rarely, diamond. The mineral chemistry of primary and accessory phases is used to classify three types of eclogite (A, B, and C). The broad range of eclogitic compositions has led a longstanding debate on the origin of eclogite xenoliths as subducted, altered oceanic crust.

Contents

Origins

Eclogites typically result from high to ultrahigh pressure metamorphism of mafic rock at low thermal gradients of <10 °C/km (29 °F/mi) as it is subducted to the lower crust to upper mantle depths in a subduction zone. [1]

Classification

Eclogites are defined as bi-mineralic, broadly basaltic rocks which have been classified into Groups A, B and C based on the chemistry of their primary mineral phases, garnet and clinopyroxene. [2] [3] The classification distinguishes each group based on the jadeite content of clinopyroxene and pyrope in garnet. [3] The rocks are gradationally less mafic (as defined by SiO2 and MgO) from group A to C, where the least mafic Group C contains higher alkali contents. [4]

The transitional nature between groups A, B and C correlates with their mode of emplacement at the surface. [3] Group A derive from cratonic regions of earth's crust, brought to the surface as xenoliths from depths greater than 150 km during kimberlite eruptions. [2] [3] Group B show strong compositional overlap with Group A, but are found as lenses or pods surrounded by peridotitic mantle material. [3] Group C are commonly found between layers of mica or glaucophane schist, primarily exemplified by the New Caledonia tectonic block off the coast of California. [5]

Surface versus mantle origin

The broad range in composition has led a longstanding debate on the origin of eclogite xenoliths as either mantle or surface derived, where the latter is associated with the gabbro to eclogite transition as a major driving force for subduction. [6] [7] [8]

Group A eclogite xenoliths remain the most enigmatic in terms of their origin due to metasomatic overprinting of their original composition. [9] [10] Models proposing a primary surface origin as seafloor protoliths strongly rely on the wide range in oxygen isotope composition, which overlaps with obducted oceanic crust, such as the Ibra section of the Samail ophiolite. [11] [12] The variation found in some eclogite xenoliths at the Roberts Victor kimberlite pipe are a result of hydrothermal alteration of basalt on the seafloor. [13] This process is attributed to both low- and high-temperature seawater exchange, resulting in large fractionations in oxygen isotope space relative to the upper mantle value typical of mid ocean ridge basalt glasses. [14] [15] Other mechanisms proposed for the origin of Group A eclogite xenoliths rely on a cumulate model, where garnet and clinopyroxene bulk compositions derive from residues of partial melting within the mantle. [16] Support of this process is result of metasomatic overprinting of the original oxygen isotope composition, driving them back towards the mantle range. [17]

Eclogite facies

Eclogites containing lawsonite (a hydrous calcium-aluminium silicate) are rarely exposed at Earth's surface, although they are predicted from experiments and thermal models to form during normal subduction of oceanic crust at depths between about 45–300 km (28–186 mi). [18]

Importance of eclogite

Photomicrograph of a thin section of eclogite from Turkey. Green omphacite (+ late chlorite) + pink garnet + blue glaucophane + colorless phengite. Eclogite dlw.jpg
Photomicrograph of a thin section of eclogite from Turkey. Green omphacite (+ late chlorite) + pink garnet + blue glaucophane + colorless phengite.

Formation of igneous rocks from eclogite

Eclogite Eclogite.jpg
Eclogite

Partial melting of eclogite has been modeled to produce tonalite-trondhjemite-granodiorite melts. [19] Eclogite-derived melts may be common in the mantle, and contribute to volcanic regions where unusually large volumes of magma are erupted. [20] The eclogite melt may then react with enclosing peridotite to produce pyroxenite, which in turn melts to produce basalt. [21]

Distribution

Eclogite from Almenning, Norway. The red-brown mineral is garnet, green omphacite and white quartz. Eclogite Almenning, Norway.jpg
Eclogite from Almenning, Norway. The red-brown mineral is garnet, green omphacite and white quartz.

Occurrences exist in western North America, including the southwest [22] and the Franciscan Formation of the California Coast Ranges. [23] Transitional granulite-eclogite facies granitoid, felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of the Petermann Orogeny, central Australia. Coesite- and glaucophane-bearing eclogites have been found in the northwestern Himalaya. [24] The oldest coesite-bearing eclogites are about 650 and 620 million years old and they are located in Brazil and Mali, respectively. [25] [26]

Related Research Articles

Lithosphere Outermost shell of a terrestrial-type planet or natural satellite

A lithosphere is the rigid, outermost shell of a terrestrial-type planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years or more. The crust and upper mantle are distinguished on the basis of chemistry and mineralogy.

Andesite Intermediate volcanic rock

Andesite is an extrusive volcanic rock of intermediate composition. In a general sense, it is the intermediate type between basalt and rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.

Xenolith

A xenolith is a rock fragment that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock entrained during magma ascent, emplacement and eruption. Xenoliths may be engulfed along the margins of a magma chamber, torn loose from the walls of an erupting lava conduit or explosive diatreme or picked up along the base of a flowing body of lava on the Earth's surface. A xenocryst is an individual foreign crystal included within an igneous body. Examples of xenocrysts are quartz crystals in a silica-deficient lava and diamonds within kimberlite diatremes. Xenoliths can be non-uniform within individual locations, even in areas which are spatially limited, e.g. rhyolite-dominated lava of Niijima volcano (Japan) contains two types of gabbroic xenoliths which are of different origin - they were formed in different temperature and pressure conditions.

Coesite Silica mineral, rare polymorph of quartz

Coesite is a form (polymorph) of silicon dioxide SiO2 that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C, 1,300 °F), are applied to quartz. Coesite was first synthesized by Loring Coes Jr., a chemist at the Norton Company, in 1953.

Peridotite A coarse-grained ultramafic igneous rock

Peridotite ( PERR-ih-doh-tyte, pə-RID-ə-) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.

Craton Old and stable part of the continental lithosphere

A craton is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates; the exceptions occur where geologically recent rifting events have separated cratons and created passive margins along their edges. They are characteristically composed of ancient crystalline basement rock, which may be covered by younger sedimentary rock. They have a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into Earth's mantle.

The Mesoarchean is a geologic era in the Archean Eon, spanning 3,200 to 2,800 million years ago, which contains the first evidence of modern-style plate subduction and expansion of microbial life. The era is defined chronometrically and is not referenced to a specific level in a rock section on Earth.

Omphacite Member of the clinopyroxene group of silicate minerals

Omphacite is a member of the clinopyroxene group of silicate minerals with formula: (Ca, Na)(Mg, Fe2+, Al)Si2O6. It is a variably deep to pale green or nearly colorless variety of clinopyroxene. It normally appears in eclogite, which is the high-pressure metamorphic rock of basalt. Omphacite is the solid solution of Fe-bearing diopside and jadeite. It crystallizes in the monoclinic system with prismatic, typically twinned forms, though usually anhedral. Its space group can be P2/n or C2/c depending on the thermal history. It exhibits the typical near 90° pyroxene cleavage. It is brittle with specific gravity of 3.29 to 3.39 and a Mohs hardness of 5 to 6.

Lunar magma ocean Theorized historical geological layer on the Moon

The Lunar Magma Ocean (LMO) is the layer of molten rock that is theorized to have been present on the surface of the Moon. The Lunar Magma Ocean was likely present on the Moon from the time of the Moon's formation to tens or hundreds of millions years after that time. It is a thermodynamic consequence of the Moon's relatively rapid formation in the aftermath of a giant impact between the proto-Earth and another planetary body. As the Moon accreted from the debris from the giant impact, gravitational potential energy was converted to thermal energy. Due to the rapid accretion of the Moon, thermal energy was trapped since it did not have sufficient time to thermally radiate away energy through the lunar surface. The subsequent thermochemical evolution of the Lunar Magma Ocean explains the Moon's largely anorthositic crust, europium anomaly, and KREEP material.

North China Craton continental crustal block in northeast China, Inner Mongolia, the Yellow Sea, and North Korea

The North China Craton is a continental crustal block with one of Earth's most complete and complex records of igneous, sedimentary and metamorphic processes. It is located in northeast China, Inner Mongolia, the Yellow Sea, and North Korea. The term craton designates this as a piece of continent that is stable, buoyant and rigid. Basic properties of the cratonic crust include being thick, relatively cold when compared to other regions, and low density. The North China Craton is an ancient craton, which experienced a long period of stability and fitted the definition of a craton well. However, the North China Craton later experienced destruction of some of its deeper parts (decratonization), which means that this piece of continent is no longer as stable.

Ultra-high-pressure metamorphism refers to metamorphic processes at pressures high enough to stabilize coesite, the high-pressure polymorph of SiO2. It is important because the processes that form and exhume ultra-high-pressure (UHP) metamorphic rocks may strongly affect plate tectonics, the composition and evolution of Earth's crust. The discovery of UHP metamorphic rocks in 1984 revolutionized our understanding of plate tectonics. Prior to 1984 there was little suspicion that continental rocks could reach such high pressures.

Noronha hotspot

Noronha hotspot is a hypothesized hotspot in the Atlantic Ocean. It has been proposed as the candidate source for volcanism in the Fernando de Noronha archipelago of Brazil, as well as of other volcanoes also in Brazil and even the Bahamas and the Central Atlantic Magmatic Province.

Roberta Rudnick American geologist

Roberta L. Rudnick is an American earth scientist and professor of geology at the University of California, Santa Barbara. She was elected a member of the National Academy of Sciences in 2010 and was awarded the Dana Medal by the Mineralogical Society of America. Rudnick is a world expert in the continental crust and lithosphere.

Tonalite-trondhjemite-granodiorite Intrusive rocks with typical granitic composition

Tonalite-trondhjemite-granodiorite rocks or TTG rocks are intrusive rocks with typical granitic composition but containing only a small portion of potassium feldspar. Tonalite, trondhjemite, and granodiorite often occur together in geological records, indicating similar petrogenetic processes. Post Archean TTG rocks are present in arc-related batholiths, as well as in ophiolites, while Archean TTG rocks are major components of Archean cratons.

South China Craton

The South China Craton or South China Block is one of the Precambrian continental blocks in China. It is traditionally divided into the Yangtze Block in the NW and the Cathaysia Block in the SE. The Jiangshan–Shaoxing Fault represents the suture boundary between the two sub-blocks. Recent study suggests that the South China Block possibly has one more sub-block which is named the Tolo Terrane. The oldest rocks in the South China Block occur within the Kongling Complex, which yields zircon U–Pb ages of 3.3–2.9 Ga.

Stanley Robert Hart is an American geologist, geochemist, leading international expert on mantle isotope geochemistry, and pioneer of chemical geodynamics.

Peter H. Barry

Peter H. Barry is an American geochemist who is an assistant scientist in the Marine Chemistry and Geochemistry Department at the Woods Hole Oceanographic Institution. He uses noble gases and stable isotopes to understand the volatile history and chemical evolution of Earth, including the dynamic processes of subduction, mantle convection and surface volcanism, which control the redistribution of chemical constituents between the crust and mantle reservoirs. Barry’s main research focus has been on high-temperature geochemistry, crust-mantle interactions and the behavior of volatile fluids in the lithosphere, He also studies crustal systems, the origin of high helium deposits, including hydrocarbon formation and transport mechanisms.

Janet Margaret Hergt is an Australian geochemist. She is a Redmond Barry Distinguished Professor in the School of Earth Sciences at the University of Melbourne, Victoria, Australia. The main focus of her research has been in the chemical analysis of rocks and minerals to explore the exquisite record of Earth processes preserved within them. Hergt is best known for her geochemical investigations of magmatic rocks although she has employed similar techniques in interdisciplinary projects including areas of archaeological and biological science.

Navajo volcanic field Volcanic field in southwestern United States

The Navajo volcanic field is a monogenetic volcanic field located in the Four Corners region of the United States, in the central part of the Colorado Plateau. The volcanic field consist of over 80 volcanoes and associated intrusions of unusual potassium-rich compositions, with an age range of 26.2 to 24.7 million years (Ma).

Catherine Chauvel is a geochemist at the Institut de Physique du Globe de Paris known for her research on the impact of volcanic activity on the chemistry of the mantle, continental crust, and island arc geochemistry.

References

  1. Zheng, Yong-Fei; Chen, Ren-Xu (September 2017). "Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins". Journal of Asian Earth Sciences. 145: 46–73. Bibcode:2017JAESc.145...46Z. doi:10.1016/j.jseaes.2017.03.009. ISSN   1367-9120.
  2. 1 2 Jacob, D. E. (2004-09-01). "Nature and origin of eclogite xenoliths from kimberlites". Lithos. Selected Papers from the Eighth International Kimberlite Conference. Volume 2: The J. Barry Hawthorne Volume. 77 (1): 295–316. doi:10.1016/j.lithos.2004.03.038. ISSN   0024-4937.
  3. 1 2 3 4 5 COLEMAN, R. G; LEE, D. E; BEATTY, L. B; BRANNOCK, W. W (1965-05-01). "Eclogites and Eclogites: Their Differences and Similarities". GSA Bulletin. 76 (5): 483–508. doi:10.1130/0016-7606(1965)76[483:EAETDA]2.0.CO;2. ISSN   0016-7606.
  4. COLEMAN, R. G; LEE, D. E; BEATTY, L. B; BRANNOCK, W. W (1965-05-01). "Eclogites and Eclogites: Their Differences and Similarities". GSA Bulletin. 76 (5): 483–508. doi:10.1130/0016-7606(1965)76[483:EAETDA]2.0.CO;2. ISSN   0016-7606.
  5. COLEMAN, R. G; LEE, D. E; BEATTY, L. B; BRANNOCK, W. W (1965-05-01). "Eclogites and Eclogites: Their Differences and Similarities". GSA Bulletin. 76 (5): 483–508. doi:10.1130/0016-7606(1965)76[483:EAETDA]2.0.CO;2. ISSN   0016-7606.
  6. Jacob, D. E. (2004-09-01). "Nature and origin of eclogite xenoliths from kimberlites". Lithos. Selected Papers from the Eighth International Kimberlite Conference. Volume 2: The J. Barry Hawthorne Volume. 77 (1): 295–316. doi:10.1016/j.lithos.2004.03.038. ISSN   0024-4937.
  7. O'Hara, M. J. (1968-01-01). "The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks". Earth-Science Reviews. 4: 69–133. doi:10.1016/0012-8252(68)90147-5. ISSN   0012-8252.
  8. Ringwood, A. E.; Green, D. H. (1966-10-01). "An experimental investigation of the Gabbro-Eclogite transformation and some geophysical implications". Tectonophysics. 3 (5): 383–427. doi:10.1016/0040-1951(66)90009-6. ISSN   0040-1951.
  9. "Chemical variations in upper mantle nodules from southern African kimberlites". Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. 297 (1431): 273–293. 1980-07-24. doi:10.1098/rsta.1980.0215. ISSN   0080-4614.
  10. Jacob, D. E. (2004-09-01). "Nature and origin of eclogite xenoliths from kimberlites". Lithos. Selected Papers from the Eighth International Kimberlite Conference. Volume 2: The J. Barry Hawthorne Volume. 77 (1): 295–316. doi:10.1016/j.lithos.2004.03.038. ISSN   0024-4937.
  11. MacGregor, Ian D.; Manton, William I. (1986). "Roberts victor eclogites: Ancient oceanic crust". Journal of Geophysical Research: Solid Earth. 91 (B14): 14063–14079. doi:10.1029/JB091iB14p14063. ISSN   2156-2202.
  12. Gregory, Robert T.; Taylor, Hugh P. (1981). "An oxygen isotope profile in a section of Cretaceous oceanic crust, Samail Ophiolite, Oman: Evidence for δ18O buffering of the oceans by deep (>5 km) seawater-hydrothermal circulation at mid-ocean ridges". Journal of Geophysical Research: Solid Earth. 86 (B4): 2737–2755. doi:10.1029/JB086iB04p02737. ISSN   2156-2202.
  13. MacGregor, Ian D.; Manton, William I. (1986). "Roberts victor eclogites: Ancient oceanic crust". Journal of Geophysical Research: Solid Earth. 91 (B14): 14063–14079. doi:10.1029/JB091iB14p14063. ISSN   2156-2202.
  14. Muehlenbachs, Karlis (1998-04-15). "The oxygen isotopic composition of the oceans, sediments and the seafloor". Chemical Geology. 145 (3): 263–273. doi:10.1016/S0009-2541(97)00147-2. ISSN   0009-2541.
  15. Mattey, David; Lowry, David; Macpherson, Colin (1994-12-01). "Oxygen isotope composition of mantle peridotite". Earth and Planetary Science Letters. 128 (3): 231–241. doi:10.1016/0012-821X(94)90147-3. ISSN   0012-821X.
  16. O'Hara, M. J. (1968-01-01). "The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks". Earth-Science Reviews. 4: 69–133. doi:10.1016/0012-8252(68)90147-5. ISSN   0012-8252.
  17. Huang, Jin-Xiang; Gréau, Yoann; Griffin, William L.; O'Reilly, Suzanne Y.; Pearson, Norman J. (2012-06-01). "Multi-stage origin of Roberts Victor eclogites: Progressive metasomatism and its isotopic effects". Lithos. 142–143: 161–181. doi:10.1016/j.lithos.2012.03.002. ISSN   0024-4937.
  18. Hacker, Bradley R. (2008). "H2O subduction beyond arcs" (PDF). Geochemistry, Geophysics, Geosystems. 9 (3). Bibcode:2008GGG.....9.3001H. CiteSeerX   10.1.1.513.829 . doi:10.1029/2007GC001707.
  19. Rapp, Robert P.; Shimizu, Nobumichi; Norman, Marc D. (2003). "Growth of early continental crust by partial melting of eclogite". Nature. 425 (6958): 605–609. Bibcode:2003Natur.425..605R. doi:10.1038/nature02031. PMID   14534583.
  20. Foulger, G.R. (2010). Plates vs. Plumes: A Geological Controversy. Wiley-Blackwell. ISBN   978-1-4051-6148-0.
  21. Sobolev, Alexander V.; Hofmann, Albrecht W.; Sobolev, Stephan V.; Nikogosian, Igor K. (March 2005). "An olivine-free mantle source of Hawaiian shield basalts". Nature. 434 (7033): 590–597. Bibcode:2005Natur.434..590S. doi:10.1038/nature03411. ISSN   0028-0836. PMID   15800614.
  22. William Alexander Deer, R. A. Howie and J. Zussman (1997) Rock-forming Minerals, Geological Society, 668 pages ISBN   1-897799-85-3
  23. C. Michael Hogan (2008) Ring Mountain, The Megalithic Portal, ed. Andy Burnham
  24. Wilke, Franziska D.H.; O'Brien, Patrick J.; Altenberger, Uwe; Konrad-Schmolke, Matthias; Khan, M. Ahmed (January 2010). "Multi-stage reaction history in different eclogite types from the Pakistan Himalaya and implications for exhumation processes". Lithos. 114 (1–2): 70–85. Bibcode:2010Litho.114...70W. doi:10.1016/j.lithos.2009.07.015.
  25. Jahn, Bor-ming; Caby, Renaud; Monie, Patrick (2001). "The oldest UHP eclogites of the World: age of UHP metamorphism, nature of protoliths and tectonic implications". Chemical Geology. 178 (1–4): 143–158. Bibcode:2001ChGeo.178..143J. doi:10.1016/S0009-2541(01)00264-9.
  26. Santos, Ticiano José Saraiva; Amaral, Wagner Silva; Ancelmi, Matheus Fernando; Pitarello, Michele Zorzetti; Fuck, Reinhardt Adolfo; Dantas, Elton Luiz (2015). "U–Pb age of the coesite-bearing eclogite from NW Borborema Province, NE Brazil: Implications for western Gondwana assembly". Gondwana Research. 28 (3): 1183–1196. Bibcode:2015GondR..28.1183D. doi:10.1016/j.gr.2014.09.013.