Eclogite

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 ( /ˈɛklədʒaɪt/) is a metamorphic rock containing 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 chemistry of primary and accessory minerals is used to classify three types of eclogite (A, B, and C). The broad range of eclogitic compositions has led to a longstanding debate on the origin of eclogite xenoliths as subducted, altered oceanic crust.

The name eclogite is derived from the Ancient Greek word for 'choice' (εκλογή, eklogḗ), meaning 'chosen rock' on account of its perceived beauty. It was first named by René Just Haüy in 1822 in the second edition of his work Traité de mineralogie.[1]

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.[2]

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.[3][4] The classification distinguishes each group based on the jadeite content of clinopyroxene and pyrope in garnet.[4] 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.[5]

The transitional nature between groups A, B and C correlates with their mode of emplacement at the surface.[4] 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.[3][4] Group B show strong compositional overlap with Group A, but are found as lenses or pods surrounded by peridotitic mantle material.[4] 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.[6]

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.[7][8][9]

Group A eclogite xenoliths remain the most enigmatic in terms of their origin due to metasomatic overprinting of their original composition.[10][3] 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.[11] 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.[13][14] 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.[8] Support of this process is result of metasomatic overprinting of the original oxygen isotope composition, driving them back towards the mantle range.[15]

Eclogite facies

This facies reflects metamorphism at high pressure (at or over 12kbar) and moderately high to very high temperatures. The pressures exceed those of greenschist, blueschist, amphibolite or granulite 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).[16]

Importance

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

Partial melting of eclogite has been modeled to produce tonalite-trondhjemite-granodiorite melts.[17] Eclogite-derived melts may be common in the mantle, and contribute to volcanic regions where unusually large volumes of magma are erupted.[18] The eclogite melt may then react with enclosing peridotite to produce pyroxenite, which in turn melts to produce basalt.[19]

Distribution

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

Occurrences exist in western North America, including the southwest[20] and the Franciscan Formation of the California Coast Ranges.[21] 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.[22] The oldest coesite-bearing eclogites are about 650 and 620 million years old and they are located in Brazil and Mali, respectively.[23][24]

References

  1. ^ International Eclogite Conference. "Eclogite History". Retrieved 25 May 2024.
  2. ^ 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.
  3. ^ a b c 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.
  4. ^ a b c d e 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. Archived from the original on 2022-02-12. Retrieved 2021-11-30.
  6. ^ 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. Archived from the original on 2022-02-12. Retrieved 2021-11-30.
  7. ^ 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. Archived from the original on 2022-02-12. Retrieved 2021-11-30.
  8. ^ a b 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.
  9. ^ 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.
  10. ^ "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. S2CID 123640184. Archived from the original on 2021-11-04. Retrieved 2021-11-30.
  11. ^ a b 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. S2CID 46321182.
  13. ^ 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.
  14. ^ 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.
  15. ^ 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.
  16. ^ 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. S2CID 135327696. Archived (PDF) from the original on 2010-06-17. Retrieved 2019-09-24.
  17. ^ 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. S2CID 4333290.
  18. ^ Foulger, G.R. (2010). Plates vs. Plumes: A Geological Controversy. Wiley-Blackwell. ISBN 978-1-4051-6148-0. Archived from the original on 2017-11-25. Retrieved 2011-03-16.
  19. ^ 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. S2CID 1565886.
  20. ^ William Alexander Deer, R. A. Howie and J. Zussman (1997) Rock-forming Minerals, Geological Society, 668 pages ISBN 1-897799-85-3
  21. ^ "C. Michael Hogan (2008) Ring Mountain, The Megalithic Portal, ed. Andy Burnham". Archived from the original on 2011-06-10. Retrieved 2009-01-14.
  22. ^ 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.
  23. ^ 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.
  24. ^ 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.
Wikimedia Commons has media related to Eclogite.