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Allen P Nutman - One of the best experts on this subject based on the ideXlab platform.
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Abyssal peridotites >3,800 Ma from southern West Greenland: field relationships, petrography, geochronology, whole-rock and mineral chemistry of dunite and Harzburgite inclusions in the Itsaq Gneiss Complex
Contributions to Mineralogy and Petrology, 2020Co-Authors: Clark R L Friend, Victoria C Bennett, Allen P NutmanAbstract:Within the northern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on the southern side of the Isua supracrustal belt, enclaves up to ~500 m long of variably altered ultramafic rocks contain some relics of unaltered dunite–Harzburgite. These are associated with mafic supracrustal and plutonic rocks and siliceous metasediments. SHRIMP U/Pb zircon geochronology on non-igneous zircons in altered ultramafic rocks and on igneous zircons from components of the surrounding orthogneisses intruding them, indicate an absolute minimum age for the ultramafic rocks of ca. 3,650 Ma, but with an age of ca. 3,800 Ma most likely. The diverse ultramafic and mafic rocks with rarer metasediment were all first tectonically intercalated and then became enclosed in much more voluminous tonalitic rocks dated at ca. 3,800 Ma. This is interpreted to have occurred during the development of a 3,810–3,790-Ma composite magmatic arc early in the evolution of the Itsaq Gneiss Complex. This northern part of the Itsaq Gneiss Complex is the most favourable for the geochemical study of early Archaean protoliths because it experienced peak metamorphism only within the amphibolite facies with little or no in-situ melt segregation, and contains some areas that have undergone little deformation since ca. 3,800 Ma. Most of the ultramafic enclaves are thoroughly altered, and largely comprise secondary, hydrous phases. However, the centres of some enclaves have escaped alteration and comprise dunite and Harzburgite with >95% olivine (Fo89–91) + orthopyroxene (En89) + Al-spinel (Cr8–20) assemblages. The dunites and Harzburgites are massive or irregularly layered and are olivine-veined on 5–10-m to 10-cm scales. Their whole rock major and rare earth element, and olivine and spinel compositions differ significantly from xenoliths representing the Archaean cratonic lithospheric mantle, but are typical of some modern abyssal peridotites. The Harzburgites and dunites show both LREE depleted and enriched patterns; however, none show the massive REE depletion associated with the modelled removal of a komatiite. They are interpreted as being the products of small degrees of melt extraction, with some showing evidence of refertilisation. These Greenland dunites and Harzburgites described here are currently the best characterised 'sample' of the early Archaean upper mantle.
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abyssal peridotites 3 800 ma from southern west greenland field relationships petrography geochronology whole rock and mineral chemistry of dunite and Harzburgite inclusions in the itsaq gneiss complex
Contributions to Mineralogy and Petrology, 2002Co-Authors: Clark R L Friend, Victoria C Bennett, Allen P NutmanAbstract:Within the northern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on the southern side of the Isua supracrustal belt, enclaves up to ~500 m long of variably altered ultramafic rocks contain some relics of unaltered dunite–Harzburgite. These are associated with mafic supracrustal and plutonic rocks and siliceous metasediments. SHRIMP U/Pb zircon geochronology on non-igneous zircons in altered ultramafic rocks and on igneous zircons from components of the surrounding orthogneisses intruding them, indicate an absolute minimum age for the ultramafic rocks of ca. 3,650 Ma, but with an age of ca. 3,800 Ma most likely. The diverse ultramafic and mafic rocks with rarer metasediment were all first tectonically intercalated and then became enclosed in much more voluminous tonalitic rocks dated at ca. 3,800 Ma. This is interpreted to have occurred during the development of a 3,810–3,790-Ma composite magmatic arc early in the evolution of the Itsaq Gneiss Complex. This northern part of the Itsaq Gneiss Complex is the most favourable for the geochemical study of early Archaean protoliths because it experienced peak metamorphism only within the amphibolite facies with little or no in-situ melt segregation, and contains some areas that have undergone little deformation since ca. 3,800 Ma. Most of the ultramafic enclaves are thoroughly altered, and largely comprise secondary, hydrous phases. However, the centres of some enclaves have escaped alteration and comprise dunite and Harzburgite with >95% olivine (Fo89–91) + orthopyroxene (En89) + Al-spinel (Cr8–20) assemblages. The dunites and Harzburgites are massive or irregularly layered and are olivine-veined on 5–10-m to 10-cm scales. Their whole rock major and rare earth element, and olivine and spinel compositions differ significantly from xenoliths representing the Archaean cratonic lithospheric mantle, but are typical of some modern abyssal peridotites. The Harzburgites and dunites show both LREE depleted and enriched patterns; however, none show the massive REE depletion associated with the modelled removal of a komatiite. They are interpreted as being the products of small degrees of melt extraction, with some showing evidence of refertilisation. These Greenland dunites and Harzburgites described here are currently the best characterised 'sample' of the early Archaean upper mantle.
Ahmed H Ahmed - One of the best experts on this subject based on the ideXlab platform.
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highly depleted Harzburgite dunite chromitite complexes from the neoproterozoic ophiolite south eastern desert egypt a possible recycled upper mantle lithosphere
Precambrian Research, 2013Co-Authors: Ahmed H AhmedAbstract:Abstract Five Neoproterozoic ophiolitic complexes from the southern Eastern Desert of Egypt have been petrologically examined for their upper mantle Harzburgite–dunite–chromitite associations. Three of them are exceptionally fresh (Abu Dahr, Abu Siayil and Arays), while the other two localities (Belamhandeit and Umm Thagar) are severely serpentinized. Although the upper mantle Harzburgite hosts in these complexes are highly depleted, they contain frequent large-sized chromitite pods with metallurgical grade. Orthopyroxene, olivine and to a lesser extent clinopyroxene with highly refractory nature, are the primary silicates found in the Harzburgites and dunites. The forsterite content of olivine is slightly higher in dunites (Fo94) than those in Harzburgites (Fo92). Chromian spinels in Harzburgites, dunites and chromitites are very refractory with restricted chemical compositions of high-Cr varieties. The average Cr-ratio (=Cr/(Cr + Al) atomic ratio) of chromian spinel in the chromitites and dunite envelopes ranges from 0.76 to 0.87, while it ranges from 0.67 to 0.86 in the Harzburgite hosts. Platinum-group elements (PGEs) in chromitites exhibit steep negatively sloped distribution patterns, being highly enriched in IPGEs (Os, Ir, Ru) and strongly depleted in PPGEs (Rh, Pt, Pd). The estimated chemical composition of the primitive parental magma in equilibrium with podiform chromitites and associated ultramafic rocks are characterized by Al2O3 contents and FeO/MgO ratio that is fairly similar to a boninitic source generated at supra-subduction zone settings. The studied complexes show constantly high oxygen fugacities (fO2), averaging Δlog fO2 + 3.33, +2.42 and +1.80 in chromitites, dunites and Harzburgites, respectively. The high oxidation state of the studied upper mantle ophiolitic complexes also suggests a boninitic source in the mantle wedge of an arc setting. The highly depleted nature of the Harzburgite hosts in the studied complexes is inconsistent with the consensus that podiform chromitite is hosted most commonly by moderately refractory Harzburgites. This can be achieved if the podiform chromitites and associated ultramafic rocks have been subsequently modified during deep recycling process within the upper mantle where the Harzburgite hosts and dunite envelopes become more refractory than the original compositions. Hence, the uncommon presence of podiform chromitites hosted by highly refractory ultramafic rocks were most probably due to a second stage melting of a depleted mantle Harzburgite at a fore-arc setting, or alternatively due to deep recycling processes within the upper mantle.
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Highly depleted Harzburgite–dunite–chromitite complexes from the Neoproterozoic ophiolite, south Eastern Desert, Egypt: A possible recycled upper mantle lithosphere
Precambrian Research, 2013Co-Authors: Ahmed H AhmedAbstract:Abstract Five Neoproterozoic ophiolitic complexes from the southern Eastern Desert of Egypt have been petrologically examined for their upper mantle Harzburgite–dunite–chromitite associations. Three of them are exceptionally fresh (Abu Dahr, Abu Siayil and Arays), while the other two localities (Belamhandeit and Umm Thagar) are severely serpentinized. Although the upper mantle Harzburgite hosts in these complexes are highly depleted, they contain frequent large-sized chromitite pods with metallurgical grade. Orthopyroxene, olivine and to a lesser extent clinopyroxene with highly refractory nature, are the primary silicates found in the Harzburgites and dunites. The forsterite content of olivine is slightly higher in dunites (Fo94) than those in Harzburgites (Fo92). Chromian spinels in Harzburgites, dunites and chromitites are very refractory with restricted chemical compositions of high-Cr varieties. The average Cr-ratio (=Cr/(Cr + Al) atomic ratio) of chromian spinel in the chromitites and dunite envelopes ranges from 0.76 to 0.87, while it ranges from 0.67 to 0.86 in the Harzburgite hosts. Platinum-group elements (PGEs) in chromitites exhibit steep negatively sloped distribution patterns, being highly enriched in IPGEs (Os, Ir, Ru) and strongly depleted in PPGEs (Rh, Pt, Pd). The estimated chemical composition of the primitive parental magma in equilibrium with podiform chromitites and associated ultramafic rocks are characterized by Al2O3 contents and FeO/MgO ratio that is fairly similar to a boninitic source generated at supra-subduction zone settings. The studied complexes show constantly high oxygen fugacities (fO2), averaging Δlog fO2 + 3.33, +2.42 and +1.80 in chromitites, dunites and Harzburgites, respectively. The high oxidation state of the studied upper mantle ophiolitic complexes also suggests a boninitic source in the mantle wedge of an arc setting. The highly depleted nature of the Harzburgite hosts in the studied complexes is inconsistent with the consensus that podiform chromitite is hosted most commonly by moderately refractory Harzburgites. This can be achieved if the podiform chromitites and associated ultramafic rocks have been subsequently modified during deep recycling process within the upper mantle where the Harzburgite hosts and dunite envelopes become more refractory than the original compositions. Hence, the uncommon presence of podiform chromitites hosted by highly refractory ultramafic rocks were most probably due to a second stage melting of a depleted mantle Harzburgite at a fore-arc setting, or alternatively due to deep recycling processes within the upper mantle.
Clark R L Friend - One of the best experts on this subject based on the ideXlab platform.
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Abyssal peridotites >3,800 Ma from southern West Greenland: field relationships, petrography, geochronology, whole-rock and mineral chemistry of dunite and Harzburgite inclusions in the Itsaq Gneiss Complex
Contributions to Mineralogy and Petrology, 2020Co-Authors: Clark R L Friend, Victoria C Bennett, Allen P NutmanAbstract:Within the northern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on the southern side of the Isua supracrustal belt, enclaves up to ~500 m long of variably altered ultramafic rocks contain some relics of unaltered dunite–Harzburgite. These are associated with mafic supracrustal and plutonic rocks and siliceous metasediments. SHRIMP U/Pb zircon geochronology on non-igneous zircons in altered ultramafic rocks and on igneous zircons from components of the surrounding orthogneisses intruding them, indicate an absolute minimum age for the ultramafic rocks of ca. 3,650 Ma, but with an age of ca. 3,800 Ma most likely. The diverse ultramafic and mafic rocks with rarer metasediment were all first tectonically intercalated and then became enclosed in much more voluminous tonalitic rocks dated at ca. 3,800 Ma. This is interpreted to have occurred during the development of a 3,810–3,790-Ma composite magmatic arc early in the evolution of the Itsaq Gneiss Complex. This northern part of the Itsaq Gneiss Complex is the most favourable for the geochemical study of early Archaean protoliths because it experienced peak metamorphism only within the amphibolite facies with little or no in-situ melt segregation, and contains some areas that have undergone little deformation since ca. 3,800 Ma. Most of the ultramafic enclaves are thoroughly altered, and largely comprise secondary, hydrous phases. However, the centres of some enclaves have escaped alteration and comprise dunite and Harzburgite with >95% olivine (Fo89–91) + orthopyroxene (En89) + Al-spinel (Cr8–20) assemblages. The dunites and Harzburgites are massive or irregularly layered and are olivine-veined on 5–10-m to 10-cm scales. Their whole rock major and rare earth element, and olivine and spinel compositions differ significantly from xenoliths representing the Archaean cratonic lithospheric mantle, but are typical of some modern abyssal peridotites. The Harzburgites and dunites show both LREE depleted and enriched patterns; however, none show the massive REE depletion associated with the modelled removal of a komatiite. They are interpreted as being the products of small degrees of melt extraction, with some showing evidence of refertilisation. These Greenland dunites and Harzburgites described here are currently the best characterised 'sample' of the early Archaean upper mantle.
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abyssal peridotites 3 800 ma from southern west greenland field relationships petrography geochronology whole rock and mineral chemistry of dunite and Harzburgite inclusions in the itsaq gneiss complex
Contributions to Mineralogy and Petrology, 2002Co-Authors: Clark R L Friend, Victoria C Bennett, Allen P NutmanAbstract:Within the northern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on the southern side of the Isua supracrustal belt, enclaves up to ~500 m long of variably altered ultramafic rocks contain some relics of unaltered dunite–Harzburgite. These are associated with mafic supracrustal and plutonic rocks and siliceous metasediments. SHRIMP U/Pb zircon geochronology on non-igneous zircons in altered ultramafic rocks and on igneous zircons from components of the surrounding orthogneisses intruding them, indicate an absolute minimum age for the ultramafic rocks of ca. 3,650 Ma, but with an age of ca. 3,800 Ma most likely. The diverse ultramafic and mafic rocks with rarer metasediment were all first tectonically intercalated and then became enclosed in much more voluminous tonalitic rocks dated at ca. 3,800 Ma. This is interpreted to have occurred during the development of a 3,810–3,790-Ma composite magmatic arc early in the evolution of the Itsaq Gneiss Complex. This northern part of the Itsaq Gneiss Complex is the most favourable for the geochemical study of early Archaean protoliths because it experienced peak metamorphism only within the amphibolite facies with little or no in-situ melt segregation, and contains some areas that have undergone little deformation since ca. 3,800 Ma. Most of the ultramafic enclaves are thoroughly altered, and largely comprise secondary, hydrous phases. However, the centres of some enclaves have escaped alteration and comprise dunite and Harzburgite with >95% olivine (Fo89–91) + orthopyroxene (En89) + Al-spinel (Cr8–20) assemblages. The dunites and Harzburgites are massive or irregularly layered and are olivine-veined on 5–10-m to 10-cm scales. Their whole rock major and rare earth element, and olivine and spinel compositions differ significantly from xenoliths representing the Archaean cratonic lithospheric mantle, but are typical of some modern abyssal peridotites. The Harzburgites and dunites show both LREE depleted and enriched patterns; however, none show the massive REE depletion associated with the modelled removal of a komatiite. They are interpreted as being the products of small degrees of melt extraction, with some showing evidence of refertilisation. These Greenland dunites and Harzburgites described here are currently the best characterised 'sample' of the early Archaean upper mantle.
Andrew Putnis - One of the best experts on this subject based on the ideXlab platform.
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The role of reacting solution and temperature on compositional evolution during Harzburgite alteration: Constraints from the Mesoarchean Nuasahi Massif (eastern India)
Lithos, 2020Co-Authors: Alik S Majumdar, Jorn Hovelmann, Sisir K Mondal, Andrew PutnisAbstract:We investigate the microtextural-chemical features of partially serpentinized Harzburgites from the lower ultramafic unit of the Mesoarchean Nuasahi Massif, eastern India, in order to understand the role of reacting fluid composition and temperature on the phase evolution across replacement interfaces during progressive alteration. Two distinct types of pseudomorphic replacement textures are identified. Type-1 replacement texture was developed after primary orthopyroxene and is composed of talc + olivine + lizardite + tremolite + magnetite. Primary olivine was replaced by mesh-textured Mg-rich lizardite + magnetite at the center of the olivine grains and successive layers of relatively Fe-rich lizardite, magnesite, and calcite toward olivine rims, defining type-2 replacement texture. The Nuasahi Harzburgite was initially out-of-equilibrium with respect to H2O-CO2-bearing reacting solution and the secondary compositions have mainly evolved in the CaO-MgO-FeO-SiO2-Al2O3-H2O-CO2 system as a result of fluid-rock interaction. The alteration process across orthopyroxene interfaces has started at relatively higher temperature conditions (400 < T < 675 degrees C) than that at primary olivine interface (T 330 degrees C). Each replacement process across reaction interfaces was controlled via an interface coupled dissolution-precipitation mechanism. The sequential development of different secondary compositions in these replacement rims indicates a micrometer-scale variation in silica activity and H2O/CO2(aq) ratio in the solution across replacement interfaces. The Fe2+Mg-1 chemical exchange potential of the equilibrating system plays an important role in dictating the Fe/Mg ratio of the secondary compositions and the molar proportions of magnetite. The precipitation of tremolite and calcite in some isolated areas reflects a variation in Ca activity in the reacting fluid. The precipitation of carbonates may also be associated with an increase in pH in the interfacial solution. The pattern in phase evolution across type-1 interfaces suggests a greater extent of diffusion of solutes toward inner interfaces relative to the rim center during progressive alteration of the Nuasahi Harzburgite, possibly due to an increase in permeability in replaced phases. With increasing magnitude of diffusion toward type-1 inner reacting interfaces, the widespread presence of arrested replacement features suggests an alteration event under low fluid/rock ratio and/or sluggish reaction kinetics with decreasing temperature. (C) 2016 Elsevier B.V. All rights reserved
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the role of reacting solution and temperature on compositional evolution during Harzburgite alteration constraints from the mesoarchean nuasahi massif eastern india
Lithos, 2016Co-Authors: Alik S Majumdar, Jorn Hovelmann, Sisir K Mondal, Andrew PutnisAbstract:Abstract We investigate the microtextural–chemical features of partially serpentinized Harzburgites from the lower ultramafic unit of the Mesoarchean Nuasahi Massif, eastern India, in order to understand the role of reacting fluid composition and temperature on the phase evolution across replacement interfaces during progressive alteration. Two distinct types of pseudomorphic replacement textures are identified. Type-1 replacement texture was developed after primary orthopyroxene and is composed of talc + olivine + lizardite + tremolite + magnetite. Primary olivine was replaced by mesh-textured Mg-rich lizardite + magnetite at the center of the olivine grains and successive layers of relatively Fe-rich lizardite, magnesite, and calcite toward olivine rims, defining type-2 replacement texture. The Nuasahi Harzburgite was initially out-of-equilibrium with respect to H 2 O–CO 2 -bearing reacting solution and the secondary compositions have mainly evolved in the CaO–MgO–FeO–SiO 2 –Al 2 O 3 –H 2 O–CO 2 system as a result of fluid–rock interaction. The alteration process across orthopyroxene interfaces has started at relatively higher temperature conditions (400 H 2 O C O 2 aq ratio in the solution across replacement interfaces. The Fe 2 + Mg − 1 chemical exchange potential of the equilibrating system plays an important role in dictating the Fe/Mg ratio of the secondary compositions and the molar proportions of magnetite. The precipitation of tremolite and calcite in some isolated areas reflects a variation in Ca activity in the reacting fluid. The precipitation of carbonates may also be associated with an increase in pH in the interfacial solution. The pattern in phase evolution across type-1 interfaces suggests a greater extent of diffusion of solutes toward inner interfaces relative to the rim center during progressive alteration of the Nuasahi Harzburgite, possibly due to an increase in permeability in replaced phases. With increasing magnitude of diffusion toward type-1 inner reacting interfaces, the widespread presence of arrested replacement features suggests an alteration event under low fluid/rock ratio and/or sluggish reaction kinetics with decreasing temperature.
Dante Canil - One of the best experts on this subject based on the ideXlab platform.
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Constraints on the origin of mantle-derived low Ca garnets
Contributions to Mineralogy and Petrology, 1992Co-Authors: Dante CanilAbstract:Current hypotheses for the source rock of low Ca garnets hosted in mantle-derived diamonds and xenoliths range from residues of komatiite generation, to subducted serpentinite, to subducted mid-ocean ridge (MORB) Harzburgite. Experiments designed to test these hypotheses were undertaken. The stability and compositional variation of garnets at pressures above 4 GPa through the melting interval of hydrous peridotite, in the subsolidus of depleted Harzburgite and peridotite compositions, and along the liquidus of aluminium-undepleted and aluminium-depleted komatiites were examined, and compared with petrological data for natural low Ca garnets. Partitioning of Cr between garnet and ultramafic liquid along the liquidus of komatiites and within the melting interval of peridotite, indicates that garnets in mantle residues after single stage Archean ultramafic liquid removal would contain 2 to 4 wt% Cr_2O_3. Thus, the more Cr-poor population of mantle-derived low Ca garnets, with Cr_2O_3 less than 4 wt%, could have originated by such a process. Experimental results for other compositions indicate that average cratonic peridotite or its hydrated equivalent is typically too Cr-poor to be the protolith from which low Ca garnets containing greater than 4 wt% Cr_2O_3 could have crystallized in the upper mantle. Experiments on a spinel Harzburgite composition indicate that an extremely Cr-rich protolith (Cr/Cr+Al>0.3) is required to crystallize spinel and Cr-rich low Ca garnets, at pressures deduced for the ultramafic inclusion suite in diamonds (5 to 7 GPa). Natural examples of such Cr-rich protoliths are represented in some ophiolite Harzburgites. All the experimental data taken together require that low Ca garnets with greater than 4 wt% Cr_2O_3 originated from residues that underwent multiple melt extraction. Whether such multi-stage events formed protoliths for low Ca garnets at shallow (i.e. MORB source region) or deep (i.e. komatiite source region) levels in the Precambrian mantle is not completely resolvable. The former environment can better account for the abundance of spinel in many diamonds hosting low Ca garnets, but the latter scenario best explains the presence of low Ca garnets in Harzburgite xenoliths with ‘cratonic’ bulk compositions well removed from typical MORB residues.
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experimental evidence for the exsolution of cratonic peridotite from high temperature Harzburgite
Earth and Planetary Science Letters, 1991Co-Authors: Dante CanilAbstract:Phase equilibrium experiments were performed on typical ‘oceanic’ and ‘cratonic’ peridotite compositions and a Ca, Al-rich orthopyroxene composition, to test the proposal that garnet lherzolites exsolved from high-temperature Harzburgites, and to further our understanding of the origin of ancient cratonic lithospheres. ‘Oceanic’ peridotites crystallize a garnet Harzburgite assemblage at pressures above 5 GPa in the temperature range 1450–1600°C, but at 5 GPa and temperatures less than 1450°C, crystallize clinopyroxene to become true lherzolites. ‘Cratonic’ peridotites crystallize a garnet Harzburgite assemblage at pressures above 5 GPa in the temperature range 1300–1600°C. Garnet-free Harzburgite crystallizes from both ‘cratonic’ and ‘oceanic’ peridotite at temperatures above 1450°C and pressures below 4.5–5 GPa. Phase relations for the high Ca, Al-rich orthopyroxene composition essentially mirror those for ‘oceanic’ peridotite. The complete solution of garnet and clinopyroxene into orthopyroxene observed in all three starting compositions at temperatures near or above the mantle solidus at pressures less than 6 GPa supports the hypothesis that garnet lherzolite could have exsolved from Harzburgite. The inferred cooling path for the original high-temperature Harzburgite protoliths of garnet lherzolites differs depending on bulk composition. The precursor Harzburgite protoliths of garnet lherzolites and Harzburgites with ‘cratonic’ bulk compositions apparently experienced simple isobaric cooling from formation temperatures near the peridotite solidus to those at which most of these peridotites were sampled in the mantle (< 1200°C). The cooling histories for Harzburgite protoliths of sheared garnet lherzolites with ‘oceanic’ compositional affinity are speculated to have involved convective circulation of mantle material to depths deeper than those at which it was originally formed. Phase equilibria and compositional relationships for orthopyroxenes produced in phase equilibrium experiments on peridotite and komatiite are consistent with an origin for ‘cratonic’ peridotite as a residue of Archean komatiite extraction, which has since cooled and exsolved clinopyroxene and garnet to become the common low-temperature, coarse-grained peridotite thought to comprise the bulk of the mantle lithosphere beneath the Archean Kaapvaal craton.
