Major- and trace-element and Sr–Nd–Hf isotopic compositions of garnet and clinopyroxene in kimberlite-borne eclogite and pyroxenite xenoliths were used to establish their origins and evolution in the subcontinental lithospheric mantle beneath the central Slave Craton, Canada. The majority of eclogites can be assigned to three groups (high-Mg, high-Ca or low-Mg eclogites) that have distinct trace-element patterns. Although post-formation metasomatism involving high field strength element (HFSE) and light rare earth element (LREE) addition has partially obscured the primary compositional features of the high-Mg and high-Ca eclogites, trace-element features, such as unfractionated middle REE (MREE) to heavy REE (HREE) patterns suggestive of garnet-free residues and low Zr/Sm consistent with plagioclase accumulation, could indicate a subduction origin from a broadly gabbroic protolith. In this scenario, the low ∑REE and small positive Eu anomalies of the high-Mg eclogites suggest more primitive, plagioclase-rich protoliths, whereas the high-Ca eclogites are proposed to have more evolved protoliths with higher (normative) clinopyroxene/plagioclase ratios plus trapped melt, consistent with their lower Mg-numbers, higher ∑REE and absence of Eu anomalies. In contrast, the subchondritic Zr/Hf and positive slope in the HREE of the low-Mg eclogites are similar to Archaean second-stage melts and point to a previously depleted source for their precursors. Low ratios of fluid-mobile to less fluid-mobile elements and of LREE to HREE are consistent with dehydration and partial melt loss for some eclogites. The trace-element characteristics of the different eclogite types translate into lower εNd for high-Mg eclogites than for low-Mg eclogites. Within the low-Mg group, samples that show evidence for metasomatic enrichment in LREE and HFSE have lower εNd and εHf than a sample that was apparently not enriched, pointing to long-term evolution at their respective parent–daughter ratios. Garnet and clinopyroxene in pyroxenites show different major-element relationships from those in eclogites, such as an opposite CaO–Na2O trend and the presence of a CaO–Cr2O3 trend, independent of whether or not opx is part of the assemblage. Therefore, these two rock types are probably not related by fractionation processes. The presence of opx in about half of the samples precludes direct crystallization from eclogite-derived melts. They probably formed from hybridized melts that reacted with the peridotitic mantle.
Heavy-mineral concentrates (garnets, chromites) and xenoliths from 21 Cretaceous–Tertiary kimberlite intrusions have been used to map the lithospheric mantle beneath the Lac de Gras area in the central part of the Slave Province. Analyses of Nickel Temperature (TNi) and Zinc Temperature (TZn) have been used to place garnet and chromite xenocrysts, respectively, in depth context. Paleogeotherms derived from both xenoliths and concentrates lie near a 35 mW/m2 conductive model at T ≤ 900°C, and near a 38 mW/m2 model at higher T, implying a marked change in conductivity and/or a thermal transient. Plots of garnet composition vs TNi also show a sharp discontinuity in mantle composition at 900°C. Garnets from <145 km depth are ultradepleted in Y, Zr, Ti and Ga, whereas those from greater depths (to ≥ 200 km) are similar to garnets from Archean mantle world-wide. Relative abundances of garnet types indicate that the shallow layer consists of ∼60% (clinopyroxene-free) harzburgite and 40% lherzolite, whereas the deeper layer contains 15–20% harzburgite and 80–85% lherzolite. T estimates on eclogite xenoliths show that all were derived from the deeper layer. Xenolith data and garnet compositions indicate that the shallow layer is more magnesian (Fo92–94) than the deeper layer (Fo91–92), and both layers are more olivine rich than South African or Siberian Archean peridotite xenoliths. The composition and sharply defined structure of the Lac de Gras lithosphere are unique within our current knowledge of Archean mantle sections. The shallow layer of this lithosphere section is similar to peridotites from some highly depleted ophiolites from convergent-margin settings, and may have formed in a similar situation during the accretion of the Hackett and Contwoyto terranes (magmatic arc and accretionary prism, respectively) to the ancient continental Anton terrane at 2.6–2.7 Ga. The deeper layer is interpreted as a plume head, which rose from the lower mantle and underplated the existing lithosphere at 2.6 Ga; evidence includes a high proportion of the superdeep inclusion assemblage (ferropericlase–perovskite) in the diamond population. This event could have provided heat for generation of the widespread 2.6 Ga post-tectonic granites. Proterozoic subduction from east and west may have modified the cratonic root, mainly by introduction of eclogites near its base.
Research Article| August 01, 2002 Subduction signature for quenched carbonatites from the deep lithosphere Esmé van Achterbergh; Esmé van Achterbergh 1Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia Search for other works by this author on: GSW Google Scholar William L. Griffin; William L. Griffin 2Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia, and Commonwealth Scientific and Industrial Research Organisation, Exploration and Mining, P.O. Box 136, North Ryde, New South Wales 1670, Australia Search for other works by this author on: GSW Google Scholar Chris G. Ryan; Chris G. Ryan 3Commonwealth Scientific and Industrial Research Organisation, Exploration and Mining, P.O. Box 136, North Ryde, New South Wales 1670, Australia Search for other works by this author on: GSW Google Scholar Suzanne Y. O'Reilly; Suzanne Y. O'Reilly 4Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia Search for other works by this author on: GSW Google Scholar Norman J. Pearson; Norman J. Pearson 4Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia Search for other works by this author on: GSW Google Scholar Kevin Kivi; Kevin Kivi 5Kennecott Canada Exploration Inc., 1300 Walsh Street, Thunder Bay, Ontario P7E 4X4, Canada Search for other works by this author on: GSW Google Scholar Buddy J. Doyle Buddy J. Doyle 6Kennecott Canada Exploration Inc., 354-200 Granville Street, Vancouver, British Columbia V6C 154, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Esmé van Achterbergh 1Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia William L. Griffin 2Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia, and Commonwealth Scientific and Industrial Research Organisation, Exploration and Mining, P.O. Box 136, North Ryde, New South Wales 1670, Australia Chris G. Ryan 3Commonwealth Scientific and Industrial Research Organisation, Exploration and Mining, P.O. Box 136, North Ryde, New South Wales 1670, Australia Suzanne Y. O'Reilly 4Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia Norman J. Pearson 4Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia Kevin Kivi 5Kennecott Canada Exploration Inc., 1300 Walsh Street, Thunder Bay, Ontario P7E 4X4, Canada Buddy J. Doyle 6Kennecott Canada Exploration Inc., 354-200 Granville Street, Vancouver, British Columbia V6C 154, Canada Publisher: Geological Society of America Received: 07 Jan 2002 Revision Received: 22 Apr 2002 Accepted: 30 Apr 2002 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2002) 30 (8): 743–746. https://doi.org/10.1130/0091-7613(2002)030<0743:SSFQCF>2.0.CO;2 Article history Received: 07 Jan 2002 Revision Received: 22 Apr 2002 Accepted: 30 Apr 2002 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Esmé van Achterbergh, William L. Griffin, Chris G. Ryan, Suzanne Y. O'Reilly, Norman J. Pearson, Kevin Kivi, Buddy J. Doyle; Subduction signature for quenched carbonatites from the deep lithosphere. Geology 2002;; 30 (8): 743–746. doi: https://doi.org/10.1130/0091-7613(2002)030<0743:SSFQCF>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Quenched carbonate-silicate inclusions in lherzolitic clinopyroxene macrocrysts, derived from 200 km beneath the Slave craton in northern Canada, are interpreted as natural samples of mantle carbonatites. Oxygen, carbon, and strontium isotope data provide evidence for the involvement of subducted crustal material in the origin of these carbonatites, supporting suggestions that carbon recycling by subduction is an important prerequisite for carbonatite magmatism. The compositional range of the inclusions suggests that the parent melt was decreasing in silica content as it was trapped in the host crystal, a trend that is predicted experimentally. Isotopic disequilibrium between the carbonatitic inclusions and the host clinopyroxene indicates that they were trapped shortly before kimberlite eruption, suggesting a temporal link between the entrapment of the carbonatite in the host and the Paleocene eruption of the kimberlite. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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