Instability of perovskite in a CO 2 -rich environment; examples from carbonatite and kimberlite
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RECEIVED JULY 16, 2004; ACCEPTED MARCH 16, 2005ADVANCE ACCESS PUBLICATION APRIL 29, 2005We introduce a modification to the current IUGS classificationsystem for igneous rocks to include ultramafic lamprophyres, whicharecurrentlyentirelyomitted.Thisisdonebyincludinganewstepinthe sequential system, after the assignment of pyroclastic rocks andcarbonatites, that considers ultramafic inequigranular textured rockswith olivine and phlogopite macrocrysts and/or phenocrysts. At thisstepultramaficlamprophyresareconsideredtogetherwithkimberlites,orangeites (former Group 2 kimberlites) and olivine lamproites.This proposal allows the correct identification and classificationof ultramafic lamprophyres within the IUGS scheme. Only threeend-members are required for describing the petrographic and com-positional continuum of ultramafic lamprophyres: alno¨ite (essentialgroundmass melilite), aillikite (essential primary carbonate) anddamtjernite (essentialgroundmass nepheline and/or alkali feldspar).It is argued that all ultramafic lamprophyre rock types canbe relatedto a common magma type which differs in important petrogeneticaspects from kimberlites, orangeites, olivine lamproites and theremainder of lamprophyres such as alkaline and calc-alkaline vari-eties. Ultramafic lamprophyres can be readily distinguished fromolivine lamproites by the occurrence of primary carbonates, and fromkimberlites by the presence of groundmass clinopyroxene. In othercases distinction between aillikites, kimberlites and orangeites mustrely on mineral compositions in order to recognize their petrogeneticaffinities.
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Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite, damtjernite) and subordinate dolomite-bearing carbonatite with U–Pb perovskite emplacement ages of ∼590–555 Ma occur in the vicinity of Aillik Bay, coastal Labrador. The ultramafic lamprophyres principally consist of olivine and phlogopite phenocrysts in a carbonate- or clinopyroxene-dominated groundmass. Ti-rich primary garnet (kimzeyite and Ti-andradite) typically occurs at the aillikite type locality and is considered diagnostic for ultramafic lamprophyre–carbonatite suites. Titanian aluminous phlogopite and clinopyroxene, as well as comparatively Al-enriched but Cr–Mg-poor spinel (Cr-number < 0.85), are compositionally distinct from analogous minerals in kimberlites, orangeites and olivine lamproites, indicating different magma geneses. The Aillik Bay ultramafic lamprophyres and carbonatites have variable but overlapping 87Sr/86Sri ratios (0·70369–0·70662) and show a narrow range in initial ɛNd (+0·1 to +1·9) implying that they are related to a common type of parental magma with variable isotopic characteristics. Aillikite is closest to this primary magma composition in terms of MgO (∼15–20 wt %) and Ni (∼200–574 ppm) content; the abundant groundmass carbonate has δ13CPDB between −5·7 and −5‰, similar to primary mantle-derived carbonates, and δ18OSMOW from 9·4 to 11·6‰. Extensive melting of a garnet peridotite source region containing carbonate- and phlogopite-rich veins at ∼4–7 GPa triggered by enhanced lithospheric extension can account for the volatile-bearing, potassic, incompatible element enriched and MgO-rich nature of the proto-aillikite magma. It is argued that low-degree potassic silicate to carbonatitic melts from upwelling asthenosphere infiltrated the cold base of the stretched lithosphere and solidified as veins, thereby crystallizing calcite and phlogopite that were not in equilibrium with peridotite. Continued Late Neoproterozoic lithospheric thinning, with progressive upwelling of the asthenosphere beneath a developing rift branch in this part of the North Atlantic craton, caused further veining and successive remelting of veins plus volatile-fluxed melting of the host fertile garnet peridotite, giving rise to long-lasting hybrid ultramafic lamprophyre magma production in conjunction with the break-up of the Rodinia supercontinent. Proto-aillikite magma reached the surface only after coating the uppermost mantle conduits with glimmeritic material, which caused minor alkali loss. At intrusion level, carbonate separation from this aillikite magma resulted in fractionated dolomite-bearing carbonatites (δ13CPDB −3·7 to −2·7‰) and carbonate-poor mela-aillikite residues. Damtjernites may be explained by liquid exsolution from alkali-rich proto-aillikite magma batches that moved through previously reaction-lined conduits at uppermost mantle depths.
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Abstract In hypabyssal and crater-facies kimberlites of the Lac de Gras kimberlite field, perovskite occurs as reaction-induced rims on earlier-crystallized Ti-bearing minerals (magnesian ilmenite and priderite), inclusions in atoll spinels and discrete crystals in a serpentine-calcite mesostasis. The mineral is associated with spinels, apatite, monticellite, phlogopite, baryte, Fe-Ni sulphides, ilmenite, diopside and zircon. Uncommon accessory phases found in an assemblage with perovskite include titanite, monazite- (Ce), witherite, strontium-apatite, khibinskite, djerfisherite, wollastonite, pectolite, suolunite, hydroxyapophyllite and bultfonteinite. Three types of perovskite can be distinguished on the basis of composition: (I) REE -Nb-Al-poor perovskite with relatively high Sr and K contents (up to 2.2 and 0.6 wt.% oxides, respectively) occurring as mantles on priderite and inclusions in atoll spinels; (II) perovskite with elevated Al, Fe, Nb and LREE (up to 1.4, 8.3, 9.1 and 17.0 wt.% oxides, respectively) found as discrete crystals and rims on macrocrystic ilmenite; (III) perovskite significantly enriched in Na, Sr, Nb and LREE (up to 3.3, 3.4, 13.0 and 22.6 wt.% oxides, respectively) found as rims on perovskite I and II. The overwhelming majority of perovskite is represented by discrete crystals of type II. In some occurrences, this type of perovskite also has high Th contents (up to 5.5 wt.% ThO 2 ) and Zr contents (up to 3.7 wt.% ZrO 2 ). Textural evidence indicates that perovskite shows an overall evolutionary trend from the most primitive type I towards type III showing the highest Na, Nb and LREE contents. Perovskite of type I probably crystallized under relatively high pressures prior to the precipitation of MUM spinels. Perovskite II crystallized after magnesiochromite, pleonaste and MUM (magnesian ulvöspinel-magnetite) spinels, under increasing f O 2 . The most compositionally evolved type III formed during near-solidus re-equilibration of the earlier-crystallized perovskite. The compositional variation of the Lac de Gras perovskite can be adequately characterized in terms of five major end-members: CaTiO 3 (perovskite), CeFeO 3 , NaNbO 3 (lueshite), Na 0.5 LREE 0.5 TiO 3 (loparite), and CaFe 0.5 Nb 0.5 O 3 (latrappite).
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Research Article| November 01, 2007 New insights into the genesis of Indian kimberlites from the Dharwar Craton via in situ Sr isotope analysis of groundmass perovskite Chad Paton; Chad Paton 1University of Melbourne, School of Earth Sciences, McCoy Building, University of Melbourne, Parkville, Victoria 3010, Australia Search for other works by this author on: GSW Google Scholar Janet M. Hergt; Janet M. Hergt 1University of Melbourne, School of Earth Sciences, McCoy Building, University of Melbourne, Parkville, Victoria 3010, Australia Search for other works by this author on: GSW Google Scholar David Phillips; David Phillips 1University of Melbourne, School of Earth Sciences, McCoy Building, University of Melbourne, Parkville, Victoria 3010, Australia Search for other works by this author on: GSW Google Scholar Jon D. Woodhead; Jon D. Woodhead 1University of Melbourne, School of Earth Sciences, McCoy Building, University of Melbourne, Parkville, Victoria 3010, Australia Search for other works by this author on: GSW Google Scholar Simon R. Shee Simon R. Shee 2Shee & Associates Pty Limited, 74 Liston Street, Glen Iris, Victoria 3146, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Chad Paton 1University of Melbourne, School of Earth Sciences, McCoy Building, University of Melbourne, Parkville, Victoria 3010, Australia Janet M. Hergt 1University of Melbourne, School of Earth Sciences, McCoy Building, University of Melbourne, Parkville, Victoria 3010, Australia David Phillips 1University of Melbourne, School of Earth Sciences, McCoy Building, University of Melbourne, Parkville, Victoria 3010, Australia Jon D. Woodhead 1University of Melbourne, School of Earth Sciences, McCoy Building, University of Melbourne, Parkville, Victoria 3010, Australia Simon R. Shee 2Shee & Associates Pty Limited, 74 Liston Street, Glen Iris, Victoria 3146, Australia Publisher: Geological Society of America Received: 30 Apr 2007 Revision Received: 06 Jul 2007 Accepted: 10 Jul 2007 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2007) 35 (11): 1011–1014. https://doi.org/10.1130/G24040A.1 Article history Received: 30 Apr 2007 Revision Received: 06 Jul 2007 Accepted: 10 Jul 2007 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Chad Paton, Janet M. Hergt, David Phillips, Jon D. Woodhead, Simon R. Shee; New insights into the genesis of Indian kimberlites from the Dharwar Craton via in situ Sr isotope analysis of groundmass perovskite. Geology 2007;; 35 (11): 1011–1014. doi: https://doi.org/10.1130/G24040A.1 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 In situ Sr isotopic analyses of kimberlitic perovskite are more representative of primary magmatic compositions than conventional bulk-rock analyses of the same samples, because the latter are variably compromised by contamination and alteration processes. Bulk-rock Sr isotopic data obtained for 18 intrusions from the adjacent Narayanpet and Wajrakarur kimberlite fields of the Dharwar Craton, India, exhibit a high degree of scatter (∼0.701–0.709) and have indistinguishable initial isotope ratios. In contrast, laser ablation perovskite results display strikingly uniform and distinct initial 87Sr/86Sr compositions for each field of 0.70312–0.70333 and 0.70234–0.70251, respectively. The increased resolution provided by these new data permits the evaluation of key aspects of kimberlite genesis. It is argued that lithospheric and crustal contamination had a negligible impact on the perovskite Sr isotope compositions, and that these values are representative of the primary melt component in each field. The results cast doubt on models of kimberlite formation that invoke either the small degree melting of metasomatized subcontinental lithospheric mantle, or derivation from unusually enriched asthenospheric mantle. The data are more compatible with a source region for the Dharwar kimberlites that was similar to a common mantle component such as prevalent mantle (PREMA) or focal zone (FOZO). 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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