DOCSLIB.ORG

Redox State of Southern Tibetan Upper Mantle and Ultrapotassic Magmas

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Redox State of Southern Tibetan Upper Mantle and Ultrapotassic Magmas https://doi.org/10.1130/G47411.1 Manuscript received 10 January 2020 Revised manuscript received 11 March 2020 Manuscript accepted 15 March 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 29 April 2020 Redox state of southern Tibetan upper mantle and ultrapotassic magmas Weikai Li1,2,3, Zhiming Yang1,4*, Massimo Chiaradia3, Yong Lai2, Chao Yu1 and Jiayu Zhang1 1 Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 2 School of Earth and Space Sciences, Peking University, Beijing 100871, China 3 Department of Earth Sciences, University of Geneva, Geneva 1205, Switzerland 4 University of Science and Technology Beijing, Beijing 100083, China ABSTRACT Fig. 1), lithospheric mantle xenoliths have been The redox state of Earth’s upper mantle in several tectonic settings, such as cratonic mantle, entrained by postcollisional ultrapotassic vol- oceanic mantle, and mantle wedges beneath magmatic arcs, has been well documented. In canic rocks (UVRs) in the Sailipu area (Zhao contrast, oxygen fugacity ( fO2 ) data of upper mantle under orogens worldwide are rare, and et al., 2008; Liu et al., 2011). The UVRs were the mechanism responsible for the mantle fO2 condition under orogens is not well constrained. derived from low-degree partial melting of the In this study, we investigated the fO2 of mantle xenoliths derived from the southern Tibetan highly metasomatized Tibetan upper mantle lithospheric mantle beneath the Himalayan orogen, and that of postcollisional ultrapotassic during the Miocene (Turner et al., 1996; Miller volcanic rocks hosting the xenoliths. The fO2 of mantle xenoliths ranges from ΔFMQ = +0.5 et al., 1999; Zhao et al., 2009; Guo et al., 2015). to +1.2 (where ΔFMQ is the deviation of log fO2 from the fayalite-magnetite-quartz buffer), Volcanic rock–hosted mantle xenoliths are com- indicating that the southern Tibetan lithospheric mantle is more oxidized than cratonic and monly considered to preserve the physicochemi- oceanic mantle, and it falls within the typical range of mantle wedge fO2 values. Mineralogical cal features of their mantle source (Coltorti and evidence suggests that water-rich fluids and sediment melts liberated from both the subduct- Grégoire, 2008), and therefore the mantle xeno- ing Neo-Tethyan oceanic slab and perhaps the Indian continental plate could have oxidized liths in southern Tibet may provide insights into the southern Tibetan lithospheric mantle. The fO2 conditions of ultrapotassic magmas show the fO2 condition of the upper mantle beneath a shift toward more oxidized conditions during ascent (from ΔFMQ = +0.8 to +3.0). Crustal the Himalayan-Tibetan orogen. evolution processes (e.g., fractionation) could influence magmatic fO2 , and thus the redox Here, we report for the first time: (1) fO2 state of mantle-derived magma may not simply represent its mantle source. values of lithospheric mantle xenoliths from southern Tibet; and (2) fO2 values of ultrapo- INTRODUCTION references therein). In contrast, the fO2 data of tassic volcanic magmas and their variation dur- The redox state of Earth’s upper mantle upper mantle beneath orogens worldwide are ing ascent. The main objectives of this study controls partial melting, the valence state of rare and indicate a broader range of variabil- were to identify the redox state of the southern multivalent and chalcophile elements, and the ity (ΔFMQ = –4.5 to +2.6; Foley, 2010). The Tibetan upper mantle and reveal the fO2 evolu- speciation of volatiles, and therefore it strong- mechanism responsible for the orogenic mantle tion of mantle-derived magmas along the path ly affects subsequent magma generation, ore- fO2 condition is not well constrained, except for from mantle source to shallow crust. forming process, and atmospheric evolution a few studies based on orogenic peridotite mas- (e.g., Frost and McCammon, 2008). The oxy- sifs (e.g., Woodland et al., 2006, and references GEOLOGICAL SETTING gen fugacity ( fO2 ) of the upper mantle in several therein). However, orogenic peridotite massifs The Lhasa terrane in southern Tibet is bound- tectonic settings has been well documented by were tectonically emplaced into the crust at con- ed by the Indus–Yarlung Zangbo suture (IYS) mantle-derived magmas and mantle peridotites vergent margins. Compared to mantle xenoliths, to the south and the Bangong-Nujiang suture (e.g., xenoliths, orogenic massifs), particularly their original geodynamic setting cannot be ex- (BNS) to the north (Fig. 1A). The final closure using ΔFMQ, which is the deviation of log fO2 actly constrained (Menzies and Dupuy, 1991). of the Neo-Tethys Ocean and collision of India from the fayalite-magnetite-quartz buffer. For Moreover, mantle structures and fO2 conditions with Asia along the southernmost margin of the instance, typical cratonic mantle (ΔFMQ = −4 may be obscured by deformation and metamor- Lhasa terrane occurred during the early Cenozo- to −2; Frost and McCammon, 2008) and oceanic phic recrystallization during tectonic emplace- ic (ca. 65–55 Ma; Leech et al., 2005; Mo et al., mantle (ΔFMQ = −1.2 to −0.4; Bézos and Hum- ment (Bodinier and Godard, 2014). 2007). After the collision, the Lhasa terrane in ler, 2005) are generally reduced. Mantle wedges The Himalayan-Tibetan orogen, created by the Himalayan-Tibetan orogen became part of a beneath magmatic arcs are relatively oxidized the collision between India and Asia after sub- postcollisional setting. Recent geophysical stud- (e.g., ΔFMQ = 0 to +3; Bénard et al., 2018, and duction of the Neo-Tethyan oceanic slab (e.g., ies (Zhao et al., 2011) have suggested that the Mo et al., 2007), is arguably one of the young- Indian continental plate subducted northward est collisional belt on Earth. In the southern beneath the Tibetan Plateau after collision (e.g., *E-mail: zm.yang@hotmail.com margin of the Tibetan Plateau (Lhasa terrane; since the Eocene; Leech et al., 2005). Since the CITATION: Li, W., et al., 2020, Redox state of southern Tibetan upper mantle and ultrapotassic magmas: Geology, v. 48, p. 733–736, https://doi.org/10.1130/G47411.1 Geological Society of America | GEOLOGY | Volume 48 | Number 7 | www.gsapubs.org 733 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/7/733/5073406/733.pdf by guest on 28 September 2021 o o xenoliths based on olivine-spinel pairs 80 E 90 E A Figure 1. (A) Sketch 100 km (Fig. S5C). The ferric iron content of spinel in Shiquanhe Qiangtang terrane map of tectonic units in Xiongba Tibet, and distributions lherzolite xenoliths was determined by electron Zabuye BNS of ultrapotassic volcanic microprobe analysis (EMPA), because the very Sailipu Garwa rocks (UVRs; red-filled Maiga small size and volume of the spinel (Fig. S5) Yaqian Mibale IYS Lhasa terrane o areas) and adakite-like Chazi 30 N rocks (blue-filled areas). were not suitable for the Mössbauer spectros- Lhasa Qulong copy analysis. Due to the disequilibrium texture Pabbai Stars represent sampling Zhunuo Bairong IYS Tibet Xigaze locations. STDS—South of the two pyroxenes in lherzolite xenoliths (Fig. STDS Tibetan detachment S5B), the single-clinopyroxene thermometer and system; IYS—Indus– barometer (Nimis and Taylor, 2000; Putirka, India Himalaya Yarlung Zangbo suture; BNS—Bangong-Nujiang 2008; see Equations 1 and 2 in the Supplemen- suture. (B) Tectonic frame- tal Material) were used to calculate temperature India 25oN B 400 km work of Tibetan Plateau. (T) and pressure (P), respectively. For compari- son, results calculated by other thermometers are mid-Oligocene, postcollisional magmatism has chyandesite in the Sailipu area (Fig. 1; Fig. S4). also provided in Table S2. Olivine phenocrysts been widespread in the Lhasa terrane, mainly They display a porphyroclastic texture, with larg- in all UVRs were used to calculate fO2 by the ol/melt consisting of ca. 25–8 Ma potassic to ultrapotas- er grains (0.1–1 mm) of olivine, orthopyroxene, DV oxybarometer of Mallmann and O’Neill sic volcanic rocks and ca. 21–12 Ma adakite-like clinopyroxene, and phlogopite, and a few fine- (2013). The whole-rock compositions were used intrusions (Fig. 1A; Yang et al., 2016). grained (<50 μm) spinel and apatite grains (Fig. as the melt compositions. The primary melt may The UVRs are mainly distributed along S5A). Clinopyroxenes show embayed texture have higher V content than whole rock because north-south–trending rifts within the Lhasa ter- filled by orthopyroxene (Fig. S5B), suggesting V is a compatible element, which may lead to rane west of 87°E (Fig. 1A), ranging in compo- disequilibrium between the two pyroxene types. a slight underestimation of the fO2 (Equation 6 sition from trachyandesite to trachyte (see Fig. Orthopyroxenes in lherzolite xenoliths host tiny in the Supplemental Material). Clinopyroxene S1 in the Supplemental Material1). two-phase melt inclusions, composed of silicate inclusions in olivine phenocrysts were used to glass (≤5 μm) and CO2 + N2 gas bubbles (Figs. calculate crystallization T and P of host olivines SAMPLES AND PETROGRAPHY S3G–S3J). These melt inclusions are isolated by the clinopyroxene–liquid thermobarometer The UVRs reported here were collected from without healed fracture trails (Figs. S3G and (Putirka et al., 2003; see Equations 3 and 4 in the Sailipu, Chazi, and Mibale areas (Fig. 1A). S3I). Phlogopite is the only hydrous mineral the Supplemental Material). Detailed methods of These samples show porphyritic textures, with phase. Spinel in the lherzolite is Cr-rich and often all calculations and error estimations are given the main phenocrysts (0.2–0.8 mm in size) be- occurs as inclusions in large crystals (Figs. S5A– in the Supplemental Material. ing olivine, clinopyroxene, phlogopite, and sani- S5C). Detailed descriptions of lherzolite xeno- On the basis of 60 sets of EMPA data, cal- dine (Figs. S2A–S2C). Some of the subhedral liths can be found in the Supplemental Material. culated T, P, and ΔFMQ values of lherzolite clinopyroxenes in these samples are enclosed xenoliths were 1316–1355 °C, 1.7–2.1 GPa or poikilitically enclosed in olivine pheno- METHODS AND RESULTS (corresponding to a continental paleodepth crysts (Fig.
Load more

Recommended publications
  • An Hypothesis for the Origin of Kimberlite 51
    Mineral. Soc. Amer. Spec. Pap. 3,51-62 (1970). AN HYPOTHESIS FOR THE ORIGIN OF KIMBERLITE IAN D. MACGREGOR Department of Geology, University of California, Davis, California 95616 ABSTRACT Kimberlites are characteristically associated with a suite of mafic and ultramafic xenoliths whose mineralogy indicates an origin within the upper mantle. The phase chemistry of the xenoliths may be reconciled with known experimental data at high pressures, as suites of crystal cumulates, or residual phases, that have formed during the high-pressure fractional crystallization of a mafic magma. The geological association of kimberlites with specificsuites of xenoliths, and the com- parison with experimental data, give support to this cognate hypothesis previously proposed by a number of other authors. Models of the Earth's thermal history indicate that the upper mantle heated up for the first few billions of years after which time it has slowly cooled to its present state. It is postulated that the kimberlites are formed by the closed system fractional crystallization of mafic magmas which have formed by fractional fusion during the early heating of the upper mantle. During the cooling cycle in the last few billions of years, the mafic liquids have cooled through fractional crystal- lization to the ambient mantle temperatures, and changed composition towards a kimberlite. Sequential primary phase assemblages are represented by harzburgite, garnet harzburgite, garnet lherzolite, hypersthene eclogite, eclogite, kyanite eclogite to an olivine-diopside-perovskite rock. Parallel changes in the liquid composition are through picrite, tholeiite, alkali basalt, a diopside-ilmenite composition to kimberlite. INTRODUCTION logical and geophysical characteristics common to the gen- Although kimberlites occupy only a very small propor- eral description included in the following section.
    [Show full text]
  • Origin of Lherzolite Inclusions in the Malapai Hill Basalt, Joshua Tree National Monument, California
    ROBERT J. STULL Department of Geology, California State University, Los Angeles, Los Angeles, California 90032 KENT McMILLAN Department of Geological Sciences, Stanford University, Stanford, California 94305 Origin of Lherzolite Inclusions in the Malapai Hill Basalt, Joshua Tree National Monument, California ABSTRACT dence that the nodules could be the end product of more than one partial melting. Alkali olivine basalt at Malapai Hill, Cali- fornia, occurs as a late Cenozoic stock that has INTRODUCTION intruded the Cretaceous White Tank Monzon- Basalts and ultramafic nodules from the ite. The basalt is chemically and mineralogi- Mojave Desert have been described in several cally similar to other alkaline basalts in the papers. Ross and others (1954) reported min- Mojave Desert, although it is devoid of zeolites. eral compositions of peridotite inclusions from The strontium isotopic composition of the Dish Hill and noted the world-wide similarity basalt (Sr87/Sr86 = 0.7030 ± 0.0006) suggests of nodules. Hess (1955) published one analysis that it is derived from a mantle that has of a nodule from Dish Hill (Fig. 1). Wise already experienced one period of partial (1966, 1969) described the characteristics of ba- melting. OUvine-rich lherzolite nodules in the salts in the Mojave Desert and provided a basalt are high in Mg and low in Si, Al, Ca, viable theory for the sequence of lava at Na, and K. The nodules are xenomorphic Pisgah Crater. Most petrologists agree that granular with a tectonite fabric and forsteritic basalt forms in the upper mantle (for example, olivine (F094-88). The Sr87/Sr86 ratio of the Green and Ringwood, 1967; Jackson and nodules is 0.7043 + 0.0008.
    [Show full text]
  • Trace Element Fractionation in Alkaline OIB
    Genesis of primitive Hawaiian rejuvenated-stage lavas: Evidence for carbonatite metasomatism and implications for ancient eclogite source Anastassia Y. Borisova 1,2* 1 Géosciences Environnement Toulouse, GET - UMR 5563 - OMP - CNRS, 14 Avenue E. Belin, 31400 Toulouse, France 2 Geological Department, Lomonosov Moscow State University, MGU, Vorobievu Gori, 119991, Moscow, Russia (revised version, 12/03/2020, for Geochemistry Geophysics Geosystems) *Corresponding author: E-mail: anastassia.borisova@get.omp.eu; *Corresponding adresse: Géosciences Environnement Toulouse UMR 5563, Observatoire Midi Pyrénées, 14 Avenue E. Belin, 31400 Toulouse, France; Tel: +33(0)5 61 54 26 31; Fax: +33(0)5 61 33 25 60 Abstract – To constrain a contribution of deep carbonated mantle, to fractionation of Hf relative to rare earth elements (REE) in volcanic series, we examine available high-quality data on major, trace element and Nd-Hf isotope compositions of ~280 primitive lavas and glasses (MgO = 8.5 – 21 wt%, SiO2 = 37 - 50 wt%) erupted during preshield, postshield and mostly rejuvenated stage of the Hawaiian hot spot (Pacific Ocean). Strong variations of Hf/Sm, Zr/Sm, Ti/Eu, K/Th, Nb/Th, La/K and Ba/K in the lavas are not features of the melt equilibration with residual amphibole or phlogopite, and cannot be due to variable degrees of batch or dynamic melting of uncarbonated lherzolite source. Enrichment in REE, Th and Ba relative to K, Hf, Zr, Ti and Nb together and low Si, high Na, K and Ca contents in the Hawaiian lavas are compositional features of carbonated mantle lithospheric to asthenospheric peridotite source affected by carbonatite metasomatism at temperatures higher than 1100°C and pressures higher than 2 GPa.
    [Show full text]
  • The Ronda Peridotite: Garnet-, Spinel-, and Plagioclase-Lherzolite Facies and the P—T Trajectories of a High-Temperature Mantle Intrusion
    The Ronda Peridotite: Garnet-, Spinel-, and Plagioclase-Lherzolite Facies and the P—T Trajectories of a High-Temperature Mantle Intrusion by MASAAKI OBATA* Institutfiir Kristallographie und Petrographie, Eidgenossische Technische Hochschule, Zurich, CH-8092, Zurich, Switzerland (Received 18 October 1978; in revised form 28 June 1979) ABSTRACT The Ronda peridotite is a high-temperature, alpine-type peridotite emplaced in the internal Zone of the Betic Cordilleras, southern Spain. Using the mineral assemblages of the peridotite and mafic layers, the peridotite mass has been subdivided into 4 zones of mineral facies: (l)garnet-lherzolite facies, (2) ariegite subfacies of spinel-lherzolite facies, (3) seiland subfacies of spinel-lherzolite facies, and (4) plagioclase-lherzolite facies. It is proposed that this mineralogical zonation developed through a syntectic recrystallization of a hot (1100 to 1200 °C), solid mantle peridotite during its ascent into the Earth's crust. Coexisting minerals from 12 peridotites covering all the mineral facies above were analysed with an electron microprobe. Core compositions of pyroxene porphyroclasts are constant in all mineral facies and indicate that the peridotite was initially equilibrated at temperatures of 1100 to 1200 °C and pressures of 20 to 25 kb. In contrast, the compositions of pyroxene neoblasts and spinel grains (which appear to have grown during later recrystallization) are well correlated with mineral facies. They indicate that the recrystallization temperature throughout the mass is more or less constant, 800 to 900 °C, but that the pressure ranges from 5-7 kb in the plagioclase-lherzolite facies to 12-15 kb in the garnet-lherzolite facies. Therefore, variation in pressure appears to be primarily responsible for the four mineral facies types.
    [Show full text]
  • Conditions for Melting and Metasomatism in the Earth's Mantle
    GEOLOGICKÝ ZBORNÍK — GEOLOGICA CARPATHICA, 36, 3, BRATISLAVA, JUNE 10(15, Pp. 323—335 PETER J. WYLLIE* CONDITIONS FOR MELTING AND METASOMATISM IN THE EARTH'S MANTLE (Figs. 6) Abstract: If we know the compositions of mantle rocks at various depths, ajnd the geotherm in different tectonic environments, then the conditions for melting are defined by experimentally determined solidus curves. The term "metasomatism" in crustal processes is defined as reac­ tion by solution or vapors, not by melts or magmas, and the same de­ finition should apply to mantle processes (reaction with magmas is "hybridization"). HiO, C02, or both are made available for metasomatic reaction at deep dissociation fronts, or by solidification of volatile-charged magmas. Regions eligible for metasomatism are limited by solidus curves above which the melts dissolve volatile components. Beneath the litho- sphere, there can be no metasomatism between about 120 and 260 km, because melting intervenes. Solidification of kimberlitic magmas at the base of continental lithosphere is a source of metasomatic fluids. Mantle metasomatism is expected in several regions above subducted oceanic lithosphere, interspersed with magmatic events. Major differentiation of the Earth is accomplished by melting, but metasomatism may cause signi­ ficant redistribution of some elements. Pe3K)Me: ECJIH H3BecTHM cocraBbi noKpbmaioiUHX nopofl B pa3Hbix rjiyÔHHax H reoTepMa B pa3iibix TeicroHHiecKHX cpeaax, noTOM ycjioBHH nJiaBnemw onpeflejiH- K3TCH 3KCnepHMeHTaJTbH0 Ha3HaieHHblMH KpHBblMH COJIHfla. TepMHH „MeTacoMa- TH3M" B npoueccax Kopu onpeaejiaeTCH KaK peaKuHH npoxofl^inaa npw noMomif pacTBopenHS HJIH nap, He paconaBOB HJIH Martu, H TaKyio 5Ke flettiHHHqHio MOÄHO npHMeHHTb HJIH npoueccoB MaHTHH (peaKUHeR c ManviaMH !\Bnftejcn ,,rH6pH/;H3auHH"). H„0, CO; HJIH 06a coe/THHenHsi npHroflHM una MeTacoiviaTHMeCKHX peaKunři B rxiy- ÔOKHX cjjpoHTax flHccouHauHH HJIH 3aTBepneBanHeM jieTywx Manvi.
    [Show full text]
  • Lesson 6 Part II Rocks of the Earth, Cont'd
    Lesson 6 Part II Rocks of the earth, cont’d A. M. Celâl Şengör What kind of rock makes up the table and the shield volcanoes? It is basalt. The word is derived from the Latin basanites used by Pliny the elder in his Historia Naturalis for the very hard black rock and is ultimately derived from the Greek βάσανος (basanos=touchstone) Basalt is defined as a black to dark grey, aphanitic rock consisting of some 65% of plagioclase (anorthite to labradorite), about 30% or less pyroxene and less than 10% feldspathoids. Its total silica (SiO2) content fluctuates between 45 to 55%. Basalt is the most widespread volcanic rock in the Solar System. We know it from the earth, Moon, Mars, Mercury and Venus. On earth it covers the ocean floors, makes up almost all the oceanic islands and plateaux, many of the island arcs, and, on the continents, the vast flood basalt provinces plus it occurs commonly in rifts. A massive basalt sample Basalt forms by the solidification of basaltic lava at the surface or by the solidification of basaltic magma close to the surface. In both cases, the solidification, i.e. crystallisation, is fast enough to create an aphanitic texture. Lava is molten rock at the surface. The word lava comes from Italian probably from the Latin root “labes” meaning a “fall” or “slide”. It seems that as a technical term it was first used in 1737 by Francesco Serao to describe the eruption of Vesuvius in 14th May to 4th June 1737. Magma is molten rock in the earth.
    [Show full text]
  • Mineralogical Evidence for Partial Melting and Melt-Rock Interaction Processes in the Mantle Peridotites of Edessa Ophiolite (North Greece)
    minerals Article Mineralogical Evidence for Partial Melting and Melt-Rock Interaction Processes in the Mantle Peridotites of Edessa Ophiolite (North Greece) Aikaterini Rogkala 1,* , Petros Petrounias 1 , Basilios Tsikouras 2 , Panagiota P. Giannakopoulou 1 and Konstantin Hatzipanagiotou 1 1 Section of Earth Materials, Department of Geology, University of Patras, 265 04 Patras, Greece; Geo.plan@outlook.com (P.P.); peny_giannakopoulou@windowslive.com (P.P.G.); k.hatzipanagiotou@upatras.gr (K.H.) 2 Physical and Geological Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Bandar Seri Begawan, Brunei Darussalam; basilios.tsikouras@ubd.edu.bn * Correspondence: krogkala@upatras.gr; Tel.: +30-2610996288 Received: 10 December 2018; Accepted: 14 February 2019; Published: 17 February 2019 Abstract: The Edessa ophiolite complex of northern Greece consists of remnants of oceanic lithosphere emplaced during the Upper Jurassic-Lower Cretaceous onto the Palaeozoic-Mesozoic continental margin of Eurasia. This study presents new data on mineral compositions of mantle peridotites from this ophiolite, especially serpentinised harzburgite and minor lherzolite. Lherzolite formed by low to moderate degrees of partial melting and subsequent melt-rock reaction in an oceanic spreading setting. On the other hand, refractory harzburgite formed by high degrees of partial melting in a supra-subduction zone (SSZ) setting. These SSZ mantle peridotites contain Cr-rich spinel residual after partial melting of more fertile (abyssal) lherzolite with Al-rich spinel. Chromite with Cr# > 60 in harzburgite resulted from chemical modification of residual Cr-spinel and, along with the presence of euhedral chromite, is indicative of late melt-peridotite interaction in the mantle wedge. Mineral compositions suggest that the Edessa oceanic mantle evolved from a typical mid-ocean ridge (MOR) oceanic basin to the mantle wedge of a SSZ.
    [Show full text]
  • Thermodynamic Modelling of Cr-Bearing Garnets with Implications for Diamond Inclusions and Peridotite Xenoliths
    Lithos 112S (2009) 986–991 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Thermodynamic modelling of Cr-bearing garnets with implications for diamond inclusions and peridotite xenoliths S. Klemme a,⁎, T.J. Ivanic b,e, J.A.D. Connolly d, B. Harte b,c a Institut für Mineralogie, Corrensstr. 24, Universität Münster, 48149 Münster, Germany b School of GeoSciences, King's Buildings, West Mains Road, University of Edinburgh, Edinburgh EH9 3JW, United Kingdom c Centre for Science at Extreme Conditions, University of Edinburgh, Erskine Williamson Building, Mayfield Road, Edinburgh EH9 3JZ, United Kingdom d Institut für Mineralogie und Petrographie, ETH Zentrum, Sonneggstrasse 5, CH-8082, Zürich, Switzerland e Geological Survey of Western Australia, Mineral House, 100 Plain Street, East Perth, WA 6004, Australia article info abstract Article history: A new approach is presented for modelling Cr-rich peridotite compositions and garnet–spinel compositions Received 8 September 2008 found in diamonds and xenoliths at conditions relevant to the deep continental Earth. Using recent Accepted 10 May 2009 experimental data, it is now possible to calculate phase relations and mineral compositions relevant to Available online 6 June 2009 pressures, temperatures, and compositions of the deep lithospheric Earth using free energy minimization techniques. Here we present calculated phase relations in Cr-rich mantle compositions from pressures of 20– Keywords: 60 kbar, and temperatures 800–1400 °C. The model is successful at modelling a wide range of natural mineral Garnet–spinel transition Peridotite compositions which are found as xenoliths in diamond-bearing kimberlites from South Africa, and is Diamond inclusion illustrated using suites of Cr-rich xenoliths from near Kimberley, South Africa.
    [Show full text]
  • A Diamondiferous Lherzolite from the Premier Diamond Mine, South Africa
    A DIAMONDIFEROUS LHERZOLITE FROM THE PREMIER DIAMOND MINE, SOUTH AFRICA K.S. (Fanus) Viljoen and Rene Dobbe GeoScience Centre, De Beers Consolidated Mines, South Africa INTRODUCTION PETROGRAPHY This paper reports on the discovery of a diamondiferous The xenolith is altered with pervasive serpentinisation peridotite from the Premier diamond mine. Such of constituent minerals (Figure 1). Garnets are generally xenoliths are surprisingly rare on the Kaapvaal Craton, partially kelyphitised. They range in size up to 2mm. with 4 specimens having been decribed from Finsch These occur interstitially with serpentine pseudomorphs (Shee et al 1982; Viljoen et al 1992), 8 from Roberts after possible olivine and/or orthopyroxene. The Victor (Viljoen et al 1994), and one from Mothae in pervasive alteration complicates petrography and it is Lesotho (Dawson and Smith 1975). A number of not possible to accurately determine volume diamond-bearing peridotitic garnet macrocrysts have percentages of these two minerals, assuming that both also been recognised at the Newlands kimberlite are present. Other minerals typical of peridotites such (Daniels et al 1995). as clinopyroxene and accessory spinel are not seen. The The x-ray diamond recovery circuit (the Sortex Plant) at xenolith texture is coarse (i.e. not sheared). the Premier diamond mine in South Africa typically Inclusions in Diamond 16 generates about 3 specimens of diamond partially Premier Xenolith enclosed in kimberlite each day. These are sent to the Other Xenoliths sorthouse at the mine where a small press is used to 14 liberate the diamonds from the kimberlite matrix. Maggie Mahlase, a diamond sorter, recognised the 12 importance of the specimen when crushing revealed numerous diamonds.
    [Show full text]
  • Mantle Melting and Origin of Basaltic Magma
    Lecture 18 - Mantle Melting Monday, 28th, March, 2005 Mantle Melting and Origin of Basaltic Magma 1 Two principal types of basalt in the ocean basins Tholeiitic Basalt and Alkaline Basalt Table 10-1 Common petrographic differences between tholeiitic and alkaline basalts Tholeiitic Basalt Alkaline Basalt Usually fine-grained, intergranular Usually fairly coarse, intergranular to ophitic Groundmass No olivine Olivine common Clinopyroxene = augite (plus possibly pigeonite) Titaniferous augite (reddish) Orthopyroxene (hypersthene) common, may rim ol. Orthopyroxene absent No alkali feldspar Interstitial alkali feldspar or feldspathoid may occur Interstitial glass and/or quartz common Interstitial glass rare, and quartz absent Olivine rare, unzoned, and may be partially resorbed Olivine common and zoned Phenocrysts or show reaction rims of orthopyroxene Orthopyroxene uncommon Orthopyroxene absent Early plagioclase common Plagioclase less common, and later in sequence Clinopyroxene is pale brown augite Clinopyroxene is titaniferous augite, reddish rims after Hughes (1982) and McBirney (1993). Each is chemically distinct Evolve via FX as separate series along different paths ● Tholeiites are generated at mid-ocean ridges ✦ Also generated at oceanic islands, subduction zones ● Alkaline basalts generated at ocean islands ✦ Also at subduction zones 2 Sources of mantle material ● Ophiolites ✦ Slabs of oceanic crust and upper mantle ✦ Thrust at subduction zones onto edge of continent ● Dredge samples from oceanic fracture zones ● Nodules and xenoliths in some basalts ● Kimberlite xenoliths ✦ Diamond-bearing pipes blasted up from the mantle carrying numerous xenoliths from depth Lherzolite is probably fertile unaltered mantle Dunite and harzburgite are refractory residuum after basalt has been extracted by partial melting 15 Tholeiitic basalt 3 10 O 2 Partial Melting Wt.% Al Wt.% 5 Figure 10-1 Brown and Mussett, A.
    [Show full text]
  • Melt-Rock Interactions and Melt-Assisted Deformation in The
    Melt-rock interactions and melt-assisted deformation in the Lherz peridodite, with implications for the structural, chemical and isotopic evolution of the lithospheric mantle Véronique Le Roux To cite this version: Véronique Le Roux. Melt-rock interactions and melt-assisted deformation in the Lherz peridodite, with implications for the structural, chemical and isotopic evolution of the lithospheric mantle. Geo- chemistry. Université Montpellier 2, 2008. English. tel-00431325 HAL Id: tel-00431325 https://tel.archives-ouvertes.fr/tel-00431325 Submitted on 12 Nov 2009 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. UNIVERSITÉ MONTPELLIER II SCIENCES ET TECHNIQUES DU LANGUEDOC THÈSE Pour obtenir le grade de DOCTEUR DE L’UNIVERSITÉ MONTPELLIER II Discipline : Structure et évolution de la Terre et des autres planètes École Doctorale : SIBAGHE Présentée et soutenue publiquement Par VÉRONIQUE LE ROUX Le 7 novembre 2008 MELT-ROCK INTERACTIONS AND MELT-ASSISTED DEFORMATION IN THE LHERZ PERIDOTITE, WITH IMPLICATIONS FOR THE STRUCTURAL, CHEMICAL AND ISOTOPIC EVOLUTION OF THE LITHOSPHERIC MANTLE JURY M. Jean-Louis Bodinier Directeur de Recherche (CNRS UMII) Directeur de thèse M. Alain Vauchez Maître de Conférences (UM II) Co-directeur de thèse Mme Suzanne Y.
    [Show full text]
  • Petrogenesis of Dunite Xenoliths from Koolau Volcano, Oahu, Hawaii: Implications for Hawaiian Volcanism
    Petrogenesis of Dunite Xenoliths from Koolau Volcano, Oahu, Hawaii: Implications for Hawaiian Volcanism by GAUTAM SEN* AND D. C. PRESNALL Department of Geosciences, The University of Texas at Dallas, P.O. Box 830688, Richardson, Texas 75083-0688 (Received 11 July 1984; revised typescript accepted 3 July 1985) ABSTRACT Ultramafic xenoliths from Koolau Volcano on the island of Oahu, Hawaii, are divided into spinel lherzolite, pyroxenite, and dunite suites. On the basis of a study of the petrography and mineral compositions of 43 spinel lherzolites, 12 pyroxenites, and 20 dunites, the following characteristics of the dunites in relation to the other nodule types and to Hawaiian lavas emerge. (1) The forsterite content of olivines in the Koolau dunites (Fo82.6-Fo89 7) overlap those of Hawaiian tholeiitic and alkalic lavas and are generally lower than those in abyssal lherzolites and dunites and in Koolau spinel lherzolites. (2) Most of the dunites contain no orthopyroxene, all except two contain chrome spinel, and a few contain interstitial plagioclase and clinopyroxene. (3) Chrome spinels from the Koolau dunites are 2+ distinctly higher in Cr/(Cr +Al), lower in Mg/(Mg + Fe ), and higher in TiO2 than those from abyssal basalts and peridotites. Chrome spinels in the dunites correspond closely in composition to chrome spinels in Hawaiian tholeiitic and alkalic lavas. (4) The abundance of dunite relative to other nodule types decreases outward from the central part of the volcano. The dunites are interpreted not as residues of partial fusion of the mantle but as crystal accumulations stored at shallow depths beneath the central part of Koolau Volcano and derived from picritic magmas parental to the shield-building tholeiitic lavas.
    [Show full text]