DOCSLIB.ORG

Crystallization History of a Pyroxenite Xenolith in a Granulite Inferred from Chemical and Single-Crystal X-Ray Data

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Crystallization History of a Pyroxenite Xenolith in a Granulite Inferred from Chemical and Single-Crystal X-Ray Data Fluid-Mineral Interactions: A Tribute to H. P. Eugster © The Geochemical Society, Special Publication No.2, 1990 Editors: R. J. Spencer and I-Ming Chou Crystallization history of a pyroxenite xenolith in a granulite inferred from chemical and single-crystal X-ray data JrRI FRYDA Geological Survey, Malostranske namesti 19, 11821 Praha 1, Czechoslovakia and MILANRIEDER Institute of Geological Sciences, Charles University, Albertov 6, 12843 Praha 2, Czechoslovakia Abstract-Porphyroclasts of orthopyroxene and clinopyroxene in the pyroxenite have bulk com- positions corresponding to an equilibrium at about 1260°C and their content of Fe3+ indicates a relatively high oxygen fugacity during their crystallization. Later they were corroded during recrys- tallization by a reaction with olivine that yielded spinel grains and pyroxene neoblasts. This reaction proceeded in the subsolidus, under increasingly reducing conditions, and produced spinel of pro- gressively smaller grain size and increasing Al content. Finally, at about 910°C, the recrystallization ceased, at approximately the same time as the porphyroclasts underwent exsolution. Orthopyroxene porphyroclasts exsolved clinopyroxene and spinel lamellae, crystallographically oriented in the host. Clinopyroxene porphyroclasts probably exsolved enstatite first and then spinel, as a result of a strong reduction probably caused by the diffusion of hydrogen. Oxygen, thus freed, combined with hydrogen, yielding about 2.5 OR per seven R2+in the exsolved enstatite and converted it into an orthoamphibole- like phase that is perfectly crystallographically aligned in the host. It appears that the unmixing of spinel lamellae may be viewed as a redox reaction and that concomitantly forming hydrated phases may serve as a proof of the role of hydrogen during the evolution of mantle xenoliths. GEOLOGICAL SETTING PETROGRAPHY THEXENOLITHof olivine websterite studied comes The studied xenolith contains orthopyroxene from the abandoned Lichtenstein quarry located (OPX), clinopyroxene (CPX), olivine (OL), and on the SSW slope of Mount Kid, about 1.5 km spinel (SPN). The minerals vary in proportion, so from the village of Vysny near Cesky Krumlov, the volume percentages (OPX 45-65%, CPX 15- southwestern Czechoslovakia. The xenolith is about 25%,OL 15-30%, SPN < 3%)are only approximate. 70 em in diameter, embedded in a relatively ho- Nevertheless, according to Streckeisen's (1973) mogeneous, partly recrystallized kyanite-garnet fel- classification, the rock can be referred to as olivine sic granulite, which belongs to the Blansky les websterite. Mountains Massif, one of the largest granulite mas- Orthopyroxene is present in what appears to be sifs of the Moldanubian crystalline complex. While two generations. The first is large lamellar porphy- numerous ideas on the genesis of the granulites in roclasts (up to 2 em in size, Fig. 1), the second, the area have been put forth, it is generally agreed smaller lamellae-free neoblasts «2 mm). The por- that they formed at high pressures, near the base of phyroclasts are irregularly shaped because of cor- the Earth's crust (VRANA,1987;FIALAet al., 1987). rosion during recrystallization, but remain elon- Ultramafic rocks such as dunites, lherzolites, harz- gated parallel to [001]. They are intensely bent and burgites, and olivine websterites (most of them car- often kinked along deformation planes roughly rying pyrope garnet) represent about 5 vol.% of the perpendicular to [001], along which smaller frag- rocks of the Blansky les Mountains Massif, and so ments separated (Fig. 1). This deformation cannot far they have received relatively little attention. be younger than the exsolution of lamellae; if it Further details about the geology of the area can be were, we would not observe SPN grains to follow found in papers by FEDIUKOVA(1965, 1978), ROST deformation zones nor would the lamellae there be (1966), KODYM (1972), KODYM et at. (1978), by about an order of magnitude thicker than in VRANA(1979, 1987), SLABY(1983), or FIALAet at. unaffected parts of the porphyroclasts (Fig. 2). The (1987). The papers by VANBREEMENet at. (1982) density oflamellae is uniform throughout the grains and AFTALIONet at. (1989) deal with geochro- all the way to the border with other pyroxene grains. nology. However, along contacts with olivine, there may be 85 86 J. Fryda and M. Rieder FIG. I. a. Orthopyroxene porphyroclast, deformed and broken, surrounded by recrystallization products. Lamellae-free zones in the porphyroclast rim recrystallized olivine grains. Crossed polars, width of photograph = 4 mm. b. A plastically deformed OPX porphyroclast corroded during re- crystallization. Olivine in the lower left-hand corner exhibits wavy extinction. The lamellae in OPX do not seem to disappear along [100]opxas readily as along directions in the (lOO)opxplane. Crossed polars, width of photograph = 4 mm. c. Lamellae of spinel (SPN) in OPX porphyroclast seen in a section roughly perpendicular to [00l]opx. The SPN platelets lie parallel to (I OO)opxand apparently contribute to the easy parting along that plane. Plane-polarized light, width of photograph = 0.5 mm. a lamellae-free margin (0.5-1.0 mm thick). Con- (Fig. 3), but appear like COr or CO-containing fluid trary to porphyroclasts, neoblasts do not show de- inclusions known from mantle xenoliths (Fig. 3; formation, and occur jointly with recrystallized ol- KIRBY and GREEN, 1980, pI. 4-D; ANDERSON et ivine and large (vermicular) grains of spinel. al., 1984, Fig. l-E, F; BERGMAN and DUBESSY, Clinopyroxene porphyroclasts reach up to 7 mm 1984, Fig. 2a, b). The zones of inclusions seem to in size and contain abundant lamellae (Figs. 2 and have no crystallographic orientation and appear not 3) evenly distributed throughout except for lamel- to have a systematic relation to kink bands. The lae-free zones, about 0.5 mm wide, that follow the rock also contains small OL grains free from signs border with OL grains. Clinopyroxene porphyro- of deformation or inclusions which always accom- clasts do not show signs of plastic deformation, but pany OPX neoblasts and vermicular spinel. many are fragmented. Small lamellae-free fragments Spinel is red-brown, and its large (vermicular) surrounded by olivine cannot be distinguished from grains or aggregates, which represent the vast ma- possible CPX neoblasts. jority of spinel in the rock, may reach 5 mm across. The mean size ofOL grains is about 4 mm, larger They do not seem to associate preferentially with grains are elongate and often penetrated by kink any of the three primary minerals in the xenolith. bands. The larger grains also contain zones or veils Contacts of OPX and CPX porphyroclasts are of isometric inclusions that were not studied further punctuated with chains ofSPN grains (Fig. 3) rang- Crystallization history of pyroxenite xenolith in granulite 87 FIG. 2. Back-scattered electron images of pyroxene porphyroclasts. a, b. Section across a CPX porphyroclast, approximately perpendicular to [OOI]cpx.Most spinel lamellae (white) are parallel to (OIO)cpxand lamellae of the orthoamphibole phase (OA, black) are mostly parallel to (100)cpx, but there are some exceptions. The 'wrong' orientation of SPN lamellae seems to be associated with the OA phase. The (100)cpx is vertical in a. and horizontal in b. c. OPX porphyroclast, cut roughly perpendicular to [OlO]opx,showing a deformation zone with swollen CPX lamellae (light grey) and enriched in SPN grains (white). d. A section ofOPX porphyroclast, approximately parallel to [OOI]opx, illustrating the distribution of lamellae of SPN (white) and CPX (light grey). ing between 10-3 and 10-1 mm in size. Similar recorded on a DRON-2,0 diffractometer (Burevestnik, 0 grains of spinel « 10-2 mm) occur along defor- Leningrad), with scanning speed 0.5 (21.1)/minute,and processed by BURNHAM'S(1962) least squares program. mation zones in OPX porphyroclasts (Fig. 2) or The cell edge of spinel was based on film data extrapolated inside OL crystals. against the Nelson-Riley function (RIEDER,1971). In the nomenclature of MERCIER and NICOLAS An ARL-SEMQ microprobe was used to obtain chem- (1975), the texture of the Lichtenstein quarry xeno- ical compositions. The accelerating voltage was 15 kV, lith is porphyroclastic. These authors cite the inclu- current 50 nA, and counting time 20 seconds. Natural silicate minerals were used as standards for Ti, Cr, Mn, sion of spinel in olivine as evidence of a recrystal- Si, Fe, K, Na, Mg, Ca, and AI, and synthetic Ni olivine lization of olivine, following an intensive defor- was used for Ni. Analytical points were selected after ob- mation. However, such an effect is typical of the servation in back-scattered electron mode (accelerating genetically younger equigranular texture, and thus voltage 30-35 kV). Five to ten grains of each type of min- we may refer to the present texture as transitional eral (porphyroclast, neoblast, lamella) were analyzed. If homogeneous, the mineral was analyzed at three to six between porphyroclastic and equigranular. points, otherwise, up to 25 analyses were made. The Ca in olivine and Ni in OL, OPX, CPX, SP were determined EXPERIMENTAL with 100 nA current and 100 seconds counting time (used for Ca by ADAMSand BISHOP,1986). The bulk compo- Single-crystal X-ray work was done on a Dioptra sition ofOPX and CPX porphyroclasts was analyzed either precession camera (HANICet al., 1955). Powder data were using a defocussed electron beam (35 µm) or by allowing 88 J. Fryda and M. Rieder the beam to scan over rasters, 50 X 50 um, that were suits. After corrections for drift of the instrument and for combined so as to pave stretches several hundred µm in dead time of the detectors, the data were reduced according diameter. Both methods yielded practically identical re- to BENCEand ALBEE(1968). When assessing the bulk chemical compositions, one must be aware that the un- certainties may exceed the errors indicated because the assumption of a homogeneous distribution of each element in the X-ray generation range is not fulfilled.
Load more

Recommended publications
  • Mantle Peridotite Xenoliths
    Earth and Planetary Science Letters 260 (2007) 37–55 www.elsevier.com/locate/epsl Mantle peridotite xenoliths in andesite lava at El Peñon, central Mexican Volcanic Belt: Isotopic and trace element evidence for melting and metasomatism in the mantle wedge beneath an active arc ⁎ Samuel B. Mukasa a, , Dawnika L. Blatter b, Alexandre V. Andronikov a a Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA b Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA Received 6 July 2006; received in revised form 3 May 2007; accepted 7 May 2007 Available online 13 May 2007 Editor: R.W. Carlson Abstract Peridotites in the mantle wedge and components added to them from the subducting slab are thought to be the source of most arc magmas. However, direct sampling of these materials, which provides a glimpse into the upper mantle beneath an active margin, is exceedingly rare. In the few arc localities where found, peridotite xenoliths are usually brought to the surface by basaltic magmas. Remarkably, the hornblende-bearing ultramafic xenoliths and clinopyroxene megaxenocrysts from El Peñon in the central Mexican Volcanic Belt were brought to the surface by a Quaternary high-Mg siliceous andesite, a rock type usually considered too evolved to be a direct product of mantle melting. The xenoliths and megaxenocrysts from El Peñon represent lithospheric mantle affected by significant subduction of oceanic lithosphere since as early as the Permian. Trace element and radiogenic isotope data we report here on these materials suggest a history of depletion by melt extraction, metasomatism involving a fluid phase, and finally, limited reaction between the ultramafic materials and the host andesite, probably during transport.
    [Show full text]
  • Depleted Spinel Harzburgite Xenoliths in Tertiary Dykes from East Greenland: Restites from High Degree Melting
    Earth and Planetary Science Letters 154Ž. 1998 221±235 Depleted spinel harzburgite xenoliths in Tertiary dykes from East Greenland: Restites from high degree melting Stefan Bernstein a,), Peter B. Kelemen b,1, C. Kent Brooks a,c,2 a Danish Lithosphere Centre, éster Voldgade 10, DK-1350 Copenhagen K, Denmark b Woods Hole Oceanographic Institution, Woods Hole, City, MA 02543, USA c Geological Institute, UniÕersity of Copenhagen, éster Voldgade 10, DK-1350 Copenhagen K, Denmark Received 28 April 1997; revised 19 September 1997; accepted 4 October 1997 Abstract A new collection of mantle xenoliths in Tertiary dykes from the Wiedemann Fjord area in Southeast Greenland shows that this part of the central Greenland craton is underlain by highly depleted peridotites. The samples are mostly spinel harzburgites with highly forsteritic olivinesŽ. Fo87± 94 , average Fo 92.7 . This, together with unusually high modal olivine contentsŽ. 70±)95% , places the Wiedemann harzburgites in a unique compositional field. Relative to depleted Kaapvaal harzburgites with comparable Fo in olivine, the Wiedemann samples have considerably lower bulk SiO2 Ž average 42.6 wt% versus 44±49 wt%. Spinel compositions are similar to those in other sub-cratonic harzburgites. Pyroxene equilibrium temperatures average 8508C, which is above an Archaean cratonic geotherm at an inferred pressure of 1±2 GPa, but low enough so that it is unlikely that the xenoliths represent residual peridotites created during Tertiary magmatism. Among mantle samples, the Wiedemann harzburgites are, in terms of their bulk composition, most similar to harzburgites from the ophiolites of Papua New GuineaŽ. PNG and New Caledonia Ž.
    [Show full text]
  • Mantle Origin and Flow Sorting of Megacryst-Xenolith Inclusions in Mafic Dikes of Black Canyon, Arizona
    Mantle Origin and Flow Sorting of Megacryst-Xenolith Inclusions in Mafic Dikes of Black Canyon, Arizona U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1541 AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the current- year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey publications released prior to the current year are listed in the most recent annual "Price and Availability List." Publications that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability List" are no longer available. Reports released through the NTIS may be obtained by writing to the National Technical Information Service, U.S. Department of Commerce, Springfield, VA 22161; please include NTIS report number with inquiry. Order U.S. Geological Survey publications by mail or over the counter from the offices given below. BY MAIL OVER THE COUNTER Books Books and Maps Professional Papers, Bulletins, Water-Supply Papers, Tech­ Books and maps of the U.S. Geological Survey are available niques of Water-Resources Investigations, Circulars, publications over the counter at the following U.S. Geological Survey offices, of general interest (such as leaflets, pamphlets, booklets), single all of which are authorized agents of the Superintendent of Docu­ copies of Earthquakes & Volcanoes, Preliminary Determination of ments. Epicenters, and some miscellaneous reports, including some of the foregoing series that have gone out of print at the Superin­ • ANCHORAGE, Alaska—4230 University Dr., Rm.
    [Show full text]
  • Geology of the Saline County Xenolith and Surrounding Area
    A.G.E.S. Brochure Series 005 State of Arkansas Arkansas Geological Survey Bekki White, State Geologist Geology of the Saline County Xenolith and surrounding area By J. Michael Howard Illustrations and photos by Angela Chandler _______________________________________________________ _______________________________________________________ Xenolith – “ a foreign inclusion in an igneous rock.” Glossary of Geology American Geological Institute 1987 (from the Greek words Xenos, meaning guest or stranger, and Lithos, meaning stone.) _______________________________________________________ _______________________________________________________ Introduction Located in Saline County, Arkansas, at the south edge of the community of Bauxite, this natural outcrop of nepheline syenite contains several geologically interesting features, including a xenolith. Sloping west, the outcrop encompasses about one-quarter acre near the center of section 21, Township 2 South, Range 14 West. In early 1990, the Aluminum Company of America (ALCOA) donated the outcrop along with approximately five surrounding acres of land to the Arkansas Geological Commission so that the site can be preserved for educational purposes. Outcrop of nepheline syenite at xenolith locality. History of the site The outcrop and its geologic features were first described by J. Francis Williams in 1891 in The Igneous Rocks of Arkansas, Arkansas Geological Survey Annual Report for 1890, Volume II. Williams discussed the outcrop and xenolith in some detail and included a sketch of the xenolith (see title page). However, for many years the outcrop location remained unknown to most scientists. In the late 2 1960’s employees in the mining division of ALCOA, suspecting that the site was on their property, began a concerted search. Soon afterward the outcrop was rediscovered and was visited by a staff member of the Arkansas Geological Commission, who in turn told Dr.
    [Show full text]
  • Title of Thesis
    Additions and Modifications to the Igneous Rock Classification Scheme Senior Thesis Submitted in partial fulfillment of the requirements for the Bachelor of Science Degree in Geological Sciences At The Ohio State University By Matthew R. H. Dugan The Ohio State University 2010 Approved by Anne E. Carey, Advisor School of Earth Sciences T ABLE OF C ONTENTS Abstract………………………………………………………………………....3 Acknowledgements……………………………………………………….…….4 Introduction……………………………………………………………………..5 Discussion……………….………………………………………………………5 Application……………….……………………………………………………..10 References Cited….……….……………………………………………………18 2 Abstract Igneous rocks as they are currently defined are in a sloppy state. Vague wording is throughout the whole of the definition, and there is not even any clear consensus on what it should be defined as. In this paper, I redefine igneous rocks in such a way as to remove a great deal of imprecision, and I go through some of the logical implications of the refined definition. I do not seek to change the intent of the definition, and I do not believe that I have. The most interesting implication of this change is that water, as it occurs on Earth, is an igneous rock, and I construct a basic classification scheme for it. 3 Acknowledgements I wish to thank Dr. Anne Carey for her intense support of the writing of this thesis, her wonderful edits and her dedication to keeping me on this. I also wish to acknowledge my lab group, also lead by Dr. Steve Goldsmith, for their wonderful feedback and encouragement. Dr. Fritz Graf, Professor and chair of the Department of Greek and Latin, was a great asset to me and he deserves recognition for his help in coining neologisms.
    [Show full text]
  • Zeolites in Tasmania
    Mineral Resources Tasmania Tasmanian Geological Survey Record 1997/07 Tasmania Zeolites in Tasmania by R. S. Bottrill and J. L. Everard CONTENTS INTRODUCTION ……………………………………………………………………… 2 USES …………………………………………………………………………………… 2 ECONOMIC SIGNIFICANCE …………………………………………………………… 2 GEOLOGICAL OCCURRENCES ………………………………………………………… 2 TASMANIAN OCCURRENCES ………………………………………………………… 4 Devonian ………………………………………………………………………… 4 Permo-Triassic …………………………………………………………………… 4 Jurassic …………………………………………………………………………… 4 Cretaceous ………………………………………………………………………… 5 Tertiary …………………………………………………………………………… 5 EXPLORATION FOR ZEOLITES IN TASMANIA ………………………………………… 6 RESOURCE POTENTIAL ……………………………………………………………… 6 MINERAL OCCURRENCES …………………………………………………………… 7 Analcime (Analcite) NaAlSi2O6.H2O ……………………………………………… 7 Chabazite (Ca,Na2,K2)Al2Si4O12.6H2O …………………………………………… 7 Clinoptilolite (Ca,Na2,K2)2-3Al5Si13O36.12H2O ……………………………………… 7 Gismondine Ca2Al4Si4O16.9H2O …………………………………………………… 7 Gmelinite (Na2Ca)Al2Si4O12.6H2O7 ……………………………………………… 7 Gonnardite Na2CaAl5Si5O20.6H2O ………………………………………………… 10 Herschelite (Na,Ca,K)Al2Si4O12.6H2O……………………………………………… 10 Heulandite (Ca,Na2,K2)2-3Al5Si13O36.12H2O ……………………………………… 10 Laumontite CaAl2Si4O12.4H2O …………………………………………………… 10 Levyne (Ca2.5,Na)Al6Si12O36.6H2O ………………………………………………… 10 Mesolite Na2Ca2(Al6Si9O30).8H2O ………………………………………………… 10 Mordenite K2.8Na1.5Ca2(Al9Si39O96).29H2O ………………………………………… 10 Natrolite Na2(Al2Si3O10).2H2O …………………………………………………… 10 Phillipsite (Ca,Na,K)3Al3Si5O16.6H2O ……………………………………………… 11 Scolecite CaAl2Si3O10.3H20 ………………………………………………………
    [Show full text]
  • Komatiites of the Weltevreden Formation, Barberton Greenstone
    Louisiana State University LSU Digital Commons LSU Doctoral Dissertations Graduate School 2005 Komatiites of the Weltevreden Formation, Barberton Greenstone Belt, South Africa: implications for the chemistry and temperature of the Archean mantle Keena Kareem Louisiana State University and Agricultural and Mechanical College, [email protected] Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_dissertations Part of the Earth Sciences Commons Recommended Citation Kareem, Keena, "Komatiites of the Weltevreden Formation, Barberton Greenstone Belt, South Africa: implications for the chemistry and temperature of the Archean mantle" (2005). LSU Doctoral Dissertations. 3249. https://digitalcommons.lsu.edu/gradschool_dissertations/3249 This Dissertation is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Doctoral Dissertations by an authorized graduate school editor of LSU Digital Commons. For more information, please [email protected]. KOMATIITES OF THE WELTEVREDEN FORMATION, BARBERTON GREENSTONE BELT, SOUTH AFRICA: IMPLICATIONS FOR THE CHEMISTRY AND TEMPERATURE OF THE ARCHEAN MANTLE A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy in The Department of Geology and Geophysics by Keena Kareem B.S., Tulane University, 1999 May 2005 ACKNOWLEDGMENTS The author wishes to thank, Dr. Gary Byerly, the author’s major advisor for guidance and support in defining the project, technical assistance with instrumentation, and practical advice concerning non-dissertation related issues. Appreciation goes out to the other members of the advisory committee: Drs. Huiming Bao, Lui-Heung Chan, Ray Ferrell, Darrell Henry, of the Department of Geology and Geophysics, and Dr.
    [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]
  • Oxygen Fugacity of Hotspot Xenoliths: a Window Into the Earth's Mantle
    Oxygen fugacity of hotspot xenoliths: A window into the Earth’s mantle Kellie Wall1,2, Fred Davis1, and Elizabeth Cottrell1 1Department of Mineral Sciences, Smithsonian National Museum of Natural History, Washington, DC 2School of Earth and Environmental Sciences, Washington State University, Pullman, WA Introduction Results Discussion Mantle xenoliths—chunks of the Earth’s Peridotite xenoliths recorded fO2 ranges of -1.4 to 0.9 (Savai’i), 0.6 to 0.7 (Tahiti), and -1.0 to 0.2 We observe an intriguing dichotomy in apparent fO2 between spinels of different habit interior carried to the surface by volcanic (Tubuai) log units relative to the quartz-fayalite-magnetite (QFM) buffer (Fig. 9). These values are within a single sample. In these samples, there is no indication that melt has interacted eruptions—provide a rare glimpse into the within the range of oxygen fugacities for peridotites from other tectonic settings: more oxidized than with the rock. We hypothesize that an increase in temperature induced by lithosphere- those from ridges but more reduced than those from subduction zones. Within some samples we plume interaction has resulted in major element diffusion and an apparent shift in fO . We chemistry of the deep Earth (Fig. 1). Of 2 observe two trends: TiO2 increasing with fO2, and a link between fO2 and spinel habit, such that fine, suggest that these xenoliths erupted before the interiors of large spinels could diffusively particular interest is the oxygen fugacity “wispy” spinel records higher apparent fO2 than large blocky grains. reequilibrate. O (f 2; the effective ‘partial pressure’ of An outstanding mystery in the fO2 realm is that ridge peridotites are an order of xenoliths’ magnitude more reduced than lavas erupted at ridges.
    [Show full text]
  • Glossary of Geological Terms
    GLOSSARY OF GEOLOGICAL TERMS These terms relate to prospecting and exploration, to the regional geology of Newfoundland and Labrador, and to some of the geological environments and mineral occurrences preserved in the province. Some common rocks, textures and structural terms are also defined. You may come across some of these terms when reading company assessment files, government reports or papers from journals. Underlined words in definitions are explained elsewhere in the glossary. New material will be added as needed - check back often. - A - A-HORIZON SOIL: the uppermost layer of soil also referred to as topsoil. This is the layer of mineral soil with the most organic matter accumulation and soil life. This layer is not usually selected in soil surveys. ADIT: an opening that is driven horizontally (into the side of a mountain or hill) to access a mineral deposit. AIRBORNE SURVEY: a geophysical survey done from the air by systematically crossing an area or mineral property using aircraft outfitted with a variety of sensitive instruments designed to measure variations in the earth=s magnetic, gravitational, electro-magnetic fields, and/or the radiation (Radiometric Surveys) emitted by rocks at or near the surface. These surveys detect anomalies. AIRBORNE MAGNETIC (or AEROMAG) SURVEYS: regional or local magnetic surveys that measures deviations in the earth=s magnetic field and carried out by flying a magnetometer along flight lines on a pre-determined grid pattern. The lower the aircraft and the closer the flight lines, the more sensitive is the survey and the more detail in the resultant maps. Aeromag maps produced from these surveys are important exploration tools and have played a major role in many major discoveries (e.g., the Olympic Dam deposit in Australia).
    [Show full text]
  • A Partial Glossary of Spanish Geological Terms Exclusive of Most Cognates
    U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY A Partial Glossary of Spanish Geological Terms Exclusive of Most Cognates by Keith R. Long Open-File Report 91-0579 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1991 Preface In recent years, almost all countries in Latin America have adopted democratic political systems and liberal economic policies. The resulting favorable investment climate has spurred a new wave of North American investment in Latin American mineral resources and has improved cooperation between geoscience organizations on both continents. The U.S. Geological Survey (USGS) has responded to the new situation through cooperative mineral resource investigations with a number of countries in Latin America. These activities are now being coordinated by the USGS's Center for Inter-American Mineral Resource Investigations (CIMRI), recently established in Tucson, Arizona. In the course of CIMRI's work, we have found a need for a compilation of Spanish geological and mining terminology that goes beyond the few Spanish-English geological dictionaries available. Even geologists who are fluent in Spanish often encounter local terminology oijerga that is unfamiliar. These terms, which have grown out of five centuries of mining tradition in Latin America, and frequently draw on native languages, usually cannot be found in standard dictionaries. There are, of course, many geological terms which can be recognized even by geologists who speak little or no Spanish.
    [Show full text]
  • An Unusual Tasmanian Tertiary Basalt Sequence, Near Boat Harbour, Northwest Tasmania
    AUSTRALIAN MUSEUM SCIENTIFIC PUBLICATIONS Sutherland, F. L., D. F. Hendry, B. J. Barron, W. L. Matthews and J. D. Hollis, 1996. An unusual Tasmanian Tertiary basalt sequence, near Boat Harbour, northwest Tasmania. Records of the Australian Museum 48(2): 131–161. [18 September 1996]. doi:10.3853/j.0067-1975.48.1996.285 ISSN 0067-1975 Published by the Australian Museum, Sydney naturenature cultureculture discover discover AustralianAustralian Museum Museum science science is is freely freely accessible accessible online online at at www.australianmuseum.net.au/publications/www.australianmuseum.net.au/publications/ 66 CollegeCollege Street,Street, SydneySydney NSWNSW 2010,2010, AustraliaAustralia Records of the Australian Museum (1996) Vo!. 48: 131-161. ISSN 0067-1975 An Unusual Tasmanian Tertiary Basalt Sequence, Near Boat Harbour, Northwest Tasmania RL. SUTHERLAND,! D.R HENDRY,z* B.J. BARRON,1 W.L. MATTHEWS3 AND J.D. HOLLIS! I Division of Earth and Environmental Sciences, The Australian Museum, 6 College Street, Sydney NSW 2000, Australia Internet: [email protected] 2 Department of Geology and Geophysics, The University of Sydney NSW 2006, Australia 3 Department of Mines, PO Box 56, Rosny Park TAS 7018, Australia * Present address: 16 Sunnyside Street, Gladesville NSW 2111, Australia ABSTRACT. The mineralogy and petrology of basalts near Boat Harbour, NW Tasmania, are described as this sequence is unusual for Tasmanian Tertiary basalts. The rocks are more sodic and evolved basalts carry more prolific anorthoclase and zircon megacrysts than is normal in Tasmania. Older nephelinites and melilite-nephelinites (26-27 Ma) and younger nepheline hawaiites and mugearites (14-15 Ma) are present and fission track zircon ages (l3-14 Ma and 8-9 Ma) demonstrate that zircon was erupted during and after the evolved basalts.
    [Show full text]