scholarly journals Intracrystalline melt migration in deformed olivine revealed by trace element compositions and polyphase solid inclusions

2021 ◽  
Vol 33 (4) ◽  
pp. 463-477
Author(s):  
Valentin Basch ◽  
Martyn R. Drury ◽  
Oliver Plumper ◽  
Eric Hellebrand ◽  
Laura Crispini ◽  
...  
Keyword(s):  
Grain Boundaries ◽  
Trace Element ◽  
Precipitation Process ◽  
Chemical Compositions ◽  
Rock Interaction ◽  
Crystallographic Preferred Orientation ◽  
Melt Migration ◽  
Host Olivine ◽  
Ligurian Alps ◽  
Polyphase Inclusions

Abstract. Melt transport mechanisms have an important impact on the chemical composition of the percolated host rock and the migrating melts. Melt migration is usually assumed to occur at grain boundaries. However, microstructural studies revealed the occurrence of polyphase inclusions along dislocations, subgrain boundaries and microcracks in single mineral grains. The inclusions are interpreted as crystallized melt pockets suggesting that melts can migrate within deformed crystals. Intracrystalline melt migration and diffusive re-equilibration can lead to significant mineral trace element enrichments when associated with dissolution–precipitation reactions. In this contribution, we study a body of replacive troctolites associated with the Erro-Tobbio ophiolitic mantle peridotites (Ligurian Alps, Italy). The replacive formation of the olivine-rich troctolite involved extensive impregnation of a dunitic matrix, i.e. partial dissolution of olivine and concomitant crystallization of interstitial phases. The olivine matrix is characterized by two distinct olivine textures: (i) coarse deformed olivine, representing relicts of the pre-existing mantle dunite matrix (olivine1), and (ii) fine-grained undeformed olivine, a product of the melt–rock interaction process (olivine2). Previous studies documented a decoupling between olivine texture and trace element composition, namely enriched trace element compositions in olivine1 rather than in olivine2, as would be expected from the dissolution–precipitation process. Notably, the trace element enrichments in deformed olivines are correlated with the occurrence of elongated 10 µm size polyphase inclusions (clinopyroxene, Ti-pargasite, chromite) preferentially oriented along olivine crystallographic axes. These inclusions show irregular contacts and have no crystallographic preferred orientation with the host olivine, and the phases composing the inclusions show similar chemical compositions to the vermicular phases formed at the grain boundaries during late-stage reactive crystallization of the troctolite. This suggests that the investigated inclusions did not form as exsolutions of the host olivine but rather by input of metasomatic fluids percolating through the deformed olivine grains during closure of the magmatic system. We infer that strongly fractionated volatile-rich melts were incorporated in oriented microfractures within olivine1 and led to the crystallization of the polyphase inclusions. The presence of intracrystalline melt greatly enhanced diffusive re-equilibration between the evolved melt and the percolated olivine1, in turn acquiring the enriched character expected in neoformed olivine crystals. Intracrystalline melt percolation can have strong geochemical implications and can lead to efficient re-equilibration of percolated minerals and rocks.

Magnetite texture and trace-element geochemistry fingerprint of pulsed mineralization in the Xinqiao Cu-Fe-Au deposit, Eastern China

2020 ◽  
Vol 105 (11) ◽  
pp. 1712-1723
Author(s):  
Yu Zhang ◽  
Pete Hollings ◽  
Yongjun Shao ◽  
Dengfeng Li ◽  
Huayong Chen ◽  
...  
Keyword(s):  
Trace Element ◽  
Triple Junction ◽  
Eastern China ◽  
Black Shales ◽  
Hydrothermal Fluids ◽  
Oscillatory Zoning ◽  
Trace Element Geochemistry ◽  
Element Geochemistry ◽  
Rock Interaction ◽  
Multiple Pulses

Abstract The origin of stratabound deposits in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYRB), Eastern China, is the subject of considerable debate. The Xinqiao Cu-Fe-Au deposit in the Tongling ore district is a typical stratabound ore body characterized by multi-stage magnetite. A total of six generations of magnetite have been identified. Mt1 is commonly replaced by porous Mt2, and both are commonly trapped in the core of Mt3, which is characterized by both core-rim textures and oscillatory zoning. Porous Mt4 commonly truncates the oscillatory zoning of Mt3, and Mt5 is characterized by 120° triple junction texture. Mt1 to Mt5 are commonly replaced by pyrite that coexists with quartz, whereas Mt6, with a fine-grained foliated and needle-like texture, commonly cuts the early pyrite as veins and is replaced by pyrite that coexists with calcite. The geochemistry of the magnetite suggests that they are hydrothermal in origin. The microporosity of Mt2 and Mt4 magnetite, their sharp contacts with Mt1 and Mt3, and lower trace-element contents (e.g., Si, Ca, Mg, and Ti) than Mt1 and Mt3 suggest that they formed via coupled dissolution and reprecipitation of the precursor Mt1 and Mt3 magnetite, respectively. This was likely caused by high-salinity fluids derived from intensive water-rock interaction between the magmatic-hydrothermal fluids associated with the Jitou stock and Late Permian metalliferous black shales. The 120° triple junction texture of Mt5 suggests it is the result of fluid-assisted recrystallization, whereas Mt6 formed by replacement of hematite as a result of fracturing. The geochemistry of the magnetite suggests that the temperature increased from Mt2 to Mt3 and implies that there were multiple pulses of fluids from a magmatic-hydrothermal system. Therefore, we propose that the Xinqiao stratiform mineralization was genetically associated with multiple influxes of magmatic hydrothermal fluids derived from the Early Cretaceous Jitou stock. This study demonstrates that detailed texture examination and in situ trace-elements analysis under robust geological and petrographic frameworks can effectively constrain the mineralization processes and ore genesis.

scholarly journals Geochemical signatures of mineralizing events in the Juomasuo Au–Co deposit, Kuusamo belt, northeastern Finland

2021 ◽  
Author(s):  
Mikael Vasilopoulos ◽  
Ferenc Molnár ◽  
Hugh O’Brien ◽  
Yann Lahaye ◽  
Marie Lefèbvre ◽  
...  
Keyword(s):  
Trace Element ◽  
Sulfur Isotope ◽  
Mineral Chemistry ◽  
Chemical Compositions ◽  
Metasedimentary Rocks ◽  
Metavolcanic Rocks ◽  
Icp Ms ◽  
Stage 1 ◽  
Host Rocks ◽  
Relationship Of

AbstractThe Juomasuo Au–Co deposit, currently classified as an orogenic gold deposit with atypical metal association, is located in the Paleoproterozoic Kuusamo belt in northeastern Finland. The volcano-sedimentary sequence that hosts the deposit was intensely altered, deformed, and metamorphosed to greenschist facies during the 1.93–1.76 Ga Svecofennian orogeny. In this study, we investigate the temporal relationship between Co and Au deposition and the relationship of metal enrichment with protolith composition and alteration mineralogy by utilizing lithogeochemical data and petrographic observations. We also investigate the nature of fluids involved in deposit formation based on sulfide trace element and sulfur isotope LA-ICP-MS data together with tourmaline mineral chemistry and boron isotopes. Classification of original protoliths was made on the basis of geochemically immobile elements; recognized lithologies are metasedimentary rocks, mafic, intermediate-composition, and felsic metavolcanic rocks, and an ultramafic sill. The composition of the host rocks does not control the type or intensity of mineralization. Sulfur isotope values (δ34S − 2.6 to + 7.1‰) and trace element data obtained for pyrite, chalcopyrite, and pyrrhotite indicate that the two geochemically distinct Au–Co and Co ore types formed from fluids of different compositions and origins. A reduced, metamorphic fluid was responsible for deposition of the pyrrhotite-dominant, Co-rich ore, whereas a relatively oxidized fluid deposited the pyrite-dominant Au–Co ore. The main alteration and mineralization stages at Juomasuo are as follows: (1) widespread albitization that predates both types of mineralization; (2) stage 1, Co-rich mineralization associated with chlorite (± biotite ± amphibole) alteration; (3) stage 2, Au–Co mineralization related to sericitization. Crystal-chemical compositions for tourmaline suggest the involvement of evaporite-related fluids in formation of the deposit; boron isotope data also allow for this conclusion. Results of our research indicate that the metal association in the Juomasuo Au–Co deposit was formed by spatially coincident and multiple hydrothermal processes.

scholarly journals TephraNZ: a major and trace element reference dataset for prominent Quaternary rhyolitic tephras in New Zealand and implications for correlation

2020 ◽  
Author(s):  
Jenni L. Hopkins ◽  
Janine E. Bidmead ◽  
David J. Lowe ◽  
Richard J. Wysoczanski ◽  
Bradley J. Pillans ◽  
...  
Keyword(s):  
New Zealand ◽  
Trace Element ◽  
Major Element ◽  
Chemical Compositions ◽  
Volcanic Centre ◽  
Reference Dataset ◽  
Future Studies ◽  
Glass Shard ◽  
Tephra Deposits ◽  
Glass Shards

Abstract. Although analyses of tephra-derived glass shards have been undertaken in New Zealand for nearly four decades (pioneered by Paul Froggatt), our study is the first to systematically develop a formal, comprehensive, open access, reference dataset of glass-shard compositions for New Zealand tephras. These data will provide an important reference tool for future studies to identify and correlate tephra deposits and for associated petrological and magma-related studies within New Zealand and beyond. Here we present the foundation dataset for TephraNZ, an open access reference dataset for selected tephra deposits in New Zealand. Prominent, rhyolitic, tephra deposits from the Quaternary were identified, with sample collection targeting original type sites or reference locations where the tephra's identification is unequivocally known based on independent dating or mineralogical techniques. Glass shards were extracted from the tephra deposits and major and trace element geochemical compositions were determined. We discuss in detail the data reduction process used to obtain the results and propose that future studies follow a similar protocol in order to gain comparable data. The dataset contains analyses of twenty-three proximal and twenty-seven distal tephra samples characterising 45 eruptive episodes ranging from Kaharoa (636 ± 12 cal. yrs BP) to the Hikuroa Pumice member (2.0 ± 0.6 Ma) from six or more caldera sources, most from the central Taupō Volcanic Zone. We report 1385 major element analyses obtained by electron microprobe (EMPA), and 590 trace element analyses obtained by laser ablation (LA)-ICP-MS, on individual glass shards. Using PCA, Euclidean similarity coefficients, and geochemical investigation, we show that chemical compositions of glass shards from individual eruptions are commonly distinguished by major elements, especially CaO, TiO2, K2O, FeOt (Na2O+ K2O and SiO2/K2O), but not always. For those tephras with similar glass major-element signatures, some can be distinguished using trace elements (e.g. HFSEs: Zr, Hf, Nb; LILE: Ba, Rb; REE: Eu, Tm, Dy, Y, Tb, Gd, Er, Ho, Yb, Sm), and trace element ratios (e.g. LILE / HFSE: Ba / Th, Ba / Zr, Rb / Zr; HFSE / HREE: Zr / Y, Zr / Yb, Hf / Y; LREE / HREE: La / Yb, Ce / Yb). Geochemistry alone cannot be used to distinguish between glass shards from the following tephra groups: Taupō (Unit Y in the post-Ōruanui eruption sequence of Taupō volcano) and Waimihia (Unit S); Poronui (Unit C) and Karapiti (Unit B); Rotorua and Rerewhakaaitu; and Kawakawa/Ōruanui, Okaia, and Unit L (of the Mangaone subgroup eruption sequence). Other characteristics can be used to separate and distinguish all of these otherwise-similar eruptives except Poronui and Karapiti. Bimodality caused by K2O variability is newly identified in Poihipi and Tahuna tephras. Using glass shard compositions, tephra sourced from Taupō Volcanic Centre (TVC) and Mangakino Volcanic Centre (MgVC) can be separated using bivariate plots of SiO2/K2O vs. Na2O+K2O. Glass shards from tephras derived from Kapenga Volcanic Centre, Rotorua Volcanic Centre, and Whakamaru Volcanic Centre have similar major- and trace-element chemical compositions to those from the MgVC, but can overlap with glass analyses from tephras from Taupō and Okataina volcanic centres. Specific trace elements and trace element ratios have lower variability than the heterogeneous major element and bimodal signatures, making them easier to geochemically fingerprint.

Trace element partitioning during high-P partial melting and melt-rock interaction; an example from northern Fiordland, New Zealand

2004 ◽  
Vol 22 (5) ◽  
pp. 443-457 ◽  
Author(s):  
F. C. Schröter ◽  
J. A. Stevenson ◽  
N. R. Daczko ◽  
G. L. Clarke ◽  
N. J. Pearson ◽  
...  
Keyword(s):  
New Zealand ◽  
Trace Element ◽  
Partial Melting ◽  
Rock Interaction ◽  
Trace Element Partitioning ◽  
Element Partitioning

scholarly journals Recrystallization processes, microstructure and crystallographic preferred orientation evolution in polycrystalline ice during high-temperature simple shear

2019 ◽  
Vol 13 (5) ◽  
pp. 1495-1511 ◽  
Author(s):  
Baptiste Journaux ◽  
Thomas Chauve ◽  
Maurine Montagnat ◽  
Andrea Tommasi ◽  
Fabrice Barou ◽  
...  
Keyword(s):  
High Temperature ◽  
Grain Boundary ◽  
Dynamic Recrystallization ◽  
Grain Boundaries ◽  
Simple Shear ◽  
Shear Plane ◽  
Preferred Orientation ◽  
Dislocation Slip ◽  
Crystallographic Preferred Orientation ◽  
Polycrystalline Ice

Abstract. Torsion experiments were performed in polycrystalline ice at high temperature (0.97 Tm) to reproduce the simple shear kinematics that are believed to dominate in ice streams and at the base of fast-flowing glaciers. As clearly documented more than 30 years ago, under simple shear ice develops a two-maxima c axis crystallographic preferred orientation (CPO), which evolves rapidly into a single cluster CPO with a c axis perpendicular to the shear plane. Dynamic recrystallization mechanisms that occur in both laboratory conditions and naturally deformed ice are likely candidates to explain the observed CPO evolution. In this study, we use electron backscatter diffraction (EBSD) and automatic ice texture analyzer (AITA) to characterize the mechanisms accommodating deformation, the stress and strain heterogeneities that form under torsion of an initially isotropic polycrystalline ice sample at high temperature, and the role of dynamic recrystallization in accommodating these heterogeneities. These analyses highlight an interlocking microstructure, which results from heterogeneity-driven serrated grain boundary migration, and sub-grain boundaries composed of dislocations with a [c]-component Burgers vector, indicating that strong local stress heterogeneity develops, in particular, close to grain boundaries, even at high temperature and high finite shear strain. Based on these observations, we propose that nucleation by bulging, assisted by sub-grain boundary formation and followed by grain growth, is a very likely candidate to explain the progressive disappearance of the c axis CPO cluster at low angle to the shear plane and the stability of the one normal to it. We therefore strongly support the development of new polycrystal plasticity models limiting dislocation slip on non-basal slip systems and allowing for efficient accommodation of strain incompatibilities by an association of bulging and formation of sub-grain boundaries with a significant [c] component.

scholarly journals Rare sapphire-bearing syenitoid pegmatites and associated granitoids of the Hamedan region, Sanandaj–Sirjan zone, Iran: analysis of petrology, lithogeochemistry and zircon geochronology / trace element geochemistry

2020 ◽  
Vol 157 (9) ◽  
pp. 1499-1525 ◽  
Author(s):  
Ali A Sepahi ◽  
Hamed Vahidpour ◽  
David R Lentz ◽  
Chris RM McFarlane ◽  
Mohammad Maanijou ◽  
...  
Keyword(s):  
Trace Element ◽  
Trace Element Geochemistry ◽  
Chemical Compositions ◽  
Zircon Geochronology ◽  
Element Geochemistry ◽  
High K ◽  
Igneous Origin ◽  
Plutonic Complex ◽  
Sanandaj Sirjan Zone ◽  
Zircon Thermometry

AbstractPegmatites and associated granitoids are integral parts of the Alvand plutonic complex in the Sanandaj–Sirjan zone, Iran. Whole rock major- and trace-element lithogeochemistry together with zircon U–Pb geochronology and zircon geochemistry are examined to evaluate the petrogenesis of sapphire-bearing pegmatites and other peraluminous pegmatites in the region. Pegmatites vary in their chemical compositions from mostly peraluminous, high-K calc-alkaline to shoshonitic signatures. A rare variety of extremely peraluminous sapphire-bearing syenitoid pegmatite (Al2O3 > 30 wt %; A/CNK > 2) exists. This silica-undersaturated pegmatite and its sapphire crystals have a primary igneous origin. U–Pb zircon geochronology of three separate samples from this pegmatite indicates the following ages: 168 ± 1 Ma, 166 ± 1 Ma and 164 ± 1 Ma. The zircon grains have notable amounts of Hf (up to 17 200 ppm), U (up to 13 580 ppm), Th (up to 5148 ppm), Y (up to 4764 ppm) and ∑REE (up to 2534 ppm). There is a positive correlation between Hf and Th, Nb and Ta, U and Th, and Y and HREE and a negative correlation between Hf and Y values in the zircons. These zircons exhibit pronounced positive Ce anomalies (Ce/Ce* = 1.15–68.06) and negative Eu anomalies (Eu/Eu* = 0.001–0.56), indicative of the relatively oxidized conditions of the parent magma. Ti-in-zircon thermometry reveals temperatures from as low as ~683 °C up to ~828 °C (average = 755° ± 73 °C). Zircon and monazite saturation equilibria are also consistent with these temperatures. Zircon grains are magmatic (average La < 1.5, (Sm/La)N > 100 and Th/U > 0.7), with chemical characteristics similar to zircons from continental crust.

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