Petrology and oxygen-isotope geochemistry of the Yamba Lake kimberlite rocks, N.W.T.

2000 ◽  
Vol 37 (7) ◽  
pp. 1053-1071 ◽  
Author(s):  
Pauline Orr ◽  
Robert W Luth
Keyword(s):  
Major Element ◽  
Isotope Geochemistry ◽  
Isotopic Fractionation ◽  
Kimberlite Pipe ◽  
Spinel Lherzolite ◽  
Garnet Lherzolite ◽  
Slave Province ◽  
Low Concentrations ◽  
Subducted Oceanic Crust ◽  
Indicator Minerals

The Torrie, Sputnik, and Eddie kimberlite rocks, located near Yamba Lake, central Slave province, N.W.T., are volcaniclastic, macrocrystic, heterolithic, olivine-rich tuff, and olivine-rich tuff breccia. Torrie and Sputnik kimberlite rocks contain pyroxene and garnet xenocrysts and megacrysts with major-element compositions consistent with derivation mostly from disaggregated garnet lherzolite, with subordinate contributions from eclogite, spinel lherzolite, garnet harzburgite, and websterite. The presence of primary groundmass phlogopite and compositionally evolved spinel, and the absence of mantle xenocrysts, xenoliths, and megacrystic ilmenite distinguish the Eddie kimberlite pipe from the other two kimberlite pipes. Large variations in δ18O of garnet and clinopyroxene in xenocrysts and xenoliths (+3.98 to +6.36‰), nonequilibrium intermineral isotopic fractionation, and major-element heterogeneity are interpreted as resulting from infiltration of fluids or melts produced by dehydration or melting of subducted oceanic crust into overlying peridotite. Although the timing is unconstrained for the xenocysts, the xenolith must have experienced this metasomatic interaction shortly before entrainment in the kimberlite. Variable δ18O values for magnesian ilmenite are also interpreted to result indirectly from such metasomatic activity in the mantle as well. The Torrie and Sputnik kimberlite rocks have low concentrations of diamond indicator minerals consistent with their low-diamond grades. These kimberlite rocks did not sample a significant amount of garnet harzburgite, the rock type commonly associated with high-diamond grades in other kimberlite rocks. Furthermore, metasomatism just prior to kimberlite eruption may have caused the resorption of any diamond present.

Kyanite-bearing eclogite xenoliths from the Udachnaya kimberlite, Siberian craton, Russia

2017 ◽  
Vol 188 (1-2) ◽  
pp. 7 ◽  
Author(s):  
Ioana-Bogdana Radu ◽  
Bertrand Moine ◽  
Dmitri Ionov ◽  
Andrey Korsakov ◽  
Alexander Golovin ◽  
...  
Keyword(s):  
Siberian Craton ◽  
Major Element ◽  
Kimberlite Pipe ◽  
West African ◽  
Coarse Grained ◽  
Chemical Compositions ◽  
Type I ◽  
Type Ii ◽  
Major Element Composition ◽  
Subducted Oceanic Crust

Xenoliths brought up by kimberlite magmas are rare samples of otherwise inaccessible lithospheric mantle. Eclogite xenoliths are found in most cratons and commonly show a range of mineral and chemical compositions that can be used to better understand craton formation. This study focuses on five new kyanite-bearing eclogites from the Udachnaya kimberlite pipe (367±5 Ma). They are fine-to coarse-grained and consist mainly of “cloudy” clinopyroxene (cpx) and garnet (grt). The clinopyroxene is Al,Na-rich omphacite while the garnet is Ca-rich, by contrast to typical bi-mineral (cpx+grt) eclogites that contain Fe- and Mg-rich garnets. The Udachnaya kyanite eclogites are similar in modal and major element composition to those from other cratons (Dharwar, Kaapvaal, Slave, West African). The kyanite eclogites have lower REE concentrations than bi-mineral eclogites and typically contain omphacites with positive Eu and Sr anomalies, i.e. a “ghost plagioclase signature”. Because such a signature can only be preserved in nonmetasomatised samples, we infer that they were present in the protoliths of the eclogites. It follows that subducted oceanic crust is present at the base of the Siberian craton. Similar compositions and textures are also seen in kyanite eclogites from other cratons, which we view as evidence for an Archean, subduction-like formation mechanism related to craton accretion. Thus, contrary to previous work that classifies all kyanite eclogites as type I (IK), metasomatized by carbonatite/kimberlitic fluids, we argue that some of them, both from this work and those from other cratons, belong to the non-metasomatized type II (IIB). The pristine type IIB is the nearest in composition to protoliths of mantle eclogites because it contains no metasomatic enrichments.

Characterization and dispersal of indicator minerals associated with the Pine Point Mississippi Valley-type (MVT) district, Northwest Territories, Canada

2015 ◽  
Vol 52 (9) ◽  
pp. 776-794 ◽  
Author(s):  
N.M. Oviatt ◽  
S.A. Gleeson ◽  
R.C. Paulen ◽  
M.B. McClenaghan ◽  
S. Paradis
Keyword(s):  
Northwest Territories ◽  
Major Element ◽  
Bimodal Distribution ◽  
Mississippi Valley ◽  
Element Geochemistry ◽  
Element Concentrations ◽  
Pine Point ◽  
Indicator Minerals ◽  
Mississippi Valley Type ◽  
Valley Type

A glacial dispersal study was conducted around a subcropping Pb–Zn deposit (O28) in the Pine Point Mississippi Valley-type (MVT) district, Northwest Territories, Canada, with the intent of characterizing and documenting the indicator minerals and their dispersal from a known orebody. Mapping of striations adjacent to deposit O28, and throughout the Pine Point district, along with observed glacial stratigraphy, indicate that there are three phases of ice flow that have affected the Pine Point district. Sphalerite, galena, and pyrite were identified in mineralized bedrock samples at deposit O28, and sphalerite and galena were recovered from the sand fraction of till samples up to 500 m from the mineralized subcrop. The majority of sphalerite and galena grains recovered from till samples down-ice of deposit O28 were 0.25–0.5 mm in size. Size and morphology of sphalerite grains in till demonstrate relative proximity to their bedrock source, with the largest and more angular grains being closer to the ore zone (<50 m) whereas smaller and more rounded grains occur further down-ice (∼250 m). The paragenesis, textures, major-element concentrations, and S and Pb isotopic compositions of bedrock samples from deposit O28 and from newly drilled core from four other deposits were characterized. Concentrations of Zn in bedrock sphalerite grains range from 43.95 to 67.48 wt.%, concentrations of S range from 32.03 to 34.01 wt.%, and concentrations of Fe range from 0.02 to 16.94 wt.%. The Fe concentration in bedrock sphalerite decreases from east to west across the district. Concentrations of S in galena grains in bedrock range from 12.50 to 14.00 wt.% and have a bimodal distribution. Generally, the geochemistry of sphalerite grains recovered from till were statistically similar to bedrock grains recovered from deposits O28 and L65. Major-element concentrations were statistically the same between the sphalerite grains recovered from till and the honey-brown and cleiophane varieties in the bedrock samples. Galena grains recovered from till samples were similar to the cubic and fracture-fill varieties of grains recovered from bedrock in the R190 and M67 deposits. Sulphur isotopic values for sphalerite grains from bedrock range from 20.6‰ to 24.2‰, while those from till samples range from −5.3‰ to 24.4‰. Lead isotopic ratios for galena grains from bedrock and till samples had very little variation, which is a characteristic of the Pine Point district. The S and Pb isotopic studies as well as major-element geochemistry suggest that indicator minerals derived from Pine Point-type mineralization can be distinguished from those sourced from other types of carbonate-hosted mineralized systems (e.g., Cordilleran zinc–lead deposits) and that the methods here can be used as exploration tools for identifying MVT deposit provenance or potential. The results of this study present criteria and highlights additional methods for exploration of MVT deposits in glaciated terrain.

The effects of solid-solid phase equilibria on the oxygen fugacity of the upper mantle

2020 ◽  
Vol 105 (10) ◽  
pp. 1445-1471
Author(s):  
Edward M. Stolper ◽  
Oliver Shorttle ◽  
Paula M. Antoshechkina ◽  
Paul D. Asimow
Keyword(s):  
Phase Equilibria ◽  
Oxygen Fugacity ◽  
Upper Mantle ◽  
Phase Changes ◽  
Primitive Mantle ◽  
Spinel Lherzolite ◽  
Garnet Lherzolite ◽  
Constant Composition ◽  
Al Content ◽  
Mantle Sources

Abstract Decades of study have documented several orders of magnitude variation in the oxygen fugacity (fO2) of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe3+/Fe2+) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe3+/Fe2+) can also lead to systematic variations in fO2 in the shallowest ~100 km of the mantle. Two different thermodynamic models were used to calculate fO2 vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an fO2 decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in fO2 and increasing pressure in this facies has little influence on fO2 (normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in fO2 (normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized fO2 reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ⇄ Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe3+-bearing components in pyroxene and therefore to decreases in normalized fO2, whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe3+-bearing components in pyroxene and to increases in normalized fO2 (although this is counteracted to some degree by progressive partitioning of Fe3+ from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized fO2 inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase → spinel → garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations. Observed variations in FMQ-normalized fO2 of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe3+/Fe2+ or bulk O2 content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the fO2 of peridotites of constant composition are likely to be superimposed on variations in fO2 that reflect differences in the whole-rock Fe3+/Fe2+ ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.

Nature and evolution of the Slave Province subcontinental lithospheric mantleThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.

2010 ◽  
Vol 47 (4) ◽  
pp. 369-388 ◽  
Author(s):  
Larry M. Heaman ◽  
D. Graham Pearson
Keyword(s):  
Great Depth ◽  
Peridotite Xenolith ◽  
Special Issue ◽  
Mantle Material ◽  
Slave Craton ◽  
Multiple Origins ◽  
Slave Province ◽  
Abundant Evidence ◽  
Subducted Oceanic Crust ◽  
Continental Lithospheric Mantle

A review of the ages determined for mantle material (xenoliths and xenocrysts entrained in kimberlite) derived from the Slave Province continental lithospheric mantle (CLM) indicates that a portion of the central Slave lithosphere may be ancient (3.5–3.3 Ga) harzburgite, but the majority of this lithosphere is much younger (2.9–2.0 Ga). Relying on the most robust chronometers, the majority of Slave lithosphere peridotite formed in the Neoarchean (peak at 2.75 Ga), whereas the majority of eclogite formed in the Paleoproterozoic (2.2–2.0 Ga). The northern Slave lithosphere contains evidence of peridotite xenolith ages that young with depth. The Paleoproterozoic eclogites may have multiple origins including remnants of subducted oceanic crust and mafic–ultramafic magmas that crystallized at great depth (100–200 km). Re–Os studies of sulfide inclusions in diamond indicate that some diamonds currently mined are ancient (∼3.5 Ga), but many Slave diamonds could be considerably younger. Most eclogitic diamonds recovered from the Slave craton are interpreted to be related to the formation of Paleoproterozoic eclogite. There is abundant evidence for Mesoproterozoic modification of the Slave lithosphere (e.g., heating by magma emplacement at great depth and metasomatism) and possible new addition to the lithosphere at that time. The Canadian Slave and African Kaapvaal lithospheres have similar peaks in cratonic peridotite formation ages at about 2.8 Ga, indicating that a large portion of the CLM in these two cratons formed and stabilized in the Neoarchean. One difference is that the Slave peridotites are much less enriched in SiO2, possibly reflecting the more metasomatized nature of the Kaapvaal CLM. The dominance of Paleoproterozoic formation ages for Slave mantle eclogites contrasts with the dominance of Neoarchean formation ages for Kaapvaal mantle eclogites.

Ages of detrital and metamorphic zircons and monazites from a pre-Taltson magmatic zone basin at the western margin of Rae Province

1994 ◽  
Vol 31 (8) ◽  
pp. 1353-1364 ◽  
Author(s):  
H. H. Bostock ◽  
O. van Breemen
Keyword(s):  
Isotope Geochemistry ◽  
Granulite Facies ◽  
Metamorphic Grade ◽  
Detrital Zircons ◽  
Ultramafic Rocks ◽  
Crustal Accretion ◽  
The West ◽  
Slave Province ◽  
Basin Formation ◽  
High Grade Metamorphism

The western edge of Rae Province, prior to indentation of Slave Province, is conceived as a compressional tectonic margin in which Archean plutonic rocks were intruded by syntectonic granites of 2.4–2.3 Ga age as a result of eastward subduction. Subsequently this margin was intruded by the 2.0–1.90 Ga granites that characterize the Taltson magmatic zone. The latter granites engulf remnants of a widespread supracrustal assemblage of lower granulite facies metamorphic grade, the age of which has heretofore been unknown. We use U–Pb zircon and monazite geochronology to limit the age of cessation of deposition of these metasediments in a pre-Taltson granite basin to between 2.13 and 2.09 Ga.Similarities in geochronology and isotope geochemistry between western Rae Province and Buffalo Head domain, together with the presence of mafic to ultramafic rocks both within the basin and along the western Rae margin, suggest that basin formation was by rifting. Influx of 2.15 Ga detrital zircons probably from the west, and high-grade metamorphism accompanying basin closure at 2.09 Ga, suggest an eastward (inward) movement of magmatism at that time. A second similar eastward migration of magmatism occurred in association with the Slave–Churchill collision (2.0–1.9 Ga). These relations suggest a complex record of crustal accretion within Buffalo Head and Chinchaga domains, the details of which remain to be established.

Using the silica isotope composition of biogenic materials in marine sediments to reconstruct ocean chemistry

2020 ◽  
Author(s):  
Daniel Conley ◽  
Katherine Hendry
Keyword(s):  
Isotope Composition ◽  
Isotopic Fractionation ◽  
Isotope Ratios ◽  
Silicon Isotope ◽  
Skeletal Morphology ◽  
Biological Communities ◽  
Low Concentrations ◽  
Sponge Spicules ◽  
Global Cycles ◽  
Key Events

&lt;div&gt;&lt;span&gt;The silicon isotopic composition of sedimentary biogenic opal can be used to track shifts in the balance between silicon inputs to the ocean and outputs by burial. In addition to biosilicification and opal burial, the global cycles of climate (hydrology, weathering, glaciation, etc.), tectonics (volcanoes, LIPs, mountain building, etc.) and geochemistry (reverse weathering, inorganic Si precipitation, etc.) have driven variations in the global Si cycle over geologic time. Prior to the start of the Phanerozoic it is thought that burial in the global oceans was controlled inorganically through chert formation. The evolution of the Si depositing organisms, radiolarians and sponges, reduced oceanic dissolved Si, but the largest reductions occurred with the evolution of the diatoms bringing dissolved Si to the low concentrations (relative to saturating concentrations) observed today. However, the timing of the depletion of dissolved Si by diatoms is currently under debate.&lt;/span&gt;&lt;/div&gt;&lt;div&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/div&gt;&lt;div&gt;&lt;span&gt;Our understanding of the biological components of the Si cycle has grown enormously. In the last decade, silicon isotope ratios (expressed as &amp;#948;30Si) in marine microfossils are becoming increasingly recognised for their ability to provide insight into silicon cycling. In particular, the &amp;#948;30Si of deep-sea sponge spicules has been demonstrated to be a useful proxy for past dissolved Si concentrations. However, more recent studies find anomalies in the isotopic fractionation of sponge spicules that relate to skeletal morphology: reliable reconstructions of past dissolved Si can only be obtained using silicon isotope ratios derived from sponges with certain spicule types. We are applying &amp;#948;30Si proxies from biosiliceous material contained in sediments to generate robust estimates of the timing and magnitude of dissolved Si drawdown. We will provide fundamental new insights into the drawdown of dissolved Si and other key events, which reorganized the distribution of carbon and nutrients in seawater, with implications for productivity of the biological communities within the ancient oceans.&amp;#160;&lt;/span&gt;&lt;/div&gt;

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