scholarly journals Garnet exsolution from stressed orthopyroxene in garnet lherzolite from the Monastery Mine

2019 ◽  
Keyword(s):  
Garnet Lherzolite

Unique spinel-garnet lherzolite inclusion in kimberlite from Australia

Geology ◽  
1977 ◽  
Vol 5 (5) ◽  
pp. 278 ◽  
Author(s):  
John Ferguson ◽  
D. J. Ellis ◽  
R. N. England
Keyword(s):  
Garnet Lherzolite

Lherzolite - carbonatite melt interaction in the presence of additive CO2 and H2O: Experimental data at 5.5 GPa and 1200-1450°C

2021 ◽  
Author(s):  
Aleksei Kruk ◽  
Alexander Sokol
Keyword(s):  
Experimental Data ◽  
Lithospheric Mantle ◽  
Fluid Phase ◽  
Experimental Results ◽  
Silicate Melt ◽  
Silicate Melts ◽  
Garnet Lherzolite ◽  
Russian Science ◽  
Continental Lithospheric Mantle

<p>We study the reaction of garnet lherzolite with carbonatitic melt rich in molecular CO<sub>2</sub> and/or H<sub>2</sub>O in experiments at 5.5 GPa and 1200-1450°C. The experimental results show that carbonation of olivine with formation of orthopyroxene and magnesite can buffer the CO<sub>2</sub> contents in the melt, which impedes immediate separation of CO<sub>2</sub> fluid from melt equilibrated with the peridotite source. The solubility of molecular CO<sub>2</sub> in melt decreases from 20-25 wt.% at 4.5-6.8 wt.% SiO<sub>2</sub> typical of carbonatite to 7-12 wt.% in more silicic kimberlite-like melts with 26-32 wt.% SiO<sub>2</sub>. Interaction of garnet lherzolite with carbonatitic melt (2:1) in the presence of 2-3 wt.% H<sub>2</sub>O and 9-13 wt.% molecular CO<sub>2</sub> at 1200-1450°С yields low SiO<sub>2</sub> (<10 wt.%) alkali‐carbonatite melts, which shows multiphase saturation with magnesite-bearing garnet harzburgite. Thus, carbonatitic melts rich in volatiles can originate in a harzburgite source at moderate temperatures common to continental lithospheric mantle (CLM).</p><p>Having separated from the source, carbonatitic magma enriched in molecular CO<sub>2</sub> and H<sub>2</sub>O can rapidly acquire a kimberlitic composition with >25 wt.% SiO<sub>2 </sub>by dissolution and carbonation of entrapped peridotite. Furthermore, interaction of garnet lherzolite with carbonatitic melt rich in K, CO<sub>2</sub>, and H<sub>2</sub>O at 1350°С produces immiscible kimberlite-like carbonate-silicate and K-rich silicate melts. Quenched silicate melt develops lamelli of foam-like vesicular glass. Differentiation of immiscible melts early during ascent may equalize the compositions of kimberlite magmas generated in different CLM sources. The fluid phase can release explosively from ascending magma at lower pressures as a result of SiO<sub>2</sub> increase which reduces the solubility of CO<sub>2</sub> due to decarbonation reaction of magnesite and orthopyroxene.</p><p>The research was performed by a grant of the Russian Science Foundation (19-77-10023).</p>

Chapter 5.2b Erebus Volcanic Province: petrology

2021 ◽  
pp. M55-2018-80 ◽  
Author(s):  
Adam P. Martin ◽  
Alan F. Cooper ◽  
Richard C. Price ◽  
Philip R. Kyle ◽  
John A. Gamble
Keyword(s):  
Mantle Convection ◽  
Sensu Stricto ◽  
Igneous Rocks ◽  
Compositional Variation ◽  
Garnet Lherzolite ◽  
Geochronological Data ◽  
Crustal Component ◽  
Volcanic Province ◽  
Mixed Spinel ◽  
Source Component

AbstractIgneous rocks of the Erebus Volcanic Province have been investigated for more than a century but many aspects of petrogenesis remain problematic. Current interpretations are assessed and summarized using a comprehensive dataset of previously published and new geochemical and geochronological data. Igneous rocks, ranging in age from 25 Ma to the present day, are mainly nepheline normative. Compositional variation is largely controlled by fractionation of olivine + clinopyroxene + magnetite/ilmenite + titanite ± kaersutite ± feldspar, with relatively undifferentiated melts being generated by <10% partial melting of a mixed spinel + garnet lherzolite source. Equilibration of radiogenic Sr, Nd, Pb and Hf is consistent with a high time-integrated HIMU sensu stricto source component and this is unlikely to be related to subduction of the palaeo-Pacific Plate around 0.5 Ga. Relatively undifferentiated whole-rock chemistry can be modelled to infer complex sources comprising depleted and enriched peridotite, HIMU, eclogite-like and carbonatite-like components. Spatial (west–east) variations in Sr, Nd and Pb isotopic compositions and Ba/Rb and Nb/Ta ratios can be interpreted to indicate increasing involvement of an eclogitic crustal component eastwards. Melting in the region is related to decompression, possibly from edge-driven mantle convection or a mantle plume.

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.

The transition between spinel lherzolite and garnet lherzolite, and its use as a Geobarometer

1981 ◽  
Vol 77 (2) ◽  
pp. 185-194 ◽  
Author(s):  
Hugh St. C. O'Neill
Keyword(s):  
Spinel Lherzolite ◽  
Garnet Lherzolite

scholarly journals Layered Lithospheric Mantle Beneath the Ontong Java Plateau: Implications from Xenoliths in Alnöite, Malaita, Solomon Islands

2004 ◽  
Vol 45 (10) ◽  
pp. 2011-2044 ◽  
Author(s):  
AKIRA ISHIKAWA ◽  
SHIGENORI MARUYAMA ◽  
TSUYOSHI KOMIYA
Keyword(s):  
Mantle Plume ◽  
Solomon Islands ◽  
Oceanic Lithosphere ◽  
Mantle Xenoliths ◽  
Garnet Lherzolite ◽  
Ontong Java Plateau ◽  
Basaltic Melt ◽  
Normative Quartz ◽  
Intermediate Part ◽  
Compositional Diversity

Abstract A varied suite of mantle xenoliths from Malaita, Solomon Islands, was investigated to constrain the evolution of the mantle beneath the Ontong Java Plateau. Comprehensive petrological and thermobarometric studies make it possible to identify the dominant processes that produced the compositional diversity and to reconstruct the lithospheric stratigraphy in the context of a paleogeotherm. P–T estimates show that both peridotites and pyroxenites can be assigned to a shallower or deeper origin, separated by a garnet-poor zone of 10 km between 90 and 100 km. This zone is dominated by refractory spinel harzburgites (Fo91–92), indicating the occurrence of an intra-lithospheric depleted zone. Shallower mantle (∼Moho to 95 km) is composed of variably metasomatized peridotite with subordinate pyroxenite derived from metacumulates. Deeper mantle (∼95–120 km) is represented by pyroxenite and variably depleted peridotites that are unevenly distributed; the least-depleted garnet lherzolite (Fo90–91) lies just below the garnet-poor depleted zone (∼100–110 km), whereas the presence of pyroxenite is restricted to the deepest region (∼110–120 km), together with relatively Fe-enriched garnet lherzolite (Fo87–88). This depth-related variation (including the depleted zone) can be explained by assuming that the degree of melting for a basalt–peridotite hybrid source was systematically different at each level of arrival depth within a single adiabatically ascending mantle plume: (1) the depleted zone at the top of the mantle plume, where garnet was totally consumed in the residual solid; (2) an intermediate part of the plume dominated by the least-depleted garnet lherzolite just above the depth of the peridotite solidus; (3) the deepest pyroxenite-rich zone, whose petrochemical variation is best explained by the interaction between peridotite and normative quartz-rich basaltic melt, below the solidus of peridotite and liquidus of basalt. We explain the obvious lack of pyroxenites at shallower depths as the effective extraction of hybrid melt from completely molten basalt through the partially molten ambient peridotite, which caused the voluminous eruption of the Ontong Java Plateau basalts. From these interpretations, we conclude that the lithosphere forms a genetically unrelated two-layered structure, comprising shallower oceanic lithosphere and deeper impinged plume material, which involved a recycled basaltic component, now present as a pyroxenitic heterogeneity. This interpretation for the present lithospheric structure may explain the seismically anomalous root beneath the Ontong Java Plateau.

A study of REE and Pb, Sr and Nd isotopes in garnet-lherzolite xenoliths from Mingxi, Fujian Province

1993 ◽  
Vol 12 (2) ◽  
pp. 97-109 ◽  
Author(s):  
Huang Wankang ◽  
Wang Junwen ◽  
A. R. Basu ◽  
M. Tatsumoto
Keyword(s):  
Garnet Lherzolite ◽  
Nd Isotopes ◽  
Fujian Province ◽  
Sr And Nd Isotopes
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