scholarly journals Redox control on chromium isotope behaviour in silicate melts in contact with magnesiochromite

2020 ◽  
Author(s):  
Pierre Bonnand ◽  
Emilie Bruand ◽  
Andrew Matzen ◽  
Matthew Jerram ◽  
Federica Schiavi ◽  
...  
Keyword(s):  
Oxygen Fugacity ◽  
Redox Conditions ◽  
Silicate Melt ◽  
Silicate Melts ◽  
Early Earth ◽  
Isotopic Compositions ◽  
High Temperature Processes ◽  
Lower Oxygen ◽  
Volatile Loss ◽  
Over Time

<p>Transition metals are of special interest for understanding the conditions of differentiation processes such as core formation. Those that have more than one oxidation state can also provide powerful constraints on changing redox conditions in the mantle over time. The ability to determine isotopic fractionations associated with differentiation processes has provided a new dimension to exploration of the conditions in the early Earth in particular. It has been recently shown that Cr isotope variations in igneous systems are strongly affected by redox conditions and chromite crystallisation.</p><p>In this study, we have investigated the variations in chemical composition and Cr isotopic compositions in both magnesiochromite and silicate melts during experiments performed under controlled redox conditions. The Cr isotopic compositions measured in the silicate melts in our experiments are strongly influenced by oxygen fugacity and experiments performed at 1300 °C and -12 < logfO<sub>2</sub> < -6 are correlated with fO<sub>2</sub>. This suggests that Cr isotopes are a powerful tool to study changes in redox conditions in high temperature processes. The Cr isotopic composition of silicate melt reacted under more oxidising conditions (logfO<sub>2</sub> > -6) are isotopically much lighter compared to melts reacted at lower oxygen fugacity. Three hypotheses are proposed to explain such variations: (i) a change in Cr bonding environment in the silicate melt (ii) a change in Cr bonding environment in the chromite (iii) volatile loss of Cr from the silicate melt. More work is needed to definitively determine the factors that control the isotopic behaviour of Cr in silicate melts.</p>

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>

scholarly journals The effect of melt composition and oxygen fugacity on manganese partitioning between apatite and silicate melt

2019 ◽  
Vol 506 ◽  
pp. 162-174 ◽  
Author(s):  
T.N. Stokes ◽  
G.D. Bromiley ◽  
N.J. Potts ◽  
K.E. Saunders ◽  
A.J. Miles
Keyword(s):  
Oxygen Fugacity ◽  
Silicate Melt ◽  
Melt Composition

The paragenesis of upper-mantle xenoliths from the Quaternary volcanics south-east of Gees, West Eifel, Germany

1991 ◽  
Vol 55 (378) ◽  
pp. 95-112 ◽  
Author(s):  
F. E. Lloyd ◽  
A. D. Edgar ◽  
D. M. Forsyth ◽  
R. L. Barnett
Keyword(s):  
Upper Mantle ◽  
Free Subgroup ◽  
Mantle Xenoliths ◽  
Silicate Melt ◽  
Silicate Melts ◽  
Hydrous Silicate ◽  
Group I ◽  
Group Ii ◽  
West Eifel ◽  
K Activity

AbstractGroup I xenoliths, orthopyroxene-rich and orthopyroxene-free, contain Cr-spinel and clinopyroxene ± phlogopite, and occur together with Group II clinopyroxenites ± Ti-spinel ± phlogopite in K-mafic pyroclastics southeast of Gees. The petrography and clinopyroxene chemistry of orthopyroxene-rich (opx-rich sub-group) Group I xenoliths is consistent with an ‘original’ harzburgitic mantle that has been transformed to lherzolite by the addition of endiopside. In harzburgites, orthopyroxenes are reacting to diopside + olivine + alkali-silicate melt, and, by inference, the orthopyroxene-free (opx-free subgroup) Group I, dunite-wehrlite series can be linked to the opx-rich sub-group via this reaction. Progressive enrichment of dunitic material in endiopside-diopside has resulted in the formation of wehrlite. Phlogopite is titaniferous and occurs as a trace mineral in opx-rich, Group I xenoliths, whereas substantial phlogopite vein-networks are confined to the opx-free sub-group (dunite-wehrlite series). Interstitial, alkali-felsic glass occurs are veins within, and as extensions of, the phlogopite networks. Clinopyroxenes in phlogopite-veined xenoliths are decreased in Mg/(Mg + FeTotal) (mg) and Cr and increased in Ti, Al and Ca, compared with clinopyroxenes in xenoliths which have trace phlogopite. It is proposed that harzburgitic and dunitic mantle has been infiltrated by a Ca- and alkalirich, hydrous silicate melt rather than an ephemeral carbonatite melt. Dunite has been transformed to phlogopite wehrlite by the invasion of a Ca-, Al-, Ti- and K-rich, hydrous silicate melt. Ca-activity was high initially in the melt and was reduced by clinopyroxene precipitation. This resulted in enhanced K-activity which led to phlogopite veining of clinopyroxene-rich mantle. Group II phlogopite clinopyroxenites contain Ti-spinel and salites that are distinct in their Ti, Al and Cr contents from endiopsides and diopsides in Group I xenoliths. It is unlikely that these Group II xenoliths represent the culmination of the infiltration processes that have transformed dunite to wehrlite, nor can they be related to the host melt. These xenoliths may have crystallised from Ca- and K-bearing, hydrous silicate melts in mantle channelways buffered by previously precipitated clinopyroxene and phlogopite. Gees lherzolites contain pyroxenes and spinel with distinctly lower Al contents than these same minerals in lherzolites described previously from other West Eifel localities, which may reflect a distinctive lithology and/or processes of modification for the Gees mantle.

scholarly journals An experimental study of the partitioning of trace elements between rutile and silicate melt as a function of oxygen fugacity

2014 ◽  
Vol 86 (4) ◽  
pp. 1609-1629 ◽  
Author(s):  
GUILHERME MALLMANN ◽  
RAÚL O.C. FONSECA ◽  
ADOLFO B. SILVA
Keyword(s):  
Oxygen Fugacity ◽  
Silicate Melt ◽  
Accessory Mineral ◽  
High Field Strength ◽  
Arc Magmas ◽  
Melt Polymerization ◽  
Incompatible Elements ◽  
Order Of Magnitude ◽  
Melt Composition ◽  
Partitioning Behavior

Subduction zone or arc magmas are known to display a characteristic depletion of High Field Strength Elements (HFSE) relative to other similarly incompatible elements, which can be attributed to the presence of the accessory mineral rutile (TiO2) in the residual slab. Here we show that the partitioning behavior of vanadium between rutile and silicate melt varies from incompatible (∼0.1) to compatible (∼18) as a function of oxygen fugacity. We also confirm that the HFSE are compatible in rutile, with D(Ta)> D(Nb)>> (D(Hf)>/∼ D(Zr), but that the level of compatibility is strongly dependent on melt composition, with partition coefficients increasing about one order of magnitude with increasing melt polymerization (or decreasing basicity). Our partitioning results also indicate that residual rutile may fractionate U from Th due to the contrasting (over 2 orders of magnitude) partitioning between these two elements. We confirm that, in addition to the HFSE, Cr, Cu, Zn and W are compatible in rutile at all oxygen fugacity conditions.

scholarly journals Mixed incorporation of carbon and hydrogen in silicate melts under varying pressure and redox conditions

2020 ◽  
Vol 549 ◽  
pp. 116520
Author(s):  
Bijaya B. Karki ◽  
Dipta B. Ghosh ◽  
Dipendra Banjara
Keyword(s):  
Redox Conditions ◽  
Silicate Melts

The oxidation state of europium in silicate melts as a function of oxygen fugacity, composition and temperature

2015 ◽  
Vol 411 ◽  
pp. 248-259 ◽  
Author(s):  
A.D. Burnham ◽  
A.J. Berry ◽  
H.R. Halse ◽  
P.F. Schofield ◽  
G. Cibin ◽  
...  
Keyword(s):  
Oxygen Fugacity ◽  
Oxidation State ◽  
Silicate Melts
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