Accuracy in the determination of the compositions of main rockforming silicates and oxides on a JXA-8100 microanalyzer

Dunite–harzburgite–olivine-bearing orthopyroxenite successions in the subvolcanic Chapesvara-I and Chapesvara-II intrusions in the Serpentinite Belt, western Kola Peninsula, are notably magnesian. The mean Mg# value (whole-rock) is 86.6, and the olivine is Fo84−89. The upper contact facies (UCF) displays a lower Mg# (81.6). It consists of grains of Fo92 and abundant chromian spinel, implying rapid crystallization of an almost unfractionated melt. On average, the whole-rock Al2O3/TiO2 value is 22.45, close to 22.9 (UCF) and to the primitive mantle, ~22. The rise of primitive ultramafic magma presumably occurred in a special tectonic setting at the boundary of the Paleoproterozoic Lapland Granulite Terrane and the Belomorian Composite Terrane of Archean age. The Chapesvara suite resembles examples of the Al-undepleted komatiites in the Barberton Belt, South Africa, with magmas of up to 30–35% MgO. The UCF rock yields an anomalously low molar MgO/SiO2 value, close to that of dunitic rocks located at the center of the Chapesvara-II body. This rock is the most primitive, as indicated by the maximum Fo content of olivine, the lowest value of (Gd/Yb)N, 0.52, and the lowest abundances of middle to heavy rare-earth elements (REE) in the chondrite-normalized spectrum. The crystallization of the Chapesvara-II sill-like intrusion likely proceeded in two stages, which are evident from the olivine compositions varying from the maximum Fo92 (UCF) to Fo≤89.5 (the central dunite zone). At Stage 1, the UCF rock (Fo92) crystallized first, close to the upper contact. The area of crystallization then shifted to a central portion of the Chapesvara-II body, in which the dunitic zone (Fo89.5) formed in situ (Stage 2). The compositional variations in chromian spinel are consistent with this suggestion. Two crystallization trends were recognized. The type-1 trend displays a relative maximum or minimum close to the center, and then diverges into two linear subtrends directed upward and downward. This pattern is manifested in the variations of Mg# in olivine and chromian spinel, the whole-rock contents of Al and Ca, and in levels of incompatible elements: Ti, V, Zr, Y, and Hf. The type-2 trend decreases or increases uniformly from top to bottom. Variations in amount of Ni in olivine, the Fe3+# index in chromian spinel, and in values of Mg# (rocks), follow a type-2 trend. Variations in total amounts of REE, Nb, and Th, which gradually increase downward, are also related to a type-2 trend. Thus, a contrasting development and possible interference of the two types of evolutionary trends were observed in the crystallization history of the Chapesvara-II sill-like body. A double-front crystallization, hitherto unreported, involved two fronts moving upward and downward, respectively. The upward subtrend appeared to be of subordinate importance, whereas the extent of fractional crystallization of the downward front was much greater. Crystallization proceeded from the top to the bottom, presumably because of the preferential loss of heat at the roof. Variations in the Fe3+# index indicate that the level of fO2 also increased downward with progressive crystallization. Convection cells were presumably the key mechanism of accumulation of the crystallizing olivine grains to form the central dunite zone close to the center of the sill-like intrusion. The observed characteristics of the Chapesvara complex indicate the existence of a primitive-mantle source and imply a highly magnesian composition of intruding magma not only for Chapesvara, but also for the Pados-Tundra layered complex and associated suites of the Serpentinite Belt in the Kola Peninsula.

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“…% (Na and Al). Special tests [16,17] were done to evaluate the accuracy and reproducibility of the analytical procedures.…”

Section: Methodsmentioning

confidence: 99%

Double-Front Crystallization in the Chapesvara Ultramafic Subvolcanic Complex, Serpentinite Belt, Kola Peninsula, Russia

et al. 2019

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“…All the data in the tables are listed according to the detection limits of the elements. The accuracy and reproducibility of the analytical procedures were evaluated with special tests (Korolyuk et al ., 2009; Lavrent'ev et al ., 2015).…”

Section: Methodsmentioning

confidence: 99%

Conditions of formation of Au–Se–Te mineralization in the Gaching ore occurrence (Maletoyvayam ore field), Kamchatka, Russia

et al. 2018

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The Gaching high-sulfidation epithermal deposit in the Maletoyvayam ore field features a wide range of Se-containing minerals and selenides, as well as complex gold oxides, Au tellurides (calaverite, krennerite) and native gold typical for epithermal deposits. Pyrite included in quartzites and quartz-alunite rocks was probably formed during an early stage of the ore-forming process. During the following Au-rich stage, the $f_{{\rm S}{\rm e}_{\rm 2}}$/$f_{{\rm S}_{\rm 2}}$ increased with $f_{{\rm O}_{\rm 2}}$ being relatively high, resulting in the formation of very rare compounds that have not been previously described in nature. These include Au2Te4(Se,S)3, Se3Te2, AuSe and Au(Te,Se,S) phases. The Au2Te4(Se,S)3 compounds have some variations in composition: the complete isomorphic series between Au2Te4Se3 and Au2Te4S3 was observed. The gold and Au-minerals at the main ore stage can be stable within a range of log$f_{{\rm O}_{\rm 2}}$ of −27.3 and atmospheric oxygen (?); log$f_{{\rm S}{\rm e}_{\rm 2}}$ between −12.4 and −5.7; log$f_{{\rm T}{\rm e}_{\rm 2}}$ between −10.5 and −7.8; and log$f_{{\rm S}_{\rm 2}}$ between −12.8 and −6.8 (at 250°C). The increasing oxygen fugacity during the final stage of mineralization resulted in the formation of complex Sb,As,Te,S-bearing Au oxides. Gold-oxide formation occurs due to oxidation of Au-tellurides. The final products of this process are newly-formed secondary mustard gold and Te–Se solid solutions.

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“…An acceleration voltage of 15 kV and a variable beam current and beam size (7 nA and 10-20 lm for glasses and 20 nA and 1 lm for minerals) were used to minimize alkali migration during analyses of hydrous glasses. Peak and background counting times were 20 s. All data were corrected with the PRZ model (Goldstein et al 1992;Korolyuk et al 2009). For glasses, H 2 O was used as additional element for PRZ corrections and calculated as difference to 100 wt%.…”

Section: Electron Probe Micro-analyzer (Epma)mentioning

confidence: 99%

Fractional crystallization of primitive, hydrous arc magmas: an experimental study at 0.7 GPa

Nandedkar¹,

Ulmer²,

Müntener

2014

Contrib Mineral Petrol

382

273

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Differentiation of mantle-derived, hydrous, basaltic magmas is a fundamental process to produce evolved intermediate to SiO 2-rich magmas that form the bulk of the middle to shallow continental and island arc crust. This study reports the results of fractional crystallization experiments conducted in a piston cylinder apparatus at 0.7 GPa for hydrous, calc-alkaline to arc tholeiitic magmas. Fractional crystallization was approached by synthesis of starting materials representing the liquid composition of the previous, higher temperature experiment. Temperatures ranged from near-liquidus at 1,170°C to near-solidus conditions at 700°C. H 2 O contents varied from 3.0 to more than 10 wt%. The liquid line of descent covers the entire compositional range from olivine-tholeiite (1,170°C) to high-silica rhyolite (700°C) and evolves from metaluminous to peraluminous compositions. The following crystallization sequence has been established: olivine ? clinopyroxene ? plagioclase, spinel ? orthopyroxene, amphibole, titanomagnetite ? apatite ? quartz, biotite. Anorthite-rich plagioclase and spinel are responsible for a marked increase in SiO 2-content (from 51 to 53 wt%) at 1,040°C. At lower temperatures, fractionation of amphibole, plagioclase and Fe-Ti oxide over a temperature interval of 280°C drives the SiO 2 content continuously from 53 to 78 wt%. Largest crystallization steps were recorded around 1,040°C and at 700°C. About 40 % of ultramafic plutonic rocks have to crystallize to generate basaltic-andesitic liquids, and an additional 40 % of amphibole-gabbroic cumulate to produce granitic melts. Andesitic liquids with a liquidus temperature of 1,010°C only crystallize 50 % over an 280°C wide range to 730°C implying that such liquids form mobile crystal mushes (\50 % crystals) in long-lived magmatic systems in the middle crust, allowing for extensive fractionation, assimilation and hybridization with periodic replenishment of more mafic magmas from deeper magma reservoirs. Keywords Liquid line of descent Á Fractional crystallization Á Calc-alkaline magmas Á Mid-crustal magma reservoirs Communicated by T. L. Grove.

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Accuracy in the determination of the compositions of main rockforming silicates and oxides on a JXA-8100 microanalyzer

Cited by 28 publications

References 3 publications

Double-Front Crystallization in the Chapesvara Ultramafic Subvolcanic Complex, Serpentinite Belt, Kola Peninsula, Russia

Double-Front Crystallization in the Chapesvara Ultramafic Subvolcanic Complex, Serpentinite Belt, Kola Peninsula, Russia

Conditions of formation of Au–Se–Te mineralization in the Gaching ore occurrence (Maletoyvayam ore field), Kamchatka, Russia

Fractional crystallization of primitive, hydrous arc magmas: an experimental study at 0.7 GPa

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