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Vol 31, No 5 (2023)
Articles
Variations in the Composition of MORB Chilled Glasses from the Mid-Atlantic Ridge, 12°–31° N: Reflection of Composition Evolution of Parent Melts and the Influence of the Hydrothermal Component
Abstract
The geochemical peculiarities of sample collection of MORB chilled glasses obtained in six areas of the axial zone of the Mid-Atlantic Ridge (MAR), 12°–31° N have been studied. The results of this study provided information on the composition of the parent melts for these glasses and made it possible to assess probable geochemical effects reflecting the interaction of magmatic melts with hydrothermal systems of the MAR axial zone or with altered oceanic crust (AOC). It is shown that basalts of the E-MORB family which includes most of samples are localized mainly in the “cold” segments of the MAR in the crustal section of which serpentinites predominate. On the other hand, samples with depleted signature (N-MORB) belong to segments of the MAR in which serpentinite outcrops are either absent or play a subordinate role. The E-MORB chilled glasses from “cold” segments of the MAR show signs of contamination of basaltic melts with components borrowed either from the host serpentinites or from aqueous-saline fluids circulating in hydrothermal systems located in serpentinites (“serpentinite hosted”). Judging by the data obtained on the nature of variations in the content of Cl, U, and Sr in the studied chilled glasses belonging to the N-MORB family, there are no signs of intracrustal contamination. It is assumed that relics of the ancient continental lithosphere preserved under axial zone of the MAR and involved in the process of partial melting of the shallow mantle took part in the formation of E-MORB parental melts in some segments of the MAR.
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Lower Talnakh Type Intrusions of the Norilsk Ore Region
Abstract
Troctolites, olivine and picrite gabbrodolerites account for up to 75% of the Lower Talnakh type intrusions in places of their increased thickness whereas reduced thickness sections consist of olivine-free and olivine-bearing gabbrodolerites. Differentiation is not obvious within these high-Mg cumulates, although the content of TiO2 and alkalis increases towards the upper endocontacts. The transitions between the rock types are gradational, and the composition of low Ni olivine in different rocks (Fo70–83, 0.01–0.2 wt % NiO) overlap significantly. Clinopyroxene (Fs7–13, Mg# 68–89) is characterized by both the lowest contents and variation ranges of Cr2O3 (0.01–0.5 wt %) and TiO2 (0.05–1.0 wt %) among all types of the intrusions of the Norilsk complex that is consistent with the Cr-depleted (0.002–0.051 wt % Cr2O3) bulk rock compositions. Later orthopyroxene (Fs15–30) is crystallised by the reaction of the residual melt with early olivine. Plagioclase forms porphyritic phenocrysts and their intergrowths along with ophitic laths as well as dominates in schlieren and fragments of leucocratic rocks in taxitic and picritic gabbrodolerites with a poorly sorted layered texture. In olivine-rich rocks, sulfides are represented by the association of troilite ± hexagonal pyrrhotite + Fe- and Co-rich pentlandite + Fe-enriched chalcopyrite (± putoranite, talnakhite) ± cubanite. In the upper and lower parts of the intrusions, the association of hexagonal pyrrhotite + chalcopyrite + pentlandite occurs, while monoclinic pyrrhotite + chalcopyrite + Ni-enriched pentlandite are formed in the endo- and exocontacts. The concentration of base (0.077–0.21 wt % Ni, 0.05–0.38 wt % Cu) and platinum metals (0.03–0.26 to 0.40 g/t total PGE) in mineralized rocks is very low. Upon small amounts of sulfides and extremely low base and platinum metal tenors, the heterogeneous S isotopic composition of Lower Talnakh type sulfides (mainly 3.8–8.6‰, but up to 11.8%) most likely reflects the achievement of repeated sulfide saturation during the assimilation of sulfate S by magma that has previously experienced loss of chalcophile metals into a coexisting sulfide fluid at depth. The Sr-Nd isotopic compositions of the Lower Talnakh intrusions (Sri – from 0.7073 to 0.7087 and εNd(Т) from –1.8 at –5.9 recalculated to 250 Ma) show the predominant contamination with Proterozoic material, in contrast to the ore-bearing intrusions, which Sr-Nd isotope compositions indicate contamination with upper crustal sedimentary matter of the Paleozoic age.
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Early Paleozoic Tectonothermal Evolution of the Khan-Khukhay Metamorphic Complex (Northern Mongolia)
Abstract
A reconstruction of metamorphic stages of Moren and Erzin metamorphic Complexes in the Khan-Khuhay block of Nothern Mongolia presented in the paper. Based on it a general geodynamic history of Sangilen terrain of Tuva-Mongolia Massif discussed. Reconstructed “clock-wise” P-T trend demonstrated two stage of metamorphism, the first of which is due to collision metamorphism with P-T parameters of 9 kbar, 740°C, the second one is due to regional magmatic episode with P-T parameters of 6-7 kbar, 860–880°C. Geochemical and petrological peculiarities of rocks in the Khan-Khuhay block demonstrates resemblance with metamorphic block of Western Sangilen in South-East Tuva. In the first time quartz monzodiorite intrusive bodies founded within Khan-Khuhay block. Their geochemical peculiarities are similar to Ordovician gabbro-monzodiorite intrusives of Western Sangilen. Quartz monzodiorite intrusives of Khan-Khuhay are likely a small apophysis of a deep-crust intermediate magma chamber and can present probable thermal source of metamorphic stage M2 in the Khan-Khuhay block. Numerical thermo-mechanical model of metamorphism in the Khan-Khuhay block explain heating in the thickening crust during collision due to higher radiogenic heat source of 1.52 μW/m3. Using computer modeling was show that the P-T parameters of second metamorphic stage can realize only by means of magmatic heat from monzodiorite intrusion. Reconstruction of P-T parameters using mineral thermobarometry, similarity of geochemical characteristics, as well as the results of thermomechanical modeling allow us to conclude that the Erzin and Moren complexes have undergone a joint tectono-metamorphic evolution.
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Stages and Conditions of Formation of Carbonate-Silicate Veins and Near-Vein Aureoles in the Early Proterozoic Complexes of the Belomorian Mobile Belt, North Karelia
Abstract
On the islands and the coast of the White Sea in North Karelia, the bodies of Early Proterozoic metamorphosed gabbroids are widely distributed in Archean gneiss. Carbonate-silicate veins with sulfide mineralization are confined to these bodies of metabasites, as well as to their contacts with gneiss. The main vein minerals are plagioclase, quartz, carbonates and chlorite. The stages of vein formation correspond to the transition from early quartz-plagioclase to late quartz-carbonate associations with chlorite and sulfides. The early (high-temperature) stage is fixed by the amphibolite aureoles around the vein with temperature estimates of about 550–650°C by the TWQ method. This stage corresponds to the quartz-plagioclase association of the marginal zones of the veins. The transition to the late stage with the formation of veined quartz-carbonate associations (± biotite) occurred at temperatures of 540°C and lower, judging by the calcite-dolomite associations. Further development of quartz–chlorite–carbonate and sulfide associations in veins and wall amphibolites corresponds to a decrease in temperature to 350°C and below, judging by chlorite thermometers. Vein formation and near-vein amphibolitization are presumably associated with the impact of metamorphic fluids at the latest retrograde stage of metamorphism in the Early Proterozoic.
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Updated COMAGMAT-5: Modeling the Effects of Sulfide Precipitation in Parallel to Crystallization Of Alumino-Chromian Spinel
Abstract
An updated version of the COMAGMAT-5.3 program is presented, which enables simulations of the silicate-sulfide immiscibility in parallel to crystallization of Al-Cr spinel and other rock-forming minerals. Main changes include a completed recalibration of the previous Fe-Ni sulfide solubility model (Ariskin et al., 2013) and incorporation of equations describing spinel-melt equilibria in a wide range of magmatic systems (Nikolaev et al., 2018а, 2018b). This allowed us to specify more accurately the link between compositions of immiscible sulfides and magma crystallization temperatures, as well as to correct partitioning of alumina between the model spinel and crystallizing melt. The updated COMAGMAT-5.3 can be used for calculations of the crystallization of basaltic to komatiitic magmas, as well as the history of solidification of mafic to ultramafic cumulates, including relative proportions of Al-Cr spinel and immiscible sulfides. Application example includes solidification of sulfide-bearing primitive olivine cumulate from the endocontact of the Yoko-Dovyren intrusion in Northern Transbaikalia (Russia). It is established that maximum crystallization proportions of Al-Cr spinel as much as 3.5 wt % are observed at Ol-Spl cotectic, following an abrupt decrease to slightly negative values during crystallization of plagioclase-bearing assemblages. This results in the inflection point on the trend of evolution of the spinel compositions, which changes from descending the Cr/Al ratio in the field of olivine to its increase when plagioclase starts to crystallize. As compared to previous version COMAGMAT-5.2, the updated model predicts somewhat higher proportions of precipitated sulfides.
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Microprobe Analysis of Titanium in Zircon: an Estimation of Secondary Fluorescence
Abstract
Microprobe study of the effects of secondary fluorescence from high-titania glass (TiO2 ≈ 16 wt %) on apparent titanium content in Ti-free zircon was conducted. It was demonstrated that small crystals (<10 µm) are not suitable for analysis. In boundary parts of large zircon crystals (20–30 µm) the error in Ti estimation may reach 10–23%. However, in central parts of large crystals (> 30 µm) the microprobe analysis gives practically undistorted titanium contents.
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