The lower part of lithosphere in collisional orogens may delaminate due to density inversion between the asthenosphere and the cold thickened lithospheric mantle.Generally, standard delamination models have neglected density changes within the crust and the lithospheric mantle, which occur due to phase transitions and compositional variations upon changes of P-T parameters.Our attention is focused on effects of phase and density changes that may be very important and even dominant when compared with the effect of a simple change of the thermal mantle structure.The paper presents the results of numerical modeling for eclogitization of basalts of the lower crust as well as phase composition changes and density of underlying peridotite resulted from tectonic thickening of the lithosphere and its foundering into the asthenosphere.As the thickness of the lower crust increases, the mafic granulite (basalt) passes into eclogite, and density inversion occurs at the accepted crust-mantle boundary (P=20 kbar) because the newly formed eclogite is heavier than the underlying peridotite by 6 % (abyssal peridotite, according to [Boyd, 1989]).The density difference is a potential energy for delamination of the eclogitic portion of the crust.According to the model, P=70 kbar and T=1300 °C correspond to conditions at the lower boundary of the lithosphere.Assuming the temperature adiabatic distribution within the asthenosphere, its value at the given parameters ranges from 1350 °C to 1400 °C.Density inversion at dry conditions occurs with the identical lithospheric and asthenospheric compositions at the expense of the temperature difference at 100 °C with the density difference of only 0.0022 %.Differences of two other asthenospheric compositions (primitive mantle, and lherzolite KH) as compared to the lithosphere (abyssal peridotite) are not compensated for by a higher temperature.The asthenospheric density is higher than that of the lithospheric base.Density inversion occurs if one assumes the presence of the asthenosphereic material in the composition similar to that of the primitive mantle or lherzolite KH in amounts no less than 1.40 and 0.83 wt.%, respectively, of the conventionally neutral fluid.This amount of the fluid seems to be overestimated and thus does not fully correlate with the current estimates of the fluid content in the mantle.Therefore, the most appropriate material for delamination of the thickened lithosphere is only the fluid-bearing asthenosphere which composition corresponds to that of the depleted mantle of middle-ocean ridges (DMM) being the reservoir existing from the Precambrian.In our model, abyssal peridotite is most similar to DMM as compared with other more fertile compositions of the lithosphere.Heat advection due to uplift of fluid-bearing plumes that occurred much time after collisional events may initiate repeated delamination of gravitationally instable parts of the orogenic and cratonic lithosphere.
Modified equations of state (EoS) of forsterite, wadsleyite, ringwoodite, akimotoite, bridgmanite and post-perovskite based on the Helmholtz free energy are described using Microsoft Excel spreadsheets. The equations of state were set up by joint analysis of reference experimental data and can be used to calculate thermodynamic and thermoelastic parameters and P–V–T properties of the Mg-silicates. We used Visual Basic for Applications module in Microsoft Excel and presented a simultaneous calculation of full set of thermodynamic and thermoelastic functions using only T–P and T–V data as input parameters. Phase transitions in the MgSiO3–MgO system play an important role in the interpretation of the seismic boundaries of the upper Earth’s mantle and in the D″ layer. Therefore, proposed EoSes of silicates in the MgSiO3–MgO system have clear geophysical implications. The developed software will be interesting to specialists who are engaged to study the mantle mineralogy and Earth’s interior.
The study of the Bol'shaya Tagna alkaline-carbonatite massif and adjacent areas was focused on the mineral and chemical compositions of minerals, the distribution of petrogenic and trace elements in pyroxene-free alkaline picrites in veins and dikes dated at the late Riphean (circa 645 Ma), and comparison with the Bushkanai kimberlite-picrite dike. Phenocrysts in the pyroxene-free picrites are represented by olivine (replaced with serpentine) and phlogopite; the bulk is formed by serpentine, phlogopite, monticellite, calcite, etc .; xenocrysts of pyrope and chrome diopside are absent. Phlogopite and Cr-spinel from the picrites are chemically similar to these minerals in kimberlites, but the evolution of the spinel compositions corresponds to the titanomagnetite trend; monticellite is depleted in forsterite (Mg 2 SiO 4 ). The rocks contain strontianite, burbankite, titanium andradite, calcirtite and Mn-ilmenite, which are not typical of kimberlites, but are inherent in carbonate-bearing ultramafic lamprophyres, ayllikites. The pyroxene-free picrites have low contents (wt %) of SiO 2 (28.4‒33.2), Al 2 O 3 (3.2‒5.6), and Na 2 O (0.01‒0.05); relatively high contents of TiO 2 (2.0‒3.3), and К 2 О (0.45‒1.33); varying contents of MgO (16.1‒24.1), СаО (12.9‒22.8), СО 2 (1.1‒12.2), Ni (260‒850 ppm), and Cr (840‒2200 ppm); and Mg#=0.73‒0.80. The contents of Th, U, Nb, Ta, La, and Ce in the veins are approximately two orders higher than those in the primitive mantle; the spectra of trace elements differ from the spectra of the South African and Yakuian kimberlites. In the pyroxene-free picrites and the rocks of the Bushkanai dike, the Nb/U, Nb/Th, Th/Ce, La/Nb, and Zr/Nb ratios are similar to those in ocean island basalts (OIB) and thus give evidence of the leading contribution of the recycled component into the source melt. In experiments conducted to investigate melting of carbonated garnet lherzolite, the pyroxene-free alkaline picrites melted at 5–6 GPa.
Barium-bearing micas (BaO content from 1.2 to 18.7 wt %) were found in hydrothermally altered ijolites and alkaline syenite of the Sredneziminsky ijolite-syenite-carbonatite massif (Eastern Siberia, Russia). It occurs in the products of low-temperature replacement of cancrinite, in association with natrolite, analcime, calcite, diaspore/boehmite, celsian, and strontianite. Ba-bearing micas are represented by grains up to 1 mm, heterogeneous in chemical composition. The amount of Ba increases in the marginal parts of grains; enrichment of some layers in mica grains with barium is also observed. The main isomorphic substitution in muscovite corresponds to the scheme K++Si4+↔Ba2++IVAl3+. The empirical formula of barium-richest areas in one of the mica grains is (Ba0.54–0.56Sr0–0.09K0.46)∑1.02–1.06Al1.98–2.01(Si2.37–2.40All.60–1.63)∑4.00 O10(OH1.70–2.00F0–0.30)∑2.00, which corresponds to ganterite. The maximum content of BaO in the majority of muscovite grains of the Sredneziminsky massif is 14.0–14.9 wt % that is equal to 0.41‒0.44 apfu Ba. It is assumed that the source of barium in the hydrothermal solution was orthoclase containing 0.5–0.9 wt % BaO which underwent albitization at the post-magmatic stage. The widespread occurrence of sulfides in the rocks indicates low oxygen fugacity, which prevents the formation of barite and is favorable to the formation of Ba-bearing muscovite and celsian.
The isotope composition of carbon and oxygen was studied in calcite of dykes and veins of ultramafic lamprophyres, kimberlite, alkaline mica picrites from the Yarma above-intrusion zone, and pyroxene-free picrites intruding the rocks of the Bolshetagninsky carbonatite massif within the Urik-Iya graben hosted by the East Sayan Mountains. The data on δ 13 C (from −6.6 to −3.9 ‰ relative to VPDB) disclose the ideas on the mantle origin of the carbonate substance of dykes. High values of δ 18 O (from +13.9 to +11.8 ‰ relative to VSMOW) suggest the impact of deuteric fluids, i.e. magmatic fluids separated from melts, at later stage of formation of the calcite-bearing alkaline ultramafic rocks.
In this study, we present the results of U–Pb (ID-TIMS) geochronological studies of calcic garnet from the alkaline ultramafic complexes of Eastern Sayan province (eastern Siberia). New U–Pb ID–TIMS garnet ages obtained from different rocks of Bolshaya Tagna (632 ± 2 Ma) and Srednaya Zima intrusions (624 ± 5 Ma), as well as previously published garnet ages of the Belaya Zima complex (646 ± 6 Ma), allow us to constrain the timing and duration of episodes of alkaline ultramafic magmatism in Eastern Sayan province (619–651 Ma). Variations in the chemical compositions of rocks from three massifs indicate that the parental melts were separated from different magmatic chambers generated during the same episode of mantle melting. This study further highlights garnet U–Pb dating as a potentially robust, high-resolution geochronometer to constrain the evolution of the main pulse of alkaline ultramafic magmatism in the large magmatic provinces.
