Epigenetic aragonite in a sheared lherzolite xenolith from the Udachnaya kimberlite pipe

Author(s):  
Konstantin Solovev ◽  
Alexander Golovin ◽  
Igor Sharygin ◽  
Dmitriy Rezvukhin ◽  
Alexey Tarasov

<p>Here we report the first finding of the high-pressure polymorph of calcium carbonate (aragonite) in the interstitial space of a sheared lherzolite xenolith from kimberlites of the Udachanaya diamond deposit (Siberian craton, Russia). Xenoliths with a sheared texture are the deepest mantle rocks sampled by kimberlite magma from 180-230 km depth. According to experimental data, aragonite is the high-pressure polymorph of calcium carbonate, which is stable at upper mantle pressure and temperature. Thereby aragonite is used as a reliable geobarometer in studies of magmatic and ultrahigh-pressure metamorphic rocks.<br>Aragonite was determined by Raman spectroscopy study. The Raman bands at 208 cm<sup>-1</sup>, 702-706 cm<sup>-1</sup> and 1462 cm<sup>-1</sup> are the identification features of aragonite. Chemical analyses of aragonite were obtained by scanning electron microscope with an energy dispersive system. Some analyses were verified by electron microprobe as well. The concentration of SrO in aragonite ranges from 0.5 to 8.8 wt.%. Aragonite has a Na<sub>2</sub>O concentration of 0.1-1.1 wt.%.<br>Aragonite (up to 100 µm) is the most common subordinate mineral from the interstitial space of this xenolith. It occupies on average 70 vol.% of the interstitial space. Aragonite grains consist of three chemically distinct zones. The first zone (core) is characterized by a low content of SrO (<1.5 wt.%) and low Mg# (~15). The second zone has roughly the same SrO but noticeably higher Mg# (~50). The third zone (rim) contains much higher concentration of SrO (up to 8.81 wt.%) and high Mg# (~50).<br>Sheared peridotite are located in the lithospheric mantle significantly below the aragonite-calcite equilibrium line. In particular, the investigated peridotite equilibrated at 1350°С and 69 kbar (~215 km). The presence of zoned aragonite from this peridotite means that this rock has been infiltrated by metasomatic agent. Numerical calculations reveals that such zoning can be preserved for 1 year at 1300°С (~equilibrium temperature of sheared peridotites) and for 10 years at 1000°С (~temperature of kimberlite magma at subsurface conditions). The short preservation time of zoning in aragonite (1-10 years) proves that aragonite could be formed immediately prior to kimberlite magmatism or after the capturing of the xenolith by kimberlite magma. Using adiabats of kimberlite magma and P-T parameters of aragonite stability in the upper mantle, aragonite in the studied sample was formed at the depth range of 80-215 km.<br>As the preservation time of zoning in aragonite is noticeably short (taking into account high temperatures), the best candidate for the role of an agent, which infiltrated the xenolith, is a primitive kimberlite melt of the Udachnaya pipe. The high percentage (70%) of aragonite in the interstitial space of the studied sheared lherzolite xenolith proves that such primitive kimberlite melt had carbonatitic composition. Our results show that not only different silicate-rich melts, but also carbonate or cabonated silicate melts might play a key role in mantle modifications. Carbonate melts are very suitable diamond-forming media and may support the idea of a genetic link between some diamonds and kimberlite magmatism.<br>This study was supported by the Russian Science Foundation (grant No 18-77-10062).</p>

The observed density distribution of the lower mantle is compared with density measurements of the (M g,Fe)SiO 3 perovskite and (Mg,Fe)O magnesiowtistite highpressure phases as functions of pressure, tem perature and composition. We find that for plausible bounds on the composition of the upper mantle (ratio of magnesium to iron + magnesium components x M g ^ 0.88) and the temperature in the lower mantle ( T ^ 2000 K), the high-pressure mineral assemblage of upper-mantle composition is at least 2 .6 ( ± 1 ) % less dense than the lower m antle over the depth range 1000-2000 km. Thus, we find that a model of uniform m antle composition is incompatible with the existing mineralogical and geophysical data. Instead, we expect that the mantle is stratified, with the upper and lower m antle convecting separately, and we estimate that the compositional density difference between these regions is about 5 ( + 2) %. The stratification may not be perfect (‘leaky layering’), but significant intermixing and homogenization of the upper and lower m antle over geological timescales are precluded.


Mineralogy ◽  
2020 ◽  
pp. 39-48
Author(s):  
Meng Dawei ◽  
Wu Xiuling ◽  
Han Yujing ◽  
Liu Rong

2019 ◽  
Vol 159 ◽  
pp. 428-431 ◽  
Author(s):  
Peifang Li ◽  
Tingting Mei ◽  
Zhiwen Lu ◽  
Lian Xiang ◽  
Xin Zhang ◽  
...  

Solid Earth ◽  
2016 ◽  
Vol 7 (2) ◽  
pp. 425-439 ◽  
Author(s):  
Hanna Silvennoinen ◽  
Elena Kozlovskaya ◽  
Eduard Kissling

Abstract. The POLENET/LAPNET (Polar Earth Observing Network) broadband seismic network was deployed in northern Fennoscandia (Finland, Sweden, Norway, and Russia) during the third International Polar Year 2007–2009. The array consisted of roughly 60 seismic stations. In our study, we estimate the 3-D architecture of the upper mantle beneath the northern Fennoscandian Shield using high-resolution teleseismic P wave tomography. The P wave tomography method can complement previous studies in the area by efficiently mapping lateral velocity variations in the mantle. For this purpose 111 clearly recorded teleseismic events were selected and the data from the stations hand-picked and analysed. Our study reveals a highly heterogeneous lithospheric mantle beneath the northern Fennoscandian Shield though without any large high P wave velocity area that may indicate the presence of thick depleted lithospheric “keel”. The most significant feature seen in the velocity model is a large elongated negative velocity anomaly (up to −3.5 %) in depth range 100–150 km in the central part of our study area that can be followed down to a depth of 200 km in some local areas. This low-velocity area separates three high-velocity regions corresponding to the cratonic units forming the area.


2016 ◽  
Vol 2 (1) ◽  
Author(s):  
Annette Bussmann-Holder ◽  
Jürgen Köhler ◽  
M.-H. Whangbo ◽  
Antonio Bianconi ◽  
Arndt Simon

AbstractThe recent report of superconductivity under high pressure at the record transition temperature of Tc =203 K in pressurized H2S has been identified as conventional in view of the observation of an isotope effect upon deuteration. Here it is demonstrated that conventional theories of superconductivity in the sense of BCS or Eliashberg formalisms cannot account for the pressure dependence of the isotope coefficient. The only way out of the dilemma is a multi-band approach of superconductivity where already small interband coupling suffices to achieve the high values of Tc together with the anomalous pressure dependent isotope coefficient. In addition, it is shown that anharmonicity of the hydrogen bonds vanishes under pressure whereas anharmonic phonon modes related to sulfur are still active.


2000 ◽  
Vol 137 (3) ◽  
pp. 235-255 ◽  
Author(s):  
M. KRABBENDAM ◽  
A. WAIN ◽  
T. B. ANDERSEN

The Western Gneiss Region of Norway is a continental terrane that experienced Caledonian high-pressure and ultrahigh-pressure metamorphism. Most rocks in this terrane show either peak-Caledonian eclogite-facies assemblages or are highly strained and equilibrated under late-Caledonian amphibolite-facies conditions. However, three kilometre-size rock bodies (Flatraket, Ulvesund and Kråkenes) in Outer Nordfjord preserve Pre-Caledonian igneous and granulite-facies assemblages and structures. Where these assemblages are preserved, the rocks are consistently unaffected by Caledonian deformation. The three bodies experienced high-pressure conditions (20–23 kbar) but show only very localized (about 5%) eclogitization in felsic and mafic rocks, commonly related to shear zones. The preservation of Pre-Caledonian felsic and mafic igneous and granulite-facies assemblages in these bodies, therefore, indicates widespread (∼ 95%) metastability at pressures higher than other metastable domains in Norway. Late-Caledonian amphibolite-facies retrogression was limited. The degree of reaction is related to the protolith composition and the interaction of fluid and deformation during the orogenic cycle, whereby metastability is associated with a lack of deformation and lack of fluids, either as a catalyst or as a component in hydration reactions. The three bodies appear to have been far less reactive than the external gneisses in this region, even though they followed a similar pressure–temperature evolution. The extent of metastable behaviour has implications for the protolith of the Western Gneiss Region, for the density evolution of high-pressure terranes and hence for the geodynamic evolution of mountain belts.


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