Abstract
The paper provides details of local geology and mineralogy of the Tazheran Massif, which was the sampling site of perovskite used as an external standard in perovskite U-Pb dating by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively-coupled plasma (LA–ICP–MS) mass spectrometry. The Tazheran Massif is a complex of igneous (mafic dikes, syenite, nepheline syenite), metamorphic (marble), and metasomatic (skarn, calc–silicate veins) rocks. Metasomatites are thin and restricted to the complex interior being absent from the margins. Perovskite has been studied at four sites of metasomatic rocks of three different types: forsterite–spinel calc–silicate veins in brucite marble (1); skarn at contacts between nepheline syenite and brucite marble (2), and skarn-related forsterite–spinel (Fo-Spl) calc–silicate veins (3). Pervoskite from Fo-Spl calc–silicate veins (type 1) is almost free from impurities (<1 wt.% in total: 0.06%–0.4% REE2O3, 0.10%–0.22% Nb2O5, ≤0.1% ThO2). The U contents are from 0.1 to 1.9 wt.% UO2 and are relatively uniform in perovskites from the same vein but differ from vein to vein of this type. Perovskite from the contact skarn (type 2) contains 1.5 to 4.5 wt.% REE2O3 but is poor in other impurities. Perovskite grains from skarn-related Fo-Spl calc–silicate rocks (type 3) belong to two generations that differ in REE, Nb, Th, Fe, and Na concentrations. Early-generation perovskites occurs as compositionally homogeneous octahedral or cubic-octahedral crystals with contents of impurities higher than in other varieties (3.6 wt.% REE2O3, 1.6 wt.% Fe2O3, 1.3 wt.% Nb2O5, 0.7 wt.% ThO2, 0.6 wt.% UO2, and 0.6 wt.% Na2O) but the lowest is at the respective site. Late-generation varieties show highly variable impurity concentrations of 1.5 to 22.7 wt.% REE2O3, 0.4 to 8.4 wt.% Nb2O5, and 0.8 to 4.5% ThO2, while the perovskite component may be as low as 65%. In addition to the lueshite and loparite, components, they contain REEFeO3 and Th0.5TiO3 endmembers which have no natural analogs.
Highlights
Perovskite is a widespread phase in SiO2 -undersaturated magmatic rocks, and is a frequent mineral in metasomatic rocks at contacts with alkaline intrusions (e.g., [1,2])
The recommended 463 Ma age of the Tazheran perovskite was determined from 206 Pb/238 U by the ID-TIMS method in the early 1980s [11], and the mineral was used many times as a standard for dating kimberlites from different areas worldwide, by sensitive high-resolution ion microprobe (SHRIMP), as well as by laser ablation inductively-coupled plasma mass spectrometry (LA–ICP–MS), and the classic TIMS method [3,5,12,13,14]
All cited publications lack geological and mineralogical details of the Tazheran deposit, which were reported in the book by Konev and Samoilov [15] published only in Russian, while the crystals used as reference material were sampled during international geological field trips
Summary
Perovskite is a widespread phase in SiO2 -undersaturated magmatic rocks (alkaline and Ne–syenite, kimberlite, lamprophyre), and is a frequent mineral in metasomatic rocks at contacts with alkaline intrusions (e.g., [1,2]). South African kimberlites by the classical U-Pb mass spectrometry, while Ireland et al [10] were pioneers in ion-ion mass spectrometry (SHRIMP) with reference to high-U perovskite from the “Tazheran skarn deposit in the Lake Baikal area of eastern Siberia” [3,9]. The recommended 463 Ma age of the Tazheran perovskite was determined from 206 Pb/238 U by the ID-TIMS method in the early 1980s [11], and the mineral was used many times as a standard for dating kimberlites from different areas worldwide, by SHRIMP, as well as by laser ablation inductively-coupled plasma mass spectrometry (LA–ICP–MS), and the classic TIMS method [3,5,12,13,14]. This paper provides a synopsis of local geology and mineralogy of the Tazheran Massif, as well as mineral chemistry of perovskite
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