Abstract The study discusses the mineralogical, geochemical and thermometric properties of rock-forming blue quartz from eight worldwide occurrences. Compared to non-blue quartz, blue quartz contains significant amounts of submicron-sized (1 μm—100 nm) and nanometre-sized (<100 nm) inclusions. Mica, ilmenite and rutile constitute the most abundant submicron-sized inclusions, and are formed probably by syngenetic precipitation in the boundary layer between quartz and melt (entrapment model). Nanometre-sized rutile possibly originated by epigenetic exsolution of Ti from the quartz structure (exsolution model). Rayleigh scattering of light by nano-particulate inclusions best explains the origin of the blue colour. Blue quartz is generally Ti-rich (∼100—300 ppm) and formed at high temperatures (∼700°C—900°C). The large number, and high spatial density, of tiny xenocrystic inclusions of Ti-bearing minerals make calculations of crystallization temperatures using the Ti-in-quartz thermometer unreliable. The geological significance of blue quartz remains obscure.
La giraudite et la hakite, ainsi que leurs analogues a dominance de soufre plus repandus, la tennantite et la tetraedrite, sont localement abondants, mais de facon generale, ce sont des especes rares au gisement d'uranium de Niederschlema-Alberoda, dans la region de l'Erzgebirge, en Allemagne. Les solutions solides continues entre giraudite et hakite mercuriennes, giraudite et tennantite, et entre tennantite riche en Se zincifere et ferreuse et la tetraedrite zincifere font partie de la mineralisation selenifere jurassique. Une grande variabilite en composition a l'echelle d'une section polie est observee dans le cas de la serie girauditetennantite, comme l'exprime la formule structurale suivante, calculee sur une base de 29 atomes par unite formulaire: (Cu 9 . 8 6 - 1 0 . 0 0 Ag 0 . 0 0 - 0 . 1 4 ) Σ 1 0 (Cu 2 + 0 . 1 8 - 1 . 8 8 Fe 0 . 0 9 - 1 . 7 7 Zn 0 . 0 0 - 1 . 2 6 Hg 0 . 0 0 - 0 . 1 0 Cd 0 . 0 0 - 0 . 0 6 ) Σ 1 . 9 5 - 2 . 1 6 (As 2 . 2 3 - 4 . 0 5 Sb 0 . 0 0 - 1 . 6 8 Te 0 . 0 0 - 0 . 1 4 Bi 0 . 0 0 - 0 . 0 1 ) Σ 3 . 8 5 - 4 . 0 7 (Se 0 . 0 0 - 1 2 . 6 4 S 0 . 3 2 - 1 2 . 9 7 ) Σ 1 2 . 9 0 - 1 3 . 0 9 (n = 58). Les solutions solides vont de gir 0 tn 1 0 0 a gir 9 8 tn 2 avec seulement deux lacunes mineures. ce qui nous mene a proposer une miscibilite complete entre giraudite et tennantite dans la nature. La giraudite peut contenir soit Hg, Cu, Zn, ou Fe comme cation bivalent predominant. La tennantite cretacee, deposee avec les mineraux de Bi-Co-Ni-Ag, accuse un deficit en Se, contient de quantites mineures de Co et de Ni, est enrichie en Ag, et pourrait contenir des quantites importantes de Bi. La tennantite bismuthifere et zincifere, produit de remplacement de la wittichenite, contient jusqu'a 12.6% de Bi (en poids, l'equivalent de 0.97 atomes par unite formulaire). Nous evaluons l'origine des associations variees des associations de tennantite et de tetraedrite en fonction de la sequence temporelle et de l'activite du selenium et du soufre au cours de leur formation. Les sulfosels jurassiques porteurs de Se, voire riches en Se, seraient des especes tardives, deposees a faible temperature a partir de fluides hydrothermaux ayant une activite en selenium plusieurs ordres de grandeur plus faible que celle qui a caracterise la cristallisation de l'umangite et de la klockmannite, phases qui ont marque le maximum en f (Se 2 ) atteint a NiederschlemaAlberoda.
Abstract Thermal conductivity (λ) is an essential physical property of minerals and rocks and fundamental in constraining the thermal field of the lithosphere. In case that adequate samples to measure λ are not available, it could be indirectly inferred from calculation. One of the most widely applied indirect methods for rocks involve modal mineralogy and porosity as parameters that are incorporated into mathematical mean or mixing models. Robust inferences from these approaches for crystalline rocks were impeded by a small number of studied samples or restriction to certain rock types. We employ this method and examine its applicability to low‐porosity plutonic rocks by calculating bulk thermal conductivity λ b for 45 samples covering the entire range from gabbro/diorite to granite. We show that the use of the harmonic‐mean model for both rock matrix and porosity provided a good match between λ b.meas and λ b.calc of <10% deviation (2σ), with relative and absolute errors amounting to 1.4 ± 9.7% and 4.4 ± 4.9%, respectively. The results of our study constitute a big step forward to a robust conclusion on the overall applicability of the harmonic‐mean model for inferring λ b of isotropic, low‐porosity, mafic to silicic plutonic and metamorphic rocks with an acceptable magnitude of error. Drill cuttings and enclaves form particularly interesting objects for application of this method, as they are poorly suited for direct λ measurement. Well‐derived λ values for those rocks would permit to calculate heat flow and to model more profoundly the thermal state of the deeper lithosphere.
A comprehensive study of monazite–cheralite–huttonite solid solutions (s.s.) and xenotime from the highly evolved, strongly peraluminous P–F–Li-rich Podlesí granite stock in the Krušné Hory Mts., Czech Republic, indicates that, with the increasing degree of magmatic and high-T early post-magmatic evolution, the content of the cheralite component in monazite increases and the relative dominance of middle rare earth elements (MREE) in xenotime becomes larger. Considering the overall compositional signatures of these two accessory minerals in the late Variscan granites of the Erzgebirge/Krušné Hory Mts., three types of granites can be distinguished: (i) chemically less evolved F-poor S(I)- and A-type granites contain monazite with a smooth, mostly symmetric chondrite-normalized (CN) rare-earth elements (REE) pattern gradually declining from La to Gd; associated xenotime is Y-rich (˃0.8 apfu Y) with a flat MREE–HREE (heavy rare earth elements) pattern; (ii) fractionated A-type granites typically contain La-depleted monazite with Th accommodated as the huttonite component, combined with usually Y-poor (0.4–0.6 apfu Y) xenotime characterized by a smoothly inclining, Yb–Lu-dominant CN-REE pattern; (iii) fractionated peraluminous Li-mica granites host monazite with a flat, asymmetric (kinked at La and Nd) CN-LREE pattern, with associated xenotime distinctly MREE (Gd–Tb–Dy)-dominant. Monazite and xenotime account for the bulk of the REE budgets in all types of granite. In peraluminous S(I)-type granites, which do not bear thorite, almost all Th is accommodated in monazite–cheralite s.s. In contrast, Th budgets in A-type granites are accounted for by monazite–huttonite s.s. together with thorite. The largest portion of U is accommodated in uraninite, if present.
Abstract Composite populations of monazite-group minerals of both metamorphic and metasomatic origin have been discovered in thin layers of granulite-facies metabasites interlayered with metapelites, located in the Val Strona di Omegna region of the Ivrea-Verbano Zone, Italy. In addition to monazite-(Ce), which is uncommonly poor in Th and is probably formed by incongruent dissolution of apatite, these populations include members of the monazite-huttonite series. The latter minerals contain between 13 and 30.1 mol.% ThSiO 4 [= huttonitic monazite-(Ce)], and are known from only half a dozen other occurrences worldwide. We propose that breakdown of primary monazite-(Ce) in the metapelites during granulite-facies metamorphism mobilized Th and the REEs , which were then transported by high-grade metamorphic fluids into the metabasite layers to form the Th-rich minerals of the monazite-huttonite series.
Among the minerals of the selenide assemblage at the Niederschlema–Alberoda uranium deposit, Erzgebirge, Germany, members of the mercurian giraudite–hakite solid solution intergrown with berzelianite and galena have been identified as rare and previously unknown phases. They form complexly zoned, anhedral, minute (<350 m) grains embedded in a dolomite matrix. Compositional variability is expressed by the following crystallochemical formula (calculated on the basis of 29 atoms per formula unit): (Cu9.92–9.99Ag0.01–0.08)10.00 (Hg0.92–1.81Cu0.06–1.12Zn0.05–0.10Fe0.00–0.15)1.98–2.06 (As0.69–3.98Sb0.02–3.29)3.91–4.08 (Se10.47– 11.53S1.47–2.61)12.90–13.09. The solid solutions span the range from gir99.5hak0.5 to gir16.2hak83.8, suggesting complete miscibility between mercurian giraudite and mercurian hakite in nature, equivalent to what already has been established for their S-bearing analogues, tennantite and tetrahedrite. The lack of thermodynamic data for both Se-rich species limits reliable inferences on the P–T–X conditions that prevailed during their formation. The assemblage mercurian giraudite–hakite + berzelianite + galena may represent a short-term equilibrium paragenesis of Jurassic age, formed at temperatures between 110 and 150°C under the conditions of low Se and S activities (i.e., –26 < logf(Se2) < –31 and –24 < logf(S2) < –28 at ~110°C), before the bulk of the selenide minerals crystallized. A second, less likely hypothesis calls upon the formation of the mercurian giraudite–hakite solid solutions during an early Cretaceous event, when pre-existing selenide minerals (berzelianite) were partially attacked by infiltrating fluids that introduced the major portion of the As and Sb into the system.
Low-temperature thermal events of Permian (ca. 265 Ma) and Triassic (ca. 215 Ma) age that predate medium-grade regional metamorphism were identified using high spatial resolution field emission–scanning electron microscopy–energy dispersive X-ray (FE-SEM-EDX) U-Th-Pb dating of uraninite microcrystals in basement rocks of the Tauern Window, Eastern Alps. Three novel points of generic geochronological importance are raised in this study. First, uraninite can be meaningfully dated with FE-SEM-EDX methods, with moderate precision. Second, uraninite is geochronologically robust, even at microcrystal scale, and can survive at least medium-grade metamorphic overprint without being reset. Third, uraninite microcrystals are powerful tools for identifying and dating discrete low-temperature thermal events in orogenic belts. Dating of uraninite microcrystals should be considered an important complementary geochronological method in the study of polymetamorphic rocks.
