186,567 research outputs found
Trapped minerals under stress
Get PDFResearch Focus on the problem of entrapment pressure estimation for mineral inclusions under residual pressure
Pressure and Temperature Data for Diamonds
Get PDFOne of the key scientific questions about diamonds is “how are they formed?” To answer this question, we need to know the diamond-forming reactions and the physicochemical conditions under which these reactions take place. The pressure (P) and temperature (T) of diamond formation are an essential part of this knowledge and their assessment is pivotal to develop predictive scenarios of diamond distribution in the Earth interior. These scenarios may contribute to our understanding of global Earth processes, such as the long-term carbon cycle, and might also eventually improve our capability to select potential targets for diamond exploration (Shirey et al. 2013; Nimis et al. 2020)
Error sources in single-clinopyroxene thermobarometry and a mantle geotherm for the Novinka kimberlite, Yakutia
Get PDFA new suite of 173 clinopyroxene grains from heavy-mineral concentrates of the diamondiferous Novinka kimberlite (Upper Muna field, Yakutia) has been analyzed for major and minor elements with an electron microprobe to perform a thermobarometric study and model the thermal structure of the Archean Upper Muna lithospheric mantle. Scrupulous evaluation of propagation of analytical uncertainties on pressure estimates revealed that (1) the single-clinopyroxene geobarometer can be very sensitive to analytical uncertainties for particular clinopyroxene compositions, and that (2) most clinopyroxenes from Novinka have compositions that are sensitive to analytical uncertainties, notwithstanding their apparent compositional suitability for single-clinopyroxene thermobarometry based on previously proposed application limits. A test on various mantle clinopyroxenes containing different proportions of the sensitive elements Cr, Na, and Al allowed us to identify clinopyroxene compositions that produce unacceptably high propagated errors and to define appropriate analytical conditions (i.e., higher beam currents and longer counting times for specific elements) that allow precise P-T estimates to be obtained for sensitive compositions. Based on the results of our analytical test, and taking into account the intrinsic limitations of the single-clinopyroxene thermobarometer, we have designed a new protocol for optimum thermobarometry, which uses partly revised compositional filters. The new protocol permits precise computation of the conductive paleogeotherm at Novinka with the single-clinopyroxene thermobarometer of Nimis and Taylor (2000). Thermal modeling of the resulting P-T estimates indicates a ~34 mW/m2 surface heat flow, a thermal lithosphere thickness of ~225 km, and an over 100 km thick “diamond window” beneath Novinka in the middle Paleozoic (344–361 Ma). We estimate that appropriate analytical conditions may extend the applicability of single-clinopyroxene thermobarometry to over 90% of clinopyroxene-bearing garnet peridotites and pyroxenites and to ~70% of chromian-diopside inclusions in diamonds. In all cases, application to clinopyroxenes with Cr/(Cr+Al)mol 4.5 GPa
Records of mantle-crust exchange processes during continental subduction-exhumation in the Nonsberg-Ultental garnet peridotites (Eastern Alps)
No full textIn the Nonsberg-Ultental area (Tonale nappe), Grt-bearing peridotites occur in high-P migmatitic gneisses enclosing relics of
eclogites. The peridotites record the transformation of high-T Spl-lherzolites to coronitic Grt+Spllherzolites to fine-grained Amph±Grt-peridotites. The transformation is preceeded by LILE
enrichment and intrusion of hot melts, and accompanied by deformation, hydration, LILE enrichment and LILE/HFSE fractionation. High-P metamorphism of peridotites and eclogites and early migmatization of the country gneisses were virtually
coeval and possibly isofacial. The peridotites may represent former mantle-wedge material that was subducted and cooled due to incorporation in a crustal slab and then metasomatized by hydrous fluids left after crystallization of leucosomes. Possible directions for further work on this and other sectors of the Variscan belt where broadly similar rock associations have been found are proposed
Propagation of analytical errors in single-clinopyroxene geobarometry and implications on estimated mantle palaeogeotherms
No full textThe genus Solenopsora (Lichenized Ascomycetes, Leprocaulaceae) in Italy
Get PDFThis paper includes a dichotomous key, descriptions and predictive distributional maps for all of the 9 infrageneric taxa of the lichen genus Solenopsora (Leprocaulaceae) known to occur in Italy. The genus includes obligatory saxicolous lichens with the main centre of diversity in the Mediterranean, Macaronesian, and Madrean biogeographical regions. All taxa have their opti-ma below the montane belt. Most of them have a distinctly Thyrrenian-Mediterranen distribution pattern in Italy, being most frequent in areas with a mild, suboceanic climate
Influence of tree species on epiphytic macrolichens in temperate mixed forests of northern Italy
No full textTree species is a key factor in shaping epiphytic lichen communities. In managed forests, tree species composition is mainly controlled by forest management, with important consequences on lichen diversity. The main aim of this work was to evaluate the differences at tree level in macrolichen richness and composition between Abies alba Mill. and Fagus sylvatica L. in a temperate mixed forest in northern Italy, in addition to evaluating two different proportions of the two species at the stand level. Abies alba and F. sylvatica host lichen communities including several rare and sensitive species. Our findings indicate that both tree species were important for lichen diversity, since they hosted different communities. However, F. sylvatica proved to be a more favourable hosting tree for several rare and sensitive species. Species associated with A. alba were mainly acidophytic lichens, while those associated with F. sylvatica were foliose hygrophytic lichens, mainly establishing over bryophytes. The frequency of the flagship species Lobaria pulmonaria (L.) Hoffm. was a valuable predictor of cyanolichen richness and was useful in identifying sites hosting lichen communities that are potentially more sensitive to thinning and human disturbance. The results support the relevance of mixed A. alba – F. sylvatica formations among European habitats worthy of conservation
A revision of the Ni-in-garnet geothermometer with special regard to its pressure dependence
Get PDFWe have explored the effect of temperature (T) and pressure (P) on Ni partitioning between garnet and olivine in well-equilibrated mantle xenoliths from on-craton, marginal-craton and off-craton settings and in high-P–T experiments at natural Ni abundance. Contrary to previous evaluations, the xenolith and experimental data indicate that the P effect is not negligible, consistent with the significant volume change of the garnet–olivine Mg-Ni exchange reaction. The recognition of a P effect satisfactorily resolves the discrepancies observed using previous calibrations of the Ni-in-Grt geothermometer and provides a solution to the long-standing controversy as to which Ni-in-Grt geothermometer is best applied to natural chromian pyrope compositions. A recalibrated, P-dependent Ni-in-garnet geothermometer reproduces the pyroxene T estimates for the xenoliths and the T conditions of the experiments with a standard error of estimate of 44 °C. The P dependence (ca. 40 °C/GPa) is comparable to that of the garnet–olivine Fe–Mg exchange geothermometer. A small tendency to overestimate at T < 900 °C relative to two-pyroxene thermometry is observed, which is unrelated to garnet compositional parameters. A set of simplified, geotherm-referenced calibrations permit traditional use of the Ni-in-garnet geothermometer as a single-mineral method, provided the local geotherm is known or can be estimated or inferred
Freshwater Lichens, Including New Species in the Genera Verrucaria, Placopyrenium and Circinaria, Associated with Lobothallia hydrocharis (Poelt & Nimis) Sohrabi & Nimis from Watercourses of Sardinia
Get PDFThis work summarizes the results of the exploration of freshwater lichen biota on the island of Sardinia associated with the regional flagship species Lobothallia hydrocharis, a large-sized crustose lichen from the splash zone along mountain streams, so far known from Sardinia only. Molecular data were used to confirm its distinctiveness from other taxa and its systematic placement and to identify critical taxa among its associated lichen biota. We found 25 species of lichenized fungi, including three species new to science in the genera Verrucaria, Placopyrenium, and Circinaria, and seven species new to Sardinia (Hydropunctaria rheithrophila, Ionaspis chrysophana, I. odora, Verrucaria aquatilis, V. collematodes, V. pseudovirescens), or new to Southern Europe (V. devensis). Specific traits for the freshwater lichen biota of Sardinia were identified and compared to those reported from freshwater sites in the Alps and Carpathian mountains, e.g., a relative scarcity of subgelatinous lichens. Parasitic or epilichenic interactions were found frequently but only in the splash zone and not in the permanently submerged zone, i.e., two parasitic Placopyrenium species, and clearly lichenicolous thalli of Kuettlingeria atroflava and Lobothallia hydrocharis. Due to its specific trait profile and the great potential for the discovery of new species, we recommend the inclusion of Sardinian and further Mediterranean sites in continental-scale monitoring programs for freshwater lichens
On the typification of the lichen genus Lepra Scop
Get PDFThe first typification of Lepra Scop. by Pertusaria discoidea (Pers.) Malme (= Lepra albescens (Hudson) Hafellner), made in the Paris Code (1956), is shown to be correct after studies of the original material in the Micheli Herbarium in FI. Details of the latter are given. All later statements about this case, even in the Code, are irrelevant
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