1,721,074 research outputs found
Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer
No full textRutile is an important carrier of high field strength elements (HFSE; Zr, Nb, Mo, Sn, Sb, Hf, Ta, W). Its Zr content is buffered in systems with quartz and zircon as coexisting phases. The effects of temperature ( T) and pressure ( P) on the Zr content in rutile have been empirically calibrated in this study by analysing rutile - quartz - zircon assemblages of 31 metamorphic rocks spanning a T range from 430 to 1,100 degreesC. Electron microprobe measurements show that Zr concentrations in rutile vary from 30 to 8,400 ppm across this temperature interval, correlating closely with metamorphic grade. The following thermometer has been formulated based on the maximum Zr contents of rutile included in garnet and pyroxene: T (in degreesC) = 127.8 x ln (Zr in ppm) - 10 No pressure dependence was observed. An uncertainty in absolute T of +/- 50 degreesC is inherited from T estimates of the natural samples used. A close approach to equilibrium of Zr distribution between zircon and rutile is suggested based on the high degree of reproducability of Zr contents in rutiles from different rock types from the same locality. At a given locality, the calculated range in T is mostly +/- 10 degreesC, indicating the geological and analytical precision of the rutile thermometer. Possible applications of this new geothermometer are discussed covering the fields of ultrahigh temperature (UHT) granulites, sedimentary provenance studies and metamorphic field gradients
Rutile geochemistry and its potential use in quantitative provenance studies
No full textRutile is among the most stable detrital minerals in sedimentary systems. Information contained in rutile is therefore of prime importance, especially in the study of mature sediments, where most diagnostic minerals are no longer stable. In contrast to zircon, rutile provides information about the last metamorphic cycle as rutile is not stable at greenschist facies conditions. Several known geochemical characteristics of rutile can be used to retrace provenance. The lithology of source rocks can be determined using Nb and Cr contents in rutile, because the most important source rocks for rutile, metapelites and metabasites, imprint a distinct Nb and Cr signature in rutiles. Since Zr in rutile, coexisting with zircon and quartz, is extremely temperature dependent, this relationship can be used as a geothermometer. Metapelites always contain zircon and quartz, thus the Nb and Cr signatures of metapelites indicate rutiles that can be used for thermometry. The result is effectively a single-mineral geothermometer, which is to our knowledge the first of its kind in provenance studies. Several other trace elements are variably enriched in rutile, but the processes creating these variations are so far not understood. In a case study, Al, Si, V, Cr, Fe, Zr, Nb and W contents in rutiles were obtained by electron microprobe from three sediment samples from Upstate New York. A Pleistocene glacial sand, whose source was granulite-facies rocks of the southern Adirondacks, has detrital rutile geochemical signatures which are consistent with the local Geology; a predominantly metapelitic source with a minor metabasitic contribution. Calculated temperatures for the metapelitic rutiles from the glacial sand are consistent with a predominantly granulite-facies source. The two other samples are from Paleozoic elastic wedges deposited in the foreland of the Taconian and Acadian orogenies. Here several geochemical patterns of detrital rutiles are comparable to rutiles derived from the Adirondacks, implying that rutiles eroded from the Taconian and Acadian orogens were originally derived from similar high grade gneiss terranes, like those found in the Adirondacks. The preferred tectonic scenario calls for an accretionary wedge where eroded Grenville province sediments accumulated, which were later recycled during the Taconian and Acadian orogenies. (C) 2004 Elsevier B.V All rights reserved
Cold subduction of oceanic crust: Implications from a lawsonite eclogite from the Dominican Republic
No full textLawsonite eclogite is a rare rock type that has been described from only five natural occurrences. In contrast, laboratory experiments and thermal models predict that lawsonite eclogite should be widespread in subducted oceanic crust deeper than 1.5 GPa. Here we report a new lawsonite eclogite find from the Dominican Republic that provides constraints on the conditions of subducted crust and on its return to the surface. In this sample, lawsonite coexisting with omphacite occurs as both inclusions in garnet and as porphyroblasts, the latter being partly replaced at their margins by epidote and zoisite. Peak pressure conditions estimated from lawsonite-phengite-omphacite-garnet assemblages were ca 1.6 GPa at a temperature of 360degreesC, implying formation under a geotherm of ca. 8degreesC/km. Peak temperature conditions of 410-450degreesC were in the zoisite eclogite field, suggesting that the sample crossed from the stability field of lawsonite eclogite into that of zoisite eclogite as a result of increasing temperature. A comparison with other reported occurrences indicates that most lawsonite eclogite exhumed at the Earth's surface formed in accretionary wedges. The rarity of lawsonite eclogite at the Earth's surface may be principally due to two factors: (i) that in 'normal' subduction settings lawsonite eclogite enters the subduction factory and hence is usually not exhumed; and (ii) that in accretionary wedge settings, where the PT path leaves the stability field of lawsonite eclogite due to heating, lawsonite eclogite is only preserved if the exhumation path is constrained to a narrow window where the terminal stability of lawsonite is not crossed
Cohenite (iron carbide) and native iron formation within garnet included in polycrystalline diamonds by redox freezing in the cratonic lithosphere
No full textCohenite, native iron and troilite inclusions in garnets from polycrystalline diamond aggregates
No full textSyngenetic garnet of eclogitic/pyroxenitic composition included in a polycrystalline diamond aggregate from the Venetia kimberlite, Limpopo Belt, South Africa shows multiple inclusions of spherules consisting of 61+/-5 vol% Fe3C (cohenite), 30+/-2 vol% Fe-Ni and 9+/-3 vol% FeS (troilite). Troilite forms shells around the native iron-cohenite assemblage, implying that both compositions were immiscible melts and were trapped rapidly by the silicate. It is proposed that this polycrystalline diamond-silicate-metallic spherule assemblage formed in very local pressure and fO(2) conditions in cracks at the base of the subcratonic lithosphere from a C-H-O fluid that reacted with surrounding silicate at about 1,300-1,400 degreesC. In a mantle fluid consisting of CH4>H2O>H-2 near fO(2)=IW, the H-2 activity increases rapidly when carbon from the fluid is consumed by diamond precipitation, driving the oxygen fugacity of the system to lower values along the diamond saturation curve. Water from the fluid induces melting of surrounding silicate material, and hydrogen reduces metals in the silicate melt, reflected by an unusually low Ni content of the garnet. The carbon isotopic composition of delta(13)C=-13.69parts per thousand (PDB) and the lack of nitrogen as an impurity is consistent with formation of the diamond from non-biogenic methane, whereas delta(18)O=7.4parts per thousand (SMOW) of the garnet implies derivation of the silicate from subduction-related material. Hence, very localized and transient reducing conditions within the subcratonic lithosphere can be created by this process and do not necessarily call for involvement of fluids derived from subducted material of biogenic origin
- …
