Garnet Ca Research Articles

Pressure–Temperature–Time Evolution of a Polymetamorphic Paragneiss With Pseudomorphs After Jadeite From the HP–UHP Gneiss‐Eclogite Unit of the Variscan Erzgebirge Crystalline Complex, Germany

ABSTRACTA quartz‐rich paragneiss from the Variscan Erzgebirge Crystalline Complex (ECC) was studied in detail because of abundant millimetre‐sized and clearly oriented pseudomorphs after a sodic mineral interpreted to have been jadeite. This mineral, or pseudomorphs after it, is rarely found in extensive high‐pressure (HP)–ultrahigh‐pressure (UHP) terranes worldwide despite reported pressure–temperature (P–T) conditions suitable for the formation of jadeite in common paragneisses and orthogneisses. In the studied rock, which contains abundant large and oriented potassic white mica flakes and minor millimetre‐sized garnet grains, the pseudomorphs consist of clusters of small albite grains with thin phengitic muscovite flakes in between. X‐ray maps for Ca and Mg in garnet demonstrate that an early generation of this mineral (Gt1) was corroded and subsequently overgrown by a Ca‐richer generation (Gt2). White mica is phengite with maximum Si contents of 3.42 atoms per formula unit. P–T conditions of 0.85 GPa and 650°C and 1.7 GPa and 660°C were derived for the formation of Gt1 and Gt2 rim + Si‐rich phengite, respectively, using pseudosection modelling. The latter conditions representing the pressure peak experienced by the paragneiss are compatible with the original presence of jadeite and possibly paragonite as well. This metamorphic peak occurred at 338.4 ± 2.3 (2σ) Ma based on in situ dating of monazite grains with the electron microprobe. A single monazite age of 386.4 ± 10.5 (2σ) Ma is related to the formation of Gt1. Thus, a Late Devonian metamorphism is suggested here for the first time to have occurred in ECC gneisses before the major HP event in the Early Carboniferous. Furthermore, the study demonstrates that the eclogite‐facies gneisses of the Gneiss‐Eclogite Unit of the ECC experienced peak pressures of not more than 2 GPa in contrast to recent proposals of an extensive UHP area in this unit. In addition, it is suggested that the localized occurrence of UHP rocks surrounded by other lithologies otherwise lacking evidence for UHP conditions should be interpreted with caution with respect to their regional extent and significance.

Nanogranitoid inclusions with grandidierite in mafic granulite from Austhovde, Lützow-Holm Complex, East Antarctica

Grandidierite, (Mg,Fe)Al3O2(BO3)SiO4, was found in a garnet-clinopyroxene-ilmenite-rich mafic granulite from Austhovde in the Late Neoproterozoic to Early Cambrian Lützow-Holm Complex (LHC), East Antarctica, the first reported occurrence of this borosilicate in a mafic granulite. It occurs in one of the many nanogranitoid inclusions (NIs) in garnet. Quartz, sodic plagioclase, myrmekite, K-feldspar, epidote and biotite are also found only as inclusions in garnet. Garnet porphyroblasts show marked compositional zoning: Ca increases and Mg decreases from the core to rim with little change in Fe and Mn contents except for the outermost rim. Anorthite content of inclusion plagioclase increases from core to rim of host garnet in parallel with increase in garnet Ca towards the rim. This together with the distinctly different mineral assemblages within and exterior to garnet porphyroblasts suggests that partial melting took place and produced melts were extracted leaving a mafic and calcic restite. Partial melting also occurred locally in garnet porphyroblasts consuming different hydrous mineral inclusions to produce various NIs ranging from K-feldspar-rich to K-feldspar-free. Subsequent decompression at high temperatures resulted breakdown of garnet to orthopyroxene + calcic plagioclase with further consumption of quartz, such that none remained in the matrix of the granulite. Grandidierite may have formed by a reaction between a trapped boron-bearing aluminous granitic melt and host garnet upon cooling.

Highly efficient and thermally stable broadband near-infrared emitting garnet Ca3Sc2Ge3O12:Cr3+,Ce3+ phosphors for multiple pc-LED applications

The introduction of Ce3+ improves the NIR luminescence properties of CSGG:Cr3+ by energy transfer and lattice distortion.

Inducing octahedral distortion to enhance NIR emission in Cr-doped garnet Ca3(Al, Sc)2Ge3O12

The luminescence intensity can be enhanced by inducing octahedral distortion in Ca3Al2−yScyGe3O12:0.04Cr3+ with the ion substitution of Sc3+ for Al3+.

On exhumation velocities of high-pressure units based on insights from chemical zoning in garnet (Tianshan, NW China)

Exhumed high-pressure rocks formed during subduction provide insight into global geodynamic and plate tectonic processes. Their metamorphic history is partially encoded in their mineralogy and mineral chemistry, which can record the P-T evolution and rates of subduction and exhumation. Here, we study a garnet-glaucophane-albite schist from the SW Tianshan in China in which complexly zoned garnet displays a distinct and sharp oscillatory Mn-zonation and irregular, jagged Ca distributions with peak widths of ∼5 μm. Combined Raman quartz-in-garnet barometry and carbonaceous matter thermometry suggests a P-T path that reached ∼2.5 GPa and was followed by ∼0.9 GPa of nearly-isothermal decompression at temperatures of 530–540°C, associated with final garnet growth. This P-T path is largely confirmed by thermodynamic modeling. Three different approaches to modeling coupled Ca, Mn, Mg and Fe diffusion in garnet were used to estimate the duration between establishment of high-resolution Mn, Fe and Ca zoning during growth and cooling below nominal closure conditions upon exhumation. All models agree best with conditions of ∼530°C maintained for around 100 kyr, though slight differences are found depending on published diffusion coefficients. For the constrained exhumation P-T path, we infer that the studied sample experienced exhumation velocities in the range of 40–45 cm/yr from ∼80 km to ∼50 km depth. We propose an exhumation model involving diapiric Stokes flow along the plate interface and low viscosities around the exhumed unit that may facilitate such high exhumation velocities.

Spectroscopic Analysis for Harnessing the Quality and Potential of Gemstones for Small and Medium-Sized Enterprises (SMEs)

Introduction of modern technologies and methods and quality analysis for the gemstone industry are the main strategic initiatives of the Small and Medium Development Authority (SMEDA) of Pakistan. In this regard, four natural gemstones Quartz, Pyrope-Almandine Garnet, Black tourmaline, and Amethyst brought from Hunza valley Pakistan were analyzed by state-of-the-art spectroscopic techniques including EDX, UV-VIS, and FTIR spectroscopy. EDX revealed the traces of Fe, Mg, and Ca in Pyrope-Almandine garnet, Mg and Fe in Black tourmaline, Au and Ca in Amethyst. UV-VIS data revealed the values of Urbach energies 520, 210, 460, and 430 meV, and the values of direct bandgap energies 5.14, 6.12, 5.54, 5.74 eV, respectively. The higher structural disorder due to the presence of Fe and other impurities in stones except Quartz was attributed to the higher values of Urbach energies and decrease in band gaps: FTIR data Fe-O and Si-O stretching vibration in Pyrope-Almandine garnet, Si-O bending vibrations and O-H stretching vibration in Quartz, Si-O-Si bending and stretching vibrations and C=O stretching vibrations in Black tourmaline, Ca-O stretching vibrations and Si-OH weak-vibrations in Amethyst. Photoluminescence results also showed useful information in investigating the properties of gemstones.

Age, duration and mineral markers of magma interactions in the deep crust: an example from the Pyrenees

We report in-situ dating, and Hf and O-isotope studies of zircon crystals, together with electron microprobe imaging of garnet crystals from a migmatitic diorite containing noritic globules, in the Ursuya massif (French Pyrenees). Diorite zircon crystals have old inherited cores and 295 ± 2 Ma rims, while those in the norite are homogeneous, with lower U and Pb, and higher Th and Th/U ratios than in the diorite zircon rims. The 295 Ma dates represent the crystallization age of hybridised crustal melts. Norite zircon crystals have similar ages (298 ± 3 Ma) to the diorite zircon rims, interpreted as the crystallization age of the norite. Hf isotopes indicate that the noritic magmas were already crustally contaminated prior to intrusion at their present locations. The contrasting behaviours of slow- and fast-diffusing species within garnet (e.g. P vs Ca) caused sharp P and diffuse Ca core-rim transitions. Modelling of garnet Ca diffusion profiles suggests a duration of 2 ± 1 Myr for the magma interactions in the deep crust. Integrated with granite dating, these data provide insights into pulsations in the regional thermal event that concluded the Hercynian orogenic cycle.

Mineralogy of a High-Temperature Skarn, in High CO2 Activity Conditions: The Occurrence from Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania)

A shallow-level monzodioritic to quartz-monzodioritic pluton of the Upper Cretaceous age caused contact metamorphism of Tithonic–Kimmeridgian reef limestones at Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania). The preserved peak metamorphic assemblage includes gehlenite (Ak 33.64–38.13), monticellite, wollastonite-2M, Ti-poor calcic garnet, and Ca-Tschermak diopside (with up to 11.15 mol.% Ca-Tschermak molecule). From the monzodioritic body to the calcitic marble, the periplutonic zoning can be described as: monzodiorite/agpaitic syenite-like inner endoskarn/wollastonite + perovskite + Ti-poor grossular + Al-rich diopside/wollastonite + Ti-poor grossular + diopside + vesuvianite/gehlenite + wollastonite + Ti poor grossular + Ti-rich grossular (outer endoskarn)/wollastonite + vesuvianite + garnet (inner exoskarn)/wollastonite + Ti-rich garnet + vesuvianite + diopside (outer exoskarn)/calcitic marble. Three generations of Ca garnets could be identified, as follows: (1) Ti-poor grossular (Grs 53.51–81.03 mol.%) in equilibrium with gehlenite; (2) Ti-rich grossular (Grs 51.13–53.47 mol.%, with up to 19.97 mol.% morimotoite in solid solution); and (3) titanian andradite (Grs 32.70–45.85 mol.%), with up to 29.15 mol.% morimotoite in solid solution. An early hydrothermal stage produced retrogression of the peak paragenesis toward vesuvianite, hydroxylellestadite (or Si-substituted apatite), clinochlore, “hibschite” (H4O4-substituted grossular). A late hydrothermal event induced the formation of lizardite, chrysotile, dickite, thaumasite, okenite and tobermorite. A weathering paragenesis includes allophane, C-S-H gels and probably portlandite, unpreserved because of its transformation in aragonite then calcite. Overprints of these late events on the primary zoning are quite limited.

The use of heavy mineral chemistry in reconstructing provenance: A case study from Mesozoic sandstones of Kutch Basin, India

Petrography and geochemistry of the Mesozoic sedimentary rocks of the Kutch Basin in western India reveal the predominance of felsic source rocks. Single‐grain chemistry of the detrital heavy minerals in these sediments refines the existing provenance interpretations by incorporating additional data related to source lithology. The study presents information on type, composition, and metamorphic grade of the source rocks for the Mesozoics of the Kutch Basin, which comprises Jhurio, Jhumara, Jhuran, and Bhuj formations. Chemistry of rutile, garnet, and tourmaline supports multiple sources of sediments. The ratio between Cr and Nb in rutile suggests the dominance of sediment input from metapelites. The relationship between (Fe + Mn)–Mg–Ca in garnets and Ca–Fetotal–Mg in tourmalines indicates the dominance of Ca‐poor felsic metamorphic source along with intermediate‐acidic plutonic rocks. The multivariate discrimination of the detrital garnets reveals the dominance of granulite and amphibolite facies source rocks. Rutile compositions indicate subordinate inputs from mafic sources across the investigated sedimentary succession. The single‐grain analysis of heavy minerals, therefore, infers a wide variation in source rock types.

Can we extract ultrahigh-temperature conditions from Fe-rich metapelites? An example from the Khondalite Belt, North China Craton

In this study, garnet–sillimanite gneisses at Hongshaba in the eastern segment of the Khondalite Belt, North China Craton (NCC) are interpreted to have experienced ultrahigh-temperature (UHT) metamorphism (980–1040 °C) followed by post-Tmax cooling at pressures of 8–9 kbar to the solidus (810–830 °C), consistent with rare sapphirine-bearing assemblages in surrounding regions. This interpretation is mainly based on the combination of P–T fields and garnet Xgr (=Ca/(Ca + Mg + Fe2+)) isopleths on the pseudosection of three garnet–sillimanite gneiss samples. Spinel tends to be enclosed in the outer margins of garnet, commonly closely associated with quartz. We interpret this to reflect the partial break down of garnet along the prograde path during heating with decompression followed by new garnet growth during cooling along an overall clockwise P–T evolution. Although Fe-rich UHT metapelites tend to contain neither diagnostic mineral assemblages nor orthopyroxene from which to extract T via Al-in-orthopyroxene thermometry, isopleths of Ca in garnet may aid in retrieving UHT conditions from these compositions. This is attributed to Ca diffusion in garnet being much slower than Fe and Mg diffusion, leading to little change in Ca contents during post-Tmax cooling. LA-ICP-MS U-Pb dating of metamorphic zircon in one garnet–sillimanite gneiss sample yields a mean 207Pb/206Pb age of ca. 1.91 Ga, which is interpreted to record the timing of cooling of the UHT rocks to the solidus. This UHT metamorphism is interpreted to have been generated by mantle-derived magma during a tectonic extension from ca. 1.95 to 1.92 Ga within a post-orogenic setting.

Mass-transfer and differential element mobility in metapelites during multistage metamorphism of the Yenisey Ridge, Siberia

Abstract Pelitic gneisses and schists from the Garevka Complex (Yenisey Ridge) at the western margin of the Siberian craton contain zoned garnet porphyroblasts that show clear evidence of multistage growth. The three discrete stages define a counter-clockwise pressure–temperature ( P–T ) path involving initial prograde low pressure heating followed by near-isothermal medium-pressure compression and post-peak retrograde synexhumation decompression and cooling. The combined study of the variations of in-situ major and trace element mineral compositions of coexisting minerals and mineral modes with changing P and T conditions and metamorphic reactions in rocks allowed detailed investigation of metamorphic processes. A significant increase in pressure during prograde collision-related metamorphism correlates with an abrupt increase of Ca in garnets, which in turn is accompanied by a significant decrease in heavy rare earth elements (HREE) and Y contents. Decrease in temperature and pressure during retrograde metamorphism led to an increase in the HREE and Y content in garnets with concomitant decrease of grossular component of garnet. A pronounced systematic negative correlation between HREE including Y content and Ca content in garnet can be attributed to substitution of trivalent rare earth elements (REE) and Y for divalent Ca cations on eightfold sites. The main reasons for the sharp increase in Ca content in garnets during collisional metamorphism are either (1) the redistribution between garnet and plagioclase, which led to less calcium in composition, or (2) consequence of epidote breakdown. A mass balance of major and trace elements between the reactants and products of metamorphic reactions yield very good matches between the measured and reconstructed modal abundances in all cases, indicating that collisional metamorphism was essentially isochemical with respect to most elements, with the exception of HREE. The minimum equilibration volume for the major elements was c. 1 mm 3 . The total HREE balance requires a rather greater reaction volume involved in the redistribution of HREEs, of the order of 3–8 mm 3 which provides evidence for their relatively higher migration mobility during metamorphism in comparison with other REE.

Analysis of experimental data on divalent cation diffusion kinetics in aluminosilicate garnets with application to timescales of peak Barrovian metamorphism, Scotland

We present a new statistical framework to analyze the diffusion data for divalent cations (Fe, Mg, Mn, and Ca) in aluminosilicate garnet using published experimental data and an Arrhenius relationship that accounts for dependence on temperature, pressure, garnet unit-cell dimension, and oxygen fugacity. The regression is based on Bayesian statistics and is implemented by the Markov chain Monte Carlo approach. All reported experimental uncertainties are incorporated, and the data are weighted by the precision of the experimental conditions. We also include a new term, the inter-experiment bias, to compensate for possible inconsistencies among experiments and to represent any experimental variability not explicitly presented in the Arrhenius relationship (e.g., water content, defect density). At high temperatures where most experiments were conducted, the diffusion coefficients calculated with the new parameters agree well with previous diffusion models (e.g., Chakraborty and Ganguly in Contrib Mineral Petrol, 111:74–86, 1992; Carlson in Am Mineral, 91:1–11, 2006). However, the down-temperature extrapolation leads to notable differences at lower temperatures for common petrological applications. For example, at 600 °C, the diffusion coefficients of Fe and Mn are one half-to-one order of magnitude faster and the diffusion coefficient of Ca is about one order of magnitude slower than calculated with parameters in Carlson (2006). Our statistical analysis also provides well-defined uncertainty bounds for timescale estimates based on garnet diffusion profiles. Application of the newly derived coefficients indicates that the timescale of the thermal peak of Barrovian metamorphism (Dalradian belt of Scotland) is about four to seven times longer than that estimated using previously published diffusion coefficients. The peak is still geologically brief, however—of the order of 106 years (0.75 Myr +0.70/−0.36 Myr; ±1σ). This brevity requires pulsed advective heat input, as provided by syn-orogenic mafic magmatism in these rocks.

Layering of the lithospheric mantle beneath the Siberian Craton: Modeling using thermobarometry of mantle xenolith and xenocrysts

Single-grain thermobarometric studies of xenocrysts were used to compile local SCLM transects through the major regions of kimberlite magmatism in Siberia and longer transects through the subcontinental mantle lithosphere (SCLM) beneath the Siberian craton. The mantle structure was obtained using P-Fe#, Ca in garnets, oxygen fugacity values fO2 and calculated temperatures T°C.The most detail transect obtained for the Daldyn field on the Udachnaya-Zarnitsa reveals layering showing an inclination of >35° to Udachnaya. Mantle layering beneath the Alakit field determined from the Krasnopresnenskaya-Sytykanskaya transect shows a moderate inclination from N to S. The inflection near Yubileinaya-Aykhal is also supported by the extreme depletion in peridotites with low-Fe sub-Ca garnets. Beneath the Malo-Botuobinsky field the sharply layered mantle section starts from 5.5GPa and reveals step-like P-Fe#Ol trends for garnets and ilmenites. The deeper part of SCLM in this field was originally highly depleted but has been regenerated by percolation of protokimberlites and hybrid melts especially beneath Internationalnaya pipe.The three global transects reveal flat layering in granite-greenstone terranes and fluctuations in the granulite-orthogneiss Daldyn collision terranes. The mantle layering beneath the Daldyn - Alakite region may have been created by marginal accretion. Most of southern fields including the Malo-Botuobinsky field reveal flat layering. The primary subduction layering is smoothed beneath the Alakit field.Lower Jurassic kimberlites from the Kharamai-Anabar kimberlite fields reveal a small decrease of the thickness of the SCLM and heating of its base. The Jurassic Kuoyka field shows an uneven base of the SCLM inclined from west to east. SCLM sequences sampled at this time started mainly from depths of 130km, but some pipes still showed mantle roots to 250km. The garnet series demonstrates an inclined straight line pyroxenite P-Fe# trend due to interaction with superplume melts.

Assessing Detrital Garnet Chemical Composition As A Quantitative Provenance Tool: A Multivariate Statistical Approach

Abstract Chemical analysis of detrital garnet is a widely utilized quantitative tool for determining the provenance of sediments. The goal of this study is to investigate the ability of the compositions of detrital garnet to provide a direct linkage to the specific source rocks from which the garnet was derived. To achieve this goal, we analyzed detrital garnet isolated from alluvial samples collected from the French Broad River and several of its tributaries, a watershed that drains portions of the multiply deformed southern Appalachian Orogen, USA. To test whether potential linkages between detrital and potential source-rock garnet could be achieved, garnet crystals were chemically characterized from a wide variety of lithologies that crop out in the French Broad watershed, and thus are the most likely local sources of detrital garnet. Mahalanobis distances measured on canonical discriminant functions successfully differentiated garnet compositions among the 18 analyzed source rocks. These metrics were then applied to ∼ 2,300 detrital-garnet compositions to link each grain to its potential source rock. Approximately 94% of the detrital garnet crystals were linked to their potential source regions. Thus only 6% of the detrital crystals were treated as “unclassified.” We interpret the “unclassified” high Ca garnets to have been derived from minor sub-map-scale lenses and boudins of calc-silicate or skarns from exposed Ocoee or Ashe metaclastic units within the French Broad River watershed. The “unclassified” high-Mn detrital garnet grains are interpreted to have been derived from the various felsic granitiods and pegmatites that intrude the region of study.

Garnet and spinel in fertile and depleted mantle: insights from thermodynamic modelling

We performed thermodynamic calculations based on model and natural peridotitic compositions at pressure and temperature conditions relevant to the Earth's upper mantle, using well-established free energy minimi- zation techniques. The model is consistent with the avail- able experimental data in Cr-bearing peridotitic systems and can therefore be used to predict phase relations and mineral compositions in a wide range of realistic mantle compositions. The generated phase diagrams for six dif- ferent bulk compositions, representative of fertile, depleted and ultra-depleted peridotitic mantle, shown that the gar- net ? spinel stability field is always broad at low temper- atures and progressively narrows with increasing temperatures. In lithospheric sections with hot geotherms (ca. 60 mW/m 2 ), garnet coexists with spinel across an interval of 10-15 km, at ca. 50-70 km depths. In colder, cratonic, lithospheric sections (e.g. along a 40 mW/m 2 geotherm), the width of the garnet-spinel transition strongly depends on bulk composition: In fertile mantle, spinel can coexist with garnet to about 120 km depth, while in an ultra-depleted harzburgitic mantle, it can be stable to over 180 km depth. The formation of chromian spinel inclusions in diamonds is restricted to pressures between 4.0 and 6.0 GPa. The modes of spinel decrease rapidly to less than 1 vol % when it coexists with garnet; hence, spinel grains can be easily overlooked during the petrographical character- ization of small mantle xenoliths. The very Cr-rich nature of many spinels from xenoliths and diamonds from cratonic settings may be simply a consequence of their low modes in high-pressure assemblages; thus, their composition does not necessarily imply an extremely refractory composition of the source rock. The model also shows that large Ca and Cr variations in lherzolitic garnets in equilibrium with spinel can be explained by variations of pressure and temperature along a continental geotherm and do not necessarily imply variations of bulk composition. The slope of the Cr# (i.e. Cr/(Cr ? Al)mol) isopleths in garnet in equilibrium with spinel changes significantly at high temperatures, posing serious limitations to the applicability of empirical geo- barometric methods calibrated on cratonic mantle xenoliths in hotter, off-craton, lithospheric mantle sections.

Peak metamorphic temperatures from cation diffusion zoning in garnet

AbstractA model that relates the characteristic diffusion length and average cooling rate to peak temperature was developed for chemical diffusion in spherical geometries on the basis of geospeedometry principles and diffusion theory. The model is quantitatively evaluated for cation diffusion profiles in garnet. Important model parameters were calibrated empirically using diffusion zoning of Ca in garnet from the Pikwitonei Granulite Domain, a terrane for which the thermal history has been well characterized. The results are used: (i) to empirically test diffusion parameters for Mg and Fe(II) and (ii) to develop a tool that uses the diffusion zoning of these cations in garnet to constrain peak temperature conditions for garnet‐bearing rocks. The thermometric approach was externally tested by applying it to garnet crystals from various metamorphic terranes worldwide and comparing the results to published peak temperature estimates. The results overlap within uncertainties in all cases, but result that are based on Fe(II) and Mg chemical‐diffusion profiles are up to three times more precise than those acquired by conventional methods. The remarkable consistency of the results implies that the model is robust and provides a reliable means of estimating peak temperatures for different types of high‐grade metamorphic rock. The tool could be of particular advantage in rocks where critical assemblages for conventional thermometry do not occur or have been replaced during retrogression.

Trace element partitioning between mantle minerals and silico-carbonate melts at 6–12 GPa and applications to mantle metasomatism and kimberlite genesis

The partitioning of a number of trace elements (LILE, HFSE, REE, Cu, Pb, Co, Ni) between mantle minerals (olivine, pyroxenes and garnet) and silico-carbonate melts was experimentally studied at 6–12GPa and 1300–1700°C. The starting compositions were model kimberlitic with ~30wt.% SiO2, which differentiated to carbonatite-like melts with <10wt.% SiO2 depending on the degree of crystallization. The melts were rich in CO2 (up to ~30wt.%) and contained 0 to 30wt.% H2O. Trace elements were added to the starting mixtures to levels of ~100ppm. They were analyzed by LA ICP MS in the products of 18 experiments. The partition coefficients of Ba, La, Ta, and Nb are very low for all phases (<0.01). These elements are especially susceptible to contamination in the experimental products. We suspect that some relatively high values obtained for these elements in previous studies are overestimated. Generally, the partition coefficients (Ds/l) of the moderately incompatible and compatible elements increase in the sequence olivine–low-Ca pyroxene–high-Ca pyroxene. Garnet shows maximum fractionation of the trace elements, such that D values for the highly incompatible elements are lower than those for pyroxenes and highest for the HREE, Sc, and V. The partition coefficients of a number of incompatible elements are rather insensitive to pressure, temperature, and melt composition. There is no correlation between the partition coefficients and the content of H2O, CO2 and the overall content of trace elements. They are predominantly controlled by the composition of the crystalline phases. In particular, garnet–liquid partitioning is very sensitive to Ca in garnet, and pyroxene–liquid partitioning to Al in pyroxene.The comparison of the obtained partition coefficients with the compositions of likely kimberlite magmas shows that depleted and refertilized harzburgite from the continental lithospheric mantle is a viable candidate as the source material of kimberlites. The estimation of trace element characteristics of near-solidus mantle melts indicates that the most reliable indicator of carbonate melt metasomatism is depletion of Zr and Hf relative to Sm and Nd. Criteria based on the ratios of more incompatible elements (e.g. Nb/La) are very sensitive to the melting conditions and can be misleading.

Optimization of the garnet-biotite geothermometer: Part II. Calibration equations and accuracy of the estimation

Calibrations of the garnet-biotite geothermometer are considered with allowance for the effect of the nonideality of the coexisting garnet and biotite upon KD. The calibrations based on the model of an asymmetric subregular Prp-Alm-Grs-Sps solid solution with Wi−jGrt (after Ganguly et al. (1996) and Holdaway (2000) and taking into account the experimental data published by Perchuk et al. (1983)) are the most accurate. The same may be said about the equations additionally taking into consideration the model of the symmetric regular Phl-Ann-Ms-(Ti-Bt) solution with Wi−jBt after Holdaway et al., (1997), Kaneko and Miano (2004), and the data published by Ferry and Spear (1978) and Perchuk et al. (1983). These equations well reproduce the temperature at which the Prp + Ann = Alm + Phl equilibrium is reached in experiments, and they are the least sensitive to the variation of the Ca and Mn contents in garnet and AlVI and Ti in biotite. The uncertainty of the temperature interval is 30–46°C (2σ).

Precise estimation of pressure–temperature paths from zoned minerals using Markov random field modeling: theory and synthetic inversion

The chemical zoning profile in metamorphic minerals is often used to deduce the pressure–temperature (P–T) history of rock. However, it remains difficult to restore detailed paths from zoned minerals because thermobarometric evaluation of metamorphic conditions involves several uncertainties, including measurement errors and geological noise. We propose a new stochastic framework for estimating precise P–T paths from a chemical zoning structure using the Markov random field (MRF) model, which is a type of Bayesian stochastic method that is often applied to image analysis. The continuity of pressure and temperature during mineral growth is incorporated by Gaussian Markov chains as prior probabilities in order to apply the MRF model to the P–T path inversion. The most probable P–T path can be obtained by maximizing the posterior probability of the sequential set of P and T given the observed compositions of zoned minerals. Synthetic P–T inversion tests were conducted in order to investigate the effectiveness and validity of the proposed model from zoned Mg–Fe–Ca garnet in the divariant KNCFMASH system. In the present study, the steepest descent method was implemented in order to maximize the posterior probability using the Markov chain Monte Carlo algorithm. The proposed method successfully reproduced the detailed shape of the synthetic P–T path by eliminating appropriately the statistical compositional noises without operator’s subjectivity and prior knowledge. It was also used to simultaneously evaluate the uncertainty of pressure, temperature, and mineral compositions for all measurement points. The MRF method may have potential to deal with several geological uncertainties, which cause cumbersome systematic errors, by its Bayesian approach and flexible formalism, so that it comprises potentially powerful tools for various inverse problems in petrology.

Bulk attenuation in the earth's mantle due to phase transitions

The calculated bulk attenuation due to phase transition from mineral physics data is reported here. With relaxation time less than 1 s, the calculated value for a pyrolite mantle is consistent with the inverted bulk attenuation of the upper mantle from seismic observations. The two important mechanisms of phase transitions, diffusion-controlled and kinetics-controlled, have different relaxation time as indicated by the models here. The diffusion controlled is more likely to contribute to the observed seismic bulk attenuation than the kinetic-controlled process based on the available diffusivity and kinetics data. The correlation between the bulk attenuation and relaxation time emphasizes the importance of a number of parameters in the mineral physics database such as Fe–Mg diffusivity and kinetics in olivine–wadsleyite–ringwoodite–perovskite, Mg–Ca–Al–Si diffusivity and kinetics in pyroxene–garnet–Ca perovskite, some of which are still unknown.