Garnet in silicic liquids and its possible use as a P-T indicator (original) (raw)
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Geochemistry International, 2009
Results of experimental study at 7.0-8.5 GPa and 1300-1900@C of the systems pyrope Mg 3 Al 2 Si 3 O 12 ( Prp )-Na 2 MgSi 5 O 12 ( Na Grt modeling solid solutions of Na-bearing garnets, Prp -jadeite NaAlSi 2 O 6 ( Jd ) in a simplified mode demonstrating melting relations of Na-rich eclogite, and Prp -Na 2 CO 3 are presented. Prp -Na 2 MgSi 5 O 12 join is a pseudobinary that results from the decomposition of Na Grt on to coesite and Na-pyroxene. Synthesized garnets are characterized by Na admixture (>0.32 wt % Na 2 O). Maximal Na 2 O concentrations (1.5 wt % Na 2 O) are reached on the solidus of the system at 8.5 GPa. Clear correlation between Na and Si was established in synthesized garnets; this provides evidence for heterovalent isomorphism of the Mg + Al Na + Si type with the appearance of Na 2 MgSi 5 O 12 component as a mechanism of such garnet formation. The Prp -Jd join is also pseudobinary that results from the formation of two series of solid solutions:
Journal of Metamorphic Geology, 2011
The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re-equilibration of these garnet crystals and to reconstruct the pressure-temperature-fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100-200 lm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell-shaped profile of spessartine component (7-3 mol.%), an increase in grossular from 14 to 22 mol.% and d 18 O values between 9.5 ± 0.3& and 10.4 ± 0.2&. Garnet inner rims (90-130 lm) are fluid inclusion-rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The d 18 O values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2&. The outer rims (<10 lm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four-stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re-equilibration by coupled dissolution-reprecipitation at the temperature peak (630 < T < 650°C). This causes the creation of porosity as the coupled dissolution-reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rimÕs domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolutionreprecipitation is a possible re-equilibration process for garnet in metamorphic rocks and that intramineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements.
Based on sequence mineral assemblages in the skarns the endoskarns include garnet (andradite), pyroxene (Salite), magnetite, titanite, scapolite, chlorite, epidote, calcite and quartz and exoskarns include garnet (andradite), magnetite, diopside or hedenbergite, calcite and quartz. The skarn deposits are iron magmatic oxides and sulfides. The metamorphic complex is observed in Eocene host rocks and before formations. In the results, are considered to have been late Eocene. Based on microprobe analysis, composition range of garnets is andradite30-100. Oscillatory zoning (alternately rich and iron-poor regions) reflects a change in fluid composition during crystallization of minerals and leaking fluid continually. Major role in the evolution of the skarns is fluid composition. Calculated Thermobarometry on the garnet-clinopyroxene minerals indicate that endoskarn minerals have formed in maximum temperature of 558 ° C and a pressure of about 2 kilobars.
Trace element zoning in garnet as a monitor of crustal melting
Geology, 1996
X-ray composition maps of Sc, Y, P, and Cr in garnet have been measured using the electron microprobe. Zoning is discontinuous and is correlated with fluid-absent crustal melting reactions. The magnitude of the discontinuity is a function of the amount of muscovite that melts, and the partition coefficients of solids versus melt. Trace element zoning is apparently not modified by diffusive processes, so the zoning can be used to monitor the extent of melting and to examine kinetics of melting and the participation of accessory phases in crustal anatexis.
Lithos, 2011
Primary-appearing fluid (FI) and melt (FI) inclusions occur in peritectic garnet from restitic enclaves from El Hoyazo (Spain). The inclusions were trapped under conditions of immiscibility during partial melting of the enclaves. Trapped fluids in Bt-Grt-Sil and Spl-Crd enclaves have been characterized by microthermometric, Raman spectroscopic, electron microprobe (EMP) and transmission electron microprobe (TEM) analyses to better constrain melt and fluid products and pressure conditions of the partial melting event. In Bt-Grt-Sil enclaves, FI are one phase and contain a CO 2-N 2 mixture, sometimes with graphite as trapped phase. In Spl-Crd enclaves, FI are two phase and contain an H 2 O-rich (≤ 90 mol%), with minor amounts of CO 2 , N 2 , and traces of H 2 S and CH 4. Graphite often occurs as a trapped phase in the H 2 O-rich FI, and rare carbonates and other accessory minerals are also observed. Although decrepitation features are not recognized during examination with a petrographic microscope, FI densities based on mass balance constraints are always lower than expected at the inferred PT conditions of entrapment, 5-7 kbar and 800-900°C. Extremely low densities (≈ 0.1 g cm − 1) of FI in Bt-Grt-Sil enclaves suggest a pressure ≤ 500 bar at 800-900°C, while densities up to 0.53 g cm − 1 in Spl-Crd enclaves indicate P ≤ 3 kbar at 800-900°C. Re-equilibration is likely to have occurred via partial decrepitation, as suggested by TEM studies that show rare partially annealed sub-μm cracks, containing small cavities, which may have been the pathways for fluid movement out of the inclusions. MI coexisting with FI have a rhyolitic, peraluminous composition, with higher H 2 O contents of MI in Spl-Crd enclaves (≈ 9 wt.%) compared to MI in Bt-Grt-Sil enclaves (≈3 wt.%). Based on published data, peritectic garnet in Spl-Crd enclaves grew in the presence of a leucogranitic melt saturated in an H 2 O-rich fluid, in good agreement with the inferred garnet PT growth conditions. The composition of the fluid phase coexisting with melt in Bt-Grt-Sil enclaves cannot be evaluated owing to the almost complete decrepitation and fluid loss from FI, and may only be inferred to have been more CO 2-rich, based on the lower H 2 O content of the coexisting melt.
Garnet geotherms: Pressure-temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks
Journal of Geophysical Research, 1996
The temperatures and pressures of equilibration of single peridotitic garnet xenocrysts are estimated using a combination of major-and trace-element data, determined using electron microprobe (EMP) and proton induced X ray emission (PIXE). This new method enables the use of xenocrysts found in kimberlites and other volcanic rocks to determine the local paleogeotherm at the time of eruption of the magma which sampled and transported the xenocrysts. The "Ni thermometer" of Griffin et al. [ 1989], based on the strong temperature dependence of the partitioning of Ni between garnet and olivine, is refined using an expanded database. Pressure is calculated from garnet composition using an algorithm that combines a modification of the geobarometer of Nickel [1989], based on Cr solubility in coexisting garnet and orthopyroxene, with the composition of a hypothetical coexisting orthopyroxene. The orthopyroxene composition is estimated by inverting the geothermometry equations of Gasparik [1987], Brey and KOhler [1990], and Harley [1984], and combining these with empirical relationships describing Cr in orthopyroxene in Crsaturated peridotitc (chromite present). The derived pressure (PCr) gives the equilibration pressure of peridotic garnets provided they were in equilibrium with chromite; garnets from Cr-undersaturated rocks will produce underestimates of pressure. Therefore, the locus of maximum PCr at a given TNi defines the "garnet geotherm", and provides a method for the determination of paleogeotherms based solely on PIXE and EMP analyses of garnet grains in concentrates. The assumption of coexisting chromite is tested by comparing the temperature distributions of garnets and chromites from the same concentrate. Chromite equilibration temperature is estimated using the "Zn thermometer", based on the strong temperature dependence of the partitioning of Zn between chromite and olivine. This thermometer is calibrated against the new Ni thermometer using a suite of garnet-chromite intergrowths. The garnet geotherm technique provides an estimate of the geotherm with an accuracy comparable to xenolith-derived geotherms and provides a means of mapping the thermal state of the lithosphere where xenoliths are rare or absent.