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Rommulo Vieira Conceicao - One of the best experts on this subject based on the ideXlab platform.
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derivation of potassic shoshonitic magmas by decompression melting of phlogopite Pargasite lherzolite
Lithos, 2004Co-Authors: Rommulo Vieira Conceicao, David H GreenAbstract:A model metasomatized lherzolite composition contains phlogopite and Pargasite, together with olivine, orthopyroxene, clinopyroxene and spinel or garnet as subsolidus phases to 3 GPa. Previous works established that at ≥1.5 GPa, phlogopite is stable above the dehydration solidus, determined by the melting behaviour of Pargasite and coexisting phases. At 2.8 GPa, melts with residual phlogopite+garnet lherzolite mineralogy at 1195 °C and with garnet lherzolite mineralogy at 1250 °C are both olivine nephelinite with K/Na (atomic)=0.51 and K/Na=0.65, respectively. Recent work shows that melting along the dehydration (fluid-absent) solidus of the phlogopite+Pargasite lherzolite at pressures <1.5 GPa is very different with the presence of phlogopite, decreasing the solidus below that of Pargasite lherzolite. At 1.0 GPa, both phlogopite and Pargasite disappear at temperatures at or slightly above the solidus. The compositions of two melts at 1.0 GPa, 1075 °C (with different water contents), in equilibrium with residual spinel lherzolite mineralogy are silica-saturated trachyandesite (∼5% melt fraction, ∼3% H2O) to silica-oversaturated basaltic andesite (∼8% melt fraction, 4.5% H2O). Both compositions may be classified as ‘shoshonites’ on the basis of normative compositions, silica-saturation, and K/Na ratio. Decompression melting of metasomatized lithospheric lherzolite with minor phlogopite and Pargasite may produce primary ‘shoshonitic’ magmas by dehydration melting at ∼1 GPa, 1050–1150 °C. Such magmas may be parental to Proterozoic batholithic syenites occurring in Brazil.
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Derivation of potassic (shoshonitic) magmas by decompression melting of phlogopite+Pargasite lherzolite
Lithos, 2004Co-Authors: Rommulo Vieira Conceicao, David H GreenAbstract:A model metasomatized lherzolite composition contains phlogopite and Pargasite, together with olivine, orthopyroxene, clinopyroxene and spinel or garnet as subsolidus phases to 3 GPa. Previous works established that at ≥1.5 GPa, phlogopite is stable above the dehydration solidus, determined by the melting behaviour of Pargasite and coexisting phases. At 2.8 GPa, melts with residual phlogopite+garnet lherzolite mineralogy at 1195 °C and with garnet lherzolite mineralogy at 1250 °C are both olivine nephelinite with K/Na (atomic)=0.51 and K/Na=0.65, respectively. Recent work shows that melting along the dehydration (fluid-absent) solidus of the phlogopite+Pargasite lherzolite at pressures
David H Green - One of the best experts on this subject based on the ideXlab platform.
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derivation of potassic shoshonitic magmas by decompression melting of phlogopite Pargasite lherzolite
Lithos, 2004Co-Authors: Rommulo Vieira Conceicao, David H GreenAbstract:A model metasomatized lherzolite composition contains phlogopite and Pargasite, together with olivine, orthopyroxene, clinopyroxene and spinel or garnet as subsolidus phases to 3 GPa. Previous works established that at ≥1.5 GPa, phlogopite is stable above the dehydration solidus, determined by the melting behaviour of Pargasite and coexisting phases. At 2.8 GPa, melts with residual phlogopite+garnet lherzolite mineralogy at 1195 °C and with garnet lherzolite mineralogy at 1250 °C are both olivine nephelinite with K/Na (atomic)=0.51 and K/Na=0.65, respectively. Recent work shows that melting along the dehydration (fluid-absent) solidus of the phlogopite+Pargasite lherzolite at pressures <1.5 GPa is very different with the presence of phlogopite, decreasing the solidus below that of Pargasite lherzolite. At 1.0 GPa, both phlogopite and Pargasite disappear at temperatures at or slightly above the solidus. The compositions of two melts at 1.0 GPa, 1075 °C (with different water contents), in equilibrium with residual spinel lherzolite mineralogy are silica-saturated trachyandesite (∼5% melt fraction, ∼3% H2O) to silica-oversaturated basaltic andesite (∼8% melt fraction, 4.5% H2O). Both compositions may be classified as ‘shoshonites’ on the basis of normative compositions, silica-saturation, and K/Na ratio. Decompression melting of metasomatized lithospheric lherzolite with minor phlogopite and Pargasite may produce primary ‘shoshonitic’ magmas by dehydration melting at ∼1 GPa, 1050–1150 °C. Such magmas may be parental to Proterozoic batholithic syenites occurring in Brazil.
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Derivation of potassic (shoshonitic) magmas by decompression melting of phlogopite+Pargasite lherzolite
Lithos, 2004Co-Authors: Rommulo Vieira Conceicao, David H GreenAbstract:A model metasomatized lherzolite composition contains phlogopite and Pargasite, together with olivine, orthopyroxene, clinopyroxene and spinel or garnet as subsolidus phases to 3 GPa. Previous works established that at ≥1.5 GPa, phlogopite is stable above the dehydration solidus, determined by the melting behaviour of Pargasite and coexisting phases. At 2.8 GPa, melts with residual phlogopite+garnet lherzolite mineralogy at 1195 °C and with garnet lherzolite mineralogy at 1250 °C are both olivine nephelinite with K/Na (atomic)=0.51 and K/Na=0.65, respectively. Recent work shows that melting along the dehydration (fluid-absent) solidus of the phlogopite+Pargasite lherzolite at pressures
David M. Jenkins - One of the best experts on this subject based on the ideXlab platform.
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An Experimental Study of Chlorine Incorporation In Amphibole Synthesized Along the Pargasite–Ferro-Pargasite Join
The Canadian Mineralogist, 2017Co-Authors: Benjamin P. Campanaro, David M. JenkinsAbstract:Abstract Understanding how chlorine partitions between calcium amphiboles and an ambient fluid can be used to deduce the chlorine content of fluids involved in a wide range of geological processes, such as in high-grade metamorphism, deposition of economic minerals, chlorine metasomatism, and seawater-oceanic crust interactions. One of the most common correlations that is observed in calcium amphiboles is the increase in chlorine concentration with increasing Fe 2+ /(Fe 2+ + Mg) (= Fe#). To quantify this dependence, amphiboles with intended compositions along the Pargasite–ferro-Pargasite [NaCa 2 (Mg 4 Al)(Al 2 Si 6 )O 22 (Cl,OH) 2 –NaCa 2 (Fe 4 Al)(Al 2 Si 6 )O 22 (Cl,OH) 2 ] join were synthesized in the presence of NaCl brines ranging in concentration from 1 molal ( m ) NaCl to halite saturation. Syntheses were made over the range 700–950 °C and 0.2–0.5 GPa at oxygen fugacities ( f O 2 ) near the Co-CoO buffer and for durations of 46–576 h. The observed amphibole compositions varied between Pargasite and hastingsite with increasing Fe# for an assumed constant Fe 3+ /(Fe 2+ + Fe 3+ ) ratio of 0.18. The most common additional phases were plagioclase and Ca-rich clinopyroxene with minor davyne in some syntheses. At any given bulk composition, amphiboles increase in Cl content with increasing brine concentrations, reaching a plateau in brines of about 0.1–0.2 mole fraction of NaCl ( X NaCl , or 5–15 m NaCl). Amphiboles synthesized in brines below halite saturation showed a fairly linear increase in Cl content from essentially 0.0 to 0.11 atoms per formula unit ( apfu ) with increasing Fe content. Syntheses in brines above halite saturation showed a distinct jump in Cl contents up to 0.14–0.30 apfu for the two most Fe-rich bulk compositions. Isopleths of constant NaCl concentration have been fitted to all of the data, and a thermochemical analysis of the dependence of the Cl/OH ratio of amphibole to the activity ratio of H 2 O/HCl in the brine has been offered for amphiboles synthesized in the 1 m NaCl brines. Comparison of the Cl contents of synthetic amphiboles made here with natural pargasitic amphiboles shows that even the highest Cl contents observed here are only about 20% of the maximum Cl contents found in amphiboles from some localities. The main implication of this study is that there is very limited Cl uptake in amphiboles made in this chemical system in the presence of aqueous brines of highly variable concentrations, even for the most Fe-rich amphiboles. The high Cl contents observed in natural assemblages are likely controlled by brine, or perhaps halide or silicate melt chemistry, or crystal-chemical factors ( e.g ., K content) that exert a stronger effect than just the Fe# of the amphibole.
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an experimental study of chlorine incorporation in amphibole synthesized along the Pargasite ferro Pargasite join
Canadian Mineralogist, 2017Co-Authors: Benjamin P. Campanaro, David M. JenkinsAbstract:Abstract Understanding how chlorine partitions between calcium amphiboles and an ambient fluid can be used to deduce the chlorine content of fluids involved in a wide range of geological processes, such as in high-grade metamorphism, deposition of economic minerals, chlorine metasomatism, and seawater-oceanic crust interactions. One of the most common correlations that is observed in calcium amphiboles is the increase in chlorine concentration with increasing Fe 2+ /(Fe 2+ + Mg) (= Fe#). To quantify this dependence, amphiboles with intended compositions along the Pargasite–ferro-Pargasite [NaCa 2 (Mg 4 Al)(Al 2 Si 6 )O 22 (Cl,OH) 2 –NaCa 2 (Fe 4 Al)(Al 2 Si 6 )O 22 (Cl,OH) 2 ] join were synthesized in the presence of NaCl brines ranging in concentration from 1 molal ( m ) NaCl to halite saturation. Syntheses were made over the range 700–950 °C and 0.2–0.5 GPa at oxygen fugacities ( f O 2 ) near the Co-CoO buffer and for durations of 46–576 h. The observed amphibole compositions varied between Pargasite and hastingsite with increasing Fe# for an assumed constant Fe 3+ /(Fe 2+ + Fe 3+ ) ratio of 0.18. The most common additional phases were plagioclase and Ca-rich clinopyroxene with minor davyne in some syntheses. At any given bulk composition, amphiboles increase in Cl content with increasing brine concentrations, reaching a plateau in brines of about 0.1–0.2 mole fraction of NaCl ( X NaCl , or 5–15 m NaCl). Amphiboles synthesized in brines below halite saturation showed a fairly linear increase in Cl content from essentially 0.0 to 0.11 atoms per formula unit ( apfu ) with increasing Fe content. Syntheses in brines above halite saturation showed a distinct jump in Cl contents up to 0.14–0.30 apfu for the two most Fe-rich bulk compositions. Isopleths of constant NaCl concentration have been fitted to all of the data, and a thermochemical analysis of the dependence of the Cl/OH ratio of amphibole to the activity ratio of H 2 O/HCl in the brine has been offered for amphiboles synthesized in the 1 m NaCl brines. Comparison of the Cl contents of synthetic amphiboles made here with natural pargasitic amphiboles shows that even the highest Cl contents observed here are only about 20% of the maximum Cl contents found in amphiboles from some localities. The main implication of this study is that there is very limited Cl uptake in amphiboles made in this chemical system in the presence of aqueous brines of highly variable concentrations, even for the most Fe-rich amphiboles. The high Cl contents observed in natural assemblages are likely controlled by brine, or perhaps halide or silicate melt chemistry, or crystal-chemical factors ( e.g ., K content) that exert a stronger effect than just the Fe# of the amphibole.
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Mid-IR bands of synthetic calcic amphiboles of tremolite-Pargasite series and of natural calcic amphiboles
American Mineralogist, 2008Co-Authors: Kiyotaka Ishida, David M. Jenkins, Frank C. HawthorneAbstract:Mid-IR spectra (4000–400 cm −1 ) of synthetic calcic amphiboles in the tremolite-Pargasite series and of various natural calcic amphiboles have been investigated. The Pargasite substitution, a combination of the Tschermak (= [4] Al [6] Al [6] Mg −1 [4] Si −1 ) and edenite (= [4] Al [A] Na [4] Si −1 [A] □ −1 ) substitutions, causes the following features in the region 1200–600 cm −1 . (1) Weak [4] Al-O stretching bands appear at 895 and 815 cm −1 that are distinct from the 955 and 925 cm −1 Si-O stretching bands in tremolite. (2) There is a reduction in the intensity and frequency of the Si-O-Si symmetric bending band (=“chain breathing” mode) at 750 cm −1 in tremolite, and there is an appearance of medium-strong composite bands having a weak shoulder on the high-frequency side near 690 cm −1 . These bands are assigned to Si-O-Al deformation bands. (3) Two OH-libration bands at 690 and 650 cm −1 become weak and broad composite bands from 720 to 610 cm −1 . And (4) because the intensity and frequency of the band at 640 cm −1 in tremolite is affected neither by deuteration nor by the Pargasite substitution, this band is ascribed to O- [T2] Si-O bending. Even in Pargasite, most T2 sites are occupied by Si, so that the O2- [T2] Si-O4 bending mode will be dominant in this amphibole. The same behavior occurs for the synthetic fluorogallian tremolite-Pargasite series but with larger downward frequency shifts—Ga-O stretching bands appear at 880 and 780 cm −1 , and an Si-O-Ga bending band appears at 605 cm −1 . The major T-O-T and O-T-O deformation bands in synthetic amphiboles are readily apparent in natural calcic amphiboles whose compositions are near the tremolite-Pargasite join.
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Experimental determination of Pargasite stability relations in the presence of orthopyroxene
Contributions to Mineralogy and Petrology, 1992Co-Authors: Richard W. Lykins, David M. JenkinsAbstract:The stability and partial melting of synthetic Pargasite in the presence of enstatitic orthopyroxene (opx), forsterite, diopsidic clinopyroxene (cpx), plagioclase (An50), and water has been studied in the range of 0.4–6.0 kb and 750–1000°C in the system Na2O-CaO-MgO-Al2O3-SiO2-H2O with a fixed bulk composition of Pargasite+5 opx. The addition of orthopyroxene effectively reduces the stability field of Pargasite by approximately 200°C at 1 kb. The invariant point involving Pargasite coexisting with water-saturated liquid and anhydrous phase shifts from about 0.85 kb and 1025°C to 2.5±0.5 kb and 925±25°C with the addition of opx. Based on the solidus mineral assemblage and direct chemical analysis of quenched glass, the vapor-saturated liquid has a composition close to that of intermediate plagioclase. A layered silicate, interpreted to be Na-phlogopite, has an upper-thermal stability that nearly equals that of Pargasite in the field of partial melting and coexists with liquid, Pargasite, cpx, and forsterite at 6 kb, 1000°C. These results support the hypothesis that mantle metasomatism could involve formation of pargasitic amphibole from a silicate melt at depths as shallow as 8–10 km.
Yasuhito Osanai - One of the best experts on this subject based on the ideXlab platform.
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Stability of Pargasite during ultrahigh-temperature metamorphism: A consequence of titanium and REE partitioning?
American Mineralogist, 2009Co-Authors: Krishnan Sajeev, Yasuhito Osanai, Yoshiaki Kon, Tetsumaru ItayaAbstract:Orthopyroxene-clinopyroxene-plagioclase needles and symplectite along the cleavage planes and grain boundaries of fluorine-bearing titanian-ferroan Pargasite from the Highland Complex, Sri Lanka, are interpreted as evidence for dehydration melting at ultrahigh-temperature conditions. High Ti (up to 0.4 pfu) and F ( X F up to 0.56) content in Pargasite extends its stability to higher temperatures, and the composition indicates the dehydration melting reaction may take place at ultrahigh-temperatures (~950 °C) at a pressure around 10 kbar, close to peak metamorphic conditions. The increase of Ti content close to the grain boundaries and cleavage planes in Pargasite indicates titanium partitioning from the melt during dehydration melting enhanced the stability of the mineral toward ultrahigh-temperature conditions. The REE content in the Pargasite shows a similar behavior to that of titanium. The cores with no breakdown assemblage consist of low and flat REE concentrations with respect to the high and Eu-depleted rim. Clinopyroxene in symplectite and needle-shaped lamellae within the Pargasite porphyroblasts have similar REE patterns with slightly low-concentrations relative to that of Pargasite. In the breakdown assemblage, LREEs are partitioned mainly into plagioclase while the HREEs are partitioned into orthopyroxene. The REE enrichment in the Pargasite rims signals their relative partitioning between Pargasite rims and melt. Modeling of the partitioning of Ti and REEs associated with Pargasite breakdown demonstrates that its stability is greatly enhanced at UHT conditions. This investigation implies that the stability of hydrous minerals such as amphibole can be extended to UHT conditions, and expands our knowledge of metamorphism in the lower crust.
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High fluorine Pargasites in ultrahigh temperature granulites from Tonagh Island in the Archean Napier Complex, East Antarctica
Lithos, 2003Co-Authors: Toshiaki Tsunogae, Yasuhito Osanai, Masaaki Owada, Tsuyoshi Toyoshima, Tomokazu Hokada, Warwick A. CroweAbstract:Abstract Pargasites (F/(F+Cl+OH) ratio ( X F ) of up to 0.48) from Tonagh Island in Enderby Land, East Antarctica are closely associated with typical high-grade minerals such as orthopyroxene in quartzo-feldspathic, mafic, and ultramafic granulites, and is regarded as a stable mineral at the peak metamorphic conditions (>1100 °C) calculated for the ultrahigh-temperature Archean Napier Complex. Although experimental investigations have suggested that the upper thermal stability limit of F-free Pargasite is below 1050 °C, thermodynamic calculations for the present Pargasite+quartz assemblage indicate that the thermal stability limit of Pargasite with X F =0.5 is about 150 °C higher than that of the hydroxyl end member. Fluorine substitution in the Pargasite therefore allowed the mineral to survive the ultrahigh-temperature metamorphism at Tonagh Island. A positive correlation between the F content of Pargasite and coexisting biotite indicates that the minerals approach chemical equilibrium in terms of F–OH distribution. Although the fluorine composition of Pargasites ( X F =0.12–0.48) and bulk rock (300–2500 ppm) varies widely, the log( f H 2 O / f HF ) values calculated for these rocks are relatively constant (3.2–3.7), which is consistent with infiltration of an F-bearing fluid during prograde metamorphism. The infiltration of such a fluid is also supported by the higher bulk F content of most of the analyzed samples compared to those of continental and oceanic basaltic rocks, that is, F had been added from an external source. A positive correlation between bulk MgO and F content suggests that F may have been selectively trapped in high- X Mg Pargasite in MgO-rich rocks.
Tetsumaru Itaya - One of the best experts on this subject based on the ideXlab platform.
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Stability of Pargasite during ultrahigh-temperature metamorphism: A consequence of titanium and REE partitioning?
American Mineralogist, 2009Co-Authors: Krishnan Sajeev, Yasuhito Osanai, Yoshiaki Kon, Tetsumaru ItayaAbstract:Orthopyroxene-clinopyroxene-plagioclase needles and symplectite along the cleavage planes and grain boundaries of fluorine-bearing titanian-ferroan Pargasite from the Highland Complex, Sri Lanka, are interpreted as evidence for dehydration melting at ultrahigh-temperature conditions. High Ti (up to 0.4 pfu) and F ( X F up to 0.56) content in Pargasite extends its stability to higher temperatures, and the composition indicates the dehydration melting reaction may take place at ultrahigh-temperatures (~950 °C) at a pressure around 10 kbar, close to peak metamorphic conditions. The increase of Ti content close to the grain boundaries and cleavage planes in Pargasite indicates titanium partitioning from the melt during dehydration melting enhanced the stability of the mineral toward ultrahigh-temperature conditions. The REE content in the Pargasite shows a similar behavior to that of titanium. The cores with no breakdown assemblage consist of low and flat REE concentrations with respect to the high and Eu-depleted rim. Clinopyroxene in symplectite and needle-shaped lamellae within the Pargasite porphyroblasts have similar REE patterns with slightly low-concentrations relative to that of Pargasite. In the breakdown assemblage, LREEs are partitioned mainly into plagioclase while the HREEs are partitioned into orthopyroxene. The REE enrichment in the Pargasite rims signals their relative partitioning between Pargasite rims and melt. Modeling of the partitioning of Ti and REEs associated with Pargasite breakdown demonstrates that its stability is greatly enhanced at UHT conditions. This investigation implies that the stability of hydrous minerals such as amphibole can be extended to UHT conditions, and expands our knowledge of metamorphism in the lower crust.