We describe the new mineral species titanium, ideally Ti, found in the podiform chromitites of the Luobusha ophiolite in Tibet, People＇s Republic of China. The irregular crystals range from 0.1 to 0.6 mm in diameter ...We describe the new mineral species titanium, ideally Ti, found in the podiform chromitites of the Luobusha ophiolite in Tibet, People＇s Republic of China. The irregular crystals range from 0.1 to 0.6 mm in diameter and form an intergrowth with coesite and kyanite. Titanium is silver grey in colour, the luster is metallic, it is opaque, the streak is grayish black, and it is non-fluorescent. The mineral is malleable, has a rough to hackly fracture and has no apparent cleavage. The estimated Mohs hardness is 4, and the calculated density is 4.503 g/cm3. The composition is Ti 99.23-100.00 wt%. The mineral is hexagonal, space group P6flmmc. Unit-cell parameters are a 2.950 （2） ~, c 4.686 （1） A,V 35.32 （5） A3, Z = 2. The five strongest powder diffraction lines [d in A （hkl） （I/I0）] are： 2.569 （010） （32）, 2.254（011） （100）, 1.730 （012） （16）, 1.478 （110） （21）, and 0.9464 （121） （8）. The species and name were approved by the CNMNC （IMA 2010-044）.展开更多
A new mineral species, named naquite（FeSi）, is found in the podiform chromitites of the Luobusha ophiolite in Qusong County, Tibet, China. The detailed composition is Fe 65.65, Si 32.57 and Al 1.78 wt%. The mineral ...A new mineral species, named naquite（FeSi）, is found in the podiform chromitites of the Luobusha ophiolite in Qusong County, Tibet, China. The detailed composition is Fe 65.65, Si 32.57 and Al 1.78 wt%. The mineral is cubic, space group P213. The irregular crystals range from 15 to 50 μm in diameter and form an intergrowth with luobusaite. Naquite is steel grey in color, opaque, with a metallic lustre and gives a grayish-black streak. The mineral is brittle, has a conchoidal fracture and no apparent cleavage. The estimated Mohs hardness is 6.5, and the calculated density is 6.128 g/cm3. Unit-cell parameters are a 4.486 （4） A, V 90.28 （6）A^3, Z=4. The five strongest powder diffraction lines [d inA（hkl） （I/I0）] are： 3.1742 （110） （40）, 2.5917（111） （43）, 2.0076 （210） （100）, 1.8307 （211） （65）, and 1.1990 （321） （36）. Originally called ＇fersilicite＇, the species and new name have now been approved by the CNMNC （IMA 2010-010）.展开更多
Diamond was found in podiform chromitites of ophiolite and harzburgite from Luobusha, Tibet. There are silicate inclusions in some diamond grains from this area. In the present work, the CCD (charge coupled detector) ...Diamond was found in podiform chromitites of ophiolite and harzburgite from Luobusha, Tibet. There are silicate inclusions in some diamond grains from this area. In the present work, the CCD (charge coupled detector) technology of X-ray powder diffraction was applied to the study of the inclusion in diamond from the ophiolite of Tibet. Diffraction patterns are obtained even though the inclusion is only 20 μm in crystal size. The results show that the inclusion in diamond consists of talc and clinochrysotile. Therefore, it is clear that the diamond from the ophiolite of Luobusha, Tibet, is natural diamond rather than a synthetic one.展开更多
The tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity(fO2)(e.g., Ballhaus, 1993;Dare et al., 2009;Parkinson and ...The tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity(fO2)(e.g., Ballhaus, 1993;Dare et al., 2009;Parkinson and Arculus, 1999). Here we present results of fO2 of chromites determined by M?ssbauer spectroscopy from both the Luobusha and Dazhuqu areas along Yarlung Zangbo suture zone, Southern Tibet. The fO2 values(-1.02~0.04 log units against the FMQ buffer) and Cr#(22~54%) in chromites from lherzolites and harzburgites of both areas are similar to those of abyssal peridotites, indicating that they may be residues after partial melting at spreading centers. However, both dunite envelopes and chromitites from Luobusha have high fO2 values(0.04~2.25 log units) and Cr#(73~84%), showing an affinity to boninitic melts, and thus form in a suprasubduction zone. Dazhuqu dunites show diverse fO2 values(-0.22~2.19 log units) and Cr#(22~82%), indicating that they form in distinct settings. Chromitites and chromite dunites from Dazhuqu have low fO2 values(-0.3~0.71 log units) and Cr#(16~63%), both of which are similar to those of MORB-like basalts, inferring that they form in an extensional setting. Both high-Cr and high-Al chromitites from other typical podiform chromite ore deposits, such as Kempirsai, Oman, and Albania ophiolites, also show high fO2 values(e.g., Chashchukhin and Votyakov, 2009;Melcher et al., 1997;Quintiliani et al., 2006;Rollinson and Adetunji, 2015), while the distribution-limited small chromitites and chromite dunites from Dazhuqu exhibit low fO2 values. The phenomenon infers that the suprasubduction zone is more beneficial to the formation of podiform chromitites.展开更多
Podiform chromitites crop out in ophiolitic harzburgites as pod-like bodies associated with dunite envelopes with various thickness. It is widely accepted that the change of melt compositions caused by melt-rock react...Podiform chromitites crop out in ophiolitic harzburgites as pod-like bodies associated with dunite envelopes with various thickness. It is widely accepted that the change of melt compositions caused by melt-rock reaction, especially an increase in silica content, plays a crucial role in the generation of podiform chromitite(e.g., Arai and Yurimoto, 1994;Zhou et al., 1994). Due to the presence of ultrahigh pressure and highly reduced minerals, the genesis of some podiform chromitites was attributed to some deep processes(e.g., Arai, 2013;Yang et al., 2007). Although much progress has been achieved, the formation mechanism of podiform chromitites are still in dispute. Iron isotope may be a potential tool to give further insight to the issue, given that some high temperature processes, such as partial melting, metasomatism, magma differentiation and redox change, can result in measurable iron isotopic fractionation to different extent(e.g. Chen et al., 2014;Weyer and Ionov, 2007;Zhao et al., 2009). This study investigates the Fe isotope compositions of chromitites and chromite dunites from Dazhuqu and Luobusha ophiolites. For Dazhuqu chromite dunites, δ56 Fe(relative to the standard, IRMM-014) values range from-0.02‰ to 0.11‰ in olivines and from 0.03‰ to 0.08‰ in chromites. Chromites in Dazhuqu chromitites show δ56 Fe values varying from-0.03‰ to 0.02‰. In nodular and densely disseminated chromitites from Luobusha, olivines have δ56 Fe values of olivines and chromites are 0.09–0.35‰ and-0.15–0.08 ‰, respectively. Chromites from Luobusha massive chromitites have δ56 Fe values of 0.07–0.12 ‰. Based on theorical calculations, chromites should be heavier than olivines in Fe isotope compositions ?56 FeOl-Chr ≈-0.08‰ at 1300 ℃ according to the ionic model(e.g., Macris et al., 2015;Sossi and O’Neill, 2017). However, most of our samples, except for two samples, have ?56 FeOl-Chr values that are greater than zero, indicating a disequilibrium inter-mineral Fe isotopic fractionation. There is a positive correlation between Fo and δ56 Fe(or ?56 FeOl-Chr) of olivines but no positive correlation between Mg# and δ56 Fe(or ?56 FeOl-Chr) of chromites. This phenomenon suggests that the Fe isotopic dis-equilibration may be caused by migrating melts in dunitic channels rather than by the sub-solidus Fe-Mg exchange(Xiao et al., 2016;Zhang et al., 2019). Additionally, the wide δ56 Fe range of chromites is similar to those of the subduction-related basalts and boninites, inferring that their parental magmas form in the suprasubduction zone.展开更多
We report a combined internal and whole-rock Sm-Nd isochron age, and Nd and Pb isotopic data for gabbro dikes of the Luobusha ophiolite in Tibet. The Sm-Nd isochron of data for two whole rocks and plagioclase and clin...We report a combined internal and whole-rock Sm-Nd isochron age, and Nd and Pb isotopic data for gabbro dikes of the Luobusha ophiolite in Tibet. The Sm-Nd isochron of data for two whole rocks and plagioclase and clinopyroxene separates from one of the rocks yields a Middle Jurassic age of (177±31) Ma (with an initial εNd(t) = +8), which provides a significant bound on the time of formation of the Luobusha ophiolite. The Nd and Pb isotopic charac-teristics of the dike indicate an Indian-Ocean-type isotopic affinity, and we conclude that the Luobusha ophiolite formed in an oceanic setting during the Middle Jurassic.展开更多
基金supported by the National Natural Science Foundation of China (40472025, 40620120098,40872043)
文摘We describe the new mineral species titanium, ideally Ti, found in the podiform chromitites of the Luobusha ophiolite in Tibet, People＇s Republic of China. The irregular crystals range from 0.1 to 0.6 mm in diameter and form an intergrowth with coesite and kyanite. Titanium is silver grey in colour, the luster is metallic, it is opaque, the streak is grayish black, and it is non-fluorescent. The mineral is malleable, has a rough to hackly fracture and has no apparent cleavage. The estimated Mohs hardness is 4, and the calculated density is 4.503 g/cm3. The composition is Ti 99.23-100.00 wt%. The mineral is hexagonal, space group P6flmmc. Unit-cell parameters are a 2.950 （2） ~, c 4.686 （1） A,V 35.32 （5） A3, Z = 2. The five strongest powder diffraction lines [d in A （hkl） （I/I0）] are： 2.569 （010） （32）, 2.254（011） （100）, 1.730 （012） （16）, 1.478 （110） （21）, and 0.9464 （121） （8）. The species and name were approved by the CNMNC （IMA 2010-044）.
基金supported financially by the National Natural Science Foundation of China (Grant No. 40472025, 40620120098,40872043)
文摘A new mineral species, named naquite（FeSi）, is found in the podiform chromitites of the Luobusha ophiolite in Qusong County, Tibet, China. The detailed composition is Fe 65.65, Si 32.57 and Al 1.78 wt%. The mineral is cubic, space group P213. The irregular crystals range from 15 to 50 μm in diameter and form an intergrowth with luobusaite. Naquite is steel grey in color, opaque, with a metallic lustre and gives a grayish-black streak. The mineral is brittle, has a conchoidal fracture and no apparent cleavage. The estimated Mohs hardness is 6.5, and the calculated density is 6.128 g/cm3. Unit-cell parameters are a 4.486 （4） A, V 90.28 （6）A^3, Z=4. The five strongest powder diffraction lines [d inA（hkl） （I/I0）] are： 3.1742 （110） （40）, 2.5917（111） （43）, 2.0076 （210） （100）, 1.8307 （211） （65）, and 1.1990 （321） （36）. Originally called ＇fersilicite＇, the species and new name have now been approved by the CNMNC （IMA 2010-010）.
基金supported by the National Natural Science Foundation of China grants 4997203 and 49872019.
文摘Diamond was found in podiform chromitites of ophiolite and harzburgite from Luobusha, Tibet. There are silicate inclusions in some diamond grains from this area. In the present work, the CCD (charge coupled detector) technology of X-ray powder diffraction was applied to the study of the inclusion in diamond from the ophiolite of Tibet. Diffraction patterns are obtained even though the inclusion is only 20 μm in crystal size. The results show that the inclusion in diamond consists of talc and clinochrysotile. Therefore, it is clear that the diamond from the ophiolite of Luobusha, Tibet, is natural diamond rather than a synthetic one.
基金granted by the China Geological Survey(Grant No.121201102000150069)
文摘The tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity(fO2)(e.g., Ballhaus, 1993;Dare et al., 2009;Parkinson and Arculus, 1999). Here we present results of fO2 of chromites determined by M?ssbauer spectroscopy from both the Luobusha and Dazhuqu areas along Yarlung Zangbo suture zone, Southern Tibet. The fO2 values(-1.02~0.04 log units against the FMQ buffer) and Cr#(22~54%) in chromites from lherzolites and harzburgites of both areas are similar to those of abyssal peridotites, indicating that they may be residues after partial melting at spreading centers. However, both dunite envelopes and chromitites from Luobusha have high fO2 values(0.04~2.25 log units) and Cr#(73~84%), showing an affinity to boninitic melts, and thus form in a suprasubduction zone. Dazhuqu dunites show diverse fO2 values(-0.22~2.19 log units) and Cr#(22~82%), indicating that they form in distinct settings. Chromitites and chromite dunites from Dazhuqu have low fO2 values(-0.3~0.71 log units) and Cr#(16~63%), both of which are similar to those of MORB-like basalts, inferring that they form in an extensional setting. Both high-Cr and high-Al chromitites from other typical podiform chromite ore deposits, such as Kempirsai, Oman, and Albania ophiolites, also show high fO2 values(e.g., Chashchukhin and Votyakov, 2009;Melcher et al., 1997;Quintiliani et al., 2006;Rollinson and Adetunji, 2015), while the distribution-limited small chromitites and chromite dunites from Dazhuqu exhibit low fO2 values. The phenomenon infers that the suprasubduction zone is more beneficial to the formation of podiform chromitites.
基金granted by the China Geological Survey(Grant No.121201102000150069)
文摘Podiform chromitites crop out in ophiolitic harzburgites as pod-like bodies associated with dunite envelopes with various thickness. It is widely accepted that the change of melt compositions caused by melt-rock reaction, especially an increase in silica content, plays a crucial role in the generation of podiform chromitite(e.g., Arai and Yurimoto, 1994;Zhou et al., 1994). Due to the presence of ultrahigh pressure and highly reduced minerals, the genesis of some podiform chromitites was attributed to some deep processes(e.g., Arai, 2013;Yang et al., 2007). Although much progress has been achieved, the formation mechanism of podiform chromitites are still in dispute. Iron isotope may be a potential tool to give further insight to the issue, given that some high temperature processes, such as partial melting, metasomatism, magma differentiation and redox change, can result in measurable iron isotopic fractionation to different extent(e.g. Chen et al., 2014;Weyer and Ionov, 2007;Zhao et al., 2009). This study investigates the Fe isotope compositions of chromitites and chromite dunites from Dazhuqu and Luobusha ophiolites. For Dazhuqu chromite dunites, δ56 Fe(relative to the standard, IRMM-014) values range from-0.02‰ to 0.11‰ in olivines and from 0.03‰ to 0.08‰ in chromites. Chromites in Dazhuqu chromitites show δ56 Fe values varying from-0.03‰ to 0.02‰. In nodular and densely disseminated chromitites from Luobusha, olivines have δ56 Fe values of olivines and chromites are 0.09–0.35‰ and-0.15–0.08 ‰, respectively. Chromites from Luobusha massive chromitites have δ56 Fe values of 0.07–0.12 ‰. Based on theorical calculations, chromites should be heavier than olivines in Fe isotope compositions ?56 FeOl-Chr ≈-0.08‰ at 1300 ℃ according to the ionic model(e.g., Macris et al., 2015;Sossi and O’Neill, 2017). However, most of our samples, except for two samples, have ?56 FeOl-Chr values that are greater than zero, indicating a disequilibrium inter-mineral Fe isotopic fractionation. There is a positive correlation between Fo and δ56 Fe(or ?56 FeOl-Chr) of olivines but no positive correlation between Mg# and δ56 Fe(or ?56 FeOl-Chr) of chromites. This phenomenon suggests that the Fe isotopic dis-equilibration may be caused by migrating melts in dunitic channels rather than by the sub-solidus Fe-Mg exchange(Xiao et al., 2016;Zhang et al., 2019). Additionally, the wide δ56 Fe range of chromites is similar to those of the subduction-related basalts and boninites, inferring that their parental magmas form in the suprasubduction zone.
基金This work was supportedby both the National Natural Science Foundation of China and the National Key Project for Basic Research on the Tibetan Plateau (Grant Nos. 49772107, 49802005 and G1998040800) the U.S. National Science Foundation (EAR-9805318)
文摘We report a combined internal and whole-rock Sm-Nd isochron age, and Nd and Pb isotopic data for gabbro dikes of the Luobusha ophiolite in Tibet. The Sm-Nd isochron of data for two whole rocks and plagioclase and clinopyroxene separates from one of the rocks yields a Middle Jurassic age of (177±31) Ma (with an initial εNd(t) = +8), which provides a significant bound on the time of formation of the Luobusha ophiolite. The Nd and Pb isotopic charac-teristics of the dike indicate an Indian-Ocean-type isotopic affinity, and we conclude that the Luobusha ophiolite formed in an oceanic setting during the Middle Jurassic.
