Melting experiments on ultramafic rocks rich in the hydrous minerals phlogopite or phlogopite+K-rich terite,some including 5%of accessory phases,have been conducted at 15 and 50 kbar.The assemblages represent probable...Melting experiments on ultramafic rocks rich in the hydrous minerals phlogopite or phlogopite+K-rich terite,some including 5%of accessory phases,have been conducted at 15 and 50 kbar.The assemblages represent probable source components that contribute to melts in cratonic regions,but whose melt compositions are poorly known.A main series of starting compositions based on MARID xenoliths consisted of a third each of clinopyroxene(CPX),phlogopite(PHL)and K-richterite(KR)with or without 5%ilmenite,rutile or apatite.Additional experiments were run without KR and with higher proportions of accessory phases.Melt traps were used at near-solidus temperatures to facilitate accurate analysis of wellquenched melts,for which reversal experiments demonstrate equilibrium.Results show that KR melts rapidly and completely within 50°C of the solidus,so that melts reflect the composition of the amphibole and its melting reaction.Melts have high SiO_(2) and especially K_(2)O but low CaO and Al_(2)O_(3) relative to basaltic melts produced from peridotites at similar pressures.They have no counterparts amongst natural rocks,but most closely resemble leucite lamproites at 15 kbar.KR and PHL melt incongruently to form olivine(OL)and CPX at 15 kbar,promoting SiO2 contents of the melt,whereas orthopyroxene OPX is increasingly stable at lower lithosphere pressures,leading to an increase in Mg O and decrease in SiO_(2) in melts,which resemble olivine lamproites.Melts of mica pyroxenites without KR are richer in CaO and Al_(2)O_(3) and do not resemble lamproites.These experiments show that low CaO and Al_(2)O_(3) in igneous rocks is not necessarily a sign of a depleted peridotite source.Accessory phases produce melts exceptionally rich in P_(2)O_(5) or TiO_(2) depending on the phases present and are unlike any melts seen at the Earth’s surface,but may be important agents of metasomatism seen in xenoliths.The addition of the 5%accessory phases ilmenite,rutile or apatite result in melting temperatures a few ten of degrees lower;at least two of these appear essential to explain the compositions of many alkaline igneous rocks on cratons.Melting temperatures for CPX+PHL+KR mixtures are close to cratonic geotherms at depths>130 km:minor perturbations of the stable geotherm at>150 km will rapidly lead to 20%melting.Melts of hydrous pyroxenites with a variety of accessory phases will be common initial melts at depth,but will change if reaction with wall-rocks occurs,leading to volcanism that contains chemical components of peridotite even though the temperature in the source region remains well below the melting point of peridotite.At higher temperatures,extensive melting of peridotite will dilute the initial alkaline melts:this is recognizable as alkaline components in basalts and,in extreme cases,alkali picrites.Hydrous pyroxenites are,therefore,components of most mantle-derived igneous rocks:basaltic rocks should not be oversimplified as being purely melts of peridotite or of mixtures of peridotite and dry pyroxenite without hydrous phases.展开更多
The northeastern(NE)Tibetan Plateau is extruding eastward at a rapid rate(∼15 mm a^(−1)),but the role of the upper mantle in this process remains unclear.Early-Miocene primary melilitites from the leading edge of the...The northeastern(NE)Tibetan Plateau is extruding eastward at a rapid rate(∼15 mm a^(−1)),but the role of the upper mantle in this process remains unclear.Early-Miocene primary melilitites from the leading edge of the extruding plateau provide critical insights into the upper mantle dynamics.Geochemical and Sr–Nd–Pb–Os isotopic data,supported by experimental melt comparisons,reveal that these melilitites originate from a hybrid source of CO_(2)-bearing mantle source,probably dominated by peridotite and pyroxenite/wehrlite lithologies.This is consistent with carbonate minerals found in mantle xenoliths(peridotite+pyroxenite)entrained within the melilitites.Geothermobarometric calculations indicate magma generation at 116–135 km depth,below the lithosphere-asthenosphere boundary(∼112 km)constrained by xenoliths and seismic data.Isotopic data trace the carbon origin to a carbonated lithosphere associated with the 150-km-thick eastern tectonic blocks.Recent seismological studies suggest that eastward-flowing asthenosphere beneath the northeastern Tibetan Plateau is actively eroding the thicker lithosphere(150–200 km)of the eastern blocks.We propose that this asthenospheric flow not only thins the lithosphere but also mobilizes carbon from the eastern carbonated lithosphere into generating the melilitite,likely via edge-driven convection.The resulting melilitite compositions is therefore a petrological record of these dynamic processes.This study highlights the critical role of upper mantle processes—astenospheric flow and lithospheric erosion—in driving the eastward extrusion of the Tibetan Plateau.It also underscores the importance of carbon mobilization in understanding mantle carbon cycling during continental collision.展开更多
Mantle-derived primary magmas generally have FeOtot contents of about 11 wt%.However,some primary melts show extremely high FeO^tot(>13 wt%)with high MgO(>12 wt%)(ferropicrite)[1].Considering the influence of ch...Mantle-derived primary magmas generally have FeOtot contents of about 11 wt%.However,some primary melts show extremely high FeO^tot(>13 wt%)with high MgO(>12 wt%)(ferropicrite)[1].Considering the influence of chemical and mineral compositions of the mantle source on melt compositions,it has been argued that ferropicrites may be near-primary partial melts of pyroxenite formed in the convecting mantle[2–4],whereas others favor an origin by partial melting of an iron-rich peridotitic mantle source[5,6].Melting experiments have shown that melting conditions such as pressure and temperature can strongly influence the melt compositions,such that ferropicrites may be generated by partial melting of an olivine-dominated mantle source at^5 GPa[5].Some ferropicrites seem to be more oxidized than other magmas,as suggested from the Panzhihua intrusions in the Emeishan large igneous province[7],which may imply melting of a more oxidized mantle source.Furthermore,Johnston and Stout[8]showed that oxygen fugacity exerts great control on the compositions and stabilities of Cr-Fe-rich minerals,and may therefore significantly affect mantle-derived melt compositions.Local oxygen fugacity(fO2)and solidus temperature of the mantle can be dramatically affected by recycled sedimentary carbonates.This raises the question as to whether sedimentary carbonate recycling could have indirectly contributed to the formation of iron-rich melts in the mantle.展开更多
The late Ediacaran Shuram Excursion(SE)records the most prominent negativeδ^(13)C excursions(δ^(13)C=-12‰)during Earth’s history.It has been hypothesized to have resulted from oxidation of dissolved organic matter...The late Ediacaran Shuram Excursion(SE)records the most prominent negativeδ^(13)C excursions(δ^(13)C=-12‰)during Earth’s history.It has been hypothesized to have resulted from oxidation of dissolved organic matter,diagenetic or authigenic precipitates.However,the origin of the SE remains enigmatic;current models face challenges regarding the significant amount of atmospheric oxygen needed to balance such extensive oxidation and sustained inputs of light carbon with extremely negative C isotope compositions.Here,we show that the Doushantuo Formation at the Jiulongwan section in South China,a key stratum recording the SE event,contains mineralogical and geochemical signatures related to igneous processes.Both the occurrence of ankerite,feldspar,moissanite and euhedral quartz in the SE samples and the relatively consistent Ce anomalies of carbonate and O isotopes of quartz indicate a contribution from an igneous source.In particular,the SE samples have trace element and C isotope compositions similar to those of recycled carbonatites formed by decarbonation and melting of sedimentary carbonate rocks.These observations suggest that the deep cycle of ancient carbonate rocks,which were subjected to decarbonation during subduction,melting and eruption related to the breakup of the Rodinia supercontinent,contributed to the SE.This igneous model for the SE may provide a connection between the deep and shallow carbon cycles of the Earth.展开更多
The trace element compositions of melts and minerals from high-pressure experiments on hydrous pyroxenites containing K-richterite are presented. The experiments used mixtures of a third each of the natural minerals c...The trace element compositions of melts and minerals from high-pressure experiments on hydrous pyroxenites containing K-richterite are presented. The experiments used mixtures of a third each of the natural minerals clinopyroxene, phlogopite and K-richterite, some with the addition of 5% of an accessory phase ilmenite, rutile or apatite. Although the major element compositions of melts resemble natural lamproites, the trace element contents of most trace elements from the three-mineral mixture are much lower than in lamproites. Apatite is required in the source to provide high abundances of the rare earth elements, and either rutile and/or ilmenite is required to provide the high field strength elements Ti, Nb, Ta, Zr and Hf. Phlogopite controls the high levels of Rb, Cs and Ba.Since abundances of trace elements in the various starting mixtures vary strongly because of the use of natural minerals, we calculated mineral/melt partition coefficients (DMin/melt) using mineral modes and melting reactions and present trace element patterns for different degrees of partial melting of hydrous pyroxenites. Rb, Cs and Ba are compatible in phlogopite and the partition coefficient ratio phlogopite/K-richterite is high for Ba (136) and Rb (12). All melts have low contents of most of the first row transition elements, particularly Ni and Cu ((0.1-0.01)×primitive mantle). Nickel has high DMin/melt for all the major minerals (12 for K-richterite, 9.2 for phlogopite and 5.6 for Cpx) and so behaves at least as compatibly as in melting of peridotites. Fluorine/chlorine ratios in melts are high and DMin/melt for fluorine decreases in the order apatite (2.2) > phlogopite (1.5) > K-richterite (0.87). The requirement for apatite and at least one Ti-oxide in the source of natural lamproites holds for mica pyroxenites that lack K-richterite. The results are used to model isotopic ageing in hydrous pyroxenite source rocks: phlogopite controls Sr isotopes, so that lamproites with relatively low 87Sr/86Sr must come from phlogopite-poor source rocks, probably dominated by Cpx and K-richterite. At high pressures (>4 GPa), peritectic Cpx holds back Na, explaining the high K2O/Na2O of lamproites.展开更多
基金funded by grants from the Deutsche Forschungsgemeinschaft(Fo 181/3)the Australian Research Council(FL180100134)。
文摘Melting experiments on ultramafic rocks rich in the hydrous minerals phlogopite or phlogopite+K-rich terite,some including 5%of accessory phases,have been conducted at 15 and 50 kbar.The assemblages represent probable source components that contribute to melts in cratonic regions,but whose melt compositions are poorly known.A main series of starting compositions based on MARID xenoliths consisted of a third each of clinopyroxene(CPX),phlogopite(PHL)and K-richterite(KR)with or without 5%ilmenite,rutile or apatite.Additional experiments were run without KR and with higher proportions of accessory phases.Melt traps were used at near-solidus temperatures to facilitate accurate analysis of wellquenched melts,for which reversal experiments demonstrate equilibrium.Results show that KR melts rapidly and completely within 50°C of the solidus,so that melts reflect the composition of the amphibole and its melting reaction.Melts have high SiO_(2) and especially K_(2)O but low CaO and Al_(2)O_(3) relative to basaltic melts produced from peridotites at similar pressures.They have no counterparts amongst natural rocks,but most closely resemble leucite lamproites at 15 kbar.KR and PHL melt incongruently to form olivine(OL)and CPX at 15 kbar,promoting SiO2 contents of the melt,whereas orthopyroxene OPX is increasingly stable at lower lithosphere pressures,leading to an increase in Mg O and decrease in SiO_(2) in melts,which resemble olivine lamproites.Melts of mica pyroxenites without KR are richer in CaO and Al_(2)O_(3) and do not resemble lamproites.These experiments show that low CaO and Al_(2)O_(3) in igneous rocks is not necessarily a sign of a depleted peridotite source.Accessory phases produce melts exceptionally rich in P_(2)O_(5) or TiO_(2) depending on the phases present and are unlike any melts seen at the Earth’s surface,but may be important agents of metasomatism seen in xenoliths.The addition of the 5%accessory phases ilmenite,rutile or apatite result in melting temperatures a few ten of degrees lower;at least two of these appear essential to explain the compositions of many alkaline igneous rocks on cratons.Melting temperatures for CPX+PHL+KR mixtures are close to cratonic geotherms at depths>130 km:minor perturbations of the stable geotherm at>150 km will rapidly lead to 20%melting.Melts of hydrous pyroxenites with a variety of accessory phases will be common initial melts at depth,but will change if reaction with wall-rocks occurs,leading to volcanism that contains chemical components of peridotite even though the temperature in the source region remains well below the melting point of peridotite.At higher temperatures,extensive melting of peridotite will dilute the initial alkaline melts:this is recognizable as alkaline components in basalts and,in extreme cases,alkali picrites.Hydrous pyroxenites are,therefore,components of most mantle-derived igneous rocks:basaltic rocks should not be oversimplified as being purely melts of peridotite or of mixtures of peridotite and dry pyroxenite without hydrous phases.
基金supported by the National Natural Science Foundation of China(42425205)the Australian Research Council grant(180100134)the Opening Foundation of the State Key Laboratory of Continental Dynamics,and Northwest University(22LCD12).
文摘The northeastern(NE)Tibetan Plateau is extruding eastward at a rapid rate(∼15 mm a^(−1)),but the role of the upper mantle in this process remains unclear.Early-Miocene primary melilitites from the leading edge of the extruding plateau provide critical insights into the upper mantle dynamics.Geochemical and Sr–Nd–Pb–Os isotopic data,supported by experimental melt comparisons,reveal that these melilitites originate from a hybrid source of CO_(2)-bearing mantle source,probably dominated by peridotite and pyroxenite/wehrlite lithologies.This is consistent with carbonate minerals found in mantle xenoliths(peridotite+pyroxenite)entrained within the melilitites.Geothermobarometric calculations indicate magma generation at 116–135 km depth,below the lithosphere-asthenosphere boundary(∼112 km)constrained by xenoliths and seismic data.Isotopic data trace the carbon origin to a carbonated lithosphere associated with the 150-km-thick eastern tectonic blocks.Recent seismological studies suggest that eastward-flowing asthenosphere beneath the northeastern Tibetan Plateau is actively eroding the thicker lithosphere(150–200 km)of the eastern blocks.We propose that this asthenospheric flow not only thins the lithosphere but also mobilizes carbon from the eastern carbonated lithosphere into generating the melilitite,likely via edge-driven convection.The resulting melilitite compositions is therefore a petrological record of these dynamic processes.This study highlights the critical role of upper mantle processes—astenospheric flow and lithospheric erosion—in driving the eastward extrusion of the Tibetan Plateau.It also underscores the importance of carbon mobilization in understanding mantle carbon cycling during continental collision.
基金This work was supported by the National Natural Science Foundation of China(41530211 and 41703015)the State Administration of Foreign Expert Affairs of China(BP0719022)+4 种基金MOST Special Funds of the State Key Laboratory of Geological Processes and Mineral Resources(MSFGPMR01)and the Fundamental Research Funds for the Central Universities(CUGL170801)Mihai N.Ducea acknowledges support from US National Science Foundation(EAR 1725002)the Romanian Executive Agency for Higher Education,Research,Development and Innovation Funding Project(PN-III-P4-ID-PCCF-2016-0014)We thank anonymous reviewers and Prof.Jussi S.Heinonen for constructive commentsWe thank Drs.Junpeng Wang and Wenbin Ning for the Electron Probe Micro Analysis.
文摘Mantle-derived primary magmas generally have FeOtot contents of about 11 wt%.However,some primary melts show extremely high FeO^tot(>13 wt%)with high MgO(>12 wt%)(ferropicrite)[1].Considering the influence of chemical and mineral compositions of the mantle source on melt compositions,it has been argued that ferropicrites may be near-primary partial melts of pyroxenite formed in the convecting mantle[2–4],whereas others favor an origin by partial melting of an iron-rich peridotitic mantle source[5,6].Melting experiments have shown that melting conditions such as pressure and temperature can strongly influence the melt compositions,such that ferropicrites may be generated by partial melting of an olivine-dominated mantle source at^5 GPa[5].Some ferropicrites seem to be more oxidized than other magmas,as suggested from the Panzhihua intrusions in the Emeishan large igneous province[7],which may imply melting of a more oxidized mantle source.Furthermore,Johnston and Stout[8]showed that oxygen fugacity exerts great control on the compositions and stabilities of Cr-Fe-rich minerals,and may therefore significantly affect mantle-derived melt compositions.Local oxygen fugacity(fO2)and solidus temperature of the mantle can be dramatically affected by recycled sedimentary carbonates.This raises the question as to whether sedimentary carbonate recycling could have indirectly contributed to the formation of iron-rich melts in the mantle.
基金supported by the Key Research&Development Program of China(2019YFA0708400)the National Natural Science Foundation of China(41530211)+1 种基金MOST(Ministry of Science and Technology)Special Fund from the State Key Laboratory of Geological Processes and Mineral Resources(MSFGPMR01)Australian Research Council(FL180100134)。
文摘The late Ediacaran Shuram Excursion(SE)records the most prominent negativeδ^(13)C excursions(δ^(13)C=-12‰)during Earth’s history.It has been hypothesized to have resulted from oxidation of dissolved organic matter,diagenetic or authigenic precipitates.However,the origin of the SE remains enigmatic;current models face challenges regarding the significant amount of atmospheric oxygen needed to balance such extensive oxidation and sustained inputs of light carbon with extremely negative C isotope compositions.Here,we show that the Doushantuo Formation at the Jiulongwan section in South China,a key stratum recording the SE event,contains mineralogical and geochemical signatures related to igneous processes.Both the occurrence of ankerite,feldspar,moissanite and euhedral quartz in the SE samples and the relatively consistent Ce anomalies of carbonate and O isotopes of quartz indicate a contribution from an igneous source.In particular,the SE samples have trace element and C isotope compositions similar to those of recycled carbonatites formed by decarbonation and melting of sedimentary carbonate rocks.These observations suggest that the deep cycle of ancient carbonate rocks,which were subjected to decarbonation during subduction,melting and eruption related to the breakup of the Rodinia supercontinent,contributed to the SE.This igneous model for the SE may provide a connection between the deep and shallow carbon cycles of the Earth.
基金supported by the National Natural Science Foundation of China(42288201,41725014,and 41888101)the State Key Laboratory of Lithospheric Evolution(SKL-Z202302)the China Geological Survey Project(DD20190098).
基金funded by grants from the Deutsche Forschungsgemeinschaft,Germany (Fo 181/3)and the Australian Research Council,Australia (FL180100134).
文摘The trace element compositions of melts and minerals from high-pressure experiments on hydrous pyroxenites containing K-richterite are presented. The experiments used mixtures of a third each of the natural minerals clinopyroxene, phlogopite and K-richterite, some with the addition of 5% of an accessory phase ilmenite, rutile or apatite. Although the major element compositions of melts resemble natural lamproites, the trace element contents of most trace elements from the three-mineral mixture are much lower than in lamproites. Apatite is required in the source to provide high abundances of the rare earth elements, and either rutile and/or ilmenite is required to provide the high field strength elements Ti, Nb, Ta, Zr and Hf. Phlogopite controls the high levels of Rb, Cs and Ba.Since abundances of trace elements in the various starting mixtures vary strongly because of the use of natural minerals, we calculated mineral/melt partition coefficients (DMin/melt) using mineral modes and melting reactions and present trace element patterns for different degrees of partial melting of hydrous pyroxenites. Rb, Cs and Ba are compatible in phlogopite and the partition coefficient ratio phlogopite/K-richterite is high for Ba (136) and Rb (12). All melts have low contents of most of the first row transition elements, particularly Ni and Cu ((0.1-0.01)×primitive mantle). Nickel has high DMin/melt for all the major minerals (12 for K-richterite, 9.2 for phlogopite and 5.6 for Cpx) and so behaves at least as compatibly as in melting of peridotites. Fluorine/chlorine ratios in melts are high and DMin/melt for fluorine decreases in the order apatite (2.2) > phlogopite (1.5) > K-richterite (0.87). The requirement for apatite and at least one Ti-oxide in the source of natural lamproites holds for mica pyroxenites that lack K-richterite. The results are used to model isotopic ageing in hydrous pyroxenite source rocks: phlogopite controls Sr isotopes, so that lamproites with relatively low 87Sr/86Sr must come from phlogopite-poor source rocks, probably dominated by Cpx and K-richterite. At high pressures (>4 GPa), peritectic Cpx holds back Na, explaining the high K2O/Na2O of lamproites.
