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Mei-fu Zhou - One of the best experts on this subject based on the ideXlab platform.

  • Using chalcophile elements to constrain Crustal Contamination and xenolith-magma interaction in Cenozoic basalts of eastern China
    Lithos, 2016
    Co-Authors: Gang Zeng, Mei-fu Zhou, Xiao Wen Huang, Li-hui Chen
    Abstract:

    Abstract Continental basalts have complicated petrogenetic processes, and their chemical compositions can be affected by multi-staged geological evolution. Compared to lithophile elements, chalcophile elements including Ni, platinum-group elements (PGEs) and Cu are sensitive to sulfide segregation and fractional crystallization during the evolution of mantle-derived magmas and can provide constraints on the genesis of continental basalts. Cenozoic intra-continental alkaline basalts in the Nanjing basaltic field, eastern China, include high-Ca and low-Ca varieties. All these basalts have poor PGE contents with Ir ranging from 0.016 ppb to 0.288 ppb and high Cu/Pd ratios from 0.7 × 10 5 to 4.7 × 10 5 (5.7 × 10 3 for DMM), indicating that they were derived from sulfide-saturated mantle sources with variable amounts of residual sulfide during melting or might undergo an early-sulfide segregation in the mantle. Relatively high Cu/Pd ratios along with high Pd concentrations for the high-Ca alkaline basalts indicate an additional removal of sulfide during magma ascent. Because these basalts have high, variable Pd/Ir ratios (2.8–16.8) with low Ce/Pb (9.9–19.7) ratios and e Nd values (+ 3.6–+6.4), Crustal Contamination is proposed to be a potential process to induce the sulfide saturation and removal. Significantly increased Pd/Ir ratios for few high-Ca basalts can be explained by the fractionation of laurite or Ru-Os-Ir alloys with olivine or chromite. For low-Ca alkaline basalts, their PGE contents are well correlated with the MgO, Sc contents, incompatible element ratios (Lu/Hf, Na/Ti and Ca/Al) and Hf isotopes. Good correlations are also observed between Pd/Ir (or Rh/Ir) and Na/Ti (or Ca/Al) ratios. Variations of these elemental ratios and Hf isotopes is previously documented to be induced by the mixing of peridotite xenolith-released melts during ascent. Therefore, we suggest that such xenolith–magma interaction are also responsible for the variable PGE compositions of low-Ca alkaline basalts.

  • Heterogeneous Os isotope compositions in the Kalatongke sulfide deposit, NW China: the role of Crustal Contamination
    Mineralium Deposita, 2012
    Co-Authors: Jian-feng Gao, Mei-fu Zhou, Peter C. Lightfoot
    Abstract:

    Re–Os isotope compositions of mantle-derived magmas are highly sensitive to Crustal Contamination because the crust and mantle have very different Os isotope compositions. Crustal Contamination may trigger S saturation and thus the formation of magmatic Ni–Cu–(PGE) sulfide deposits. The ∼287-Ma Kalatongke norite intrusion of NW China are hosted in carboniferous tuffaceous rocks and contain both disseminated and massive sulfide mineralization. The Re–Os isotope compositions in the intrusion are highly variable. Norite and massive sulfide ores have γ Os values ranging from +59 to +160 and a Re–Os isochron age of 239 ± 51 Ma, whereas disseminated sulfide ores have γ Os values from +117 to +198 and a Re–Os isochron age of 349 ± 34 Ma. The variability of Os isotope compositions can be explained as the emplacement of two distinct magma pulses. Massive sulfide ores and barren norite in the intrusion formed from the same magma pulse, whereas the disseminated sulfide ores with more radiogenic Os isotopes formed from another magma pulse which underwent different degrees of Crustal Contamination. Re–Os isotopes may not be suitable for dating sulfide-bearing intrusions that underwent variable degrees of Crustal Contamination to form magmatic sulfide deposits.

  • Differentiation, Crustal Contamination and emplacement of magmas in the formation of the Nantianwan mafic intrusion of the ~260 Ma Emeishan large igneous province, SW China
    Contributions to Mineralogy and Petrology, 2012
    Co-Authors: Christina Yan Wang, Mei-fu Zhou, Yali Sun, Nicholas T. Arndt
    Abstract:

    The Nantianwan mafic intrusion in the Panxi region, SW China, part of the ~260 Ma Emeishan large igneous province, consists of the olivine gabbro and gabbronorite units, separated by a transitional zone. Olivine gabbros contain olivine with Fo values ranging from 83 to 87, indicating crystallization from a moderately evolved magma. They have 0.2 to 0.9 wt % sulfide with highly variable PGE (17–151 ppb) and variable Cu/Pd ratios (1,500–32,500). Modeling results indicate that they were derived from picritic magmas with high initial PGE concentrations. Olivine gabbros have negative eNd(t) values (−1.3 to −0.1) and positive γOs(t) values (5–15), consistent with low degrees of Crustal Contamination. Gabbronorites include sulfide-bearing and sulfide-poor varieties, and both have olivine with Fo values ranging from 74 to 79, indicating crystallization from a more evolved magma than that for olivine gabbros. Sulfide-bearing gabbronorites contain 1.9–4.1 wt % sulfide and 37–160 ppb PGE and high Cu/Pd ratios (54,000–624,000). Sulfide-poor gabbronorites have 0.1–0.6 wt % sulfide and 0.2–15 ppb PGE and very high Cu/Pd ratios (16,900–2,370,000). Both sulfide-bearing and sulfide-poor gabbronorites have eNd(t) values (−0.9 to −2.1) similar to those for olivine gabbros, but their γOs(t) values (17–262) are much higher and more variable than those of the olivine gabbros. Selective assimilation of Crustal sulfides from the country rocks is thus considered to have resulted in more radiogenic 187Os of the gabbronorites. Processes such as magma differentiation, Crustal Contamination and sulfide saturation at different stages in magma chambers may have intervened during formation of the intrusion. Parental magmas were derived from picritic magmas that had fractionated olivine under S-undersaturated conditions before entering a deep-seated staging magma chamber, where the parental magmas crystallized olivine, assimilated minor Crustal rocks and reached sulfide saturation, forming an olivine- and sulfide-laden crystal mush in the lower part and evolved magmas in the upper part of the chamber. The evolved magmas were forced out of the staging chamber and became S-undersaturated due to a pressure drop during ascent to a shallow magma chamber. The magmas re-attained sulfide saturation by assimilating external S from S-rich country rocks. They may have entered the shallow magma chamber as several pulses so that several gabbronorite layers each with sulfide segregated to the base and a sulfide-poor upper part. The olivine gabbro unit formed from a new and more primitive magma that entrained olivine crystals and sulfide droplets from the lower part of the staging chamber. A transitional zone formed along the boundary with the gabbronorite unit due to chemical interaction between the two rock units.

  • differentiation Crustal Contamination and emplacement of magmas in the formation of the nantianwan mafic intrusion of the 260 ma emeishan large igneous province sw china
    Contributions to Mineralogy and Petrology, 2012
    Co-Authors: Mei-fu Zhou, Christina Yan Wang, Yali Sun, Nicholas T. Arndt
    Abstract:

    The Nantianwan mafic intrusion in the Panxi region, SW China, part of the ~260 Ma Emeishan large igneous province, consists of the olivine gabbro and gabbronorite units, separated by a transitional zone. Olivine gabbros contain olivine with Fo values ranging from 83 to 87, indicating crystallization from a moderately evolved magma. They have 0.2 to 0.9 wt % sulfide with highly variable PGE (17–151 ppb) and variable Cu/Pd ratios (1,500–32,500). Modeling results indicate that they were derived from picritic magmas with high initial PGE concentrations. Olivine gabbros have negative eNd(t) values (−1.3 to −0.1) and positive γOs(t) values (5–15), consistent with low degrees of Crustal Contamination. Gabbronorites include sulfide-bearing and sulfide-poor varieties, and both have olivine with Fo values ranging from 74 to 79, indicating crystallization from a more evolved magma than that for olivine gabbros. Sulfide-bearing gabbronorites contain 1.9–4.1 wt % sulfide and 37–160 ppb PGE and high Cu/Pd ratios (54,000–624,000). Sulfide-poor gabbronorites have 0.1–0.6 wt % sulfide and 0.2–15 ppb PGE and very high Cu/Pd ratios (16,900–2,370,000). Both sulfide-bearing and sulfide-poor gabbronorites have eNd(t) values (−0.9 to −2.1) similar to those for olivine gabbros, but their γOs(t) values (17–262) are much higher and more variable than those of the olivine gabbros. Selective assimilation of Crustal sulfides from the country rocks is thus considered to have resulted in more radiogenic 187Os of the gabbronorites. Processes such as magma differentiation, Crustal Contamination and sulfide saturation at different stages in magma chambers may have intervened during formation of the intrusion. Parental magmas were derived from picritic magmas that had fractionated olivine under S-undersaturated conditions before entering a deep-seated staging magma chamber, where the parental magmas crystallized olivine, assimilated minor Crustal rocks and reached sulfide saturation, forming an olivine- and sulfide-laden crystal mush in the lower part and evolved magmas in the upper part of the chamber. The evolved magmas were forced out of the staging chamber and became S-undersaturated due to a pressure drop during ascent to a shallow magma chamber. The magmas re-attained sulfide saturation by assimilating external S from S-rich country rocks. They may have entered the shallow magma chamber as several pulses so that several gabbronorite layers each with sulfide segregated to the base and a sulfide-poor upper part. The olivine gabbro unit formed from a new and more primitive magma that entrained olivine crystals and sulfide droplets from the lower part of the staging chamber. A transitional zone formed along the boundary with the gabbronorite unit due to chemical interaction between the two rock units.

  • Selective Crustal Contamination and decoupling of lithophile and chalcophile element isotopes in sulfide-bearing mafic intrusions: An example from the Jingbulake Intrusion, Xinjiang, NW China
    Chemical Geology, 2012
    Co-Authors: Sheng-hong Yang, Mei-fu Zhou, Peter C. Lightfoot, John Malpas, Ji-bing Zhou, De-yi Kong
    Abstract:

    The Jingbulake mafic-ultramafic intrusion in the South Tianshan orogenic belt, Xinjiang, NW China, is a zoned intrusive body composed of gabbro-diorite, olivine gabbro and wehrlite, locally intruded by pyroxenite, within a major W-E structure. Both olivine gabbro and wehrlite contain disseminated Ni-Cu sulfides, whereas pyroxenite hosts an ore body containing massive, net-textured and disseminated sulfide ores. Both silicate rocks and sulfide ores from the Jingbulake intrusion have low Pd/Ir ratios (10-63). The silicate rocks contain Ni-poor olivine (generally 30% Crustal Contamination is required to explain the radiogenic Os isotopic composition if average upper crust is adopted as the Crustal contaminant, or 5%-25% if sulfide-rich Crustal rock is used as the Crustal contaminant. However only 5-10% Crustal Contamination is required to explain the Nd and Sr isotopes. We propose that the decoupling of Os from Sr-Nd isotopes in the silicate rocks was due to selective Crustal Contamination. Addition of external sulfur from a Crustal source may be the key factor triggering sulfide saturation. The silicate magma that underwent sulfide extraction before emplacement to shallow depth preserved a Crustal Os isotope signature. Continued reaction of sulfide melts with new pulses of mantle-derived magma increased the Os content and decreased the 187Os/ 188Os(i) ratios, effectively masking the Crustal Os contribution in the sulfides. © 2011 Elsevier B.V.link_to_subscribed_fulltex

Christina Yan Wang - One of the best experts on this subject based on the ideXlab platform.

  • Differentiation, Crustal Contamination and emplacement of magmas in the formation of the Nantianwan mafic intrusion of the ~260 Ma Emeishan large igneous province, SW China
    Contributions to Mineralogy and Petrology, 2012
    Co-Authors: Christina Yan Wang, Mei-fu Zhou, Yali Sun, Nicholas T. Arndt
    Abstract:

    The Nantianwan mafic intrusion in the Panxi region, SW China, part of the ~260 Ma Emeishan large igneous province, consists of the olivine gabbro and gabbronorite units, separated by a transitional zone. Olivine gabbros contain olivine with Fo values ranging from 83 to 87, indicating crystallization from a moderately evolved magma. They have 0.2 to 0.9 wt % sulfide with highly variable PGE (17–151 ppb) and variable Cu/Pd ratios (1,500–32,500). Modeling results indicate that they were derived from picritic magmas with high initial PGE concentrations. Olivine gabbros have negative eNd(t) values (−1.3 to −0.1) and positive γOs(t) values (5–15), consistent with low degrees of Crustal Contamination. Gabbronorites include sulfide-bearing and sulfide-poor varieties, and both have olivine with Fo values ranging from 74 to 79, indicating crystallization from a more evolved magma than that for olivine gabbros. Sulfide-bearing gabbronorites contain 1.9–4.1 wt % sulfide and 37–160 ppb PGE and high Cu/Pd ratios (54,000–624,000). Sulfide-poor gabbronorites have 0.1–0.6 wt % sulfide and 0.2–15 ppb PGE and very high Cu/Pd ratios (16,900–2,370,000). Both sulfide-bearing and sulfide-poor gabbronorites have eNd(t) values (−0.9 to −2.1) similar to those for olivine gabbros, but their γOs(t) values (17–262) are much higher and more variable than those of the olivine gabbros. Selective assimilation of Crustal sulfides from the country rocks is thus considered to have resulted in more radiogenic 187Os of the gabbronorites. Processes such as magma differentiation, Crustal Contamination and sulfide saturation at different stages in magma chambers may have intervened during formation of the intrusion. Parental magmas were derived from picritic magmas that had fractionated olivine under S-undersaturated conditions before entering a deep-seated staging magma chamber, where the parental magmas crystallized olivine, assimilated minor Crustal rocks and reached sulfide saturation, forming an olivine- and sulfide-laden crystal mush in the lower part and evolved magmas in the upper part of the chamber. The evolved magmas were forced out of the staging chamber and became S-undersaturated due to a pressure drop during ascent to a shallow magma chamber. The magmas re-attained sulfide saturation by assimilating external S from S-rich country rocks. They may have entered the shallow magma chamber as several pulses so that several gabbronorite layers each with sulfide segregated to the base and a sulfide-poor upper part. The olivine gabbro unit formed from a new and more primitive magma that entrained olivine crystals and sulfide droplets from the lower part of the staging chamber. A transitional zone formed along the boundary with the gabbronorite unit due to chemical interaction between the two rock units.

  • differentiation Crustal Contamination and emplacement of magmas in the formation of the nantianwan mafic intrusion of the 260 ma emeishan large igneous province sw china
    Contributions to Mineralogy and Petrology, 2012
    Co-Authors: Mei-fu Zhou, Christina Yan Wang, Yali Sun, Nicholas T. Arndt
    Abstract:

    The Nantianwan mafic intrusion in the Panxi region, SW China, part of the ~260 Ma Emeishan large igneous province, consists of the olivine gabbro and gabbronorite units, separated by a transitional zone. Olivine gabbros contain olivine with Fo values ranging from 83 to 87, indicating crystallization from a moderately evolved magma. They have 0.2 to 0.9 wt % sulfide with highly variable PGE (17–151 ppb) and variable Cu/Pd ratios (1,500–32,500). Modeling results indicate that they were derived from picritic magmas with high initial PGE concentrations. Olivine gabbros have negative eNd(t) values (−1.3 to −0.1) and positive γOs(t) values (5–15), consistent with low degrees of Crustal Contamination. Gabbronorites include sulfide-bearing and sulfide-poor varieties, and both have olivine with Fo values ranging from 74 to 79, indicating crystallization from a more evolved magma than that for olivine gabbros. Sulfide-bearing gabbronorites contain 1.9–4.1 wt % sulfide and 37–160 ppb PGE and high Cu/Pd ratios (54,000–624,000). Sulfide-poor gabbronorites have 0.1–0.6 wt % sulfide and 0.2–15 ppb PGE and very high Cu/Pd ratios (16,900–2,370,000). Both sulfide-bearing and sulfide-poor gabbronorites have eNd(t) values (−0.9 to −2.1) similar to those for olivine gabbros, but their γOs(t) values (17–262) are much higher and more variable than those of the olivine gabbros. Selective assimilation of Crustal sulfides from the country rocks is thus considered to have resulted in more radiogenic 187Os of the gabbronorites. Processes such as magma differentiation, Crustal Contamination and sulfide saturation at different stages in magma chambers may have intervened during formation of the intrusion. Parental magmas were derived from picritic magmas that had fractionated olivine under S-undersaturated conditions before entering a deep-seated staging magma chamber, where the parental magmas crystallized olivine, assimilated minor Crustal rocks and reached sulfide saturation, forming an olivine- and sulfide-laden crystal mush in the lower part and evolved magmas in the upper part of the chamber. The evolved magmas were forced out of the staging chamber and became S-undersaturated due to a pressure drop during ascent to a shallow magma chamber. The magmas re-attained sulfide saturation by assimilating external S from S-rich country rocks. They may have entered the shallow magma chamber as several pulses so that several gabbronorite layers each with sulfide segregated to the base and a sulfide-poor upper part. The olivine gabbro unit formed from a new and more primitive magma that entrained olivine crystals and sulfide droplets from the lower part of the staging chamber. A transitional zone formed along the boundary with the gabbronorite unit due to chemical interaction between the two rock units.

  • Permian flood basalts and mafic intrusions in the Jinping (SW China)–Song Da (northern Vietnam) district: Mantle sources, Crustal Contamination and sulfide segregation
    Chemical Geology, 2007
    Co-Authors: Christina Yan Wang, Mei-fu Zhou
    Abstract:

    The ∼ 260-Ma Emeishan large igneous province (ELIP) in SW China and northern Vietnam includes high-Ti and low-Ti volcanic rocks and coeval mafic–ultramafic plutons. In the Jinping (SW China)–Song Da (northern Vietnam) district, the southernmost part of the ELIP, basalts and gabbros of the high-Ti series displays a narrow range of eNd(t) values (+ 0.5 to + 1.1) and low primitive mantle-normalized Th/Nb ratios (∼ 1.4), similar to those of an OIB-like enriched mantle source. The rocks have chondrite-normalized REE patterns enriched in LREE and depleted in HREE. This feature, together with low Al2O3/TiO2 ratios (∼ 4), indicates that the primary melts were generated by small degrees of partial melting at garnet stable depths. The low-Ti series includes LREE-depleted and LREE-enriched picrites, basalts, olivine gabbros and gabbros. The LREE-depleted picrites have chondrite-normalized Ce/Yb ratios of 0.4–0.5, very positive eNd(t) values (+ 7.0), and very negative Th anomalies in primitive mantle-normalized trace element patterns. They were formed by high degrees of partial melting of a strongly depleted mantle source at relatively shallow mantle depths. The LREE-enriched picrites have very negative eNd(t) values (− 7.8) that indicate a mantle source locally modified by subducted oceanic crust. Highly variable chemical compositions and eNd(t) values (+ 6.4 to − 10.2) of the low-Ti basalts and gabbros are attributed to variable degrees of upper Crustal Contamination of the picritic parental magmas. Therefore, the extrusive and intrusive rocks in this region clearly formed from heterogeneous mantle sources. Crustal Contamination triggered S-saturation of some low-Ti magmas and resulted in segregation of immiscible sulfide melts and generation of magmatic Ni–Cu–(PGE) sulfide deposits in this region. Eruption of magmas after sulfide segregation formed PGE-depleted low-Ti basalts.

  • Permian flood basalts and mafic intrusions in the Jinping (SW China)–Song Da (northern Vietnam) district: Mantle sources, Crustal Contamination and sulfide segregation
    Chemical Geology, 2007
    Co-Authors: Christina Yan Wang, Mei-fu Zhou
    Abstract:

    The ∼ 260-Ma Emeishan large igneous province (ELIP) in SW China and northern Vietnam includes high-Ti and low-Ti volcanic rocks and coeval mafic-ultramafic plutons. In the Jinping (SW China)-Song Da (northern Vietnam) district, the southernmost part of the ELIP, basalts and gabbros of the high-Ti series displays a narrow range of εNd(t) values (+ 0.5 to + 1.1) and low primitive mantle-normalized Th/Nb ratios (∼ 1.4), similar to those of an OIB-like enriched mantle source. The rocks have chondrite-normalized REE patterns enriched in LREE and depleted in HREE. This feature, together with low Al 2O 3/TiO 2 ratios (∼ 4), indicates that the primary melts were generated by small degrees of partial melting at garnet stable depths. The low-Ti series includes LREE-depleted and LREE-enriched picrites, basalts, olivine gabbros and gabbros. The LREE-depleted picrites have chondrite-normalized Ce/Yb ratios of 0.4-0.5, very positive εNd(t) values (+ 7.0), and very negative Th anomalies in primitive mantle-normalized trace element patterns. They were formed by high degrees of partial melting of a strongly depleted mantle source at relatively shallow mantle depths. The LREE-enriched picrites have very negative εNd(t) values (- 7.8) that indicate a mantle source locally modified by subducted oceanic crust. Highly variable chemical compositions and εNd(t) values (+ 6.4 to - 10.2) of the low-Ti basalts and gabbros are attributed to variable degrees of upper Crustal Contamination of the picritic parental magmas. Therefore, the extrusive and intrusive rocks in this region clearly formed from heterogeneous mantle sources. Crustal Contamination triggered S-saturation of some low-Ti magmas and resulted in segregation of immiscible sulfide melts and generation of magmatic Ni-Cu-(PGE) sulfide deposits in this region. Eruption of magmas after sulfide segregation formed PGE-depleted low-Ti basalts. © 2007 Elsevier B.V. All rights reserved.link_to_subscribed_fulltex

Shane J. Cronin - One of the best experts on this subject based on the ideXlab platform.

  • rapid timescales of differentiation and evidence for Crustal Contamination at intra oceanic arcs geochemical and u th ra sr nd isotopic constraints from lopevi volcano vanuatu sw pacific
    Earth and Planetary Science Letters, 2008
    Co-Authors: Heather Handley, Simon Turner, Ian E. M. Smith, Robert B. Stewart, Shane J. Cronin
    Abstract:

    Abstract The extent and geochemical impact of Crustal Contamination during magmatic evolution in intra-oceanic subduction zone settings is often assumed to be of minimal significance but remains poorly constrained. Acquiring such information is crucial if meaningful timescales of magma generation and Crustal residence beneath volcanoes are to be determined. High-MgO basalts and differentiates erupted over the last 100 years at Lopevi volcano display a strong negative correlation between 87Sr/86Sr isotope ratio and indices of differentiation (e.g. SiO2) even though these lavas have a restricted Sr–Nd isotopic range compared to other Vanuatu arc lavas. This trend provides compelling evidence for the interaction of rising mafic magmas with ‘primitive’ sub-arc crust and so this volcano affords an excellent opportunity to investigate and ascertain timescales of Crustal interaction using U-series data. Quantitative geochemical modelling of whole-rock trace element ratios, 87Sr/86Sr isotope compositions and U-series data shows that assimilation of a relatively small-degree partial melt of > 380 Ka mafic oceanic crust (similar to Pacific- or Indian-MORB in 87Sr/86Sr isotopic composition) during fractional crystallisation of magma exerts major control on the (230Th/232Th) and (226Ra/230Th) activity ratios of the lavas. The incorporation of higher (230Th/232Th) and lower (226Ra/230Th) assimilated material drives compositions closer to secular equilibrium than simple closed-system differentiation, reducing calculated apparent timescales of closed-system differentiation from Th isotope composition (104–105) by orders of magnitude. Modelling suggests that assimilation occurs extremely rapidly at Lopevi with timescales for magma generation, differentiation and eruption on the order of 102 years.

  • Rapid timescales of differentiation and evidence for Crustal Contamination at intra-oceanic arcs: Geochemical and U–Th–Ra–Sr–Nd isotopic constraints from Lopevi Volcano, Vanuatu, SW Pacific
    Earth and Planetary Science Letters, 2008
    Co-Authors: Heather Handley, Simon Turner, Ian E. M. Smith, Robert B. Stewart, Shane J. Cronin
    Abstract:

    Abstract The extent and geochemical impact of Crustal Contamination during magmatic evolution in intra-oceanic subduction zone settings is often assumed to be of minimal significance but remains poorly constrained. Acquiring such information is crucial if meaningful timescales of magma generation and Crustal residence beneath volcanoes are to be determined. High-MgO basalts and differentiates erupted over the last 100 years at Lopevi volcano display a strong negative correlation between 87Sr/86Sr isotope ratio and indices of differentiation (e.g. SiO2) even though these lavas have a restricted Sr–Nd isotopic range compared to other Vanuatu arc lavas. This trend provides compelling evidence for the interaction of rising mafic magmas with ‘primitive’ sub-arc crust and so this volcano affords an excellent opportunity to investigate and ascertain timescales of Crustal interaction using U-series data. Quantitative geochemical modelling of whole-rock trace element ratios, 87Sr/86Sr isotope compositions and U-series data shows that assimilation of a relatively small-degree partial melt of > 380 Ka mafic oceanic crust (similar to Pacific- or Indian-MORB in 87Sr/86Sr isotopic composition) during fractional crystallisation of magma exerts major control on the (230Th/232Th) and (226Ra/230Th) activity ratios of the lavas. The incorporation of higher (230Th/232Th) and lower (226Ra/230Th) assimilated material drives compositions closer to secular equilibrium than simple closed-system differentiation, reducing calculated apparent timescales of closed-system differentiation from Th isotope composition (104–105) by orders of magnitude. Modelling suggests that assimilation occurs extremely rapidly at Lopevi with timescales for magma generation, differentiation and eruption on the order of 102 years.

Heather Handley - One of the best experts on this subject based on the ideXlab platform.

  • rapid timescales of differentiation and evidence for Crustal Contamination at intra oceanic arcs geochemical and u th ra sr nd isotopic constraints from lopevi volcano vanuatu sw pacific
    Earth and Planetary Science Letters, 2008
    Co-Authors: Heather Handley, Simon Turner, Ian E. M. Smith, Robert B. Stewart, Shane J. Cronin
    Abstract:

    Abstract The extent and geochemical impact of Crustal Contamination during magmatic evolution in intra-oceanic subduction zone settings is often assumed to be of minimal significance but remains poorly constrained. Acquiring such information is crucial if meaningful timescales of magma generation and Crustal residence beneath volcanoes are to be determined. High-MgO basalts and differentiates erupted over the last 100 years at Lopevi volcano display a strong negative correlation between 87Sr/86Sr isotope ratio and indices of differentiation (e.g. SiO2) even though these lavas have a restricted Sr–Nd isotopic range compared to other Vanuatu arc lavas. This trend provides compelling evidence for the interaction of rising mafic magmas with ‘primitive’ sub-arc crust and so this volcano affords an excellent opportunity to investigate and ascertain timescales of Crustal interaction using U-series data. Quantitative geochemical modelling of whole-rock trace element ratios, 87Sr/86Sr isotope compositions and U-series data shows that assimilation of a relatively small-degree partial melt of > 380 Ka mafic oceanic crust (similar to Pacific- or Indian-MORB in 87Sr/86Sr isotopic composition) during fractional crystallisation of magma exerts major control on the (230Th/232Th) and (226Ra/230Th) activity ratios of the lavas. The incorporation of higher (230Th/232Th) and lower (226Ra/230Th) assimilated material drives compositions closer to secular equilibrium than simple closed-system differentiation, reducing calculated apparent timescales of closed-system differentiation from Th isotope composition (104–105) by orders of magnitude. Modelling suggests that assimilation occurs extremely rapidly at Lopevi with timescales for magma generation, differentiation and eruption on the order of 102 years.

  • Rapid timescales of differentiation and evidence for Crustal Contamination at intra-oceanic arcs: Geochemical and U–Th–Ra–Sr–Nd isotopic constraints from Lopevi Volcano, Vanuatu, SW Pacific
    Earth and Planetary Science Letters, 2008
    Co-Authors: Heather Handley, Simon Turner, Ian E. M. Smith, Robert B. Stewart, Shane J. Cronin
    Abstract:

    Abstract The extent and geochemical impact of Crustal Contamination during magmatic evolution in intra-oceanic subduction zone settings is often assumed to be of minimal significance but remains poorly constrained. Acquiring such information is crucial if meaningful timescales of magma generation and Crustal residence beneath volcanoes are to be determined. High-MgO basalts and differentiates erupted over the last 100 years at Lopevi volcano display a strong negative correlation between 87Sr/86Sr isotope ratio and indices of differentiation (e.g. SiO2) even though these lavas have a restricted Sr–Nd isotopic range compared to other Vanuatu arc lavas. This trend provides compelling evidence for the interaction of rising mafic magmas with ‘primitive’ sub-arc crust and so this volcano affords an excellent opportunity to investigate and ascertain timescales of Crustal interaction using U-series data. Quantitative geochemical modelling of whole-rock trace element ratios, 87Sr/86Sr isotope compositions and U-series data shows that assimilation of a relatively small-degree partial melt of > 380 Ka mafic oceanic crust (similar to Pacific- or Indian-MORB in 87Sr/86Sr isotopic composition) during fractional crystallisation of magma exerts major control on the (230Th/232Th) and (226Ra/230Th) activity ratios of the lavas. The incorporation of higher (230Th/232Th) and lower (226Ra/230Th) assimilated material drives compositions closer to secular equilibrium than simple closed-system differentiation, reducing calculated apparent timescales of closed-system differentiation from Th isotope composition (104–105) by orders of magnitude. Modelling suggests that assimilation occurs extremely rapidly at Lopevi with timescales for magma generation, differentiation and eruption on the order of 102 years.

De-yi Kong - One of the best experts on this subject based on the ideXlab platform.

  • Selective Crustal Contamination and decoupling of lithophile and chalcophile element isotopes in sulfide-bearing mafic intrusions: An example from the Jingbulake Intrusion, Xinjiang, NW China
    Chemical Geology, 2012
    Co-Authors: Sheng-hong Yang, Mei-fu Zhou, Peter C. Lightfoot, John Malpas, Ji-bing Zhou, De-yi Kong
    Abstract:

    The Jingbulake mafic-ultramafic intrusion in the South Tianshan orogenic belt, Xinjiang, NW China, is a zoned intrusive body composed of gabbro-diorite, olivine gabbro and wehrlite, locally intruded by pyroxenite, within a major W-E structure. Both olivine gabbro and wehrlite contain disseminated Ni-Cu sulfides, whereas pyroxenite hosts an ore body containing massive, net-textured and disseminated sulfide ores. Both silicate rocks and sulfide ores from the Jingbulake intrusion have low Pd/Ir ratios (10-63). The silicate rocks contain Ni-poor olivine (generally 30% Crustal Contamination is required to explain the radiogenic Os isotopic composition if average upper crust is adopted as the Crustal contaminant, or 5%-25% if sulfide-rich Crustal rock is used as the Crustal contaminant. However only 5-10% Crustal Contamination is required to explain the Nd and Sr isotopes. We propose that the decoupling of Os from Sr-Nd isotopes in the silicate rocks was due to selective Crustal Contamination. Addition of external sulfur from a Crustal source may be the key factor triggering sulfide saturation. The silicate magma that underwent sulfide extraction before emplacement to shallow depth preserved a Crustal Os isotope signature. Continued reaction of sulfide melts with new pulses of mantle-derived magma increased the Os content and decreased the 187Os/ 188Os(i) ratios, effectively masking the Crustal Os contribution in the sulfides. © 2011 Elsevier B.V.link_to_subscribed_fulltex

  • Selective Crustal Contamination and decoupling of lithophile and chalcophile element isotopes in sulfide-bearing mafic intrusions: An example from the Jingbulake Intrusion, Xinjiang, NW China
    Chemical Geology, 2011
    Co-Authors: Sheng-hong Yang, Mei-fu Zhou, Peter C. Lightfoot, John Malpas, Ji-bing Zhou, De-yi Kong
    Abstract:

    Abstract The Jingbulake mafic–ultramafic intrusion in the South Tianshan orogenic belt, Xinjiang, NW China, is a zoned intrusive body composed of gabbro-diorite, olivine gabbro and wehrlite, locally intruded by pyroxenite, within a major W–E structure. Both olivine gabbro and wehrlite contain disseminated Ni–Cu sulfides, whereas pyroxenite hosts an ore body containing massive, net-textured and disseminated sulfide ores. Both silicate rocks and sulfide ores from the Jingbulake intrusion have low Pd/Ir ratios (10–63). The silicate rocks contain Ni-poor olivine (generally 187 Os/ 188 Os(i) ratios (0.277 to 0.916) of the silicate rocks are consistent with a major contribution of Os from Crustal materials. On the other hand, the net-textured sulfide ores from No. 8 mineralization zone have high Os contents (15.1 to 32.6 ppb), low Re/Os (2.0 to 2.3) and low 187 Os/ 188 Os(i) ratios (0.125 to 0.141), indicating a mantle-derived origin. Modeling indicates that > 30% Crustal Contamination is required to explain the radiogenic Os isotopic composition if average upper crust is adopted as the Crustal contaminant, or 5%–25% if sulfide-rich Crustal rock is used as the Crustal contaminant. However only 5–10% Crustal Contamination is required to explain the Nd and Sr isotopes. We propose that the decoupling of Os from Sr–Nd isotopes in the silicate rocks was due to selective Crustal Contamination. Addition of external sulfur from a Crustal source may be the key factor triggering sulfide saturation. The silicate magma that underwent sulfide extraction before emplacement to shallow depth preserved a Crustal Os isotope signature. Continued reaction of sulfide melts with new pulses of mantle-derived magma increased the Os content and decreased the 187 Os/ 188 Os(i) ratios, effectively masking the Crustal Os contribution in the sulfides.