The Experts below are selected from a list of 297 Experts worldwide ranked by ideXlab platform
Rajesh K. Srivastava - One of the best experts on this subject based on the ideXlab platform.
-
Nd-isotope and geochemistry of an early Palaeoproterozoic high-Si high-Mg Boninite–norite suite of rocks in the southern Bastar craton, central India: petrogenesis and tectonic significance
International Geology Review, 2016Co-Authors: Rajesh K. Srivastava, Márcio Martins Pimentel, Gulab C. GautamAbstract:ABSTRACTNd-isotope and lithogeochemistry of an early Palaeoproterozoic high-Si high-Mg Boninite–norite (BN) suite of rocks from the southern Bastar craton, central India, are presented to understand their nature, origin, and tectonic setting of emplacement. Various types of evidence, such as field relationships, radiometric metamorphic ages, and the global distribution of BN magmatism, suggest emplacement in an intracratonic rift setting, commonly around 2.4–2.5 Ga. On the basis of geochemistry these high-Si high-Mg rocks are classified as high-Ca Boninites, high-Mg norites, and high-Mg diorites. Nd-isotope data indicate that the high-Mg norite and the high-Mg diorite samples are similar, whereas the high-Ca Boninites have a different isotopic character. The high-Mg norite and the high-Mg diorite samples have younger TDM model ages than the high-Ca Boninites. Geochemical and Nd-isotopic characteristics of the studied rocks indicate some prospect of crustal contamination; however, the possibility of mantle...
-
nd isotope and geochemistry of an early palaeoproterozoic high si high mg Boninite norite suite of rocks in the southern bastar craton central india petrogenesis and tectonic significance
International Geology Review, 2016Co-Authors: Rajesh K. Srivastava, Márcio Martins Pimentel, Gulab C. GautamAbstract:ABSTRACTNd-isotope and lithogeochemistry of an early Palaeoproterozoic high-Si high-Mg Boninite–norite (BN) suite of rocks from the southern Bastar craton, central India, are presented to understand their nature, origin, and tectonic setting of emplacement. Various types of evidence, such as field relationships, radiometric metamorphic ages, and the global distribution of BN magmatism, suggest emplacement in an intracratonic rift setting, commonly around 2.4–2.5 Ga. On the basis of geochemistry these high-Si high-Mg rocks are classified as high-Ca Boninites, high-Mg norites, and high-Mg diorites. Nd-isotope data indicate that the high-Mg norite and the high-Mg diorite samples are similar, whereas the high-Ca Boninites have a different isotopic character. The high-Mg norite and the high-Mg diorite samples have younger TDM model ages than the high-Ca Boninites. Geochemical and Nd-isotopic characteristics of the studied rocks indicate some prospect of crustal contamination; however, the possibility of mantle...
-
Nd-isotope and geochemistry of an early Palaeoproterozoic high-Si high-Mg Boninite–norite suite of rocks in the southern Bastar craton, central India: petrogenesis and tectonic significance
2016Co-Authors: Rajesh K. Srivastava, Márcio Martins Pimentel, Gulab C. GautamAbstract:Nd-isotope and lithogeochemistry of an early Palaeoproterozoic high-Si high-Mg Boninite–norite (BN) suite of rocks from the southern Bastar craton, central India, are presented to understand their nature, origin, and tectonic setting of emplacement. Various types of evidence, such as field relationships, radiometric metamorphic ages, and the global distribution of BN magmatism, suggest emplacement in an intracratonic rift setting, commonly around 2.4–2.5 Ga. On the basis of geochemistry these high-Si high-Mg rocks are classified as high-Ca Boninites, high-Mg norites, and high-Mg diorites. Nd-isotope data indicate that the high-Mg norite and the high-Mg diorite samples are similar, whereas the high-Ca Boninites have a different isotopic character. The high-Mg norite and the high-Mg diorite samples have younger TDM model ages than the high-Ca Boninites. Geochemical and Nd-isotopic characteristics of the studied rocks indicate some prospect of crustal contamination; however, the possibility of mantle metasomatism during ancient subduction event cannot be ignored. Trace-element modelling suggests that the high-Ca Boninites may have crystallized from a magma generated by a comparatively greater percentage of melting of a lherzolite mantle source than the source for the other two varieties. Furthermore, the high-Ca Boninite rocks are most likely derived from an Archaean subduction process (the Whundo-type), whereas the other two types are the products of the interaction of subduction-modified refractory mantle wedge and a plume, around the Neoarchaean–Palaeoproterozoic boundary. The emplacement of the high-Mg norites and the high-Mg diorites may be linked to crustal thickening and associated cratonization at the end of the Archaean.
-
Global Intracratonic Boninite-Norite Magmatism during the Neoarchean—Paleoproterozoic: Evidence from the Central Indian Bastar Craton
International Geology Review, 2010Co-Authors: Rajesh K. SrivastavaAbstract:Although Boninites and Boninite-like rocks formed throughout Earth history and have been emplaced in different tectonic settings, they are most common in Phanerozoic settings where they are associated with subduction-related magmatism. However, Neoarchean—Paleoproterozoic noritic intrusions, which have bulk-rock geochemistry closely similar to Boninites, occur in many Archean terrains, chiefly emplaced in intracratonic settings. Thus, noritic intrusions are probably derived from melts similar to those of Boninites. The best examples of such Boninite-norite associations occur in the Bushveld, southern West Greenland, East Antarctica, Wyoming, northwest Scotland, and the eastern Fennoscandian shield. A similar Boninite-norite suite also is present in the central Indian Bastar craton. These high-Mg mafic igneous rocks were emplaced as dikes and volcanic rocks during Neoarchean—Paleoproterozoic time in an intracratonic rift. They contain high Si (>52 wt%), high Mg (>8 wt%), and low Ti (mostly
-
A new find of Boninite dyke from the Palaeoproterozoic Dongargarh Super group: inference for a fossil subduction zone in the Archaean of the Bastar craton, Central India
Neues Jahrbuch für Mineralogie - Abhandlungen, 2009Co-Authors: N. V. Chalapathi Rao, Rajesh K. SrivastavaAbstract:The Dongargarh Supergroup (DSG), a bimodal Large igneous province (LIP), is one of the Palaeoproterozoic greenschist facies-metamorphosed volcano-sedimentary belts in the Bastar craton of the Central Indian shield. Two contrasting models are in vogue for the generation of the mafic volcanics from the DSG - a continental rifting model and an arc related model. In this paper, we report the occurrence of a Boninite dyke from the Bijli rhyolite Formation, which is the lower volcanic horizon in the Nandgaon Group of the DSG. The Boninite dyke is characterised by high magnesium (MgO : 18.32-18.80 wt.%), primitive Mgnumber (Mg# > 80), abundance of silica (SiO 2 : 51.63-51.95 wt.%), high Ni (~369 ppm), Cr (~2703 ppm), extremely low titania (TiO 2 : 0.04 wt.%), enrichment of LREE over MREE and HFSE and pronounced negative anomalies in Nb, Ti and Zr on primitive mantle normalized multi-element plots. The Dongargarh Boninite dyke is inferred to have been derived from a primary magma and shares geochemical characteristics of modern- as well as Archaean-Boninites. It comes under the high-Ca Boninite category and displays distinct geochemical traits compared to the so far reported Boninites from the Bastar craton. Its petrogenesis necessitates a two stage-model involving a refractory mantle as well as fluids derived from subducted sediments. Crustal assimilation (contamination) or a direct plume-derived melt cannot account for its observed geochemical characters. Even though we cannot constrain the generation of the mafic volcanics of DSG vis-a-vis rifting vs convergence with the available data, the occurrence and geochemistry of the Boninite dyke indeed demonstrates that this domain represents a fossil subduction zone.
Julian A. Pearce - One of the best experts on this subject based on the ideXlab platform.
-
Mineral compositions and thermobarometry of basalts and Boninites recovered during IODP Expedition 352 to the Bonin forearc
American Mineralogist, 2020Co-Authors: Scott A. Whattam, Julian A. Pearce, John W. Shervais, Mark K. Reagan, Daniel A. Coulthard, Peter C. Jones, Jieun Seo, Keith Putirka, Timothy Chapman, D. E. HeatonAbstract:Abstract Central aims of IODP Expedition 352 were to delineate and characterize the magmatic stratigraphy in the Bonin forearc to define key magmatic processes associated with subduction initiation and their potential links to ophiolites. Expedition 352 penetrated 1.2 km of magmatic basement at four sites and recovered three principal lithologies: tholeiitic forearc basalt (FAB), high-Mg andesite, and Boninite, with subordinate andesite. Boninites are subdivided into basaltic, low-Si, and high-Si varieties. The purpose of this study is to determine conditions of crystal growth and differentiation for Expedition 352 lavas and compare and contrast these conditions with those recorded in lavas from mid-ocean ridges, forearcs, and ophiolites. Cr# (cationic Cr/Cr+Al) vs. TiO2 relations in spinel and clinopyroxene demonstrate a trend of source depletion with time for the Expedition 352 forearc basalt to Boninite sequence that is similar to sequences in the Oman and other suprasubduction zone ophiolites. Clinopyroxene thermobarometry results indicate that FAB crystallized at temperatures (1142–1190 °C) within the range of MORB (1133–1240 °C). When taking into consideration liquid lines of descent of Boninite, orthopyroxene barometry and olivine thermometry of Expedition 352 Boninites demonstrate that they crystallized at temperatures marginally lower than those of FAB, between ~1119 and ~1202 °C and at relatively lower pressure (~0.2–0.4 vs. 0.5–4.6 kbar for FAB). Elevated temperatures of Boninite orthopyroxene (~1214 °C for low-Si Boninite and 1231–1264 °C for high-Si Boninite) may suggest latent heat produced by the rapid crystallization of orthopyroxene. The lower pressure of crystallization of the Boninite may be explained by their lower density and hence higher ascent rate, and shorter distance of travel from place of magma formation to site of crystallization, which allowed the more buoyant and faster ascending Boninites to rise to shallower levels before crystallizing, thus preserving their high temperatures.
-
Identification, classification, and interpretation of Boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics
Geosphere, 2019Co-Authors: Julian A. Pearce, Mark K. ReaganAbstract:Abstract Boninites are rare, high-Si, high-Mg, low-Ti lavas that have considerable tectonic significance, especially for recognizing and interpreting episodes of subduction initiation in the geologic record. Formal identification and classification of Boninites may be carried out using MgO-SiO2 and MgO-TiO2 diagrams to find compositions that satisfy modified International Union of Geological Sciences (IUGS) criteria of Si8 > 52 and Ti8 < 0.5, where Si8 and Ti8 refer to concentrations of the oxides at 8 wt% MgO. However, screening of highly metasomatized rocks and accurate classification require precautions, including normalization to a 100% volatile-free basis. The MgO-SiO2 diagram can also be used for subdivision into low-Si Boninites (Si8 < 57) and high-Si Boninites (Si8 > 57). Satisfying one but not both of the Boninite criteria are rocks with Si8 > 52 but Ti8 ≥ 0.5 (siliceous high-magnesium basalts) and rocks with Si8 ≤ 52 but Ti8 < 0.5 (low-Ti basalts). We tested the classification methodologies using ∼100 low-Ti lava suites dating from the present-day back to the Eoarchean. We conclude that, of those classifying as “Boninite series,” Izu-Bonin-Mariana arc–type subduction initiation terranes provide the dominant setting only back as far as ca. 2 Ga, which marks the maximum age of extensive clinopyroxene-undersaturated melting and eruption of high-Si Boninites. From 2 to 3 Ga, most Boninites formed in intraplate settings by melting of refertilized, depleted cratonic roots. Prior to 3 Ga, hot, depleted mantle plumes provided the main Boninite sources. Nonetheless, arc-basin Boninites, though rare, do extend back to 3.8 Ga, and, together with the inherited subduction component in intracratonic Boninites, they provide evidence for some form of subduction during the Archean.
-
Mantle flow, volatiles, slab-surface temperatures and melting dynamics in the north Tonga arc-Lau back-arc basin
Journal of Geophysical Research, 2012Co-Authors: John Caulfield, Simon Turner, Richard J. Arculus, C. W. Dale, Frances E. Jenner, Julian A. Pearce, Colin G. Macpherson, Heather HandleyAbstract:The Fonualei Spreading Center affords an excellent opportunity to evaluate geochemical changes with increasing depth to the slab in the Lau back-arc basin. We present H2O and CO2concentrations and Sr, Nd, Pb, Hf and U-Th-Ra isotope data for selected glasses as well as new Hf isotope data from Boninites and seamounts to the north of the Tonga arc. The Pb and Hf isotope data are used to show that mantle flow is oriented to the southwest and that the tear in the northern end of the slab may not extend east as far as the Boninite locality. Along the Fonualei Spreading Center, key geochemical parameters change smoothly with increasing distance from the arc front and increasing slab surface temperatures. The latter may range from 720 to 866°C, based on decreasing H2O/Ce ratios. Consistent with experimental data, the geochemical trends are interpreted to reflect changes in the amount and composition of wet pelite melts or super-critical fluids and aqueous fluids derived from the slab. With one exception, all of the lavas preserve both238U excesses and 226Ra excesses. We suggest that lavas from the Fonualei Spreading Center and Valu Fa Ridge are dominated by fluid-fluxed melting whereas those from the East and Central Lau Spreading Centers, where slab surface temperatures exceed ∼850–900°C, are largely derived through decompression. A similar observation is found for the Manus and East Scotia back-arc basins and may reflect the expiry of a key phase such as lawsonite in the subducted basaltic crust.
-
Boninite and harzburgite from leg 125 bonin mariana forearc a case study of magma genesis during the initial stages of subduction
1992Co-Authors: Julian A. Pearce, Richard J. Arculus, Teruaki Ishii, Sieger R Van Der Laan, Bramley J Murton, David W Peate, Ian J ParkinsonAbstract:Holes drilled into the volcanic and ultrabasic basement of the Izu-Ogasawara and Mariana forearc terranes during Leg 125 provide data on some of the earliest lithosphere created after the start of Eocene subduction in the Western Pacific. The volcanic basement contains three Boninite series and one tholeiite series. (1) Eocene low-Ca Boninite and low-Ca bronzite andesite pillow lavas and dikes dominate the lowermost part of the deep crustal section through the outer-arc high at Site 786. (2) Eocene intermediate-Ca Boninite and its fractionation products (bronzite andesite, andesite, dacite, and rhyolite) make up the main part of the boninitic edifice at Site 786. (3) Early Oligocene intermediate-Ca to high-Ca Boninite sills or dikes intrude the edifice and perhaps feed an uppermost breccia unit at Site 786. (4) Eocene or Early Oligocene tholeiitic andesite, dacite, and rhyolite form the uppermost part of the outer-arc high at Site 782. All four groups can be explained by remelting above a subduction zone of oceanic mantle lithosphere that has been depleted by its previous episode of partial melting at an ocean ridge. We estimate that the average Boninite source had lost 10-15 wt% of melt at the ridge before undergoing further melting (5-10%) shortly after subduction started. The composition of the harzburgite (<2% clinopyroxene, Fo content of about 92%) indicates that it underwent a total of about 25% melting with respect to a fertile MORB mantle. The low concentration of Nb in the Boninite indicates that the oceanic lithosphere prior to subduction was not enriched by any asthenospheric (OIB) component. The subduction component is characterized by (1) high Zr and Hf contents relative to Sm, Ti, Y, and middle-heavy REE, (2) light REE-enrichment, (3) low contents of Nb and Ta relative to Th, Rb, or La, (4) high contents of Na and Al, and (5) Pb isotopes on the Northern Hemisphere Reference Line. This component is unlike any subduction component from active arc volcanoes in the Izu-Mariana region or elsewhere. Modeling suggests that these characteristics fit a trondhjemitic melt from slab fusion in amphibolite facies. The resulting metasomatized mantle may have contained about 0.15 wt% water. The overall melting regime is constrained by experimental data to shallow depths and high temperatures (1250°C and 1.5 kb for an average Boninite) of Boninite segregation. We thus envisage that Boninites were generated by decompression melting of a diapir of metasomatized residual MORB mantle leaving the harzburgites as the uppermost, most depleted residue from this second stage of melting. Thermal constraints require that both subducted lithosphere and overlying oceanic lithosphere of the mantle wedge be very young at the time of Boninite genesis. This conclusion is consistent with models in which an active transform fault offsetting two ridge axes is placed under compression or transpression following the Eocene plate reorganization in the Pacific. Comparison between Leg 125 Boninites and Boninites and related rocks elsewhere in the Western Pacific highlights large regional differences in petrogenesis in terms of mantle mineralogy, degree of partial melting, composition of subduction components, and the nature of pre-subduction lithosphere. It is likely that, on a regional scale, the initiation of subduction involved subducted crust and lithospheric mantle wedge of a range of ages and compositions, as might be expected in this type of tectonic setting.
-
Isotopic evidence for the origin of Boninites and related rocks drilled in the Izu-Bonin (Osagawara) Forearc, Leg 125
Proceedings of the Ocean Drilling Program 125 Scientific Results, 1992Co-Authors: Julian A. Pearce, Richard J. Arculus, Bramley J Murton, Matthew F. Thirlwall, Gerry Ingram, Sieger R Van Der LaanAbstract:Twenty-six samples representing the wide range of lithologies (low- and intermediate-Ca Boninites and bronzite andesites, high-Ca Boninites, basaltic andesites-rhyolites) drilled during Leg 125 at Sites 782 and 786 on the Izu-Bonin outer-arc high have been analyzed for Sr, Nd, and Pb isotopes. Nd-Sr isotope covariations show that most samples follow a trend parallel to a line from Pacific MORB mantle (PMM) to Pacific Volcanogenic sediment (PVS) but displaced slightly toward more radiogenic Sr. Pb isotope covariations show that all the Eocene-Oligocene samples plot along the Northern Hemisphere Reference Line, indicating little or no Pb derived from subducted pelagic sediment in their source. Two young basaltic andesite clasts within sediment do have a pelagic sediment signature but this may have been gained by alteration rather than subduction. In all isotopic projections, the samples form consistent groupings: the tholeiites from Site 782 and Hole 786A plot closest to PMM, the Boninites and related rocks from Sites 786B plot closest to PVS, and the Boninite lavas from Hole 786A and late boninitic dikes from Hole 786B occupy an intermediate position. Isotope-trace element covariations indicate that these isotopic variations can be explained by a three-component mixing model. One component (A) has the isotopic signature of PMM but is depleted in the more incompatible elements. It is interpreted as representing suboceanic mantle lithosphere. A second component (B) is relatively radiogenic (eNd = ca 4-6; 206Pb/204Pb = ca 19.0-19.3; eSr = ca -10 to -6)). Its trace element pattern has, among other characteristics, a high Zr/Sm ratio, which distinguishes it from the "normal" fluid components associated with subduction and hotspot activity. There are insufficient data at present to tie down its origin: probably it was either derived from subducted lithosphere or volcanogenic sediment fused in amphibolite facies; or it represents an asthenospheric melt component that has been fractionated by interaction with amphibole-bearing mantle. The third component (C) is characterized by high contents of Sr and high eSr values and is interpreted as a subducted fluid component. The mixing line on a diagram of Zr/Sr against e Sr suggests that component C may have enriched the lithosphere (component A) before component B. These components may also be present on a regional basis but, if so, may not have had uniform compositions. Only the boninitic series from nearby Chichijima would require an additional, pelagic sediment component. In general, these results are consistent with models of subduction of ridges and young lithosphere during the change from a ridge-transform to subduction geometry at the initiation of subduction in the Western Pacific.
Colin G. Macpherson - One of the best experts on this subject based on the ideXlab platform.
-
Mantle flow, volatiles, slab-surface temperatures and melting dynamics in the north Tonga arc-Lau back-arc basin
Journal of Geophysical Research, 2012Co-Authors: John Caulfield, Simon Turner, Richard J. Arculus, C. W. Dale, Frances E. Jenner, Julian A. Pearce, Colin G. Macpherson, Heather HandleyAbstract:The Fonualei Spreading Center affords an excellent opportunity to evaluate geochemical changes with increasing depth to the slab in the Lau back-arc basin. We present H2O and CO2concentrations and Sr, Nd, Pb, Hf and U-Th-Ra isotope data for selected glasses as well as new Hf isotope data from Boninites and seamounts to the north of the Tonga arc. The Pb and Hf isotope data are used to show that mantle flow is oriented to the southwest and that the tear in the northern end of the slab may not extend east as far as the Boninite locality. Along the Fonualei Spreading Center, key geochemical parameters change smoothly with increasing distance from the arc front and increasing slab surface temperatures. The latter may range from 720 to 866°C, based on decreasing H2O/Ce ratios. Consistent with experimental data, the geochemical trends are interpreted to reflect changes in the amount and composition of wet pelite melts or super-critical fluids and aqueous fluids derived from the slab. With one exception, all of the lavas preserve both238U excesses and 226Ra excesses. We suggest that lavas from the Fonualei Spreading Center and Valu Fa Ridge are dominated by fluid-fluxed melting whereas those from the East and Central Lau Spreading Centers, where slab surface temperatures exceed ∼850–900°C, are largely derived through decompression. A similar observation is found for the Manus and East Scotia back-arc basins and may reflect the expiry of a key phase such as lawsonite in the subducted basaltic crust.
-
tectonic setting of eocene Boninite magmatism in the izu bonin mariana forearc
Earth and Planetary Science Letters, 2001Co-Authors: Colin G. Macpherson, Robert HallAbstract:Abstract Middle Eocene Boninites were simultaneously generated over a large region during the early history of the Izu–Bonin–Mariana (IBM) arc. However, widespread Boninite magmatism is not recognised in younger subduction zones of similar dimensions. This suggests that an additional tectonic or thermal factor influenced the generation of the IBM Boninite suite. Regional uplift, ocean island basalt-style magmatism and high heat-flow also characterised the northern Philippine Sea plate (PSP) at the start of the Middle Eocene. These features are similar to those observed in large volume basaltic volcanic provinces such as the early Tertiary North Atlantic and suggest that the IBM Boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the PSP. The reconstructed Middle Eocene location of the IBM arc and West Philippine Basin lies close to the present day Manus Basin where petrological and geochemical evidence indicate the presence of a mantle plume. A calculated plume track linking these locations through time also passes close to the Eauripik Rise, an aseismic ridge on the Caroline Plate, during the Oligocene and Miocene. Therefore, we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western Pacific from the Middle Eocene until the present day.
-
Tectonic setting of Eocene Boninite magmatism in the Izu–Bonin–Mariana forearc
Earth and Planetary Science Letters, 2001Co-Authors: Colin G. Macpherson, Robert HallAbstract:Abstract Middle Eocene Boninites were simultaneously generated over a large region during the early history of the Izu–Bonin–Mariana (IBM) arc. However, widespread Boninite magmatism is not recognised in younger subduction zones of similar dimensions. This suggests that an additional tectonic or thermal factor influenced the generation of the IBM Boninite suite. Regional uplift, ocean island basalt-style magmatism and high heat-flow also characterised the northern Philippine Sea plate (PSP) at the start of the Middle Eocene. These features are similar to those observed in large volume basaltic volcanic provinces such as the early Tertiary North Atlantic and suggest that the IBM Boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the PSP. The reconstructed Middle Eocene location of the IBM arc and West Philippine Basin lies close to the present day Manus Basin where petrological and geochemical evidence indicate the presence of a mantle plume. A calculated plume track linking these locations through time also passes close to the Eauripik Rise, an aseismic ridge on the Caroline Plate, during the Oligocene and Miocene. Therefore, we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western Pacific from the Middle Eocene until the present day.
Richard J. Arculus - One of the best experts on this subject based on the ideXlab platform.
-
Compositions and classification of fractionated Boninite series melts from the Izu-Bonin-Mariana arc: A machine learning approach
Journal of Petrology, 2021Co-Authors: Matthew J. Valetich, Richard J. Arculus, Susumu Umino, Charles Le Losq, John MavrogenesAbstract:Abstract Much of the Boninite magmatism in the Izu-Bonin-Mariana (IBM) arc is preserved as evolved Boninite series compositions wherein extensive fractional crystallisation of pyroxene and spinel have obscured the diagnostic geochemical indicators of Boninite parentage, such as high-Mg and low-Ti at intermediate silica contents. As a result, the usual geochemical discriminants used for the classification of the broad range of parental Boninites are inapplicable to such highly fractionated melts. These issues are compounded by the mixing of demonstrably different whole-rock and glass analyses in classification schemes and petrological interpretations based thereon. Whole-rock compositions are compromised by entrainment of variable proportions of crystalline phases resulting in inconsistent differences with corresponding in-situ glass analyses, which arguably better reflect prior melt compositions. To circumvent such issues, we herein present a robust method for the classification of highly fractionated Boninite series glasses. This new classification leverages the analysis of trace elements, much more sensitive to evolutionary processes than major elements, and benefits from the use of unsupervised machine learning as a classification tool. The results show the most fractionated Boninite series melts preserve geochemical indicators of their parentage, and highlight the pitfalls of interpreting whole rock and glass analyses interchangeably.
-
Mantle flow, volatiles, slab-surface temperatures and melting dynamics in the north Tonga arc-Lau back-arc basin
Journal of Geophysical Research, 2012Co-Authors: John Caulfield, Simon Turner, Richard J. Arculus, C. W. Dale, Frances E. Jenner, Julian A. Pearce, Colin G. Macpherson, Heather HandleyAbstract:The Fonualei Spreading Center affords an excellent opportunity to evaluate geochemical changes with increasing depth to the slab in the Lau back-arc basin. We present H2O and CO2concentrations and Sr, Nd, Pb, Hf and U-Th-Ra isotope data for selected glasses as well as new Hf isotope data from Boninites and seamounts to the north of the Tonga arc. The Pb and Hf isotope data are used to show that mantle flow is oriented to the southwest and that the tear in the northern end of the slab may not extend east as far as the Boninite locality. Along the Fonualei Spreading Center, key geochemical parameters change smoothly with increasing distance from the arc front and increasing slab surface temperatures. The latter may range from 720 to 866°C, based on decreasing H2O/Ce ratios. Consistent with experimental data, the geochemical trends are interpreted to reflect changes in the amount and composition of wet pelite melts or super-critical fluids and aqueous fluids derived from the slab. With one exception, all of the lavas preserve both238U excesses and 226Ra excesses. We suggest that lavas from the Fonualei Spreading Center and Valu Fa Ridge are dominated by fluid-fluxed melting whereas those from the East and Central Lau Spreading Centers, where slab surface temperatures exceed ∼850–900°C, are largely derived through decompression. A similar observation is found for the Manus and East Scotia back-arc basins and may reflect the expiry of a key phase such as lawsonite in the subducted basaltic crust.
-
High-Ca Boninites from the active Tonga Arc
Journal of Geophysical Research, 2010Co-Authors: Lauren B. Cooper, Richard J. Arculus, Terry Plank, Erik H. Hauri, Paul S. Hall, Stephen W. ParmanAbstract:[1] We report the first known occurrence of high-Ca Boninites within an active submarine island arc, at Volcano A within the Tonga Arc. Both the whole rock and a population of melt inclusions (in Fo86–92 olivines) from a dredged satellite cone have compositions classified as high-Ca Boninite. All samples from Volcano A, however, may be related to parental Boninites, given the similarity in their rare earth element patterns and their coherency along a similar liquid line of descent. The primary high-Ca Boninite liquids were generated in the mantle wedge by high cumulative degrees of melting (>∼24%) at typical mantle wedge temperatures ( 4 wt % H2O in primary liquids). We propose a two-stage model for generating primary Boninite liquids at Volcano A: (1) melting of fertile peridotite within the Lau back-arc basin, followed by (2) remelting of this residual peridotite with slab-derived fluid beneath the Tonga Arc. The occurrence of high-Ca Boninites at Volcano A is related to the relative location and duration of back-arc spreading. Here, the Eastern Lau Spreading Center has been processing mantle for ∼1 Ma, and corner flow circulation brings mantle from the back-arc melting regime into the arc melting regime at a rate that is a significant fraction (>30%) of the convergence rate. On the basis of Si6.0 and Ti6.0 relationships, we argue that a significant portion of the central Tonga Arc near Volcano A, as well as several other arc volcanoes with active back-arc basins, are also erupting basaltic andesites with Boninite parentage.
-
Boninite and harzburgite from leg 125 bonin mariana forearc a case study of magma genesis during the initial stages of subduction
1992Co-Authors: Julian A. Pearce, Richard J. Arculus, Teruaki Ishii, Sieger R Van Der Laan, Bramley J Murton, David W Peate, Ian J ParkinsonAbstract:Holes drilled into the volcanic and ultrabasic basement of the Izu-Ogasawara and Mariana forearc terranes during Leg 125 provide data on some of the earliest lithosphere created after the start of Eocene subduction in the Western Pacific. The volcanic basement contains three Boninite series and one tholeiite series. (1) Eocene low-Ca Boninite and low-Ca bronzite andesite pillow lavas and dikes dominate the lowermost part of the deep crustal section through the outer-arc high at Site 786. (2) Eocene intermediate-Ca Boninite and its fractionation products (bronzite andesite, andesite, dacite, and rhyolite) make up the main part of the boninitic edifice at Site 786. (3) Early Oligocene intermediate-Ca to high-Ca Boninite sills or dikes intrude the edifice and perhaps feed an uppermost breccia unit at Site 786. (4) Eocene or Early Oligocene tholeiitic andesite, dacite, and rhyolite form the uppermost part of the outer-arc high at Site 782. All four groups can be explained by remelting above a subduction zone of oceanic mantle lithosphere that has been depleted by its previous episode of partial melting at an ocean ridge. We estimate that the average Boninite source had lost 10-15 wt% of melt at the ridge before undergoing further melting (5-10%) shortly after subduction started. The composition of the harzburgite (<2% clinopyroxene, Fo content of about 92%) indicates that it underwent a total of about 25% melting with respect to a fertile MORB mantle. The low concentration of Nb in the Boninite indicates that the oceanic lithosphere prior to subduction was not enriched by any asthenospheric (OIB) component. The subduction component is characterized by (1) high Zr and Hf contents relative to Sm, Ti, Y, and middle-heavy REE, (2) light REE-enrichment, (3) low contents of Nb and Ta relative to Th, Rb, or La, (4) high contents of Na and Al, and (5) Pb isotopes on the Northern Hemisphere Reference Line. This component is unlike any subduction component from active arc volcanoes in the Izu-Mariana region or elsewhere. Modeling suggests that these characteristics fit a trondhjemitic melt from slab fusion in amphibolite facies. The resulting metasomatized mantle may have contained about 0.15 wt% water. The overall melting regime is constrained by experimental data to shallow depths and high temperatures (1250°C and 1.5 kb for an average Boninite) of Boninite segregation. We thus envisage that Boninites were generated by decompression melting of a diapir of metasomatized residual MORB mantle leaving the harzburgites as the uppermost, most depleted residue from this second stage of melting. Thermal constraints require that both subducted lithosphere and overlying oceanic lithosphere of the mantle wedge be very young at the time of Boninite genesis. This conclusion is consistent with models in which an active transform fault offsetting two ridge axes is placed under compression or transpression following the Eocene plate reorganization in the Pacific. Comparison between Leg 125 Boninites and Boninites and related rocks elsewhere in the Western Pacific highlights large regional differences in petrogenesis in terms of mantle mineralogy, degree of partial melting, composition of subduction components, and the nature of pre-subduction lithosphere. It is likely that, on a regional scale, the initiation of subduction involved subducted crust and lithospheric mantle wedge of a range of ages and compositions, as might be expected in this type of tectonic setting.
-
Isotopic evidence for the origin of Boninites and related rocks drilled in the Izu-Bonin (Osagawara) Forearc, Leg 125
Proceedings of the Ocean Drilling Program 125 Scientific Results, 1992Co-Authors: Julian A. Pearce, Richard J. Arculus, Bramley J Murton, Matthew F. Thirlwall, Gerry Ingram, Sieger R Van Der LaanAbstract:Twenty-six samples representing the wide range of lithologies (low- and intermediate-Ca Boninites and bronzite andesites, high-Ca Boninites, basaltic andesites-rhyolites) drilled during Leg 125 at Sites 782 and 786 on the Izu-Bonin outer-arc high have been analyzed for Sr, Nd, and Pb isotopes. Nd-Sr isotope covariations show that most samples follow a trend parallel to a line from Pacific MORB mantle (PMM) to Pacific Volcanogenic sediment (PVS) but displaced slightly toward more radiogenic Sr. Pb isotope covariations show that all the Eocene-Oligocene samples plot along the Northern Hemisphere Reference Line, indicating little or no Pb derived from subducted pelagic sediment in their source. Two young basaltic andesite clasts within sediment do have a pelagic sediment signature but this may have been gained by alteration rather than subduction. In all isotopic projections, the samples form consistent groupings: the tholeiites from Site 782 and Hole 786A plot closest to PMM, the Boninites and related rocks from Sites 786B plot closest to PVS, and the Boninite lavas from Hole 786A and late boninitic dikes from Hole 786B occupy an intermediate position. Isotope-trace element covariations indicate that these isotopic variations can be explained by a three-component mixing model. One component (A) has the isotopic signature of PMM but is depleted in the more incompatible elements. It is interpreted as representing suboceanic mantle lithosphere. A second component (B) is relatively radiogenic (eNd = ca 4-6; 206Pb/204Pb = ca 19.0-19.3; eSr = ca -10 to -6)). Its trace element pattern has, among other characteristics, a high Zr/Sm ratio, which distinguishes it from the "normal" fluid components associated with subduction and hotspot activity. There are insufficient data at present to tie down its origin: probably it was either derived from subducted lithosphere or volcanogenic sediment fused in amphibolite facies; or it represents an asthenospheric melt component that has been fractionated by interaction with amphibole-bearing mantle. The third component (C) is characterized by high contents of Sr and high eSr values and is interpreted as a subducted fluid component. The mixing line on a diagram of Zr/Sr against e Sr suggests that component C may have enriched the lithosphere (component A) before component B. These components may also be present on a regional basis but, if so, may not have had uniform compositions. Only the boninitic series from nearby Chichijima would require an additional, pelagic sediment component. In general, these results are consistent with models of subduction of ridges and young lithosphere during the change from a ridge-transform to subduction geometry at the initiation of subduction in the Western Pacific.
Robert Hall - One of the best experts on this subject based on the ideXlab platform.
-
tectonic setting of eocene Boninite magmatism in the izu bonin mariana forearc
Earth and Planetary Science Letters, 2001Co-Authors: Colin G. Macpherson, Robert HallAbstract:Abstract Middle Eocene Boninites were simultaneously generated over a large region during the early history of the Izu–Bonin–Mariana (IBM) arc. However, widespread Boninite magmatism is not recognised in younger subduction zones of similar dimensions. This suggests that an additional tectonic or thermal factor influenced the generation of the IBM Boninite suite. Regional uplift, ocean island basalt-style magmatism and high heat-flow also characterised the northern Philippine Sea plate (PSP) at the start of the Middle Eocene. These features are similar to those observed in large volume basaltic volcanic provinces such as the early Tertiary North Atlantic and suggest that the IBM Boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the PSP. The reconstructed Middle Eocene location of the IBM arc and West Philippine Basin lies close to the present day Manus Basin where petrological and geochemical evidence indicate the presence of a mantle plume. A calculated plume track linking these locations through time also passes close to the Eauripik Rise, an aseismic ridge on the Caroline Plate, during the Oligocene and Miocene. Therefore, we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western Pacific from the Middle Eocene until the present day.
-
Tectonic setting of Eocene Boninite magmatism in the Izu–Bonin–Mariana forearc
Earth and Planetary Science Letters, 2001Co-Authors: Colin G. Macpherson, Robert HallAbstract:Abstract Middle Eocene Boninites were simultaneously generated over a large region during the early history of the Izu–Bonin–Mariana (IBM) arc. However, widespread Boninite magmatism is not recognised in younger subduction zones of similar dimensions. This suggests that an additional tectonic or thermal factor influenced the generation of the IBM Boninite suite. Regional uplift, ocean island basalt-style magmatism and high heat-flow also characterised the northern Philippine Sea plate (PSP) at the start of the Middle Eocene. These features are similar to those observed in large volume basaltic volcanic provinces such as the early Tertiary North Atlantic and suggest that the IBM Boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the PSP. The reconstructed Middle Eocene location of the IBM arc and West Philippine Basin lies close to the present day Manus Basin where petrological and geochemical evidence indicate the presence of a mantle plume. A calculated plume track linking these locations through time also passes close to the Eauripik Rise, an aseismic ridge on the Caroline Plate, during the Oligocene and Miocene. Therefore, we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western Pacific from the Middle Eocene until the present day.