richard arculus - Academia.edu (original) (raw)
Conference Presentations by richard arculus
The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting ... more The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting dynamic and diverse tectonic settings (e.g., rapidly opening basin, migrating microplate boundaries, within-plate discrete rear arc volcanoes, and weak on-arc magmatism, all in close spatial proximity). Frequent contemporaneous eruptions of high-MgO magmas and their differentiates from closely spaced volcanoes provide an uncommonly detailed view of the diversity of parent magma types formed in the broader suprasubduction zone environment. They show how mantle lithological variations couple with local tectonic setting to control magmatism, and influence magma accumulation, storage, and subsequent eruption. The results of 6 recent research expeditions to this relatively small region show how these attributes collectively result in systematic relationships between eruption style, inferred duration, size, intensity, location, and compositional variability. In particular, we can constrain magmatic conditions and timescales from single eruption deposits that we sampled with high-spatial-resolution in the context of detailed deposit mapping, high resolution 210 Po-210 Pb eruption chronologies, and U-Th-Ra disequilibrium. At the same time, variations between eruption deposits reveal how quickly parent melt compositions drawn from the mantle wedge can change in space and time. We will discuss and interpret diverse phenomena from the region, including high rates of temporal variability seen in magma compositions within single eruptions, ultra-high variability in source compositions sampled at sub-km scale on neighboring small Mata volcanoes, highly productive and frequent volcanism on the NE Lau Spreading center, and the petrogenetc links between boninite and high MgO basalt magmas with extensive contemporaneous dacite volcanism in the region.
Papers by richard arculus
Frontiers in Earth Science
The Amami-Sankaku Basin, located in the Philippine Sea, records approximately 50 million years of... more The Amami-Sankaku Basin, located in the Philippine Sea, records approximately 50 million years of sediment accumulation and diagenesis of volcanic ash derived from the Kyushu-Palau and Kyushu-Ryukyu arcs. Analyses of porewater and sediment samples from the 1461-m core recovered at IODP Expedition 351, Site U1438, included major and trace elements, strontium radiogenic isotope ratio (87Sr/86Sr), and taxonomic identification of archaeal classes and bacterial orders. Sediment X-ray diffraction and thin section analyses show that smectite, zeolites and chlorite are the main authigenic minerals. A multicomponent solute diffusion and reaction numerical model was developed to simulate the long-term diagenesis that took place in this sedimentary sequence and to account for the transition to a calcium chloride brine at about 670 mbsf. Numerical results indicate that 45% of the initial amount of volcanic ash has been dissolved at 750 mbsf. At this depth, 13% of the initial water (H2O) in the ...
Active Margins and Marginal Basins of the Western Pacific, 1995
... 125 ashes by Arculus and Bloomfield [1992], several more detailed analytical efforts have bee... more ... 125 ashes by Arculus and Bloomfield [1992], several more detailed analytical efforts have been undertaken by different research groups on the leg 125 ashes as well as material from previous DSDP legs in the Izu-Bonin-Mariana region [Arculus et ah, 1992; Chen and Arculus ...
Oceanic islands and seamounts of volcanic origin are numerous; e.g. population is >93000 islan... more Oceanic islands and seamounts of volcanic origin are numerous; e.g. population is >93000 islands and seamounts in the Pacific Ocean (Hillier and Watts, 2007). Most oceanic intraplate volcanoes are a product of hotspot volcanism or mantle plumes, which bring hot material from the Transition Zone, lower mantle or core-mantle boundary to melting conditions at the base of the lithosphere, and produces long chains of volcanoes that become progressively older as a function of distance from the active volcanism locus (Koppers et al., 2010a; Staudigel and Clague, 2010). Transport of the islands away from the hotspot, cessation of volcanism, thermal subsidence and flexure of the lithosphere lead to submersion of volcanic edifices (commonly <~20Ma in the Pacific Ocean). Under appropriate oceanic conditions (i.e. clean and warm waters) reefal limestones may deposit on top of drowning oceanic islands and form atolls. Further subsidence can lead to total drowning of atolls that become flat-topped guyots (Flood, 1999a, 2001). Eventually, oceanic islands and seamounts are driven into subduction zones were they are recycled into the mantle or accrete to the upper plate. Despite large abundance of oceanic islands and seamounts in the oceans, their study has remained very limited. They are however significant for several reasons. Oceanic islands and seamounts in the oceans are our principal probe into the chemical evolution of the mantle (Koppers et al., 2010a). Abundant metal-rich deposits on seamounts constitute large mineral resources to be exploited in a near future (Hein et al., 2010). Subducting oceanic islands and seamounts have been recognized for a long time to be an important disturbing factor along convergent margins: (1) incoming of seamounts (or drowned oceanic islands) in the subduction zone triggers mass-wasting events (Dominguez et al., 1998); (2) subducting seamounts may cause basal removal of large portions of the overriding plate by erosive tunnelling (Ranero and von Huene, 2000) or, alternatively, contribute to the growth of convergent margins through partial accretion (Buchs et al., 2009); (3) the seamounts act as seismic asperities at the
Geochemistry, Geophysics, Geosystems, 2007
ABSTRACT The triple junction formed by the subduction of the Woodlark spreading ridge beneath the... more ABSTRACT The triple junction formed by the subduction of the Woodlark spreading ridge beneath the New Georgia Group (NGG) of the Solomon Islands is associated with unusual arc-related volcanic centers on the subducting Australia-India plate and a foreshortened arc-trench gap (30 - 90 km). Large calc-alkaline seamounts have grown along the north-trending Simbo ridge and the ESE-trending Ghizo ridge. These two features intersect the trench system at points separated by nearly 100 km, and the absence of a deep trench in this area and the lack of a strong Benioff zone beneath the NGG suggests that subduction is being stifled by the presence of the high-standing Woodlark ridge topography. The presence of numerous arcuate faults in the triangular region bounded by the ridges and the trench suggests the area is under extreme rotational stress due to this impingement. Rotation and faulting here may have allowed magmas to rise along the zones of weakness at the Simbo and Ghizo ridges, which are a Woodlark transform and a possible former spreading segment, respectively. Arc-like volcanism to the south of the triple junction has been attributed to migration of calc-alkaline magmas from a source region under the Pacific plate. However, the mantle in this area was geochemically enriched by a previous episode of southerly subduction of the Pacific Plate. The impingement of the Ontong Java Plateau in the Miocene caused a subduction polarity reversal and initiated the current northerly subduction direction, placing mantle that was once in a back arc region under the NGG and Australia-India plate. This enrichment is observed in MORB and andesites acquired from the Woodlark spreading center up to 50 km from the triple junction, and increases with decreasing distance from the arc. We are testing the hypothesis that the anomalous lavas of the NGG and on the Australia-India plate are both generated in-situ and tap similar mantle sources that were enriched by the earlier phase of subduction. Samples from seamounts on both sides of the trench in the triple junction area were collected during the 2000 CSIRO Franklin cruise (FR04/00) by dredge and rock grab. Major and trace elements, and Sr, Nd, and Pb isotopic compositions of the rocks are being compared in this study to constrain the geochemical characteristics of the mantle sources being sampled by these volcanic centers, to identify geochemical gradients in the region, and to evaluate any contributions by the subducted Australia-India slab to the NGG lavas.
Geochimica et Cosmochimica Acta, 2021
Nature communications, Jan 29, 2018
Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attr... more Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Here we provide direct evidence for contributions of oxidising slab agents to melts trapped in the sub-arc mantle. Measurements of sulfur (S) valence state in sub-arc mantle peridotites identify sulfate, both as crystalline anhydrite (CaSO) and dissolved SO in spinel-hosted glass (formerly melt) inclusions. Copper-rich sulfide precipitates in the inclusions and increased Fe/∑Fe in spinel record a S-Fe redox coupling during melt percolation through the sub-arc mantle. Sulfate-rich glass inclusions exhibit high U/Th, Pb/Ce, Sr/Nd and δS (+ 7 to + 11‰), indicating the involvement of dehydration products of serpentinised slab rocks in their parental melt sources. These observations provide a link between l...
The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting ... more The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting dynamic and diverse tectonic settings (e.g., rapidly opening basin, migrating microplate boundaries, within-plate discrete rear arc volcanoes, and weak on-arc magmatism, all in close spatial proximity). Frequent contemporaneous eruptions of high-MgO magmas and their differentiates from closely spaced volcanoes provide an uncommonly detailed view of the diversity of parent magma types formed in the broader suprasubduction zone environment. They show how mantle lithological variations couple with local tectonic setting to control magmatism, and influence magma accumulation, storage, and subsequent eruption. The results of 6 recent research expeditions to this relatively small region show how these attributes collectively result in systematic relationships between eruption style, inferred duration, size, intensity, location, and compositional variability. In particular, we can constrain mag...
Geophysical Monograph Series, 2004
Of the volumetrically significant magma types, those emplaced in arcs at plate convergence zones ... more Of the volumetrically significant magma types, those emplaced in arcs at plate convergence zones are typically richest in dissolved volatiles (H 2 O, CO 2 , and S species). These volatiles are mostly products of large-scale recycling, derived though multistage processes involving devolatilization of variably hydrated and carbonated, sediment-bearing, subducted lithosphere, and transported towards the surface by magmas generated in the mantle overlying the subducted plate. Volatile contents of parental arc basalts are globally variable, but mostly range from ∼ 0.5 to 10 wt% H 2 O, ≤1000 ppm CO 2 , and ≤3500 ppm S (as H 2 S and/or SO 2 ). Arc magmas are also generally more oxidized than those of ridges and hot-spots. These characteristics lead to distinctive differences in the course of magmatic crystallization compared with dry, reduced types: plagioclase saturation is delayed, and Ca-rich upon appearance; olivine persists in the crystallization sequence to higher SiO 2 contents; a spinel phase appears early and persists throughout crystallization. Resultant relatively voluminous Na-K-feldspar- and SiO 2 -rich residual magmas dominate the bulk continental crust. The complementary SiO 2 -poor olivine-clinopyroxene-dominated fraction is probably recycled into the upper mantle. Volatile fluxes through the subduction cycle are not straightforwardly determined; total magma volume flux estimates range from ∼ 1.2 to 7 km 3 /year, but are not well constrained for the full diversity of global arc systems. Even at the high end of this range, more C is subducted than returned via arcs, presumably with long-term effects on the global C cycle. At the lower end, deficiencies may exist in return fluxes of H 2 O and S species.
ASEG Extended Abstracts, 2006
The SS1112004 research voyage of the Marine National Facility to the northern Tonga Arc and adjac... more The SS1112004 research voyage of the Marine National Facility to the northern Tonga Arc and adjacent Fonualei Rifts (backarc) discovered several new submarine, basalt-to-rhyolite volcanic centres (stratovolcanoes and massive caldera systems) in the Arc. Numerous sector and ...
New Zealand Journal of Geology and Geophysics, 2005
... RICHARD ARCULUS1 STEVE EGGINS3 1Department of Earth and Marine Sciences Australian National U... more ... RICHARD ARCULUS1 STEVE EGGINS3 1Department of Earth and Marine Sciences Australian National University Canberra, Australia 0200 email: carl@geology.anu ... bearing, Early Permian strata to the east of the mountains as a separate unit known as the Caravan Formation ...
International Geology Review, 2009
ABSTRACT Plate subduction and mantle plumes are two of the most important material transport proc... more ABSTRACT Plate subduction and mantle plumes are two of the most important material transport processes of the silicate Earth. Currently, a debate exists over whether the subducted oceanic crust is recycled back to the Earth's surface through mantle plumes, and can explain their derivation and major characteristics. It is also puzzling as to why plume heads have huge melting capacities and differ dramatically from plume tails both in size and chemical composition. We present data showing that both ocean island basalt and mid-ocean ridge basalt have identical supra-primitive mantle mean Nb/U values of 46.7, significantly larger than that of the primitive mantle value. From a mass balance calculation based on Nb/U we have determined that nearly the whole mantle has evolved by plate subduction-induced crustal recycling during formation of the continental crust. This mixing back of subducted oceanic crust, however, is not straightforward, because it generally would be denser than the surrounding mantle, both in solid and liquid states. A mineral segregation model is proposed here to reconcile different lines of observation. First of all, subducted oceanic crustal sections are denser than the surrounding mantle, such that they can stay in the lower mantle, for billions of years as implied by isotopic data. Parts of subducted oceanic crust may eventually lose a large proportion of their heavy minerals, magnesian-silicate-perovskite and calcium-silicate-perovskite, through density segregation in ultra-low-velocity zones as well as in very-low-velocity provinces at the core-mantle boundary due to low viscosity. The remaining minerals would thus become lighter than the surrounding mantle, and could rise, trapping mantle materials, and forming mantle plumes. Mineral segregation progressively increases the SiO2 content of the ascending oceanic crust, which enhances flux melting, and results in giant Si-enriched plume heads followed by dramatically abridged plume tails. Therefore, ancient mineral-segregated subducted oceanic crust is likely to be a major trigger and driving force for the formation of mantle plumes.
The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting ... more The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting dynamic and diverse tectonic settings (e.g., rapidly opening basin, migrating microplate boundaries, within-plate discrete rear arc volcanoes, and weak on-arc magmatism, all in close spatial proximity). Frequent contemporaneous eruptions of high-MgO magmas and their differentiates from closely spaced volcanoes provide an uncommonly detailed view of the diversity of parent magma types formed in the broader suprasubduction zone environment. They show how mantle lithological variations couple with local tectonic setting to control magmatism, and influence magma accumulation, storage, and subsequent eruption. The results of 6 recent research expeditions to this relatively small region show how these attributes collectively result in systematic relationships between eruption style, inferred duration, size, intensity, location, and compositional variability. In particular, we can constrain magmatic conditions and timescales from single eruption deposits that we sampled with high-spatial-resolution in the context of detailed deposit mapping, high resolution 210 Po-210 Pb eruption chronologies, and U-Th-Ra disequilibrium. At the same time, variations between eruption deposits reveal how quickly parent melt compositions drawn from the mantle wedge can change in space and time. We will discuss and interpret diverse phenomena from the region, including high rates of temporal variability seen in magma compositions within single eruptions, ultra-high variability in source compositions sampled at sub-km scale on neighboring small Mata volcanoes, highly productive and frequent volcanism on the NE Lau Spreading center, and the petrogenetc links between boninite and high MgO basalt magmas with extensive contemporaneous dacite volcanism in the region.
Frontiers in Earth Science
The Amami-Sankaku Basin, located in the Philippine Sea, records approximately 50 million years of... more The Amami-Sankaku Basin, located in the Philippine Sea, records approximately 50 million years of sediment accumulation and diagenesis of volcanic ash derived from the Kyushu-Palau and Kyushu-Ryukyu arcs. Analyses of porewater and sediment samples from the 1461-m core recovered at IODP Expedition 351, Site U1438, included major and trace elements, strontium radiogenic isotope ratio (87Sr/86Sr), and taxonomic identification of archaeal classes and bacterial orders. Sediment X-ray diffraction and thin section analyses show that smectite, zeolites and chlorite are the main authigenic minerals. A multicomponent solute diffusion and reaction numerical model was developed to simulate the long-term diagenesis that took place in this sedimentary sequence and to account for the transition to a calcium chloride brine at about 670 mbsf. Numerical results indicate that 45% of the initial amount of volcanic ash has been dissolved at 750 mbsf. At this depth, 13% of the initial water (H2O) in the ...
Active Margins and Marginal Basins of the Western Pacific, 1995
... 125 ashes by Arculus and Bloomfield [1992], several more detailed analytical efforts have bee... more ... 125 ashes by Arculus and Bloomfield [1992], several more detailed analytical efforts have been undertaken by different research groups on the leg 125 ashes as well as material from previous DSDP legs in the Izu-Bonin-Mariana region [Arculus et ah, 1992; Chen and Arculus ...
Oceanic islands and seamounts of volcanic origin are numerous; e.g. population is >93000 islan... more Oceanic islands and seamounts of volcanic origin are numerous; e.g. population is >93000 islands and seamounts in the Pacific Ocean (Hillier and Watts, 2007). Most oceanic intraplate volcanoes are a product of hotspot volcanism or mantle plumes, which bring hot material from the Transition Zone, lower mantle or core-mantle boundary to melting conditions at the base of the lithosphere, and produces long chains of volcanoes that become progressively older as a function of distance from the active volcanism locus (Koppers et al., 2010a; Staudigel and Clague, 2010). Transport of the islands away from the hotspot, cessation of volcanism, thermal subsidence and flexure of the lithosphere lead to submersion of volcanic edifices (commonly <~20Ma in the Pacific Ocean). Under appropriate oceanic conditions (i.e. clean and warm waters) reefal limestones may deposit on top of drowning oceanic islands and form atolls. Further subsidence can lead to total drowning of atolls that become flat-topped guyots (Flood, 1999a, 2001). Eventually, oceanic islands and seamounts are driven into subduction zones were they are recycled into the mantle or accrete to the upper plate. Despite large abundance of oceanic islands and seamounts in the oceans, their study has remained very limited. They are however significant for several reasons. Oceanic islands and seamounts in the oceans are our principal probe into the chemical evolution of the mantle (Koppers et al., 2010a). Abundant metal-rich deposits on seamounts constitute large mineral resources to be exploited in a near future (Hein et al., 2010). Subducting oceanic islands and seamounts have been recognized for a long time to be an important disturbing factor along convergent margins: (1) incoming of seamounts (or drowned oceanic islands) in the subduction zone triggers mass-wasting events (Dominguez et al., 1998); (2) subducting seamounts may cause basal removal of large portions of the overriding plate by erosive tunnelling (Ranero and von Huene, 2000) or, alternatively, contribute to the growth of convergent margins through partial accretion (Buchs et al., 2009); (3) the seamounts act as seismic asperities at the
Geochemistry, Geophysics, Geosystems, 2007
ABSTRACT The triple junction formed by the subduction of the Woodlark spreading ridge beneath the... more ABSTRACT The triple junction formed by the subduction of the Woodlark spreading ridge beneath the New Georgia Group (NGG) of the Solomon Islands is associated with unusual arc-related volcanic centers on the subducting Australia-India plate and a foreshortened arc-trench gap (30 - 90 km). Large calc-alkaline seamounts have grown along the north-trending Simbo ridge and the ESE-trending Ghizo ridge. These two features intersect the trench system at points separated by nearly 100 km, and the absence of a deep trench in this area and the lack of a strong Benioff zone beneath the NGG suggests that subduction is being stifled by the presence of the high-standing Woodlark ridge topography. The presence of numerous arcuate faults in the triangular region bounded by the ridges and the trench suggests the area is under extreme rotational stress due to this impingement. Rotation and faulting here may have allowed magmas to rise along the zones of weakness at the Simbo and Ghizo ridges, which are a Woodlark transform and a possible former spreading segment, respectively. Arc-like volcanism to the south of the triple junction has been attributed to migration of calc-alkaline magmas from a source region under the Pacific plate. However, the mantle in this area was geochemically enriched by a previous episode of southerly subduction of the Pacific Plate. The impingement of the Ontong Java Plateau in the Miocene caused a subduction polarity reversal and initiated the current northerly subduction direction, placing mantle that was once in a back arc region under the NGG and Australia-India plate. This enrichment is observed in MORB and andesites acquired from the Woodlark spreading center up to 50 km from the triple junction, and increases with decreasing distance from the arc. We are testing the hypothesis that the anomalous lavas of the NGG and on the Australia-India plate are both generated in-situ and tap similar mantle sources that were enriched by the earlier phase of subduction. Samples from seamounts on both sides of the trench in the triple junction area were collected during the 2000 CSIRO Franklin cruise (FR04/00) by dredge and rock grab. Major and trace elements, and Sr, Nd, and Pb isotopic compositions of the rocks are being compared in this study to constrain the geochemical characteristics of the mantle sources being sampled by these volcanic centers, to identify geochemical gradients in the region, and to evaluate any contributions by the subducted Australia-India slab to the NGG lavas.
Geochimica et Cosmochimica Acta, 2021
Nature communications, Jan 29, 2018
Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attr... more Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Here we provide direct evidence for contributions of oxidising slab agents to melts trapped in the sub-arc mantle. Measurements of sulfur (S) valence state in sub-arc mantle peridotites identify sulfate, both as crystalline anhydrite (CaSO) and dissolved SO in spinel-hosted glass (formerly melt) inclusions. Copper-rich sulfide precipitates in the inclusions and increased Fe/∑Fe in spinel record a S-Fe redox coupling during melt percolation through the sub-arc mantle. Sulfate-rich glass inclusions exhibit high U/Th, Pb/Ce, Sr/Nd and δS (+ 7 to + 11‰), indicating the involvement of dehydration products of serpentinised slab rocks in their parental melt sources. These observations provide a link between l...
The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting ... more The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting dynamic and diverse tectonic settings (e.g., rapidly opening basin, migrating microplate boundaries, within-plate discrete rear arc volcanoes, and weak on-arc magmatism, all in close spatial proximity). Frequent contemporaneous eruptions of high-MgO magmas and their differentiates from closely spaced volcanoes provide an uncommonly detailed view of the diversity of parent magma types formed in the broader suprasubduction zone environment. They show how mantle lithological variations couple with local tectonic setting to control magmatism, and influence magma accumulation, storage, and subsequent eruption. The results of 6 recent research expeditions to this relatively small region show how these attributes collectively result in systematic relationships between eruption style, inferred duration, size, intensity, location, and compositional variability. In particular, we can constrain mag...
Geophysical Monograph Series, 2004
Of the volumetrically significant magma types, those emplaced in arcs at plate convergence zones ... more Of the volumetrically significant magma types, those emplaced in arcs at plate convergence zones are typically richest in dissolved volatiles (H 2 O, CO 2 , and S species). These volatiles are mostly products of large-scale recycling, derived though multistage processes involving devolatilization of variably hydrated and carbonated, sediment-bearing, subducted lithosphere, and transported towards the surface by magmas generated in the mantle overlying the subducted plate. Volatile contents of parental arc basalts are globally variable, but mostly range from ∼ 0.5 to 10 wt% H 2 O, ≤1000 ppm CO 2 , and ≤3500 ppm S (as H 2 S and/or SO 2 ). Arc magmas are also generally more oxidized than those of ridges and hot-spots. These characteristics lead to distinctive differences in the course of magmatic crystallization compared with dry, reduced types: plagioclase saturation is delayed, and Ca-rich upon appearance; olivine persists in the crystallization sequence to higher SiO 2 contents; a spinel phase appears early and persists throughout crystallization. Resultant relatively voluminous Na-K-feldspar- and SiO 2 -rich residual magmas dominate the bulk continental crust. The complementary SiO 2 -poor olivine-clinopyroxene-dominated fraction is probably recycled into the upper mantle. Volatile fluxes through the subduction cycle are not straightforwardly determined; total magma volume flux estimates range from ∼ 1.2 to 7 km 3 /year, but are not well constrained for the full diversity of global arc systems. Even at the high end of this range, more C is subducted than returned via arcs, presumably with long-term effects on the global C cycle. At the lower end, deficiencies may exist in return fluxes of H 2 O and S species.
ASEG Extended Abstracts, 2006
The SS1112004 research voyage of the Marine National Facility to the northern Tonga Arc and adjac... more The SS1112004 research voyage of the Marine National Facility to the northern Tonga Arc and adjacent Fonualei Rifts (backarc) discovered several new submarine, basalt-to-rhyolite volcanic centres (stratovolcanoes and massive caldera systems) in the Arc. Numerous sector and ...
New Zealand Journal of Geology and Geophysics, 2005
... RICHARD ARCULUS1 STEVE EGGINS3 1Department of Earth and Marine Sciences Australian National U... more ... RICHARD ARCULUS1 STEVE EGGINS3 1Department of Earth and Marine Sciences Australian National University Canberra, Australia 0200 email: carl@geology.anu ... bearing, Early Permian strata to the east of the mountains as a separate unit known as the Caravan Formation ...
International Geology Review, 2009
ABSTRACT Plate subduction and mantle plumes are two of the most important material transport proc... more ABSTRACT Plate subduction and mantle plumes are two of the most important material transport processes of the silicate Earth. Currently, a debate exists over whether the subducted oceanic crust is recycled back to the Earth's surface through mantle plumes, and can explain their derivation and major characteristics. It is also puzzling as to why plume heads have huge melting capacities and differ dramatically from plume tails both in size and chemical composition. We present data showing that both ocean island basalt and mid-ocean ridge basalt have identical supra-primitive mantle mean Nb/U values of 46.7, significantly larger than that of the primitive mantle value. From a mass balance calculation based on Nb/U we have determined that nearly the whole mantle has evolved by plate subduction-induced crustal recycling during formation of the continental crust. This mixing back of subducted oceanic crust, however, is not straightforward, because it generally would be denser than the surrounding mantle, both in solid and liquid states. A mineral segregation model is proposed here to reconcile different lines of observation. First of all, subducted oceanic crustal sections are denser than the surrounding mantle, such that they can stay in the lower mantle, for billions of years as implied by isotopic data. Parts of subducted oceanic crust may eventually lose a large proportion of their heavy minerals, magnesian-silicate-perovskite and calcium-silicate-perovskite, through density segregation in ultra-low-velocity zones as well as in very-low-velocity provinces at the core-mantle boundary due to low viscosity. The remaining minerals would thus become lighter than the surrounding mantle, and could rise, trapping mantle materials, and forming mantle plumes. Mineral segregation progressively increases the SiO2 content of the ascending oceanic crust, which enhances flux melting, and results in giant Si-enriched plume heads followed by dramatically abridged plume tails. Therefore, ancient mineral-segregated subducted oceanic crust is likely to be a major trigger and driving force for the formation of mantle plumes.
Geology, 2000
ABSTRACT The Permian Greenhills Complex of Southland, New Zealand, is a small ultramafic to mafic... more ABSTRACT The Permian Greenhills Complex of Southland, New Zealand, is a small ultramafic to mafic layered intrusion that hosts primary magmatic platinum-group minerals (PGMs) as inclusions in cumulus chromian spinel grains. We have determined the major and trace element chemistry of the parent magmas of the complex, using the composition of rehomogenized melt inclusions trapped in cumulus spinel grains from dunite. These melt inclusions have compositions similar to those of primitive ankaramite dikes that cut the complex, verifying that our data are representative of the parent-melt chemistry. These magmas have high MgO and CaO and are enriched in Pb, Sr, and large ion lithophile elements compared to high field strength and rare earth elements. We suggest that the parent magmas to the complex were primitive low-K island-arc tholeiite basalts that formed by high degrees of melting of a spinel peridotite source. The platinum-group element contents of such magmas are expected to be very high. Therefore, the primary PGMs at Greenhills are suggested to have precipitated directly from the melt following differentiation and cooling in a shallow magma chamber.