Dense melt residues drive mid-ocean-ridge “hotspots”

ABSTRACT The geodynamic origin of melting anomalies found at the surface, often referred to as “hotspots,” is classically attributed to a mantle plume process. The distribution of hotspots along mid-ocean-ridge spreading systems around the globe, however, questions the universal validity of this concept. Here, the preferential association of hotspots with slow- to intermediate-spreading centers and not fast-spreading centers, an observation contrary to the expected effect of ridge suction forces on upwelling mantle plumes, is explained by a new mechanism for producing melting anomalies at shallow (<2.3 GPa) depths. By combining the effects of both chemical and thermal density changes during partial melting of the mantle (using appropriate latent heat and depth-dependent thermal expansivity parameters), we find that mantle residues experience an overall instantaneous increase in density when melting occurs at <2.3 GPa. This controversial finding is due to thermal contraction of material during melting, which outweighs the chemical buoyancy due to melting at shallow pressures (where thermal expansivities are highest). These dense mantle residues are likely to locally sink beneath spreading centers if ridge suction forces are modest, thus driving an increase in the flow of fertile mantle through the melting window and increasing magmatic production. This leads us to question our understanding of sub–spreading center dynamics, where we now suggest a portion of locally inverted mantle flow results in hotspots. Such inverted flow presents an alternative mechanism to upwelling hot mantle plumes for the generation of excess melt at near-ridge hotspots, i.e., dense downwelling of mantle residue locally increasing the flow of fertile mantle through the melting window. Near-ridge hotspots, therefore, may not require the elevated temperatures commonly invoked to account for excess melting. The proposed mechanism also satisfies counterintuitive observations of ridge-bound hotspots at slow- to intermediate-spreading centers, yet not at fast-spreading centers, where large dynamic ridge suction forces likely overwhelm density-driven downwelling. The lack of observations of such downwelling in numerical modeling studies to date reflects the generally high chemical depletion buoyancy and/or low thermal expansivity parameter values employed in simulations, which we find to be unrepresentative for melting at <2.3 GPa. We therefore invite future studies to review the values used for parameters affecting density changes during melting (e.g., depletion buoyancy, latent heat of melting, specific heat capacity, thermal expansivity), which quite literally have the potential to turn our understanding of mantle dynamics upside down.

Investigating influences on the Pb pseudo-isochron using three-dimensional mantle convection models with a continental reservoir

For mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs), measurements of Pb isotope ratios show broad linear correlations with a certain degree of scatter. In 207Pb/204Pb - 206Pb/204Pb space, the best fit line defines a pseudo-isochron age (τPb) of ~1.9 Gyr.Previous modelling suggests a relative change in the behaviours of U and Pb between 2.25-2.5 Ga, resulting in net recycling of HIMU (high U/Pb) material in the latter part of Earth's history, to explain the observed τPb. However, simulations in which fractionation is controlled by a single set of partition coefficients throughout the model runs fail to reproduce τPb and the observed scatter in Pb isotope ratios. We build on these models with 3D mantle convection simulations including parameterisations for melting, U recycling from the continents and preferential removal of Pb from subducted oceanic crust.We find that both U recycling after the great oxygenation event (GOE) and Pb extraction after the onset of plate tectonics, are required in order to fit the observed gradient and scatter of both the 207Pb/204Pb - 206Pb/204Pb and 208Pb/204Pb - 206Pb/204Pb arrays. Unlike much previous work, our model does not require accumulations of subducted oceanic crust to persist at the CMB for long periods of time in order to match geochemical observations.

Accessories in Kaiserstuhl carbonatites and related rocks as accurate and faithful recorders of whole rock age and isotopic composition

The accessories perovskite, pyrochlore, zirconolite, calzirtite and melanite from carbonatites and carbonate-rich foidites from the Kaiserstuhl are variously suited for the in situ determination of their U–Pb ages and Sr, Nd- and Hf-isotope ratios by LA-ICP-MS. The 143Nd/144Nd ratios may be determined precisely in all five phases, the 176Hf/177Hf ratios only in calzirtite and the 87Sr/86Sr ratios in perovskites and pyrochlores. The carbonatites and carbonate-rich foidites belong to one of the three magmatic groups that Schleicher et al. (1990) distinguished in the Kaiserstuhl on the basis of their Sr, Nd and Pb isotope ratios. Tephrites, phonolites and essexites (nepheline monzogabbros) form the second and limburgites (nepheline basanites) and olivine nephelinites the third. Our 87Sr/86Sr isotope data from the accessories overlap with the carbonatite and olivine nephelinite fields defined by Schleicher et al. (1990) but exhibit a much narrower range. These and the εNd and εHf values plot along the mantle array in the field of oceanic island basalts relatively close to mid-ocean ridge basalts. Previously reported K–Ar, Ar–Ar and fission track ages for the Kaiserstuhl lie between 16.2 and 17.8 Ma. They stem entirely from the geologically older tephrites, phonolites and essexites. No ages existed so far for the geologically younger carbonatites and carbonate-rich foidites except for one apatite fission track age (15.8 Ma). We obtained precise U–Pb ages for zirconolites and calzirtites of 15.66, respectively 15.5 Ma (± 0.1 2σ) and for pyrochlores of 15.35 ± 0.24 Ma. Only the perovskites from the Badberg soevite yielded a U–P concordia age of 14.56 ± 0.86 Ma while the perovskites from bergalites (haüyne melilitites) only gave 206Pb/238U and 208Pb/232Th ages of 15.26 ± 0.21, respectively, 15.28 ± 0.48 Ma. The main Kaiserstuhl rock types were emplaced over a time span of 1.6 Ma almost 1 million years before the carbonatites and carbonate-rich foidites. These were emplaced within only 0.32 Ma.

Incorporation mechanism of Fe and Al into bridgmanite in a subducting mid-ocean ridge basalt and its crystal chemistry

The compositional difference between subducting slabs and their surrounding lower-mantle can yield the difference in incorporation mechanism of Fe and Al into bridgmanite between both regions, which should cause heterogeneity in physical properties and rheology of the lower mantle. However, the precise cation-distribution has not been examined in bridgmanites with Fe- and Al-contents expected in a mid-ocean ridge basalt component of subducting slabs. Here we report on Mg0.662Fe0.338Si0.662Al0.338O3 bridgmanite single-crystal characterized by a combination of single-crystal X-ray diffraction, synchrotron 57Fe-Mössbauer spectroscopy and electron probe microanalysis. We find that the charge-coupled substitution AMg2+ + BSi4+ ↔ AFe3+(high-spin) + BAl3+ is predominant in the incorporation of Fe and Al into the practically eightfold-coordinated A-site and the sixfold-coordinated B-site in bridgmanite structure. The incorporation of both cations via this substitution enhances the structural distortion due to the tilting of BO6 octahedra, yielding the unusual expansion of mean <A–O> bond-length due to flexibility of A–O bonds for the structural distortion, in contrast to mean <B–O> bond-length depending reasonably on the ionic radius effect. Moreover, we imply the phase-transition behavior and the elasticity of bridgmanite in slabs subducting into deeper parts of the lower mantle, in terms of the relative compressibility of AO12 (practically AO8) and BO6 polyhedra.

Ridge Jumps and Mantle Exhumation in Back-Arc Basins

Back-arc basins in continental settings can develop into oceanic basins, when extension lasts long enough to break up the continental lithosphere and allow mantle melting that generates new oceanic crust. Often, the basement of these basins is not only composed of oceanic crust, but also of exhumed mantle, fragments of continental crust, intrusive magmatic bodies, and a complex mid-ocean ridge system characterised by distinct relocations of the spreading centre. To better understand the dynamics that lead to these characteristic structures in back-arc basins, we performed 2D numerical models of continental extension with asymmetric and time-dependent boundary conditions that simulate episodic trench retreat. We find that, in all models, episodic extension leads to rift and/or ridge jumps. In our parameter space, the length of the jump ranges between 1 and 65 km and the timing necessary to produce a new spreading ridge varies between 0.4 and 7 Myr. With the shortest duration of the first extensional phase, we observe a strong asymmetry in the margins of the basin, with the margin further from trench being characterised by outcropping lithospheric mantle and a long section of thinned continental crust. In other cases, ridge jump creates two consecutive oceanic basins, leaving a continental fragment and exhumed mantle in between the two basins. Finally, when the first extensional phase is long enough to form a well-developed oceanic basin (>35 km long), we observe a very short intra-oceanic ridge jump. Our models are able to reproduce many of the structures observed in back-arc basins today, showing that the transient nature of trench retreat that leads to episodes of fast and slow extension is the cause of ridge jumps, mantle exhumation, and continental fragments formation.

The Volcano‐Tectonic Evolution of the Macquarie Ridge Complex, Australia‐Pacific Plate Boundary South of New Zealand

<p>The Macquarie Ridge Complex (MRC) forms the submarine expression of the Australia‐Pacific plate boundary south of New Zealand, comprising a rugged bathymetry made up of numerous seamounts along its length. Tectonic plate reconstructions show that the plate boundary evolved from divergent to transpressional relative plate motion from ca. 40 – 6 Ma. However, only limited geological observation of the products of past seafloor spreading and present transpressional deformation has been achieved. This study presents new high-resolution multibeam, photographic, petrologic and geochemical data for 10 seamounts located along the MRC in order to elucidate the current nature and evolution of the plate boundary. Seamounts are oriented parallel to the plate boundary, characterized by elongate forms, and deformed by transform faulting. Three guyot‐type seamounts display summit plateaux that were formed by wave and current erosion. MRC seafloor is composed of alkaline to sub‐alkaline basaltic pillow, massive and sheet lava flows, lava talus, volcaniclastic breccia, diabase and gabbro. This oceanic crust was formed during effusive mid‐ocean ridge volcanism at the relic Macquarie spreading centre and has since been sheared, accreted and exhumed along the modern transpressional plate boundary. Major element systematics indicate samples originated from spatially distinct magmatic sources and have since been juxtaposed at seamounts due to transpressional relative plate motion. MRC seamounts have formed as discrete elevations as a result of dip‐slip and strike‐slip faulting of the ridge axis. Thus, MRC seamounts are volcanic in origin but are now the morphological manifestations of tectonic and geomorphic processes. Petrologic and geochemical characteristics of volcanic glass samples from the MRC indicate that both effusive and explosive eruption styles operated at the relic Macquarie spreading centre. Primitive and sub‐alkaline to transitional basaltic magma that rose efficiently to the seafloor was erupted effusively and cooled to form lava flows with low vesicle and phenocryst contents or was granulated on contact with seawater to form hyaloclasts deposited in volcaniclastic breccias. More alkaline magmas that underwent crystal fractionation and volatile exsolution in shallow reservoirs were fragmented and erupted during submarine hawaiian‐type eruptions. Such a scenario is likely to have occurred during the final stages of magmatism at the Australia‐Pacific plate boundary south of New Zealand when seafloor spreading was ultraslow or had ceased, which induced low degrees of partial melting and retarded magma ascent rates. All MRC samples display enriched mid‐ocean ridge basalt (E‐MORB) trace element characteristics. The sample suite can be divided into two groups, with Group 1 samples distinguished from Group 2 samples by their lower concentrations of highly incompatible trace elements, flatter LREE slopes, higher MgO contents and lower alkali element contents. Group 1 basalts were derived from low degree partial melting of spinel lherzolite generated during the late stages of mid‐ocean ridge volcanism at the plate boundary when seafloor spreading rates were slow to ultraslow (full spreading rate < 20 mm yr⁻¹). Group 2 basalts were derived from low degree partial melting of spinel lherzolite, mixed with small amounts of very low degree partial melting of garnet lherzolite, during post‐spreading volcanism at the MRC. Remnant heat from previous seafloor spreading induced buoyant ascent of the sub‐ridge mantle and enriched heterogeneities were preferentially tapped by the ensuing low melt fractions. Magma ascent was stalled due to the cessation of extension at the ridge and the melts underwent crystal fractionation prior to eruption, which accounts for the lower MgO contents of Group 2 basalts. The pervasive incompatible element‐enrichment of MRC basalts and similarity to lavas from fossil spreading ridges in the eastern Pacific Ocean may reflect regional enrichment of the Pacific upper mantle.</p>

The Volcano‐Tectonic Evolution of the Macquarie Ridge Complex, Australia‐Pacific Plate Boundary South of New Zealand

<p>The Macquarie Ridge Complex (MRC) forms the submarine expression of the Australia‐Pacific plate boundary south of New Zealand, comprising a rugged bathymetry made up of numerous seamounts along its length. Tectonic plate reconstructions show that the plate boundary evolved from divergent to transpressional relative plate motion from ca. 40 – 6 Ma. However, only limited geological observation of the products of past seafloor spreading and present transpressional deformation has been achieved. This study presents new high-resolution multibeam, photographic, petrologic and geochemical data for 10 seamounts located along the MRC in order to elucidate the current nature and evolution of the plate boundary. Seamounts are oriented parallel to the plate boundary, characterized by elongate forms, and deformed by transform faulting. Three guyot‐type seamounts display summit plateaux that were formed by wave and current erosion. MRC seafloor is composed of alkaline to sub‐alkaline basaltic pillow, massive and sheet lava flows, lava talus, volcaniclastic breccia, diabase and gabbro. This oceanic crust was formed during effusive mid‐ocean ridge volcanism at the relic Macquarie spreading centre and has since been sheared, accreted and exhumed along the modern transpressional plate boundary. Major element systematics indicate samples originated from spatially distinct magmatic sources and have since been juxtaposed at seamounts due to transpressional relative plate motion. MRC seamounts have formed as discrete elevations as a result of dip‐slip and strike‐slip faulting of the ridge axis. Thus, MRC seamounts are volcanic in origin but are now the morphological manifestations of tectonic and geomorphic processes. Petrologic and geochemical characteristics of volcanic glass samples from the MRC indicate that both effusive and explosive eruption styles operated at the relic Macquarie spreading centre. Primitive and sub‐alkaline to transitional basaltic magma that rose efficiently to the seafloor was erupted effusively and cooled to form lava flows with low vesicle and phenocryst contents or was granulated on contact with seawater to form hyaloclasts deposited in volcaniclastic breccias. More alkaline magmas that underwent crystal fractionation and volatile exsolution in shallow reservoirs were fragmented and erupted during submarine hawaiian‐type eruptions. Such a scenario is likely to have occurred during the final stages of magmatism at the Australia‐Pacific plate boundary south of New Zealand when seafloor spreading was ultraslow or had ceased, which induced low degrees of partial melting and retarded magma ascent rates. All MRC samples display enriched mid‐ocean ridge basalt (E‐MORB) trace element characteristics. The sample suite can be divided into two groups, with Group 1 samples distinguished from Group 2 samples by their lower concentrations of highly incompatible trace elements, flatter LREE slopes, higher MgO contents and lower alkali element contents. Group 1 basalts were derived from low degree partial melting of spinel lherzolite generated during the late stages of mid‐ocean ridge volcanism at the plate boundary when seafloor spreading rates were slow to ultraslow (full spreading rate < 20 mm yr⁻¹). Group 2 basalts were derived from low degree partial melting of spinel lherzolite, mixed with small amounts of very low degree partial melting of garnet lherzolite, during post‐spreading volcanism at the MRC. Remnant heat from previous seafloor spreading induced buoyant ascent of the sub‐ridge mantle and enriched heterogeneities were preferentially tapped by the ensuing low melt fractions. Magma ascent was stalled due to the cessation of extension at the ridge and the melts underwent crystal fractionation prior to eruption, which accounts for the lower MgO contents of Group 2 basalts. The pervasive incompatible element‐enrichment of MRC basalts and similarity to lavas from fossil spreading ridges in the eastern Pacific Ocean may reflect regional enrichment of the Pacific upper mantle.</p>

Diamond and Other Exotic Mineral-Bearing Ophiolites on the Globe: A Key to Understand the Discovery of New Minerals and Formation of Ophiolitic Podiform Chromitite

Ophiolite-hosted diamond from peridotites and podiform chromitites significantly differs from those of kimberlitic diamond and ultra-high pressure (UHP) metamorphic diamond in terms of occurrence, mineral inclusion, as well as carbon and nitrogen isotopic composition. In this review, we briefly summarize the global distribution of twenty-five diamond-bearing ophiolites in different suture zones and outline the bulk-rock compositions, mineral and particular Re-Os isotopic systematics of these ophiolitic chromitites and host peridotites. These data indicate that the subcontinental lithospheric mantle is likely involved in the formation of podiform chromitite. We also provide an overview of the UHP textures and unusual mineral assemblages, including diamonds, other UHP minerals (e.g., moissanite, coesite) and crustal minerals, which robustly offer evidence of crustal recycling in the deep mantle along the suprasubduction zone (SSZ) and then being transported to shallow mantle depths by asthenospheric mantle upwelling in mid-ocean-ridge and SSZ settings. A systematic comparison between four main genetic models provides insights into our understanding of the origin of ophiolite-hosted diamond and the formation of podiform chromitite. Diamond-bearing peridotites and chromitites in ophiolites are important objects to discover new minerals from the deep earth and provide clues on the chemical composition and the physical condition of the deep mantle.

The Mode of Trench-Parallel Subduction of the Middle Ocean Ridge

Trench-parallel subduction of mid-ocean ridges occurs frequently in plate motion history, such as along the western boundary of the Pacific plate in the early Cenozoic and along the eastern boundary of the Pacific plate at present. Such subduction may strongly alter the surface topography, volcanic activity and slab morphology in the mantle, whereas few studies have been conducted to investigate its evolutionary process. Here, we construct a 2-D viscoelastoplastic numerical model to study the modes and key parameters controlling trench-parallel subduction of mid-ocean ridges. Our model results show that the subduction modes of mid-ocean ridges can be primarily categorized into three types: the fast spreading mode, the slow spreading mode, and the extinction mode. The key factor controlling these subduction modes is the relative motion between the foregoing and the following oceanic plates, which are separated by the mid-ocean ridge. Different subduction modes exert different surface geological expressions, which may explain specific evolutionary processes related to mid-ocean ridge subduction, such as topographic deformation and the eruption gap of volcanic rocks in East Asia within 55–45 Ma and in the western North American plate during the late Cenozoic.