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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.

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.

Halogens are important tracers of various planetary formation and evolution processes, and an accurate understanding of their abundances in the Earth’s silicate reservoirs can help us reconstruct the history of interactions among mantle, atmosphere, and oceans. The previous studies of halogen abundances in the bulk silicate Earth (BSE) are based on the assumption of constant ratios of element abundances, which is shown to result in a gross underestimation of the BSE halogen budget. Here we present a more robust approach using a log-log linear model. Using this method, we provide an internally consistent estimate of halogen abundances in the depleted mid-ocean ridge basalts (MORB)-source mantle, the enriched ocean island basalts (OIB)-source mantle, the depleted mantle, and BSE. Unlike previous studies, our results suggest that halogens in BSE are not more depleted compared to elements with similar volatility, thereby indicating sufficient halogen retention during planetary accretion. According to halogen abundances in the depleted mantle and BSE, we estimate that ∼87% of all stable halogens reside in the present-day mantle. Given our understanding of the history of mantle degassing and the evolution of crustal recycling, the revised halogen budget suggests that deep halogen cycle is characterized by efficient degassing in the early Earth and subsequent net regassing in the rest of Earth history. Such an evolution of deep halogen cycle presents a major step toward a more comprehensive understanding of ancient ocean alkalinity, which affects carbon partitioning within the hydrosphere, the stability of crustal and authigenic minerals, and the development of early life.

AbstractThe 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.

The crustal evolution of Venus appears to be principally driven by intraplate processes that may be related to mantle upwelling as there is no physiographic (i.e. mid-ocean ridge, volcanic arc) evidence of Earth-like plate tectonics. Rocks with basaltic composition were identified at the Venera 9, 10, 13, and 14, and Vega 1 and 2 landing sites whereas the rock encountered at the Venera 8 landing site may be silicic. The Venera 14 rock is chemically indistinguishable from terrestrial olivine tholeiite but bears a strong resemblance to basalt from terrestrial Archean greenstone belts. Forward petrological modeling (i.e. fractional crystallization and partial melting) and primary melt composition calculations using the rock compositions of Venus can yield results indistinguishable from many volcanic (ultramafic, intermediate, silicic) and plutonic (tonalite, trondhjemite, granodiorite, anorthosite) rocks that typify Archean greenstone belts. Evidence of chemically precipitated (carbonate, evaporite, chert, banded-iron formation) and clastic (sandstone, shale) sedimentary rocks is scarce to absent, but their existence is dependent upon an ancient Venusian hydrosphere. Nevertheless, it appears that the volcanic–volcaniclastic–plutonic portion of terrestrial greenstone belts can be constructed from the known surface compositions of Venusian rocks and suggests that it is possible that Venus and Early Earth had parallel evolutionary tracks in the growth of proto-continental crust.

Abstract Successive flip-flop detachment faults in a nearly-amagmatic region of the ultraslow-spreading Southwest Indian Ridge (SWIR) at 64°30'E accommodate ~100% of plate divergence, with mostly ultramafic seafloor. As magma is the main heat carrier to the oceanic lithosphere, the nearly-amagmatic SWIR 64°30'E is expected to have a very thick lithosphere. Here, our microseismicity data shows a 15-km thick seismogenic lithosphere, actually thinner than the more magmatic SWIR Dragon Flag detachment with the same spreading rate. This challenges current models of how spreading rate and melt supply control the thermal regime of mid-ocean ridges. Microearthquakes with normal focal mechanisms are colocated with seismically imaged damage zones of the detachment and reveal hanging-wall normal faulting, which is not seen at more magmatic detachments at the SWIR or the Mid-Atlantic Ridge. We also document a two-day seismic swarm, interpret as caused by an upward-migrating melt intrusion in the detachment footwall (6-11 km), triggering a sequence of shallower (~1.5 km) tectonic earthquakes in the detachment fault plane. This points to a possible link between sparse magmatism and tectonic failure at melt-poor ultraslow ridges.

AbstractDespite progress in understanding seafloor accretion at ultraslow spreading ridges, the ultimate driving force is still unknown. Here we use 40Ar/39Ar isotopic dating of mid-ocean ridge basalts recovered at variable distances from the axis of the Gakkel Ridge to provide new constraints on the spatial and temporal distribution of volcanic eruptions at various sections of an ultraslow spreading ridge. Our age data show that magmatic-dominated sections of the Gakkel Ridge spread at a steady rate of ~11.1 ± 0.9 mm/yr whereas amagmatic sections have a more widely distributed melt supply yielding ambiguous spreading rate information. These variations in spreading rate and crustal accretion correlate with locations of hotter thermochemical anomalies in the asthenosphere beneath the ridge. We conclude therefore that seafloor generation in ultra-slow spreading centres broadly reflects the distribution of thermochemical anomalies in the upper mantle.