Subslab mantle of African provenance infiltrating the Aegean mantle wedge

The presence of a tear in the subducting African slab has a strong effect on Neogene magmatism in western Turkey, but its influence on volcanism in the Quaternary Aegean arc to the west is unknown. In order to investigate to what extent arc volcanism can be affected by slab-parallel mantle flow from a slab window, we present new trace element and Nd-Pb isotope data for Nisyros and Santorini volcanoes. Trace element modeling allows quantification of the infiltration of trace element-enriched mantle of subslab provenance through the slab tear into the depleted Aegean mantle wedge. Primitive Nisyros magmas record melting of a mixed source that contains as much as 10% of the enriched, subslab mantle component, and a contribution of this component can be traced as far west as Santorini, ~250 km away from the slab tear. We conclude that trace element and Nd-Pb isotope variations between Nisyros and Santorini do not require along-arc variations in subducting sediment composition, but reflect the heterogeneous nature of the Aegean mantle wedge related to infiltration of subslab mantle through the slab tear. Our geochemical evidence is in excellent agreement with predictions made on the basis of mantle tomography and anisotropy that indicate toroidal mantle flow around the edge of the Aegean slab. This implies that suction related to slab rollback can lead to the infiltration of subslab mantle material and slab-parallel mantle flow, thus potentially strongly influencing arc volcanism, processes that perhaps need greater assessment in other arc systems

A mineral and cumulate perspective to magma differentiation at Nisyros volcano, Aegean arc

Lavas and pyroclastic products of Nisyros volcano (Aegean arc, Greece) host a wide variety of phenocryst and cumulate assemblages that offer a unique window into the earliest stages of magma differentiation. This study presents a detailed petrographic study of lavas, enclaves and cumulates spanning the entire volcanic history of Nisyros to elucidate at which levels in the crust magmas stall and differentiate. We present a new division for the volcanic products into two suites based on field occurrence and petrographic features: a low-porphyricity andesite and a high-porphyricity (rhyo)dacite (HPRD) suite. Cumulate fragments are exclusively found in the HPRD suite and are predominantly derived from upper crustal reservoirs where they crystallised under hydrous conditions from melts that underwent prior differentiation. Rarer cumulate fragments range from (amphibole-)wehrlites to plagioclase-hornblendites and these appear to be derived from the lower crust (0.5–0.8 GPa). The suppressed stability of plagioclase and early saturation of amphibole in these cumulates are indicative of high-pressure crystallisation from primitive hydrous melts (≥ 3 wt% H2O). Clinopyroxene in these cumulates has Al2O3 contents up to 9 wt% due to the absence of crystallising plagioclase, and is subsequently consumed in a peritectic reaction to form primitive, Al-rich amphibole (Mg# > 73, 12–15 wt% Al2O3). The composition of these peritectic amphiboles is distinct from trace element-enriched interstitial amphibole in shallower cumulates. Phenocryst compositions and assemblages in both suites differ markedly from the cumulates. Phenocrysts, therefore, reflect shallow crystallisation and do not record magma differentiation in the deep arc crust

Reliability of detrital marine sediments as proxy for continental crust composition: the effects of hydrodynamic sorting on Ti and Zr isotope systematics

The isotopic composition of the detrital sediment record harbours a valuable proxy for estimating the composition of the erodible upper crust since the Archaean. Refractory elements such as titanium (Ti) and zirconium (Zr) can display systematic variations in their isotopic composition as a result of magmatic differentiation. Hence, for such elements, the isotope composition of detrital sediments could potentially be used to infer the average composition (e.g., SiO₂ content) of their source region, even when elemental systematics are obfuscated by weathering and diagenetic processes. A key premise of this approach is that the isotopic composition of sediments remains unbiased relative to their protolith. To what extent isotopic fractionation can occur during sedimentary processes, notably the hydrodynamic sorting of heavy mineral assemblages with contrasting isotopic compositions, remains poorly understood. We investigate the effects of such processes on the Ti and Zr isotope composition of a suite of detrital sediments from the Eastern Mediterranean Sea (EMS). These sediments are binary mixtures of two main provenance components, Saharan dust and Nile sediment, with strongly contrasting mineralogical and geochemical signatures. The EMS sediments display clear evidence for hydrodynamic sorting of zircon, expressed as a large variation in Zr/Al₂O₃ and deviation of εHf relative to the terrestrial εNd-εHf array. Our new data, however, do not show pronounced Zr isotope variation resulting from either hydrodynamic sorting of zircon or sediment provenance. Although this agrees with theoretical models that predict negligible equilibrium zircon-melt Zr isotope fractionation, it contrasts with recent observations suggesting that kinetic Zr isotope fractionation might be a common feature in igneous rocks. For the EMS sediments, the negligible shift in Zr isotope composition through hydrodynamic sorting means that fine-grained samples accurately reflect the composition of their source. The nearly overlapping Zr isotope compositions of Sahara- and Nile-derived sediment, however, means that Zr isotopes, in this case, have insufficient resolution to be a useful provenance proxy. Titanium behaves differently. A small but resolvable, systematic difference in Ti isotope composition is observed between the Sahara and Nile provenance components. Samples with a strong Saharan dust signature show some Ti isotope evidence for hydrodynamic sorting of oxides in tandem with zircon, but a much stronger effect is inferred for Nile sediment. Regression of the EMS sediment samples shows that the Ti isotope composition of the Nile-derived component is strongly fractionated compared to its protolith, the Ethiopian flood basalts. Whereas Ti in Nile sediment is carried in essentially unmodified concentration, and by inference isotope composition, from its sources to the delta, large-scale hydrodynamic sorting of Fe-Ti oxides occurs in the delta. This process decreases TiO₂/Al₂O₃ of the residual fine-grained sediment fraction and shifts its Ti isotope composition to heavier compositions. The potential of such an “oxide effect” in detrital sediments has implications for crustal evolution models that use Ti isotopes as a proxy for the proportion of felsic crust and can account for the observed scatter in the shale record

Sr isotopes in arcs revisited: tracking slab dehydration using δ88/86Sr and 87Sr/86Sr systematics of arc lavas

Dehydration of the subducting slab is a crucial process in the generation of hydrous convergent margin magmas, yet the exact processes of how and where the slab dehydrates and how these fluids are transported to the mantle wedge remain obscure. Strontium is a “fluid-mobile” element and as such well suited to investigate the source of slab-derived fluids. We employ mass-dependent Sr isotope systematics (δ88/86Sr; the deviation in 88Sr/86Sr of a sample relative to NIST SRM 987) of primitive arc lavas, in tandem with conventional radiogenic 87Sr/86Sr measurements, as a novel tracer of slab dehydration. To characterise the δ88/86Sr composition of subduction zone inputs, we present new δ88/86Sr data for subducting sediments, altered oceanic crust and MORB. Calcareous sediments are isotopically lighter and carbonate-free sediments are isotopically heavier than mid-ocean ridge basalts (MORB). Samples of the altered oceanic crust display elevated 87Sr/86Sr but only the most intensely altered sample has significantly higher δ88/86Sr than pristine MORB. Mafic arc lavas from the Aegean and Mariana arc invariably have a mass-dependent Sr isotope composition that is indistinguishable from MORB and lower 87Sr/86Sr than upper altered oceanic crust. This δ88/86Sr-87Sr/86Sr signature of the arc lavas, in combination with their high but variable Sr/Nd, can only be explained if it is provided by a fluid that acquired its Sr isotope signature in the deeper, less altered part of the subducted oceanic crust. We propose a model where the breakdown of serpentinite in the slab mantle releases a pulse of fluid at sub-arc depths. These fluids travel through and equilibrate with the overlying oceanic crust and induce wet partial melting of the upper altered crust and sediments. This hydrous melt is then delivered to the mantle source of arc magmas as a single metasomatic component. From mass balance it follows that the slab-derived fluid contributes >70% of the Sr budget of both Mariana and Aegean arc lavas. Whereas this fluid-dominated character is unsurprising for the sediment-poor Mariana arc, the Aegean arc sees the subduction of 3–6 km of calcareous sediments that were found to exert very little control on the Sr budget of the arc magmas and are overwhelmed by the fluid contribution

Problems in obtaining precise and accurate Sr isotope analysis from geological materials using laser ablation MC-ICPMS

This paper reviews the problems encountered in eleven studies of Sr isotope analysis using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) in the period 1995–2006. This technique has been shown to have great potential, but the accuracy and precision are limited by: (1) large instrumental mass discrimination, (2) laser-induced isotopic and elemental fractionations and (3) molecular interferences. The most important isobaric interferences are Kr and Rb, whereas Ca dimer/argides and doubly charged rare earth elements (REE) are limited to sample materials which contain substantial amounts of these elements. With modern laser (193 nm) and MC-ICPMS equipment, minerals with >500 ppm Sr content can be analysed with a precision of better than 100 ppm and a spatial resolution (spot size) of approximately 100 μm. The LA MC-ICPMS analysis of 87Sr/86Sr of both carbonate material and plagioclase is successful in all reported studies, although the higher 84Sr/86Sr ratios do suggest in some cases an influence of Ca dimer and/or argides. High Rb/Sr (>0.01) materials have been successfully analysed by carefully measuring the 85Rb/87Rb in standard material and by applying the standard-sample bracketing method for accurate Rb corrections. However, published LA-MC-ICPMS data on clinopyroxene, apatite and sphene records differences when compared with 87Sr/86Sr measured by thermal ionisation mass spectrometry (TIMS) and solution MC-ICPMS. This suggests that further studies are required to ensure that the most optimal correction methods are applied for all isobaric interferences

Metabolic compartmentalization in the human cortex and hippocampus: evidence for a cell- and region-specific localization of lactate dehydrogenase 5 and pyruvate dehydrogenase

BACKGROUND: For a long time now, glucose has been thought to be the main, if not the sole substrate for brain energy metabolism. Recent data nevertheless suggest that other molecules, such as monocarboxylates (lactate and pyruvate mainly) could be suitable substrates. Although monocarboxylates poorly cross the blood brain barrier (BBB), such substrates could replace glucose if produced locally.The two key enzymatiques systems required for the production of these monocarboxylates are lactate dehydrogenase (LDH; EC1.1.1.27) that catalyses the interconversion of lactate and pyruvate and the pyruvate dehydrogenase complex that irreversibly funnels pyruvate towards the mitochondrial TCA and oxydative phosphorylation. RESULTS: In this article, we show, with monoclonal antibodies applied to post-mortem human brain tissues, that the typically glycolytic isoenzyme of lactate dehydrogenase (LDH-5; also called LDHA or LDHM) is selectively present in astrocytes, and not in neurons, whereas pyruvate dehydrogenase (PDH) is mainly detected in neurons and barely in astrocytes. At the regional level, the distribution of the LDH-5 immunoreactive astrocytes is laminar and corresponds to regions of maximal 2-deoxyglucose uptake in the occipital cortex and hippocampus. In hippocampus, we observed that the distribution of the oxidative enzyme PDH was enriched in the neurons of the stratum pyramidale and stratum granulosum of CA1 through CA4, whereas the glycolytic enzyme LDH-5 was enriched in astrocytes of the stratum moleculare, the alveus and the white matter, revealing not only cellular, but also regional, selective distributions. The fact that LDH-5 immunoreactivity was high in astrocytes and occurred in regions where the highest uptake of 2-deoxyglucose was observed suggests that glucose uptake followed by lactate production may principally occur in these regions. CONCLUSION: These observations reveal a metabolic segregation, not only at the cellular but also at the regional level, that support the notion of metabolic compartmentalization between astrocytes and neurons, whereby lactate produced by astrocytes could be oxidized by neurons

Subduction of continental material in the Banda Arc, Eastern Indonesia: Sr-Nd-Pb isotope and trace-element evidence from volcanics and sediments

This thesis presents the results of a geochemical study of the Banda Arc (East Indonesia) where magma genesis is influenced by subducted source components that are controlled by an active arc-continent collision. The main objective of this study is to investigate the role of subducted continental material on the magma genesis using isotopic and chemical compositions of samples from volcanoes and sediments distributed along the whole stretch of the arc. Sr-Nd-Pb isotope ratios and major and trace-element contents were determined for a total of 152 rocks from seven volcanic islands, 127 surface sediments from seven locations and five sediment samples from the DSDP Site 262 hole in the Timor Trough. The composition of these sediments is considered to represent that of a subducted component with continental affinity which is currently involved in magma genesis. This data set is used to (1) assess the importance of subducted continental material as opposed to material assimilated from the arc crust; (2) quantify the contribution of subducted continental material to magma sources; (3) discuss the mode of transfer from the slab to magma sources in the overlying mantle wedge, and (4) determine the role of sediment provenance in generating isotopic heterogeneities within the arc. Large variations in the isotopic composition were found between the volcanoes. These variations are matched by similar variations in the sediments along the arc. This parallelism is most evident for Pb isotopes and is considered as strong evidence for the involvement of subducted continental material in magma genesis. Conventional bulk mixing models indicate that the contribution of subducted continental material increases along the arc from < 1% in the NE to 5-10% in the SW. Important within-suite Sr-Nd isotopic ranges were also found for individual volcanoes. Assimilation is thought to be significant in one volcano and probably occurs to some extent on most islands. However, it cannot explain the conspicuous 'continental' signatures of the more mafic Banda Arc volcanics. Trace-element compositions of the volcanics show upper continental crust signatures. Many ratios of incompatible elements (e.g. LILE/HFSE and LILE/REE) and rare-earth patterns display an increasing correspondence with those of the sediments in the same direction. REE mixing-melting models were applied using a typical MORB source mantle and a representative sediment as end members. The results are consistent with the isotopic mixing models in terms of quantities and with the NE-SW along-arc changes. These models indicate that the bulk addition of subducted continental material to mantle sources is an important characteristic of the Banda Arc which can explain many of its trace-element signatures. Addition in the form of large-degree melts rather than mechanical mixing is considered to be the most plausible bulk-addition mechanism, particularly in the SW Banda Arc. Nevertheless, some trace-element ratios (e.g. BaJNb, Th/Nb, Th/Zr) cannot be expl~ned by bulk mixing and must be due to a process of selective mobilization, probably by fluids escaping from the slab. It is suggested that bulk transfer of subducted continental material dominates and is most conspicuous in the SW whereas fluid transfer is more evident in the NE. Despite the evidence for the involvement of subducted continental material, the Banda Arc volcanics are characterized by higher 208Pbp04Pb at a given 206Pbp04Pb compared to the sediments. This difference becomes more pronounced from NE to SW along the arc. High 208Pbp04Pb is a typical characteristic of the Indian ocean MORB and alB (so-called Dupal anomaly), and the high 208Pbp04Pb component in the volcanics could thus be derived from a mantle contribution. Because of the high Pb concentration in continental material compared to the Pb concentration in any solid mantle type, it is suggested that small-degree partial melts from a mantle source are involved. The terrigenous fraction in the sediments is thought to be responsible for the overall 'continental' character of the arc. From the combined isotopic and trace-element ratios in this fraction two provenance areas can be distinguished: in the NE part of the arc the sediments originate from Phanerozoic New Guinea and in the SW they originate mainly from Proterozoic Australia. A further subdivision can be made on the basis of Th/Sc, 147Sm/l44Nd, Pb and Nd isotopes into: North New Guinea+Seram, South New Guinea, Timor and Australia. In trace-element ratios and REE patterns the sediments are generally similar to published estimates for average Upper Continental Crust and Post Archean Australian Shale. No systematic difference was found between the compositions of sediments from shelf, wedge and back arc. The results for the Banda Arc highlight the importance of the subducted component in controlling inter- and intra-arc variations in chemical and isotopic signatures. Therefore, data from local sediments are indispensable for models of arc magma genesis in individUal cases. The potential variability of subducted sediments also has important implications for the origin and scale of mantle heterogeneities

Detecting Emerging Challenges in Social Sidewalk Navigation

When mobile urban robots will share the sidewalk with people, the resulting interactions can cause unexpected undesirable outcomes to emerge – from people running away scared to people deliberately teasing and harassing such systems. How can we design such AI systems to aptly handle the unexpected? Directly anticipating and/or detecting these kinds of situations will inherently be unreliable; they are unexpected, after all. And yet, there exists a very clear signal for social slip-ups: the emotional response of people. We thus argue that such systems need to be imbued with a capacity to interpret the socio-emotional reactions to their own behavior

Isotopic evolution of the Tonga arc during Lau Basin rifting; evidence from the volcaniclastic record

New Pb, Sr and Nd isotope data from volcaniclastic sediments recovered from the Tonga forearc and Lau Basin permit the isotopic evolution of a section of this arc system, next to the modern island of Ata, to be traced through the backarc basin rifting process from 7·0 Ma to the present. The new data suggest that the isotopic character of the mantle wedge remains constant, and of Pacific mid-ocean ridge basalt (MORB) character, during the early rifting phase. The isotopic evidence supports trace element data in showing an increase in the sediment contribution to arc petrogenesis about 2-3 m.y. after the start of Lau Basin rifting. Since 0·45 Ma the sediment contribution decreased to pre-rift values with the initiation of spreading in the adjacent backarc basin, where the high sediment influence is not seen in the isotopes. The isotopes show a relative increase in the volcaniclastic compared with pelagic sediment involvement during rifting. The inferred peak in sediment subduction is probably the result of a decoupling of the two plates owing to roll-back of the Pacific lithosphere at the time of arc rifting

Trace element concentrations and Li isotope composition in DSDP and ODP sediments from island arcs

We have studied the sedimentary and basaltic inputs of lithium to subduction zones. Various sediments from DSDP and ODP drill cores in front of the Mariana, South Sandwich, Banda, East Sunda and Lesser Antilles island arcs have been analysed and show highly variable Li contents and d7Li values. The sediment piles in front of the Mariana and South Sandwich arcs largely consist of pelagic sediments (clays and oozes). The pelagic clays have high Li contents (up to 57.3 ppm) and Li isotope compositions ranging from +1.3‰ to +4.1‰. The oozes have lower Li contents (7.3-16 ppm) with d7Li values of the diatom oozes from the South Sandwich lower (+2.8‰ to +3.2‰) than those of the radiolarian oozes from the Mariana arc (+8.1‰ to +14.5‰). Mariana sediment also contains a significant portion of volcanogenic material, which is characterised by a moderate Li content (14 ppm) and a relatively heavy isotope composition (+6.4‰). Sediments from the Banda and Lesser Antilles contain considerable amounts of continental detritus, and have high Li contents (up to 74.3 ppm) and low d7Li values (around 0‰), caused by weathering of continental bedrock. East Sunda sediments largely consist of calcareous oozes. These carbonate sediments display intermediate to high Li contents (2.4-41.9 ppm) and highly variable d7Li values (-1.6‰ to +12.8‰). Basaltic oceanic crust samples from worldwide DSDP and ODP drill cores are characterised by enrichment of Li compared to fresh MORB (6.6-33.1 vs. 3.6-7.5 ppm, respectively), and show a large range in Li isotope compositions (+1.7‰ to +11.8‰). The elemental and isotopic enrichment of Li in altered basalts is due to the uptake of isotopically heavy seawater Li during weathering. However, old oceanic crust samples from Sites 417/418 exhibit lighter Li isotope compositions compared to young basaltic crust samples from Sites 332B and 504B. This lighter Li isotope signature in old crust is unexpected and further research is needed to explore this issue