Enriched Basalts at Segment Centers: The Lucky Strike (37°17′N) and Menez Gwen (37°50′N) Segments of the Mid‐Atlantic Ridge

Basalts from the Mid-Atlantic Ridge change progressively in composition with increasing distance from the Azores platform. Study of the Lucky Strike and Menez Gwen segments reveals much complexity in the gradient. Both segments contain only basalts enriched relative to normal mid-oceanic ridge basalt, but in two distinct groups. Moderately enriched basalts occur throughout the segments, with proximal Menez Gwen enriched relative to Lucky Strike. Highly enriched basalts occur at segment centers. Incompatible element ratios of the highly enriched basalts exceed those of the Azores platform, while isotopic compositions are less enriched. These observations can be explained by a low-degree melt of garnet-bearing Azores mantle added to mantle depleted by previous melt extraction. Melting this “metasomatized” mantle produces lavas that match the enriched samples. The Azores gradient cannot be explained by simple two-component mixing; rather, it reflects recent melt extraction and addition processes related to southward flow of the Azores plume. The Azores gradient also permits tests of segmentation models. Central supply models predict step functions in chemical compositions between segments. Within-segment gradients require vertical supply. Central supply is supported by robust central volcanoes, thicker crust at segment centers, and a step function in isotopes between the segments. The lava diversity at segment centers, however, requires batches of distinct magma that are preserved through melting and melt delivery. Within-segment gradients in moderately incompatible element ratios support a component of multiple supply. The data suggest partial homogenization of magma within a segment and preferential melt focusing to segment centers with some vertical transport.Earth and Planetary Science

Correlation of fluorescence microscopy, electron microscopy, and NanoSIMS stable isotope imaging on a single tissue section.

Correlative light and electron microscopy allows localization of specific molecules at the ultrastructural level in biological tissue but does not provide information about metabolic turnover or the distribution of labile molecules, such as micronutrients. We present a method to directly correlate (immuno)fluorescent microscopy, (immuno)TEM imaging and NanoSIMS isotopic mapping of the same tissue section, with nanometer-scale spatial precision. The process involves chemical fixation of the tissue, cryo sectioning, thawing, and air-drying under a thin film of polyvinyl alcohol. It permits to effectively retain labile compounds and strongly increases NanoSIMS sensitivity for 13C-enrichment. The method is illustrated here with correlated distribution maps of a carbonic anhydrase enzyme isotype, β-tubulin proteins, and 13C- and 15N-labeled labile micronutrients (and their anabolic derivates) within the tissue of a reef-building symbiotic coral. This broadly applicable workflow expands the wealth of information that can be obtained from multi-modal, sub-cellular observation of biological tissue

Surface Analysis by Secondary Ion Mass Spectrometry (SIMS): Principles and Applications from Swiss laboratories

Secondary Ion Mass Spectrometry (SIMS) extracts chemical, elemental, or isotopic information about a localized area of a solid target by performing mass spectrometry on secondary ions sputtered from its surface by the impact of a beam of charged particles. This primary beam sputters ionized atoms and small molecules (as well as many neutral particles) from the upper few nanometers of the sample surface. The physical basis of SIMS has been applied to a large range of applications utilizing instruments optimized with different types of mass analyzer, either dynamic SIMS with a double focusing mass spectrometer or static SIMS with a Time of Flight (TOF) analyzer. Here, we present a short review of the principles and major applications of three different SIMS instruments located in Switzerland

The importance of mantle wedge heterogeneity to subduction zone magmatism and the origin of EM1

The composition of the convecting asthenospheric mantle that feeds the mantle wedge can be investigated via rear-arc lavas that have minimal slab influence. This "ambient mantle wedge" composition (the composition of the wedge prior to the addition of a slab component) varies substantially both worldwide and within individual arcs. Nd-143/Nd-144 measurements of rear-arc samples that have minimal slab influence are similar to Nd-143/Nd-144 in the stratovolcanoes of the adjacent volcanic fronts, suggesting that Nd-143/Nd-144 of arc-front volcanics are largely inherited from the ambient mantle composition. Nd-143/Nd-144 correlates with ratios such as Th/U, Zr/Nb, and La/Sm, indicating that these ratios also are strongly influenced by ambient wedge heterogeneity. The same phenomenon is observed among individual volcanoes from the Chilean Southern Volcanic Zone (SVZ), where along-strike variability of the volcanic front tracks that of rear-arc monogenetic volcanics. Depleted mantle wedges are more strongly influenced by slab-derived components than are enriched wedges. This leads to surprising trace element correlations in the global dataset, such as between Pb/Nb and Zr/Nb, which are not explicable by variable compositions or fluxes of slab components. Depleted ambient mantle is present beneath arcs with back-arc spreading; relatively enriched mantle is present adjacent to continents. Ambient mantle wedge heterogeneity both globally and regionally forms isotope mixing trajectories for Sr, Nd and Hf between depleted mantle and EM1-type enriched compositions as represented by Gough Island basalts. Making use of this relationship permits a quantitative match with the SVZ data. It has been suggested that EM1-type mantle reservoirs are the result of recycled lower continental crust, though such models do not account for certain trace element ratios such as Ce/Pb and Nb/U or the surprisingly homogeneous trace element compositions of EM1 volcanics. A model in which the EM1 end-member found in continental arcs is produced by low-degree melt-metasomatism of the sub-continental lithospheric mantle may be more plausible. The Nd-143/Nd-144 maximum along the SVZ may be a consequence of either rifting and collision of two ancient lithospheric domains or a slab tear. The correspondence of mantle wedge variations with EM1 suggests a potential role for metasomatized sub-continental lithosphere in creating EM1 sources globally. (C) 2017 Elsevier B.V. All rights reserved

Short magmatic residence times of quartz phenocrysts in Patagonian rhyolites associated with Gondwana breakup

A key parameter in the study of magma evolution is the time scale on which magmatic processes occur. Using nanoscale secondary ion mass spectrometry (NanoSIMS), SIMS, and cathodoluminescence (CL) analyses, we have measured titanium (Ti) diffusion profiles in quartz phenocrysts from a Jurassic rhyolite of the El Quemado Complex (Patagonia, Argentina), providing new insights into the time scales of the associated volcanic processes. CL imaging of quartz phenocrysts reveals oscillatory magmatic zoning. We determined Ti concentrations with SIMS and acquired multiple NanoSIMS profiles across growth zones from core to rim. All transects show sharp changes in the Ti-48/Si-29 ratio, which correlate reasonably well with changes in CL intensity. Diffusion modeling of Ti in quartz yields a surprisingly short time scale for quartz crystallization of 5.6 +/- 2.2 yr and a rapid crystal growth rate of 2.3 x 10(-12) m/s. Based on the observed quartz textures, we suggest that the rhyolite erupted shortly after initial onset of crystallization, followed by decompression-driven quartz dissolution during fast magma ascent. We further argue that the observed oscillatory zoning and the variation of the Ti concentration of the quartz phenocryst does not reflect temperature, pressure, or titanium activity (a(Ti)) changes of the magmatic system, but rather is the result of growth kinetics, which has important implications for the Ti-in-quartz thermometry

Parental arc magma compositions dominantly controlled by mantle-wedge thermal structure

The processes that lead to the fourfold variation in arc-averaged compositions of mafic arc iavas remain controversial. Control by the mantle-wedge thermal structure is supported by chemical correlations with the thickness of the underlying arc crust(1-3), which affects the thermal state of the wedge. Control by down-going slab temperature is supported by correlations with the slab thermal parameter(3-7). The Chilean Southern Volcanic Zone provides a test of these hypotheses. Here we use chemical data to demonstrate that the Southern Volcanic Zone and global arc averages define the same chemical trends, both among elements and between elements and crustal thickness. But in contrast to the global arc system, the Southern Volcanic Zone is built on crust of variable thickness with a constant slab thermal parameter. This natural experiment, along with a set of numerical simulations, shows that global arc compositional variability is dominated by different extents of melting that are controlled by the thermal structure of the mantle wedge. Slab temperatures play a subordinate role. Variations in the subducting slab's fluid flux and sediment compositions, as well as mantle-wedge heterogeneities, produce second-order effects that are manifested as distinctive trace element and isotopic signatures; these can be more clearly elucidated once the importance of wedge thermal structure is recognized

Intracellular competition for nitrogen controls dinoflagellate population density in corals

The density of dinoflagellate microalgae in the tissue of symbiotic corals is an important determinant for health and productivity of the coral animal. Yet, the specific mechanism for their regulation and the consequence for coral nutrition are insufficiently understood due to past methodological limitations to resolve the fine-scale metabolic consequences of fluctuating densities. Here, we characterized the physiological and nutritional consequences of symbiont density variations on the colony and tissue level in Stylophora pistillata from the Red Sea. Alterations in symbiont photophysiology maintained coral productivity and host nutrition across a broad range of symbiont densities. However, we demonstrate that density-dependent nutrient competition between individual symbiont cells, manifested as reduced nitrogen assimilation and cell biomass, probably creates the negative feedback mechanism for symbiont population growth that ultimately defines the steady-state density. Despite fundamental changes in symbiont nitrogen assimilation, we found no density-related metabolic optimum beyond which host nutrient assimilation or tissue biomass declined, indicating that host nutrient demand is sufficiently met across the typically observed range of symbiont densities under ambient conditions

Correlation of fluorescence microscopy, electron microscopy, and NanoSIMS stable isotope imaging on a single tissue section

Correlative light and electron microscopy allows localization of specific molecules at the ultrastructural level in biological tissue but does not provide information about metabolic turnover or the distribution of labile molecules, such as micronutrients. We present a method to directly correlate (immuno)fluorescent microscopy, (immuno)TEM imaging and NanoSIMS isotopic mapping of the same tissue section, with nanometer-scale spatial precision. The process involves chemical fixation of the tissue, cryo sectioning, thawing, and air-drying under a thin film of polyvinyl alcohol. It permits to effectively retain labile compounds and strongly increases NanoSIMS sensitivity for C-13-enrichment. The method is illustrated here with correlated distribution maps of a carbonic anhydrase enzyme isotype, beta -tubulin proteins, and C-13- and N-15-labeled labile micronutrients (and their anabolic derivates) within the tissue of a reef-building symbiotic coral. This broadly applicable workflow expands the wealth of information that can be obtained from multi-modal, sub-cellular observation of biological tissue. Loussert-Fonta et al. have developed a method to directly correlate fluorescent microscopy, (immuno)TEM imaging and NanoSIMS isotopic mapping of the same tissue section, with nanometer-scale spatial precision. They illustrate the technique by imaging tissue from a symbiotic coral, Stylophora pistillata

Phenotypic heterogeneity driven by nutrient limitation promotes growth in fluctuating environments

Most microorganisms live in environments where nutrients are limited and fluctuate over time. Cells respond to nutrient fluctuations by sensing and adapting their physiological state. Recent studies suggest phenotypic heterogeneity(1) in isogenic populations as an alternative strategy in fluctuating environments, where a subpopulation of cells express a function that allows growth under conditions that might arise in the future(2-9). It is unknown how environmental factors such as nutrient limitation shape phenotypic heterogeneity in metabolism and whether this allows cells to respond to nutrient fluctuations. Here, we show that substrate limitation increases phenotypic heterogeneity in metabolism, and this heterogeneity allows cells to cope with substrate fluctuations. We subjected the N-2-fixing bacterium Klebsiella oxytoca to different levels of substrate limitation and substrate shifts, and obtained time-resolved single-cell measurements of metabolic activities using nanometre-scale secondary ion mass spectrometry (NanoSIMS). We found that the level of NH4+ limitation shapes phenotypic heterogeneity in N-2 fixation. In turn, the N-2 fixation rate of single cells during NH4+ limitation correlates positively with their growth rate after a shift to NH4+ depletion, experimentally demonstrating the benefit of heterogeneity. The results indicate that phenotypic heterogeneity is a general solution to two important ecological challenges-nutrient limitation and fluctuations-that many microorganisms face