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1. Geochemical Variability Along the Northern East Pacific Rise: Coincident Source Composition and Ridge Segmentation

   https://doi.org/10.1029/2019GC008287  

   Mallick, Soumen; Salters, Vincent J. M.; Langmuir, Charles H. (April 2019, Geochemistry, Geophysics, Geosystems)

   Abstract New trace element abundances and isotope compositions for more than 100 mid‐ocean ridge basalts from 5.5°N to 19°N on the East Pacific Rise show step function variations in isotopic composition along the ridge axis that coincide with ridge discontinuities. Transform faults, overlapping spreading centers, and devals (deviation from axial linearity) mark the separation of individual clusters of distinct isotopic composition and trace element ratios that indicate source variations. This correlated chemical clustering and morphological segmentation indicates that source composition and segmentation can be closely related even on a fine scale. Substantial chemical variations within a segment are related to source composition. This suggests that even within segments the magma transport is mainly vertical, and there is limited along‐ridge transport, and there is little evidence for magma chambers that are well mixed along strike. Trace element concentrations show good correlations with isotopic compositions on a segment scale but less so on a regional scale. The trace element and isotopic variability along the northern East Pacific Rise can be explained by three mantle components: a depleted peridotite endmember, an enriched peridotite endmember, and a recycled gabbro‐like component. The gabbroic component has an isotopic signature indicating an ancient origin. The high‐resolution sampling indicates that within a segment the chemical variability is largely binary but that the endmembers of the binary mixing change from segment to segment. The endmembers of the binary variation within a segment are a combination of three of the endmembers. 

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2. The Standardized Spectroscopic Mixture Model

   https://doi.org/10.3390/rs16203768  

   Small, Christopher; Sousa, Daniel (October 2024, Remote Sensing)

   The standardized spectral mixture model combines the specificity of a physically based representation of a spectrally mixed pixel with the generality and portability of a spectral index. Earlier studies have used spectrally and geographically diverse collections of broadband and spectroscopic imagery to show that the reflectance of the majority of ice-free landscapes on Earth can be represented as linear mixtures of rock and soil substrates (S), photosynthetic vegetation (V) and dark targets (D) composed of shadow and spectrally absorptive/transmissive materials. However, both broadband and spectroscopic studies of the topology of spectral mixing spaces raise questions about the completeness and generality of the Substrate, Vegetation, Dark (SVD) model for imaging spectrometer data. This study uses a spectrally diverse collection of 40 granules from the EMIT imaging spectrometer to verify the generality and stability of the spectroscopic SVD model and characterize the SVD topology and plane of substrates to assess linearity of spectral mixing. New endmembers for soil and non-photosynthetic vegetation (NPV; N) allow the planar SVD model to be extended to a tetrahedral SVDN model to better accommodate the 3D topology of the mixing space. The SVDN model achieves smaller misfit than the SVD, but does so at the expense of implausible fractions beyond [0, 1]. However, a refined spectroscopic SVD model still achieves small (<0.03) RMS misfit, negligible sensitivity to endmember variability and strongly linear scaling over more than an order of magnitude range of spatial resolution. 

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3. Mixing Loops, Mixing Envelopes, and Scattered Correlations among Trace Elements and Isotope Ratios Produced by Mixing of Melts Derived from a Spatially and Lithologically Heterogenous Mantle

   https://doi.org/10.1093/petrology/egac092  

   Liang, Yan (September 2022, Journal of Petrology)

   Abstract Mixing has been widely used in the interpretation of radiogenic isotope ratios and highly incompatible trace element variations in basalts produced by melting of a heterogeneous mantle. The binary mixing model is constructed by considering mass balance of endmember components, which is independent of physical state and spatial distribution of the endmembers in the mantle source. Variations of radiogenic isotope ratios and highly incompatible trace elements in basalts also depend on the size and spatial distribution of chemical and lithological heterogeneities in the mantle source. Here we present a new mixing model and a mixing scheme that take into account of the size, spatial location, and melting history of enriched mantle (EM) and depleted mantle (DM) parcels in the melting column. We show how Sr, Nd, and Hf concentrations and isotope ratios in the aggregated or pooled melt collected at the top of the melting column vary as a function of location of the EM parcel in the melting column. With changing location of the EM parcel in the upwelling melting column, compositions of the pooled melt do not follow a single mixing curve expected by the binary mixing model. Instead, they define a mixing loop that has an enriched branch and a depleted branch joined by two extreme points in composition space. The origin of the mixing loop can be traced back to four types of EM distribution or configuration in the melting column. The shape of the mixing loop depends on the relative melting rate of the EM to that of the DM and the number and spacing of EM parcels in the melting column. Probabilities of sampling the enriched and depleted branches in the pooled melt are proportional to volume fractions of the enriched and depleted materials in the mantle source. Mixing of pooled melts from a bundle of melting columns results in mixing envelopes in the isotope ratio correlation diagrams. The mixing envelope is a useful tool for studying chemical variations in mantle-derived melts. As an application, we consider scattered correlations in 87Sr/86Sr vs. 143Nd/144Nd and 143Nd/144Nd vs. 176Hf/177Hf in mid-ocean ridge basalts. We show that such correlations arise naturally from melting of a spatially heterogeneous mantle. 

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4. Changing shape of mantle heterogeneity by melt migration beneath mid-ocean ridges

   https://doi.org/10.1016/j.epsl.2024.118925  

   Liu, Boda; Liang, Yan; Liu, Chuan-Zhou (October 2024, Earth and Planetary Science Letters)

   None (Ed.)

   The common assumption that residual peridotites retain the Nd-Hf isotope ratios in the mantle source is debated because melt and solid of different isotopic compositions could undergo chemical exchange during melt migration, altering the isotopic signature of the source. By modeling the transport of chemical heterogeneities in the melting region beneath a mid-ocean ridge, we show that the shape of a chemical heterogeneity marked by Nd or Hf isotope ratio changes systematically through subvertical dispersion, stretching, compression, and shearing. The isotope ratios inside the chemical heterogeneity decay toward the values of background mantle. The amount of decay depends on the strength of dispersion, which itself is strongly dependent on the melt fraction in the melting region. When the maximum melt fraction is greater than 1%, buoyancy-driven melt flow relative to the solid causes subvertical dispersion of isotopic signals in the solid. Differential flows of the melt and solid also produce chromatography fractionation of Nd with respect to Hf, causing their isotope ratios to decouple. Compositions of the residue in Nd-Hf isotope ratio diagram do not record the endmembers in the source, instead they represent an area that covers part of the binary mixing line between the background mantle and the original heterogeneity. In the case of small melt fraction (<0.2%), the low permeability results in sluggish melt flow, weak dispersion, and negligible chromatography fractionation. Consequently, Nd and Hf isotope ratios in the residue remain coupled, representing the endmember isotope ratios in the source. The ridge model with larger melt fraction may correspond to the fast-spreading ridge, while the model with smaller melt fraction may correspond to the ultraslow-spreading ridge. The present study underscores the importance of melt migration processes beneath mid-ocean ridges on the deformation, mixing and decoupling of Nd-Hf isotope ratios in residual peridotites. 

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5. Isotopic fractionation accompanying CO2 hydroxylation and carbonate precipitation from high pH waters at The Cedars, California, USA

   https://doi.org/doi.org/10.1016/j.gca.2021.01.003  

   Christensen, J. (January 2021, Geochimica)

   Teagle, Damon A (Ed.)

   The Cedars ultramafic block hosts alkaline springs (pH > 11) in which calcium carbonate forms upon uptake of atmospheric CO2 and at times via mixing with surface water. These processes lead to distinct carbonate morphologies with ‘‘floes” forming at the atmosphere-water interface, ‘‘snow” of fine particles accumulating at the bottom of pools and terraced constructions of travertine. Floe material is mainly composed of aragonite needles despite CaCO3 precipitation occurring in waters with low Mg/Ca (<0.01). Precipitation of aragonite is likely promoted by the high pH (11.5–12.0) of pool waters, in agreement with published experiments illustrating the effect of pH on calcium carbonate polymorph selection. The calcium carbonates exhibit an extreme range and approximately 1:1 covariation in d13C (
   9 to
   28‰ VPDB) and d18O (0 to
   20‰ VPDB) that is characteristic of travertine formed in high pH waters. The large isotopic fractionations have previously been attributed to kinetic isotope effects accompanying CO2 hydroxylation but the controls on the d13C-d18O endmembers and slope have not been fully resolved, limiting the use of travertine as a paleoenvironmental archive. The limited areal extent of the springs (0.5 km2) and the limited range of water sources and temperatures, combined with our sampling strategy, allow us to place tight constraints on the processes involved in generating the systematic C and O isotope variations. We develop an isotopic reaction–diffusion model and an isotopic box model for a CO2-fed solution that tracks the isotopic composition of each dissolved inorganic carbon (DIC) species and CaCO3. The box model includes four sources or sinks of DIC (atmospheric CO2, high pH spring water, fresh creek water, and CaCO3 precipitation). Model parameters are informed by new floe D44Ca data (
   0.75 ± 0.07‰), direct mineral growth rate measurements (4.8 to 8  10
   7 mol/m2/s) and by previously published elemental and isotopic data of local water and DIC sources. Model results suggest two processes control the extremes of the array: (1) the isotopically light end member is controlled by the isotopic composition of atmospheric CO2 and the kinetic isotope fractionation factor (KFF (‰) = (a
   1)  1000) accompanying CO2 hydroxylation, estimated here to be
   17.1 ± 0.8‰ (vs. CO2(aq)) for carbon and
   7.1 ± 1.1‰ (vs. ‘CO2(aq)+H2O’) for oxygen at 17.4 ± 1.0
   C. Combining our results with revised CO2 hydroxylation KFF values based on previous work suggests consistent KFF values of
   17.0 ± 0.3‰ (vs. CO2(aq)) for carbon and
   6.8 ± 0.8‰ for oxygen (vs. ‘CO2(aq)+H2O’) over the 17–28
   C temperature range. (2) The isotopically heavy endmember of calcium carbonates at The Cedars reflects the composition of isotopically equilibrated DIC from creek or surface water (mostly HCO- 3, pH = 7.8–8.7) that occasionally mixes with the high-pH spring water. The bulk carbonate d13C and d18O values of modern and ancient travertines therefore reflect the proportion of calcium carbonate formed by processes (1) and (2), with process (2) dominating the carbonate precipitation budget at The Cedars. These results show that recent advances in understanding kinetic isotope effects allow us to model complicated but common natural processes, and suggest ancient travertine may be used to retrieve past meteoric water d18O and atmospheric d13C values. There is evidence that older travertine at The Cedars recorded atmospheric d13C that predates large-scale combustion of fossil fuels. 

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