Search for: All records

Abstract A major challenge in mantle geochemistry is determining the source composition and melt fraction involved in melting. We provide a new Rare‐Earth Element (REE) inverse model that provides source concentration, source and melt mineral modes, and melt fraction based on the difference between separate determinations of bulk distribution coefficients and constrained by boundary conditions. An analytical inverse of the batch melting equation provides expressions for source, , and bulk distribution coefficient of the mantle, , with two unknowns, the initial concentration of La in the mantle, , andPⁱ, the bulk distribution coefficient of the melt. We traverse through a range of steps and examine thousands of melt modes,Pⁱ, at each step. Thousands of trial melt modes fail by generating that are inconsistent with partition coefficients. Many surviving trials cannot be inverted to estimate a mantle mode. Other boundary conditions eliminate even more trials. Surviving trials are ordered by the difference between calculated from the REE data of a lava suite and calculated from partition coefficients and mantle mode. We select the solution with the closest fit that passes all the boundary conditions. We tested our new model with lava suites from Hawaii where different lines of evidence suggest that they melted from different mantle sources, Mauna Kea representing shield‐stage lava and submarine Kiekie representing rejuvenated stage lava. Our inverse determination of mantle composition and melting parameters was consistent with earlier models based on assumptions of HREE composition. 

Raman spectroscopy has become the tool of choice for analyzing fluid inclusions and melt inclusion (MI) vapor bubbles as it allows the density of CO2-rich fluids to be quantified. Measurements are often made at ambient temperature (Tamb ~18-25 °C), resulting in reported bulk densities between 0.2 and 0.7 g/mL despite that single-phase CO2 under these conditions is thermodynamically unstable and instead consists of a liquid (~0.7 g/mL), and a vapor phase (~0.2 g/mL). Here, we present results from experiments conducted at Tamb and 37 °C (above the CO2 critical temperature) on 14 natural CO2-rich MI bubbles from Mount Morning, Antarctica. Here, we show that at Tamb, laser power strongly affects the CO2 Raman spectrum of MI bubbles with bulk densities within the miscibility gap. High-power laser heating and low spectral resolution explain why published measurements have reported such bulk densities at Tamb even when using an instrument-specific calibration. 

Recent geochemical evidence confirms the oxidized nature of arc magmas, but the underlying processes that regulate the redox state of the subarc mantle remain yet to be determined. We established a link between deep subduction-related fluids derived from dehydration of serpentinite ± altered oceanic crust (AOC) using B isotopes and B/Nb as fluid proxies, and the oxidized nature of arc magmas as indicated by Cu enrichment during magma evolution and V/Yb. Our results suggest that arc magmas derived from source regions influenced by a greater serpentinite (±AOC) fluid component record higher oxygen fugacity. The incorporation of this component into the subarc mantle is controlled by the subduction system’s thermodynamic conditions and geometry. Our results suggest that the redox state of the subarc mantle is not homogeneous globally: Primitive arc magmas associated with flat, warm subduction are less oxidized overall than those generated in steep, cold subduction zones. 

It is well established that mantle plumes are the main conduits for upwelling geochemically enriched material from Earth's deep interior. The fashion and extent to which lateral flow processes at shallow depths may disperse enriched mantle material far (>1,000 km) from vertical plume conduits, however, remain poorly constrained. Here, we report He and C isotope data from 65 hydrothermal fluids from the southern Central America Margin (CAM) which reveal strikingly high 3 He/ 4 He (up to 8.9R A ) in low-temperature (≤50 °C) geothermal springs of central Panama that are not associated with active volcanism. Following radiogenic correction, these data imply a mantle source 3 He/ 4 He >10.3R A (and potentially up to 26R A , similar to Galápagos hotspot lavas) markedly greater than the upper mantle range (8 ± 1R A ). Lava geochemistry (Pb isotopes, Nb/U, and Ce/Pb) and geophysical constraints show that high 3 He/ 4 He values in central Panama are likely derived from the infiltration of a Galápagos plume–like mantle through a slab window that opened ∼8 Mya. Two potential transport mechanisms can explain the connection between the Galápagos plume and the slab window: 1) sublithospheric transport of Galápagos plume material channeled by lithosphere thinning along the Panama Fracture Zone or 2) active upwelling of Galápagos plume material blown by a “mantle wind” toward the CAM. We present a model of global mantle flow that supports the second mechanism, whereby most of the eastward transport of Galápagos plume material occurs in the shallow asthenosphere. These findings underscore the potential for lateral mantle flow to transport mantle geochemical heterogeneities thousands of kilometers away from plume conduits. 

Central America has a rich mix of conditions that allow comparisons of different natural experiments in the generation of arc magmas within the relatively short length of the margin. The shape of the volcanic front and this margin's architecture derive from the assemblage of exotic continental and oceanic crustal slivers, and later modification by volcanism and tectonic activity. Active tectonics of the Cocos-Caribbean plate boundary are strongly influenced by oblique subduction, resulting in a narrow volcanic front segmented by right steps occurring at ∼150-km intervals. The largest volcanic centers are located where depths to the slab are ∼90–110 km. Volcanoes that develop above deeper sections of the subducting slab are less voluminous and better record source geochemical heterogeneity. Extreme variations in isotopic and trace element ratios are derived from different components of thesubducted oceanic lithosphere. However, the extent that volcanoes sample these signatures is also influenced by lithospheric structures that control the arc segmentation. ▪  The architecture of Central America derives from the assemblage of exotic continental and oceanic crustal slivers modified by arc magmatism and tectonic processes. ▪  Active tectonics in Central America are controlled by oblique subduction. ▪  The lithospheric architecture and tectonics define the segmentation of the volcanic front, and thus the depth to the slab below a volcanic center. ▪  The composition of the subducted material is the main control of the along arc geochemical variations observed in Central American volcanoes. ▪  Geochemical heterogeneity in each segment is highlighted by extreme compositions representing the smaller centers with variations up to 65% of the total observed range.