| 1. |
- Farré-de-Pablo, Júlia, et al.
(författare)
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Ophiolite hosted chromitite formed by supra-subduction zone peridotite –plume interaction
- 2020
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Ingår i: Geoscience Frontiers. - : Elsevier. - 1674-9871. ; 11:6, s. 2083-2102
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Tidskriftsartikel (refereegranskat)abstract
- Chromitite bodies hosted in peridotites typical of suboceanic mantle (s.l. ophiolitic) are found in the northern and central part of the Loma Caribe Peridotite in the Cordillera Central of the Dominican Republic. These chromitites are massive pods of small size (less than a few meters across) and veins that intrude both dunite and harzburgite. Compositionally, they are high-Cr chromitites [Cr# = Cr/(Cr+Al) atomic ratio = 0.71–0.83] singularly enriched in TiO2 (up to 1.25 wt.%), Fe2O3 (2.77–9.16 wt.%) as well as some trace elements (Ga, V, Co, Mn, and Zn) and PGE (up to 4548 ppb in whole-rock). This geochemical signature is unknown for chromitites hosted in oceanic upper mantle but akin to those chromites crystallized from mantle plume derived melts. Noteworthy, the melt estimated to be in equilibrium with such chromite from the Loma Caribe chromitites is similar to basalts derived from different source regions of a heterogeneous Caribbean mantle plume. This mantle plume is responsible for the formation of the Caribbean Large Igneous Province (CLIP). Dolerite dykes with back-arc basin basalt (BABB) and enriched mid-ocean ridge basalt (E-MORB) affinities commonly intrude the Loma Caribe Peridotite, and are interpreted as evidence of the impact that the Caribbean plume had in the off-axis magmatism of the back-arc basin, developed after the Caribbean island-arc extension in the Late Cretaceous. We propose a model in which chromitites were formed in the shallow portion of the back-arc mantle as a result of the metasomatic reaction between the supra-subduction zone (SSZ) peridotites and upwelling plume-related melts.
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| 2. |
- Rivera, Javier, et al.
(författare)
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Platinum-group element and gold enrichment in soils monitored by chromium stable isotopes during weathering of ultramafic rocks
- 2018
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Ingår i: Chemical Geology. - : Elsevier. - 0009-2541 .- 1872-6836. ; 499, s. 84-99
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Tidskriftsartikel (refereegranskat)abstract
- Weathering of ultramafic rocks can lead to the formation of soil profiles with high contents of Fe, Ni, Co, platinum-group elements (PGE) and gold. Traditionally, these metal-rich soils are known as “laterites” and are formed under tropical climates and stable tectonic conditions. However, little is known about their possible development in cold/humid regions, and the factors governing PGE and gold mobility and enrichment under these weathering conditions are poorly constrained. In this study, five soil profiles developed on serpentinized, chromite-bearing ultramafic rocks at La Cabaña, located in the Coastal Range of south-central Chile (38° S) were studied by combining major and trace element geochemistry with chromium stable isotope data. The results show that the soils developed at La Cabaña have higher PGE and Au contents than the parent serpentinite rock, with ∑PGE and Au reaching up to 160 ppb and 29 ppb in a limonitic soil horizon and clay saprolite, respectively. Most soil samples have slightly negative δ53/52CrSRM979 values, within a range of −0.089 ± 0.012‰ to −0.320 ± 0.013‰ (average of −0.178‰), and are in agreement with previous data reported for modern soils. A noteworthy relation between δ53/52Cr data and PGE + Au contents is observed in the studied soil horizons, where isotopically lighter values of δ53/52Cr match the higher contents of PGE and gold. These results show that pedogenetic processes operating at the cold and humid La Cabaña area are capable of increasing the total PGE and Au contents of certain soil horizons. Such processes are complex and multivariate but are primarily modulated by chromite dissolution and the formation of secondary phases such as clay minerals and oxy-hydroxide phases in the soil. These findings provide evidence that important weathering and PGE + Au supergene accumulation are not only restricted to tropical latitudes, and that the chromium isotope system is a useful proxy to track surface redox process and noble metal enrichment during pedogenesis.
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