▪ Abstract The Re-Os isotope sytem, based on the long-lived β− transition of 187Re to 187Os, has matured to wide use in cosmochemistry and high-temperature geochemistry. The siderophilic/chalcophilic behavior of Re and Os is different from that of the elements that comprise most other long-lived radiogenic isotope systems. Magmatic iron meteorites (IIIAB, IIAB, IVA, and IVB) have Re-Os isochrons that indicate asteroidal core crystallization within the first 10–40 million years of Solar System evolution. Rocks from Earth's convecting mantle show generally chondritic Re/Os evolution throughout Earth history that is explained by the addition of highly siderophile elements to the mantle after core formation via late accretion. Oceanic basalts have Os-isotope systematics that improve the detailed geological interpretation of extant mantle components. Some portions of ancient subcontinental lithospheric mantle are severely depleted in Re and have correspondingly subchondritic 187Os/188Os, indicating long-term i...

THE Re-Os ISOTOPE SYSTEM IN COSMOCHEMISTRY AND HIGH-TEMPERATURE GEOCHEMISTRY

The Re-Os isotopic system in cosmochemistry and high temperature geochemistry

PRIMELT2.XLS software is introduced for calculating primary magma composition and mantle potential temperature (TP) from an observed lava composition. It is an upgrade over a previous version in that it includes garnet peridotite melting and it detects complexities that can lead to overestimates in TP by >100°C. These are variations in source lithology, source volatile content, source oxidation state, and clinopyroxene fractionation. Nevertheless, application of PRIMELT2.XLS to lavas from a wide range of oceanic islands reveals no evidence that volatile-enrichment and source fertility are sufficient to produce them. All are associated with thermal anomalies, and this appears to be a prerequisite for their formation. For the ocean islands considered in this work, TP maxima are typically ~1450–1500°C in the Atlantic and 1500–1600°C in the Pacific, substantially greater than ~1350°C for ambient mantle. Lavas from the Galapagos Islands and Hawaii record in their geochemistry high TP maxima and large ranges in both TP and melt fraction over short horizontal distances, a result that is predicted by the mantle plume model.

/pdf/petrology-of-some-oceanic-island-basalts-primelt2-xls-4ejvkk7e5f.pdf

Petrology of some oceanic island basalts: PRIMELT2.XLS software for primary magma calculation

[1] The existence, spatial distribution, and style of volcanism on terrestrial planets is an expression of their internal dynamics and evolution. On Earth a physical link has been proposed between hot spots, regions with particularly persistent, localized, and high rates of volcanism, and underlying deep mantle plumes. Such mantle plumes are thought to be constructed of large spherical heads and narrow trailing conduits. This plume model has provided a way to interpret observable phenomena including the volcanological, petrological, and geochemical evolution of ocean island volcanoes, the relative motion of plates, continental breakup, global heat flow, and the Earth's magnetic field within the broader framework of the thermal history of our planet. Despite the plume model's utility the underlying dynamics giving rise to hot spots as long-lived stable features have remained elusive. Accordingly, in this review we combine results from new and published observational, analog, theoretical, and numerical studies to address two key questions: (1) Why might mantle plumes in the Earth have a head-tail structure? (2) How can mantle plumes and hot spots persist for large geological times? We show first that the characteristic head-tail structure of mantle plumes, which is a consequence of hot upwellings having a low viscosity, is likely a result of strong cooling of the mantle by large-scale stirring driven by plate tectonics. Second, we show that the head-tail structure of such plumes is a necessary but insufficient condition for their longevity. Third, we synthesize seismological, geodynamic, geomagnetic, and geochemical constraints on the structure and composition of the lowermost mantle to argue that the source regions for most deep mantle plumes contain dense, low-viscosity material within D″ composed of partial melt, outer core material, or a mixture of both (i.e., a “dense layer”). Fourth, using results from laboratory experiments on thermochemical convection and new theoretical scaling analyses, we argue that the longevity of mantle plumes in the Earth is a consequence of the interactions between plate tectonics, core cooling, and dense, low-viscosity material within D″. Conditions leading to Earth-like mantle plumes are highly specific and may thus be unique to our own planet. Furthermore, long-lived hot spots should not a priori be anticipated on other terrestrial planets and moons. Our analysis leads to self-consistent predictions for the longevity of mantle plumes, topography on the dense layer, and composition of ocean island basalts that are consistent with observations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003RG000144

E Widom

Papers

Th–Sr–Nd–Pb isotope and trace element evidence for the origin of the São Miguel, Azores, enriched mantle source