Compatibility of rhenium in garnet during mantle melting and magma genesis

Measurements of the partitioning of rhenium (Re) between garnet and silicate liquid from 1.5 to 2.0 gigapascals and 1250 degrees to 1350 degreesC show that Re is compatible in garnet. Oceanic island basalts (OIBs) have lower Re contents than mid-ocean ridge basalt, because garnet-bearing residues of deeper OIB melting will retain Re. Deep-mantle garnetite or eclogite may harbor the missing Re identified in crust-mantle mass balance calculations. Oceanic crust recycled into the upper mantle at subduction zones will retain high Re/Os (osmium) ratios and become enriched in radiogenic 187Os. Recycled eclogite in a mantle source should be easily traced using Re abundances and Os isotopes.     

Short-lived oxygen diffusion during hot, deep-seated meteoric alteration of anorthosite

Heterogeneous oxygen isotope compositions of plagioclase from the Boehls Butte anorthosite include some of the most oxygen-18-depleted values (to -16 per mil) reported for plagioclase in meta-igneous rocks and indicate high-temperature (T > 500 degrees C) isotopic exchange between plagioclase and nearly pristine meteoric fluid. Retrograde reaction-enhanced permeability assisted influx of meteoric-hydrothermal fluids into the deep-seated anorthosite. Isotopic gradients of about 14 per mil over 600 micrometers in single crystals require short-lived (about 10(4) years) diffusional exchange of oxygen and locally large effective water:rock ratios, followed by rapid loss of water and cessation of oxygen diffusion in the anorthosite.     

Natural radioactivity of Miami soils

An important part of man's radioactive environment is the natural radioactivity of soils. This radioactivity varies with soil type and withdepth in the soil profile. The relation between gross gamma activity and soil depth for a particular soil (Miami silt loam) is presented, together with a discussion of the contribution of K(40) and the uranium and thorium series and of the effects of fallout from bomb tests in increasing the radioactivity of a thin layer of surface soil.     

Short-lived chemical heterogeneities in the archean mantle with implications for mantle convection

The neodymium isotope and samarium-neodymium systematics of 2.7-billion-year-old mantle-derived magmas indicate that the lifetime of chemical heterogeneities was much shorter in the Archean mantle than in the modern mantle. Isotopic evidence is compatible with a Rayleigh number 100 times larger and convection 10 times faster in the Late Archean compared with the present-day mantle. Modern plate tectonics thus may be an improbable analog for the Archean. Chemical heterogeneities in the mantle may originate upon magma migration and mineralogical phase changes rather than by recycling of oceanic and continental crust.     

Short-lived nuclides in hibonite grains from Murchison: evidence for solar system evolution

Records of now-extinct short-lived nuclides in meteorites provide information about the formation and evolution of the solar system. We have found excess 10B that we attribute to the decay of short-lived 10Be (half-life 1.5 million years) in hibonite grains from the Murchison meteorite. The grains show no evidence of decay of two other short-lived nuclides-26Al (half-life 700,000 years) and 41Ca (half-life 100,000 years)-that may be present in early solar system solids. One plausible source of the observed 10Be is energetic particle irradiation of material in the solar nebula. An effective irradiation dose of approximately 2 x 10(18) protons per square centimeter with a kinetic energy of >/=10 megaelectronvolts per atomic mass unit can explain our measurements. The presence of 10Be, coupled with the absence of 41Ca and 26Al, may rule out energetic particle irradiation as the primary source of 41Ca and 26Al present in some early solar system solids and strengthens the case of a stellar source for 41Ca and 26Al.     

Time-resolved seismic tomography detects magma intrusions at Mount Etna

The continuous volcanic and seismic activity at Mount Etna makes this volcano an important laboratory for seismological and geophysical studies. We used repeated three-dimensional tomography to detect variations in elastic parameters during different volcanic cycles, before and during the October 2002-January 2003 flank eruption. Well-defined anomalous low P- to S-wave velocity ratio volumes were revealed. Absent during the pre-eruptive period, the anomalies trace the intrusion of volatile-rich (>/=4 weight percent) basaltic magma, most of which rose up only a few months before the onset of eruption. The observed time changes of velocity anomalies suggest that four-dimensional tomography provides a basis for more efficient volcano monitoring and short- and midterm eruption forecasting of explosive activity.