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21 Ne depth profiles measured from amalgamated samples of sand-sized quartz from the Quaternary Bandelier Tuff were used to estimate inheritance-corrected exposure ages in stream terraces and an alluvial fan on the Pajarito Plateau. Stable alluvium and associated soils display a downward decreasing (Type I) nuclide profile pattern. Bioturbated colluvium with uniform (Type III) nuclide concentrations has buried these alluvial surfaces. Another possible profile type with downward increasing nuclide concentrations (Type II) is described but was not found on the Pajarito Plateau. We present mathematical models for each of the profile patterns relating nuclide concentration to inheritance, time-averaged bulk density, and either exposure time or burial rate. Curve fits of Type I profiles are used to estimate the age of alluvial landforms. A burial correction technique for deposits buried by younger colluvium is also presented. Inheritance- and burial-corrected 21 Ne surface exposure ages of 144±17 ka and 63±8 ka for two Pajarito Plateau stream terraces agree within error with independent soil profile development index dates of 160 ka and 68–78 ka. A soil overlying the 50–60 ka El Cajete pumice yields an inheritance-corrected 21 Ne exposure age of 44±11 ka that also agrees closely with a 14 C age on soil charcoal of 41 ka. These results demonstrate that reliable cosmogenic 21 Ne exposure ages can be obtained for fine-grained alluvial surfaces with the depth profile technique.

Dating of very young igneous activity has been hampered by lack of suitable chronometers. We report here the results of 3He surface exposure dating (Kurz, 1986a,b; Craig and Poreda, 1986) for lavas from the Potrillo volcanic field. This field is particularly suited to the technique because it has well-preserved, young (

Carbon stable isotopic composition of the major carbonaceous component in enstatite chondrites varies with petrologic type. Investigation of a suite of HF/HCl-resistant residues has shown that this variation is due to an inherent difference in δ 13 C of the carbon, and is not a result of the presence of small amounts of isotopically anomalous carbon-bearing components. These latter do occur in type EH3 and EH4 chondrites, in concentrations similar to those found in C1 and C2 carbonaceous chondrites. Combustion of the major carbon component (apparently elemental carbon, not necessarily graphite) occurs at relatively higher temperatures in enstatite chondrites of increasing petrologic type. This is considered to reflect an increase in crystallinity or ordering of the carbonaceous component, and is a measure of the degree of thermal processing to which the meteorites have been subjected during accretion and/or metamorphism.