Observation of non-lithophile behavior for U

In the Hvittis enstatite chondrite fission track radiography shows that U is highly concentrated in CaS (oldhamite), with a concentration of 400±50 ppb. The oldhamite U concentrations are equilibrated, although intergrain concentration variations of less than 15% would not be detected. Various approaches to a U material balance do not agree, but at least 50% and, more likely, nearly 100% of the U is concentrated in CaS. It is likely that CaS is the major reservoir for other actinide and lanthanide elements as well. Consequently, enstatite chondrites may provide reliable Pu/U abundance ratios. The highly reducing formation conditions for enstatite chondrites have caused U to deviate from lithophile behaviour, but K is observed to remain lithophile. Although this observation may not be generalizable, we propose that discussions of radioactive heating of planetary cores should include U and Th and not focus exclusively on K

The solar system boron abundance

The concentrations of B in seven carbonaceous chondritic meteorites have been determined by the use of two different analytical techniques. The data correspond to a B/H ratio of about 10^(-9) compared to the value of 10^(-8) previously proposed by Cameron, Colgate, and Grossman. However, the meteoritic abundance remains at least a factor of 2-10 higher than various estimates of the solar photosphere abundance. We conclude that both meteoritic and photospheric B and Be abundances must be considered in comparisons with nucleosynthesis calculations. Using our revised B abundances and assuming ^7Li was synthesized in the big bang, we find that the residual ^6Li/1°B, ^9Be/^(10)B, and ^(11)B/^(10)B abundance ratios are well matched by the production rates for bombardment of a CNO mixture of solar proportions by protons and a-particles with a kinetic energy per nucleon spectrum of the form E^(-1.8)

The Use of Boron Concentrations in Fossil Materials as a Paleosalinity Indicator

The ¹⁰B(n,α) ⁷Li nuclear reaction has been used in conjunction with alpha-sensitive plastic track detectors to determine boron concentrations in various biologically precipitated minerals. A correlation between the boron concentration in the water in which the precipitating organisms grew and the boron concentration in the materials analyzed was found in specimens of Mytilus edulis, related bivalves, diatoms, and siliceous sponge spicules. M. edulis shell boron concentrations in aragonite ranged from about 5 ppm in specimens from 5 ⁰/₀₀ salinity water to about 15 ppm in specimens from 35 ⁰/₀₀ salinity water. Salinities can not be distinguished quantitatively using carbonate shells beyond the general distinction of marine, brackish, or nearly-fresh water. Fossil shells showed evidence for some loss of boron from aragonite. Freshwater diatomites contained less than 10 ppm B, while marine diatomites and unconsolidated marine core samples contained about 100 ppm. Detailed studies of live­ collected sponge spicules established that individual spicules larger than 20-25 microns in diameter can be analyzed, and that there is a correlation with water salinity for sponges from regions of low water temperature and high productivity. Measured concentrations ranged between nearly 0 for fresh-water sponges to 600-700 for marine sponges. However, spicules from sponges from tropical, low-productivity marine locations contained markedly less boron. The exact relationships between B concentration in the spicules and concentration in the water, temperature, nutrient supply, and food sources for the sponges are not known. Pleistocene spicules from deep-sea cores tended to contain somewhat less boron than might be anticipated by comparison with live-collected spicules based on present water temperatures and nutrient supplies. It is uncertain whether the lower concentrations are the result of diagenetic processes or the unknown effects of differences in the food supply and/or environmental conditions. Until the significance of the boron concentration in spicules is understood more fully, paleosalinity determinations will not be possible. If a relationship between boron content and water temperature or nutrient supply can be defined, it may be possible to use boron concentrations in spicules to trace oceanic circulation patterns in the past

Boron concentrations in carbonaceous chondrites

We have analyzed B in carbonaceous chondrites in order to clarify a factor of 100 difference between the solar system B abundance derived from the solar photosphere and that inferred from previous meteorite data. Consistent results were obtained from two instrumental methods for B analysis: (a) counting of the high energy betas from ^(12)B produced by the ^(11)B(d,p) reaction, and (b) measurement of particle track densities from ^(10)B(n,α)^7Li in a plastic track detector affixed to a homogenized meteorite sample. Contamination is a major problem in B analyses, but extensive testing showed that our results were not seriously affected. Our B concentrations are typically 1–2 ppm and are a factor of 2–6 lower than previous carbonaceous chondrite measurements. Our data for the Cl chondrites Ivuna and Orgueil would indicate a solar system B/Si atomic abundance ratio of 58 × 10^(−6), but this is still a factor of 2–10 higher than the photospheric estimates. It may be that B is depleted in the sun by thermonuclear processes; however, the similarity of photospheric and meteoritic Be abundances is a problem for this point of view. Alternatively, B may be enhanced in carbonaceous chondrites, but this would make B a cosmochemically unique element. A mm-sized (Fe,Mn,Mg)CO_3 crystal from Orgueil shows no B enrichment. We find ^(10)B ≤ 10^(16) atoms/g in two Allende fine-grained inclusions suggesting that B is not a refractory element under solar nebula conditions. This ^(10)B limit, when taken as a limit on ^(10)Be when the inclusion formed, puts constraints on the possibility of a solar system synthesis of ^(26)Al. For a proton spectrum of E^(−a), a must be ≥ 3 if a solar gas is irradiated or a ≥1.5 if dust of solar composition is irradiated