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An analytical procedure has been developed to measure in situ the 11 B/10 B ratio in terrestrial basaltic rocks and meteoritic chondrules having B concentration of less than 1 μg g−1 using a small radius ims3f ion microprobe. The central difficulties for these measurements are (i) the removal of the trace amount of B contamination introduced in the sample during polishing, (ii) the precise calibration of instrumental mass fractionation of B isotopes and (iii) the low count rates of 10 B and 11 B. Contamination experiments conducted with isotopically labelled B enriched in 10 B showed that ultrasonic cleaning in bi‐distilled water (< 1 ng g−1 B) and pre‐sputtering of the analysed area decrease B contamination to the level of 0.01 μg g−1. Analyses of isotope standards spanning a range of 11 B/10 B between 3.93 and 4.20 showed that instrumental mass fractionation was constant within? during one session of analyses. Repeated analyses of a standard glass showed a reproducibility of instrumental mass fractionation between February 1991 and October 1996 of 1.3. Taking into account all sources of error, boron isotope measurements are accurate to within 5 for meteoritic samples having B contents in the range 0.1 to 1 μg g−1. A slightly better accuracy of 1.5 can be achieved for basaltic glasses which can be sputtered with very intense primary beams.

Most arc magmas are thought to be generated by partial melting of the mantle wedge induced by infiltration of slab-derived fluids. However, partial melting of subducting oceanic crust has also been proposed to contribute to the melt generation process, especially when young and hot lithosphere is being subducted. The isotopic composition of boron measured in situ in olivine-hosted primitive melt inclusions in a basaltic andesite from Mt. Shasta, California, is characterized by large negative values that are also highly variable (delta(11)B = -21.3 to -0.9 per mil). The boron concentrations, from 0.7 to 1.6 parts per million, are lower than in most other arc lavas. The relation between concentration and isotopic composition of boron observed here supports a hypothesis that materials left after dehydration of the subducting slab may have contributed to the generation of basaltic andesite lavas at Mt. Shasta.

[1] Abstract: The variation of B concentration in atmospheric deposition was studied from the analysis of 35 individual rain events, 17 snow packs, and 17 lichens sampled over NE North America (south from Hudson Bay) and Asia (from the coast of Bangladesh to the high Himalayas of Nepal). Rain samples show a range of B concentration between 0.3 and 9.4 mg/L (average of 1.8 1.7 mg/L), excluding two rains with higher B contents of 17 and 37.5 mg/L, most likely reflecting anthropic contamination. Snowpacks and lichens which average atmospheric deposition over periods of a few months to a few years show a smaller range of variation, from 0.1 to 2.3 mg/L (average 1.1 0.8 mg/L) for snowpacks and from 1 to 25.9 ppm for lichens. The lichens have elemental ratios (such as B/Cl) similar to the average of rains, showing that they are good monitors of atmospheric B deposition without significant biofractionation of elements. This is also demonstrated by their halogen contents, which follow the systematics of the atmospheric distribution of these elements previously derived from the study of rains and atmospheric particles. Though individual rains do not show systematic decrease in B concentration with distance to the sea, this behavior is clearly shown by samples having longer integration times, snowpacks, and lichens. The snow and lichen data show that seawater is a major source of atmospheric B. Using the lichen data, the enrichment factor (normalized to Na) of marine air masses relative to seawater is estimated to be $13 (average B/Na of 5.6 Â 10 À3 ). This implies that the fractionation factor of atmospheric suspended marine particles relative to seawater (F X = (X/Na) particle / (X/Na) seawater ) is 0.177. Thus B transfer from seawater to the atmosphere occurs mostly via degassing of sea salts, the residence time of gaseous B being estimated at 15.9-fold that of particulate B. The distribution of B in the atmosphere between gaseous B and degassed sea salts can be used to model the large range of B concentrations measured in single rain events by selective removal of one of the two B components. In this model, unfractionated marine air masses can reach the continents with very variable B concentrations, between $0.2 and $20 mg/L.