Methods for in situ SIMS microanalysis of boron and its isotopes in palagonite

• Published in: Clays and clay minerals 2014-06, Vol.62 (3), p.224-234
• Main Authors: Pauly, Bruce D, Williams, Lynda B, Hervig, Richard L, Schiffman, Peter, Zierenberg, Robert A
• Format: Article
• Language: English
• Subjects: alteration; B Content; B Exchangeability; B Isotopes; B-11/B-10; Biogeosciences; Boron; Calibration; chemical properties; chemical reactions; clastic sediments; clay; clay mineralogy; clay minerals; Earth and Environmental Science; Earth Sciences; Exchange; experimental studies; Geochemistry; glasses; igneous rocks; illite; in situ; Ion exchangers; ion probe data; Isotopes; mass spectra; mdash; Medicinal Chemistry; Mineralogy; Nanoscale Science and Technology; palagonite; Palagonitization; Secondary ion mass spectrometry; sed rocks, sediments; Sedimentary petrology; sediments; sheet silicates; Sideromelane; silicates; Similarity; Sims; smectite; Soil Science & Conservation; spectra; stable isotopes; Thin Section; volcanic rocks
• ISSN: 0009-8604; 1552-8367
• DOI: 10.1346/CCMN.2014.0620306

Boron has been shown to be a useful trace element in clay-mineralization reactions, raising the possibility that B studies may provide a means to investigate environmental controls on palagonitization. The objective of the present study was to address calibration, matrix effects, and B exchangeability issues such that meaningful secondary ion mass spectrometry (SIMS) microanalysis of B in thin sections of palagonite will be feasible. Silver Hill illite (IMt-1) was found to be a suitable calibration reference material, based on compositional similarity, relatively high B content, and ease of mounting on thin-section samples for SIMS microanalysis. Matrix effects of borated sideromelane and illite were compared and found to be similar, confirming previous studies which showed no matrix effects for B among minerals. Boron substitutes for Si in tetrahedral sites and also can be adsorbed in exchangeable sites of 2:1 clay minerals. Similarly, B can be found in tetrahedral and exchangeable sites within palagonite, which consists of both layered and amorphous volumes. In order to measure tetrahedral B content and isotopic ratio in the palagonite, exchangeable B was removed by soaking sample thin sections in a 1 M NH4Cl solution until exchangeable cation concentrations were constant. Treated samples showed decreases in B content and isotopic ratio with exchange. Extraction of exchangeable B permits the direct measurement of tetrahedral B content and isotopic ratio. The exchange technique devised and tested here should have broad applicability to thin-section microanalysis of B in clay and clay-like materials where cation exchange can be used for surface-analytical techniques. The present study represents an initial attempt to address sample-preparation, calibration, and potential matrix-effects problems for analyses by SIMS. Further refinements may improve the accuracy of the measurements, but the results presented here indicate that meaningful measurements are possible.