Methods forIn SituSIMS Microanalysis of Boron and its Isotopes in Palagonite

Abstract

Abstract-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 thinsection 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 NH 4 Cl 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 samplepreparation, 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.

Formation mechanisms of macroscopic globules in andesitic glasses from the Izu–Bonin–Mariana forearc (IODP Expedition 352)

“…9. The higher 11 B of the globules compared to the andesitic groundmass is inconsistent with the adsorption of B into secondary clays and palagonite due to low-temperature alteration, which results in an enrichment of lighter 10 B (Pauly et al 2014;Fonseca et al 2017). It is thus unlikely that the formation of the globules and the andesitic groundmass in the Unit 6 globular andesite can be explained by low-temperature seafloor alteration.…”

“…Boron isotopes, in particular, are well suited to trace processes leading to the formation of these globular samples, as B is soluble in fluids and significant stable isotope fractionation can occur even at magmatic temperatures, especially when compared to the similarly light C or O (Hervig et al 2002). Boron isotopes are also highly fractionated as a result of low-temperature alteration processes, when adsorbed into secondary clay minerals (Pauly et al 2014;Fonseca et al 2017). As such, B isotope data, coupled with other geochemical information, namely, Sr isotopes and fluid-mobile element concentrations should allow us to assess the involvement of fluid-mediated processes in the genesis of these unique samples, as well as the genetic relationship of the different textural components found in these samples.…”

“…There is also no clear evidence that these presumptive vesicles contracted due to degassing, or that resorption of H 2 O into melt or globules has taken place (globules and andesitic groundmass are seemingly anhydrous), which would be expected to result from extensive vesiculation and degassing (McIntosh et al 2014;Marxer et al 2015). Low-temperature seafloor alteration of these samples would be expected to introduce elements like Cl and B in addition to water in the form of secondary clays, hydroxy-chlorides and palagonite (Pauly et al 2014;Seyfried Jr et al 1986;Gibson et al 1996). However, Cl is depleted in the globules relative to the andesitic groundmass and, aside from the voids and the interface, and potentially alkali feldspar in the globules, neither textural domain shows evidence of extensive secondary mineralogy.…”

Element mobility and oxygen isotope systematics during submarine alteration of basaltic glass

“…Boron displays marked enrichment patterns during alteration ( Fig. 5h; Table 1), owing to its ability to substitute for Si in tetrahedral sites and being adsorbed in exchangeable sites of palagonite (Pauly et al 2014). Cu, Ni, and Zn are also strongly enriched in palagonite ( Fig.…”