In Situ Micro-analytical Investigations of Palagonitization

Geochemical parameters that potentially control palagonitization were investigated by subjecting a suite of samples that were altered to palagonite in five potentially distinct geochemical environments to several in-situ microanalytical techniques. Results of petrographic (textural and modal analysis), reflected-light Fourier transform, infrared spectroscopy (relative water content), electron microprobe (major-element composition), and laser-ablation inductively-coupled plasma mass spectroscopy (trace-element composition) analyses are presented and discussed in Chapter 1. A newly devised palagonitization extent parameter, based on modal rather than chemical analysis, varies linearly and inversely with original sample porosity. This suggests that original sample porosity plays a key role in controlling palagonitization. Palagonite major- and traceelement composition depend on original sideromelane chemistry (alkaline vs. subalkaline), indicating original glass compositional control of resulting palagonite composition. Isocon diagrams were used to determine elemental mobilities during palagonitization. Zr, Nb, Sc, La, and Nd are immobile, and all of the major-elements are mobile, with potentially large mass losses. This finding suggests that palagonitization is not an isovolumetric process, and requires extensive dissolution and creation of microporosity. For given environmental conditions, subalkaline sideromelane appears to dissolve more rapidly than alkaline sideromelane during palagonitization. REE and MgO concentrations decrease systematically through the palagonitized sideromelane inward toward the fresh sideromelane contact. The REE (immobile) concentration gradient is consistent with progressive glass dissolution; the MgO (mobile) concentration gradient may suggest progressive evolution of a phyllosilicate (i.e. smectite) layer between the gel-palagonite layer and the fluid. Two distinct palagonitization styles, burial-diagenetic and hydrothermal, are recognized.;A methodology for the analytical measurement of boron concentration and isotopic ratios in palagonitized sideromelane using secondary-ion mass-spectrometry is described in Chapter 2. The purpose of this part of the research is to address the issues of calibration, matrix effects, and cation exchange in order to demonstrate that meaningful boron isotope analyses can be performed. Clay Minerals Society Source Clay IMT-1 is shown to be a suitable calibration standard. Matrix effects on B isotopic analyses are similar in IMT-1 and sideromelane, suggesting that similar matrix effects can be expected for palagonite analyses as well, hence permitting calibrated B isotopic analyses of palagonite. A 1M NH4Cl solution exchanged B in thin-section palagonite samples, permitting analysis of both bulk and tetrahedral B concentration and isotopic ratios; interlayer B concentration and isotopic ratios can then be determined using mass balance.;In Chapter 3, the first-documented palagonite boron concentration and isotopic ratio results are presented and discussed. For samples that palagonitized in seawater, zeolitepoor (low-extent) samples were found to have relatively low delta11B compared to seawater, indicating that significant fractionation occurred during palagonitization. Conversely, zeolite-rich (high-extent) samples have delta11B values that are relatively high and close to sea-level values, indicating that insignificant fractionation occurred. Since mineral-water B isotopic fractionation depends on pH, low reaction extent samples must have palagonitized in high w/r (high porosity) environments, and high reaction extent samples must have palagonitized in low w/r (low porosity) environments. This result provides geochemical confirmation of the Chapter 1 finding that palagonitization extent is controlled at least in part by original sample porosity.