TESTING LEACHING PROTOCOLS TO REMOVE SEAWATER ALTERATION FROM BASALTS.

Seawater-derived alteration of lavas results in changes to isotopic ratios and trace elements that can impact the conclusions drawn about the origins of these magmas. Various leaching methods have been proposed to remove alteration. This study aims to test how variable leaching of basalt chips and powders impacts isotopic data. A subset of lava flows collected via remote operated submersible during Ocean Exploration Trust expedition NA134 were selected for analyses. The samples are 86-83 Ma alkali basalts to trachytes from the Voyager Seamount Chain, south of the Northwest Hawaiian Ridge. Sample powders and chips were sequentially leached in 6N HCL at 500C, 6N HCl at 1200C, 3.5N HNO3 at 1200C for one hour each, and then in cold 2N HF for 15 minutes. In a second experiment, only 6N HCL at 500C and 6N HCl+0.2N HF at 1200C were used. Leachates were collected after each step and residues were collected after the final leach. All aliquots were analyzed for trace and major elements by ICPMS. Unleached powders and leached residues were analyzed for Sr, Nd, Pb, and Hf isotopes by MC-ICPMS. Select leachates were analyzed for Sr isotopes.

The leaching tests showed that as much as 85% of the bulk rock REE, Pb, U, and Th are leached out with weak leaching (500C 6N HCl), consistent with their residence in the altered glass matrix. The 1200C 6N HCl mobilized primarily Rb, Cs, Zr, and Hf, while HNO3 leached out primarily Cs, Zr and Hf. The HF step leached out up to 60% of the Nb, Ta, and Ba and up to 40% of the Rb, Zr, and Hf. The amount of Sr that leached out in each step varied by sample, and as a function of microphenocryst mineralogy (typically plagioclase and amphibole) but was generally highest in the 1200C HCL and HF steps. Leaching decreased the Sr and increased the Nd radiogenic isotope ratios in all samples, consistent with seawater-derived alteration removal. Hf isotopes did not change with leaching, suggesting that they are not affected by seawater alteration. Pb isotopes showed a decrease in 207Pb/204Pb, but more complex behavior in 206Pb/204Pb and 208Pb/204Pb in the different leached fractions. Leached powders are always marginally lower in 87Sr/86Sr than chips, likely due to the increase of surface area and more efficient removal of alteration. The commonly used HCl or HCl/weak HF mixtures are less efficient in alteration removal.