Alteration of Crystalline and Glassy Basaltic Protolith by Seawater as Recorded by Drill Core and Drill Cutting Samples

Abstract:

The major and trace element composition of hydrothermally altered basaltic drill core and drill cutting samples from the seawater recharged Reykjanes geothermal system in Iceland are compared to unaltered surface flows from the Reykjanes Peninsula compiled from the literature. Trace element characteristics of deep (>2000 m) core samples record bimodal compositions similar to trace element enriched and trace element depleted Reykjanes Ridge basalts. Drill cuttings (350-3000 m) overwhelmingly reflect the more common trace element enriched igneous precursor. Crystalline protoliths (dolerite dykes and pillow lava cores) are depleted in Cs, Rb, K, and Ba (± Pb and Th) relative to an unaltered equivalent, despite variations in the extent of alteration ranging from from minor chloritization with intact igneous precursor minerals through to extensive chloritization and uralitization. Glassy protoliths (dyke margins, pillow edges, and hyaloclastites) show similar depletions of Cs, Rb, K, and Ba, but also show selective depletions of the light rare earth elements (LREE) La, Ce, Pr, Nd and Eu due to extensive recrystallization to hydrothermal hornblende. Lower grade alteration shows less pronounced decoupling of LREE and is likely controlled by a combination of Cl complexation in the seawater-derived recharge fluid, moderated by anhydrite and epidote precipitation. These results suggest that alteration of glassy protolith in seawater-recharged systems is an important contribution to the consistently light rare earth and Eu enriched patterns observed in seafloor hydrothermal fluids from basaltic systems. An important conclusion of this study is that that drill cuttings samples are strongly biased toward unaltered rock and more resistant alteration minerals including epidote and quartz potentially resulting in misidentification of lithology and extent of alteration.