Palagonitization of Mauna Kea Sideromelane:
New Insights From in-situ Micro Mass Spectrometry

Pauly, B¹, Schiffman, P¹, Hervig, R², Yin, Q¹, Zierenberg, R¹

¹Dept. of Geology, University of California, Davis, 1 Shields Ave., Davis, CA 95616 USA
²Dept. of Geological Sciences, Arizona State University, Box 871404, Tempe, AZ 85287 USA

Geochemically, the process of palagonitization remains incompletely understood largely because of the difficulty in physically separating adjacent fresh and palagonitized sideromelane.  We have used integrated LA-ICPMS (quadrupole and multicollector) and SIMS analysis to investigate palagonitization in a hyaloclastite from a depth of 2847 m in the Hawaii Scientific Drilling Project’s 3 km-deep corehole.  The LA-ICPMS (quadruple) at UCDavis was used to analyze trace elements along a contact-perpendicular traverse from palagonite into adjacent sideromelane.  All trace elements analyzed are mobile during palagonitization.  Concentration gradients were observed in the palagonite (more enriched further from the contact) for Cr, Cu, and Zr, while Ba and Rb gradients increased in the opposite direction.  Sc, V, Co, and Zn showed no gradient.  The REE pattern in palagonite had the same slope as for the adjacent sideromelane (smooth, LREE-enriched), and was enriched relative to sideromelane.  A pronounced REE gradient (more enriched further from the contact) was observed in the palagonite.  The LA-ICPMS (multi-collector) at UCDavis was used to analyze Sr concentration and ^87/86Sr along a similar traverse.  A concentration gradient was observed for Sr, with the highest concentration nearest the contact.  ^87/86Sr values were highest at the outer palagonite rim, decreasing inward toward the contact.  These concentration and isotopic data fit a simple Sr mixing model using sideromelane and seawater as end-members, and indicate that water circulation decreased from the outer palagonite rim toward the contact during palagonitization (assuming the palagonitization reaction front moved inward).  The Cameca 3F SIMS at Arizona State University was used to analyze B concentration, δ¹¹B, and δ¹⁸O along traverses across the palagonite-sideromelane contact.  B concentration within palagonite increased from basalt values (essentially 0 ppm) at the contact outwards toward seawater values (4.5 ppm).  Simple mixture modeling for B concentration yields water circulation results similar to those obtained for the Sr mixing model.  δ¹¹B was approximately 14 per mil throughout the palagonite.  δ¹⁸O values were constant in sideromelane and adjacent palagonite, with the palagonite values 4 per mil heavier than sideromelane.  Absolute δ¹⁸O measurements will require the creation of a palagonite δ¹⁸O standard.