The goal of this project was to carry out an integrated bathymetric, geological and geochemical study of the seafloor near 14?N on the Mid-Atlantic Ridge. This area was selected for study because it is the site where one of the most gas-rich submarine samples was recovered in 1985, by dredging the seafloor at depths of 3900 meters (12,800 feet) below the surface.  The outer glassy portions of the seafloor lavas were so gas-rich that the carbon dioxide filled bubbles ?popped? when brought from the bottom of the ocean to the deck of the ship, due to the pressure change, leading to the term ?popping rock?. The gas contents of these dredged popping rock samples have been important to estimating the gas contents and composition of the earth?s interior.  However, this section of the mid-ocean ridge is unusual in having extensive active faults on the seafloor, and it is slow spreading, meaning that there are fewer volcanic eruptions than in other areas; the exact location and geologic context of the popping rocks was not well determined.  This project involved the combined use of the research vessel Atlantis, the submersible Alvin, and the autonomous underwater vehicle Sentry, to find the ?popping rock? locations, collect a suite of sea floor basaltic glasses from the different tectonic settings at 14?N, and test the relationship between faulting and volatile contents in the glasses. The seagoing work was carried out on two cruises, beginning in March 2016 and finishing in June of 2018 (AT33-03 an AT40-02).  The two cruises resulted in 27 Alvin dives and 26 Sentry dives, which allowed collection of unique high-resolution multibeam and magnetic data over ~88 square kilometers (34 square miles), making this one of the best documented slow spreading ridges.  Sample collection on the cruises included 421 rock samples and 99 sediment push cores. Of the rock samples, 45 were classified as popping rock samples. The combination of Sentry mapping and Alvin sampling allowed a successful delineation of popping rock occurrence on the N-S trending ?Popping Rock Ridge? and the valley walls to the east. Major and trace elements were measured in the recovered samples, and a subset of the glasses were analyzed for major volatiles (CO2, H2O, Cl, F, S), noble gases (He, Ne, Ar) and radiogenic isotopes (Sr, Nd, Pb). These data have been integrated with geological observations and vesicle size distribution, providing insights into the relationship between mantle source, eruption dynamics and volatiles. An important outcome is the determination of volatiles and noble gases in the same phases (glass and vesicles), thus allowing an assessment of the mantle C/3He and 4He/40Ar* ratios, which are key indicators for evaluating fluxes, degassing, diffusion and fractionation processes. Several of the samples were collected in anaerobic pressure vessels to exclude exposure to air and to improve sample collection for understanding mantle volatiles.  One hypothesis that was developed from this research is that popping rocks are formed by gas accumulation in the compressional environment of the slow spreading ridge axis, at crustal depths, where faulting is prevalent.  This is important because it implies that the popping rocks were produced by gas accumulation and therefore may not be directly used to infer (solid) mantle gas concentrations.

This research provided important research opportunities for a number of early career scientists, and was the focus of two Master's degrees (Boise State University, University of Idaho), two Ph.D. theses (MIT/WHOI Joint Program in Oceanography, Institut de Physique du Globe) and two Postdoctoral Research projects (Woods Hole Oceanographic Institution).   The seagoing research, particularly the combined use of a human occupied vehicle with an autonomous underwater vehicle, provided unique research and career growth opportunities for all the participants.

Last Modified: 11/10/2020
Modified by: Mark D Kurz