THE GROWING PROMISE OF MN-FE OXIDE CRUSTS FOR ESTIMATING AGE OF SUBMARINE LAVA, HAWAII

A black Mn-Fe oxide crust coats most exposed rock on the ocean floor. These crusts are well developed on steep rocky lava slopes of the Hawaiian Ridge and have been sampled during dredging and submersible dives. The crusts also occur on fragments detached from bedrock, on submerged coral reefs and on lithified sediment. Measurements of the thickness of the crusts on ~1000 submarine rock samples collected on the Hawaiian Ridge from 110 dive or dredge localities show a systematic thickening of the crusts toward the northwest—toward the older volcanic features away from the Hawaiian hotspot. The maximum crust thickness at each site was selected to eliminate the presence of anomalously thin crusts on many samples—apparently resulting from post-depositional breakage and periodic burial of outcrops. Comparison of the maximum crust thickness with radiometric ages of related subaerial features suggests a crust accretion rate of about 2.5 mm/Ma. The thickness information allows a comparison of the relative exposure ages of features offshore from different volcanoes, and clarifies the ages of the major shields and the rejuvenated volcanic centers that grow upon them. Grouping of volcanic sites with similar crust thicknesses indicates that segments of the Hawaiian Ridge several hundred km long may be active during a restricted time interval. Samples described here were collected in the last 35 years and represent a large part of all deep-water, well located samples collected off Hawaii. Of the 110 sites, 58 (53%) were collected in 2001 by ROVs (remotely operated vehicles). Future sampling will, no doubt, rely heavily on ROVs, and the large number of samples that they can efficiently collect will provide a base for further evaluation of Mn-Fe oxide crusts as a dating tool.