Got glass? Glass from sediment and foraminifera tests contribute clues to volcanic history

Abstract

Volcanic glass fragments recovered from sediment collected in the Escanaba Trough and on traverses up the west walls at two locations on the Gorda Ridge greatly extend information on the petrologic diversity of the region obtained from outcrops. Compositions of glass grains found loose in sediments or agglutinated in the tests of benthic foraminifers provide information on compositions of unexposed flows that were covered by later eruptions and/or sediment. Many of the glass grains have compositions that closely match those of lava outcrops, indicating that they formed by spalling off pillow rims and quench granulation. Bubble-wall glass shards found only in the sediment give evidence of explosive eruptions that occurred despite great depth. Some of the glass fragments from the walls of the axial valley have unusually high K 2 O (to >0.7%) that is not represented by any sampled outcrop. Such enriched mid-oceanic-ridge basalt compositions have not previously been reported from the Gorda Ridge. Analyzing glass grains from the sediment in addition to rims of lava samples allows a more comprehensive characterization of the volcanic history of the region.