Time Dependent Porostiy of Very Young Upper Oceanic Crust From Ocean Bottom Gravity Measurements

H P Johnson (school of Oceanography, University of Washington, Seattle, WA 98195;206-543-8474;e-mail: johnson@ocean.washington.edu)

Porosity and density are two of the most utilized parameters in marine geological and geophysical studies of oceanic crust. In the absence of quantitative measurements of crustal densities and with a suspicion of considerable variation with age, investigators have used estimates that have varied widely. To address this problem, a recent ALVIN program has obtained 133 on-bottom gravity stations, and calculated crustal densities for three recent volcanic eruptions on the CoAxial Segment of the Juan de Fuca Ridge. A Bell-Aerospce BGM-3 gyrostabilized gravity meter was mounted inside the personnel sphere of ALVIN and used to collect fixed-station gravity data on 12 separate transect lines on (1) the recent volcanic eruption that occurred in June/July of 1993, (2) two nearby eruption sites dated post-1982, and (3) older areas on the axial ridge.

Upper crustal densities for the recent volcanic eruptions ranged between 2370 and 2530 kg/m, while the upper 70 meters of the surrounding older extrusive volcanic section had crustal densities of approximately 2470 kg/m. Using laboratory grain densities measured from 153 mini-core samples (54 distinct rocks) taken in the study area, anomalously high crustal porosities have been calculated for the recent volcanic flows when compared to the surrounding axial ridge. Porosities for the new lava flows range between 25-30%, while the surrounding axial region had porosities values of approximately 12%. This rapid decrease in porosity is not related to vesicularity, but is attributed to the collapse and filling of large-scale macroscopic voids and drain back features following a volcanic eruption. While the time scale for this initial rapid decrease in porosity cannot be precisely constrained, this physical process is clearly on the time scale of 10 to 10 years.