Porosity of very young oceanic crust from sea floor gravity measurements

Abstract

Porosity and density are two of the most important, but under‐determined parameters in studies of igneous oceanic crust. Lacking quantitative measurements of crustal densities, but with plausible expectations of substantial variation with age, previous investigators have used estimates that have varied widely. To address this problem more rigorously, a recent ALVIN program obtained 133 on‐bottom gravity stations to calculate crustal densities for three recent volcanic eruptions on the Juan de Fuca Ridge. In the summer of 1995, a Bell‐Aerospace BGM‐3 gravity meter was mounted in ALVIN and made fixed‐station gravity measurements on 11 transect lines from three distinct volcanic environments including (1) the recent CoAxial Segment volcanic eruption that occurred in 1993, (2) two nearby flows with less well‐determined ages but dated post‐1981, and (3) several older adjacent areas on the Juan de Fuca axial ridge. Densities for the recent eruptive units ranged between 2280 and 2450 kg/m³, while the upper 50 meters of the older, unfractured surrounding flows were approximately 2800 kg/m³. Laboratory grain densities measured from rocks collected in the study area allow crustal porosities to be calculated, which are anomalously high for the recent flows in comparison to the surrounding axial ridge. Porosities for the three youngest flows range between 29–36%, while the adjacent unfractured regions of the surrounding axial region had porosities near 10%. Two localized regions of intense surficial crustal fracturing in older crust, located just south of the 1993 eruption site, also had high porosities of 26–28%. This high variability indicates that the evolution of crustal porosities from young to older oceanic crust is not related only to the collapse and infilling of macroscopic voids, but that tectonic activity can significantly modify the porosity of older crustal structures.