Spatial distribution of melt conduits in the mantle beneath oceanic spreading ridges: Observations from the Ingalls and Oman ophiolites

Abstract

[1] Ophiolites are on-land exposures of igneous crust and residual upper mantle formed beneath submarine spreading ridges. Upper mantle outcrops in ophiolites provide insight into focusing of melt transport from a $100 km wide region of partial melting into an $5 km wide zone of igneous crustal accretion beneath the ridges. Dunite veins, composed of the minerals olivine and spinel, mark conduits for melt transport through at least the uppermost 30 km of the mantle. New data in this paper, on dunite veins in the Ingalls ophiolite, central Washington Cascades, show a power law relationship between frequency and width, in which frequency/m %0.02 width À3 over a size interval from $0.1 to 2 m. There may be several ways to generate this relationship, but we favor the hypothesis that the dunites represent a coalescing melt transport network. This conclusion is broadly consistent with the related hypothesis that mantle melt extraction occurs in a fractal, branching network, and with recent results on formation of a coalescing network of dissolution channels via flow of a solvent through a partially soluble, compacting porous medium.

Petrologic and microstructural constraints on focused melt transport in dunites and the rheology of the shallow mantle

“…w Extrapolation in this manner adequately predicts the abundance of the largest dunites observed in individual massifs in Oman (L=1O km) as reported by Kelemen et al (2000). (Figure 9) Estimating dunite abundance over length scales commensurate with the melting region beneath mid-ocean ridges requires one order of magnitude of additional extrapolation (L= 100 km).…”

“…technique, similar to that used by Kelemen et al (2000). Dunite widths are measured along parallel lines perpendicular to the dominant dunite strike for each image mosaic.…”

“…Reaction zones around each crack grow with the square root of time. In addition, five new dunites nucleate at each time step (unlike Kelemen et al (2000)). With time, the merging of reaction zones create increasingly wider dunites, and a weak power-law size/frequency distribution begins to emerge.…”

“…This is an improvement upon the method used by Kelemen et al (2000), exploiting greater overlap, better camera positioning, and less image distortion. The prepared outcrops ranged in size from 3x3.5m to 16x5m, with a final DunIte locations were determined by image analysis and are seen here as the lighter regions in the image.…”

“…Dunites have been hypothesized to form by (1) diffusive reaction around meltfilled hydrofractures (Nicolas, 1986;Nicolas, 1990;Suhr, 1999); (2) random merging of growing reaction zones around several hydrofractures (Kelemen et al, 2000); and (3) a reactive infiltration instability resulting in high porosity conduits. (Aharonov et aI., 1995;Kelemen and Dick, 1995;Kelemen et al, 1995b;) (Figure 1 Oa-c)…”