Robert B. Scott

Tectonic fabric and hydrothermal activity of Mid-Atlantic Ridge crest (lat 26°N)

An asymmetric tectonic fabric was delineated by narrow-beam bathymetric profiles in a 180-km 2 area of the Mid-Atlantic Ridge crest at lat 26°N. Features of the tectonic fabric are a continuous rift valley offset by small ( The rift valley has a double structure consisting of linear segments, bounded by ridges, and basins at the intersections of the minor fracture zones. The double structure of the rift valley acts like a template that programs the reproduction of the tectonic fabric. The minor fracture zones form an asymmetric V about the rift valley at variance with the symmetric small circles formed by major fracture zones. To reconcile the asymmetry of minor fracture zones with the symmetry of major fracture zones, it is proposed that the minor fracture zones have been preferentially reoriented by an external stress field attributed to interplate and intraplate motions. Major fracture zones remain symmetric under the same stress field owing to differential stability between minor and major structures of oceanic lithosphere.

Rare-earth and other trace element contents and the origin of minettes (mica-lamprophyres)

Abstract Instrumental neutron activation and X-ray fluorescence analyses of minettes from New Brunswick, Canada, indicate that these rocks are strongly enriched in REE, especially the LREE, and other incompatible elements as well as Cr and Co. The geochemistry of minettes precludes their formation by anatexis or assimilation of crustal rocks, contamination of mantle-derived basalts by non-crystalline residua of granite crystallization, or any process involving fractional crystallization of feldspar. Their peculiar geochemical characteristics must be a direct function of their origin in the mantle. Ultrapotassic rocks, kimberlites, and, to a lesser extent, carbonatites are strikingly similar to minettes in their rare earth and other trace element contents, suggesting genetic links among these rock types. It is difficult to explain the temporal and spatial constancy of this similarity by post-anatectic late enrichment of diversely produced magmas by volatile transport. We tentatively propose that the process best able to account for their unique geochemistry is limited partial melting of the subcontinental mantle following and dependent on the metasomatic introduction of K, Ti, Fe, REE, halogens, P, and other elements as well as H2O and/or CO2. If the enriched mantle is H2O-rich, minette magma is produced; if it is CO2-rich or has an intermediate CO 2 H 2 O ratio, carbonatitic-kimberlitic and/or ultrapotassic magmas result.

Duration of hydrothermal activity at an oceanic spreading center, Mid-Atlantic Ridge (lat 26°N)

Hydrothermal manganese oxide coats talus on the Mid-Atlantic Ridge at lat 26/sup 0/N until spreading moves the rock past the thermally and structurally active rift-valley wall. Hydrothermal activity is replaced by hydrogenous ferromanganese oxide precipitation on ocean crust older than 0.7 m.y. on the ridge-crest highlands.