Rare-Earth variations across ‘normal segments’ of the Reykjanes Ridge, 60°-53°N, Mid-Atlantic Ridge, 29°S, and East Pacific Rise, 2°-19°S, and evidence on the composition of the underlying low-velocity layer

Abstract

Supplement is available with entire article on microfiche. Order from American Geonhvsical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J75-002; $1.00. Payment must accompany order. Rare-earth (RE) abundance patterns in 26 basalts occurring across normal segments of the Reykjanes ridge, 60°–53°N; the mid-Atlantic ridge, 29°S (Deep-Sea Drilling Project leg 3); and the East Pacific rise, 2°–19°S, including the Nazca plate, are reported. Without exception, all the RE patterns were found to be characteristically light RE depleted, reflecting to a first-order approximation, a mantle source with uniquely defined RE content. Computer multiphase and eutectic, equilibrium-partial melting models show that the reconstituted RE pattern of the mantle underlying these regions must be more depleted in light RE than the basalts generated. This is so whether a lherzolite (spinel free or bearing), garnet peridotite, or plagioclase peridotite is considered for the primary mantle source. The source is thought to be the low velocity layer apparently depleted in light RE and other large ionic lithophile elements. A lherzolite phase assemblage is found to be preferable for the depleted low-velocity layer source under normal midocean ridge segments at a depth greater than 30–38 km. Above 30–38 km, equilibration of normal ridge basalts with a plagioclase peridotite cannot be ruled out. Rare-earth results across these three normal ridge segments further emphasize (1) the spatial and temporal RE uniformity of the depleted low-velocity layer source, (2) the widespread geographic coverage of this mantle source, (3) its RE composition contrast with mantle source regions, such as those beneath Iceland, the Azores, and the Afar, which have been assumed to represent rising plumes (or blobs), and finally (4) the passive nature of volcanism along normal ridge segments occurring in response to the spreading as a simple ‘healing’ process.