Vertical and lateral heterogeneity of the upper mantle

Abstract

Abstract Application of pyroxene geobarometry to various suites of ultramafitites (xenoliths and massifs) illustrates lateral and vertical heterogeneities of the uppermost mantle as well as its local petrologic variability throughout time. The upper mantle is best described as a mosaic of large and laterally homogeneous cells with sharp boundaries extending deep into the upper mantle. In addition, local lateral heterogeneities (10–100 km in diameter) of great vertical extent (>200 km) are present at cell boundaries (diapirs: solid-state upwelling) and within cells (plumes: magma impregnation). Progressive vertical petrologic variations may occur within cells, diapirs and plumes. Six different types of sections have been so far identified: infracratonic, suboceanic (ophiolites), Colorado-type (Si-metasomatized infracratonic), meta-somatic (paleo-orogenic; plumes), prerift-type (by differentiation of meta-somatic type) and rift-type (diapirs). Contrasting with the last two types of mantle, dominant (80%) infracratonic and suboceanic types are typically stable sections with density increasing with depth. Shallow upper-mantle cummulate dunites and wehrlites are common; their chemistry reflects some of the characteristics of the overlain metamorphic mantle and suggests genetic ties.