Role of water in magma generation and initiation of diapiric uprise in the mantle

Abstract

The content and distribution of water is a critical factor in determining mantle properties, especially in the low-velocity and Benioff zones. Estimated temperature distributions vary widely, depending on assumptions regarding the relative significance for heat transfer of conduction, radiation, and convection. Comparison of estimated geotherms with known or inferred phase relationships in the system peridotite-eclogite-water provides information about the possible physical state of the mantle at various depths for comparison with geophysical measurements. Water is stabilized in minerals such as amphibole and phlogopite in the upper mantle; at greater depths, it may exist as intergranular fluid probably adsorbed on mineral surfaces, or it is dissolved in interstitial silicate magma. The most satisfactory explanation for the low-velocity zone involves incipient melting due to traces of water. Water rising from the deep mantle in preferred zones would augment the interstitial silicate magma at the base of the low-velocity zone, initiating the diapiric uprise necessary for basaltic magma generation. A petrological model for the suboceanic mantle suggests that a lens of gabbro-rich material may account for the gravity anomaly and seismic refraction measurements at mid-oceanic ridges. A petrological model for the area beneath island arcs suggests that the seismic low-velocity zone should not be continuous across it. Magma may be generated beneath island arcs by partial fusion of down-going oceanic crust by frictional heating, by migration of water from the lithosphere into overlying mantle, or by diapiric uprise of material from the downgoing lithosphere. Diapiric uprise could be initiated by influx of water into the layer including; the mantle-upper lithosphere boundary over a down-going slab. The composition of the liquid generated depends on many variables, including the water content and the stability of hydrous minerals. Magma generation may depend on the regime of water, rising from the deep mantle in preferred zones and carried down with the lithosphere in Benioff zones.