Melting anomalies: Tp, Xc, or v?

John Maclennan

School of GeoSciences, University of Edinburgh


Melting within the mantle is controlled by the temperature, composition and flow rates of the solid mantle. If mantle upwelling under mid-ocean ridges is driven by plate spreading alone then variations in oceanic crustal thickness can be related to variations in mantle temperature and composition. Parameterisations of peridotite melting, based on the results of many experimental studies, have shown that the standard oceanic crustal thickness of 7 km can be generated by melting of mantle with a potential temperature (Tp) of close to 1300°C. Higher crustal thicknesses, such as those proposed for Iceland (20-40 km) can be generated by melting hotter peridotite, with 20 km crust produced at Tp of 1470°C and 40 km at Tp >1600°C. An alternative mechanism for generating such large oceanic crustal thicknesses is by melting mixtures of recycled basalt and peridotite. At a given Tp such mixtures melt to greater extents and start to melt deeper than peridotite alone. An initial effort was made to quantify the crustal thickness variations that may result from adiabatic decompression melting of such mixtures. Model calculations indicate that the 70/30 peridotite/MORB mixture of Yaxley (2000) can generate a crustal thickness of 20 km at a Tp of 1390°C or 40 km at a Tp of 1510°C. Addition of recycled MORB to the Icelandic source cannot account for the variation in crustal thickness reported, and is likely to be coupled to variations in mantle temperature or upwelling rate. Two important caveats must be placed upon this conclusion. Firstly, there are very few experimental constraints upon the melting peridotite/MORB mixtures. Secondly, the relationship between the Moho recovered from modelling of seismic data from Iceland and the thickness of melt generated is not understood in detail.

Yaxley, G.M., 2000. Experimental study of the phase and melting relations of homogeneous basalt + peridotite mixtures and implication for the petrogenesis of flood basalts, Cont. Min. Pet., 139, 326-338.