PARTIAL MELTING OF MAFIC LOWER CRUST: FAST AND MASH-FREE?

The effects of short term periodicity in magmatically-supplied heat on the rate and amount of partial melting of mafic, lower continental crust, has been quantified numerically for a number of key variables including maximum (average) melt fraction, height of the melting column, heat flux and melt temperature. For the well known case of a single discrete intrusion, conductive heat loss through the lower boundary will led to partial melting and possible contamination by buoyant upwelling of crustal melt from below. Multiple overaccreted intrusions demonstrate this effect, but the influence decreases with time. Average granitic (s.l.) partial melt temperatures generally exceed 900°C, and maximum temperatures may exceed 1000°C. The degree of partial melting, governed by the initial intrusion temperature and the periodicity, yields a maximum predicted average melt fraction of 0.38. There is a time lag between maximum melt production and the maximum height reached by the melt column, with melt fractions in excess of 0.2 generated only where the interval between each new intrusion is £200 years. Predicted maximum melt thicknesses do not exceed ca. 100 m, implying that large, melt-dominated (MASH-type) magma chambers will not form at depth during partial melting of mafic lower crust by intruding basalt magma.

The ratio of the period of intrusion and the characteristic timescale for heat loss defines an important variable (R) that can be used to assess the thermal behaviour of the melting column, with both melt temperature and average melt fraction maximised where R=1. These results suggest that the cumulative periodic influx of heat over a sustained, but geologically short, (c. 200 - 3 x 10⁵ yr.) interval of time, as opposed to a single (one-off) heating event, provide optimal conditions for partial melting of mafic lower crust in continental arc (and other) tectonic settings. Such a multiple intrusion mechanism is arguably in keeping with seismic evidence for a layered lower crust. Any liquid reservoir that forms at depth through partial melting of mafic lower crust in the way described is likely to be of high aspect ratio and transient.