THE MEANING OF MEAN-F - CLARIFYING THE MEAN EXTENT OF MELTING AT OCEAN RIDGES

In a recent paper, Forsyth (1993) concludes that fractional melting le ads to unexpected relationships between the degree of melting (F), cru stal thickness, and the depth of melting beneath mid-ocean ridges. Spe cifically, he suggests that a commonly cited rule of thumb, that 10% m ean melting of a 60-km column of mantle leads to 6 km of crustal thick ness (Klein et al., 1991; Langmuir et al., 1992), is incorrect for fra ctional melting of the mantle. Here we show that the rule of thumb rem ains valid for Langmuir et al.'s definition of mean F and that confusi on has arisen because there has been disagreement on the definition of mean F. Plank and Langmuir (1992) have defined mean F as the ratio of the mass flux of melt added to the oceanic crust to the mass flux of mantle entering the melting region; Forsyth (1993) has defined mean F as the average degree of melting of all pooled melt increments, with d egree calculated at the last point of chemical equilibration. We show here that both definitions of mean F are valid conceptually and mathem atically, clarify the differences between them, show how they relate d ifferently to observables such as crustal thickness and crustal compos ition, and propose nomenclature to clarify usage in the future (F-B fo r Plank and Langmuir's bulk melt fraction and F-V for Forsyth's mean v alue).