The axial melt lens as a processor of evolved melts at fast-spreading mid-ocean ridges

Abstract:

The axial melt lens is a steady-state, generally magma-rich body located at the dyke-gabbro transition at mid-crustal levels beneath intermediate- and fast-spreading ridges. It is widely believed to be the reservoir from which mid-ocean ridge basalt (MORB) erupts. Using a remotely-operated vehicle, cruise JC21 to the Hess Deep Rift recovered the first comprehensive sample suite of the uppermost plutonics from a fast-spreading ridge. We present the results of a detailed microanalytical investigation of 23 samples (8 dolerites, 14 gabbronorites, and 1 gabbro) recovered by ROV dive 78 from a section traversing the transition from the uppermost gabbros into the sheeted dykes. With the exception of a single olivine-bearing sample (78R-6), dive 78 is dominated by evolved, varitextured (both in hand sample and thin section) oxide gabbronorites. Full thin section quantitative element maps were acquired on serial thin sections from each sample using the analytical scanning electron microscope in the at Cardiff University. The resulting maps were post-processed in Matlab^TM to determine the full distribution of plagioclase compositions across entire thin sections (typically 500,000 analyses per sample); an approach we term ‘quantitative assessment of compositional distribution’ (QACD). By so doing we are able to conduct the first fully rigorous assessment of gabbro compositions, and, by extension, melt compositions present at this level beneath the ridge axis. Critically, we only found 2 grains of high-An plagioclase (An>80) in all of the samples (N = 51). These occur as cores within a sample dominated by lower-An plagioclase. Instead, the vast majority (75%) of plagioclase within the samples have compositions of An₆₅ or lower; compositions too evolved to be in equilibrium with MORB. The most primitive sample, 78R-6, is an olivine-bearing gabbronorite with Fo₆₇ olivine, and plagioclase ranging from An_52-77 (median An = 65). These data are difficult to reconcile with models in which primitive melts fed from the mantle reside in the melt lens and crystallize significant gabbroic material. If MORB was sourced from the melt lens at Hess Deep, the paucity of primitive compositions in the upper plutonics suggests that their residence time may be sufficiently limited to prevent substantial crystallization.