Cooling rates of mid-ocean ridge lava deduced from clinopyroxene spherulites

Abstract

We present a compositional analysis of spherulites and their surrounding groundmass in basaltic andesite lava samples collected from the Pacific–Antarctic Ridge. The spherulites consist of radiating dendritic branches of clinopyroxene, and contain abundant vesicles that are stretched parallel to the branches. Stretched vesicles require that the spherulites grew when the groundmass was liquid, and thus while it was hotter than its glass transition. Although chemical components incompatible in clinopyroxene should have been enriched in the melt by spherulite growth, no compositional variations occur at a scale larger than the 10μm-wide slit aperture used for laser ablation ICP-MS. The absence of large-scale gradients implies that the spherulites grew faster than the elements could diffuse ~10μm. Because diffusion is temperature controlled, cooling must have been rapid enough to prevent movement. If the spherulites nucleated when the melt was at between 1380 and 1223K (i.e., undercooled by 0 to 157K), then the constant concentrations of Li, Na, and K, the fastest diffusing elements analyzed, require temperature to have cooled faster than 106–7Kmin−1. If, instead, the spherulites nucleated at undercoolings of 357 to 465K (or as cold as 915K, equal to the glass transition temperature), then the cooling rate could have been as slow as those inferred for glassy rinds on submarine lavas by relaxation geospeedometry. At all possible cooling rates, however, the spherulites must have grown as fast as ~103–5μms−1 to reach their final sizes of 1.4 to 3.2mm. We speculate that the rapid growth resulted from low kinetic barriers to clinopyroxene growth in the basaltic andesite melt.