Evolution of textures, crystal size distributions and growth rates of plagioclase, clinopyroxene and spinel crystallized at variable cooling rates from a mid-ocean ridge basaltic melt

Mid-ocean ridge basalts (MORBs) are the most abundant eruptive tholeiitic products on Earth. Many experiments
have been performed to investigate the solidification of basalts but under limited thermal ranges of cooling (ΔTc)
and cooling rates (ΔT/Δt). We analyze the experimental charges solidified from previous studies: the BIR1A
basalt from USGS was solidified using ΔT/Δt of 1, 7, 60, 180, 1800 and 9000 °C/h, in the ΔTc between 1300 and
800 °C, at atmospheric conditions. The previous studies allowed determining the glass-forming ability (GFA) of
sub-alkaline silicate liquids, but do not give information on their textures. Here, we quantify the evolution of
sizes, shapes, number of crystals per area (#/A), CSDs and growth rates (Gs) of plg (plagioclase), cpx (clinopyroxene)
and sp. (spinel). Textures were investigated by image analysis on thousands of crystals and are one of
the most complete datasets ever obtained from laboratory studies: they reflect rapid, intermediate and sluggish
cooled parts of MORB from liquidus to solidus.
Faceted plg grows only at ΔT/Δt ≤ 60 °C/h, while cpx and sp. became dendritic at ΔT/Δt between 60 and
180 °C/h. As ΔT/Δt increase, crystal size ranges decrease from 1000 to 10 μm at 1 °C/h to 100–1 at 60 °C/h μm
for plg, from 400 to 8 μm at 1 °C/h to 25–0.5 μm at 1800 °C/h μm for cpx, and from 90 to 6 μm at 1 °C/h to 6–0.5
at 1800 °C/h μm, for sp. The #/A increases with increasing ΔT/Δt, except for cpx between 60 and 180 °C/h. As
ΔT/Δt increases, CSDs of plg, cpx and sp increase their slopes (m) and population densities per size (n0), reduce
the size ranges and tend to be log-linear. At low ΔT/Δt, CSDs are composed of several log-linear segments, which
slopes are related to different pulses of crystal nucleation, and subsequent growth by coarsening.
The CSDs parameters (slope, m, and nucleation density per size, n0) linearly scale each other and both are
highly correlated with ΔT/Δt. Maximum (Gmax) and average (GCSD) growth rates are computed respectively by
averaged major axis (Lmax) of the five longest crystals and by the m of CSDs. Both the Gs are a function of
experimental time (t) and increase with the increasing of ΔT/Δt, changing up to two orders of magnitude. The
Gmax of cpx is correlated with m and n0 and can be used in natural MORB to retrieve either ΔT/Δt and Gmax. The
plg and cpx crystals with sizes between 0.1 and 1 mm are abundant in the experimental charges obtained at low
ΔT/Δt. In volcanic rocks, these crystal sizes are generally considered representative of intra-telluric conditions
(phenocrysts and microphenocrysts). Our data demonstrate that crystals with mm-sizes may also grow in syn-topost-
depositional conditions. The continuous evolution of textures in response to ΔT/Δt variations implies that
kinetic effects can fully capture the solidification of MORBs. As a result, the widely accepted assumption that
phenocrysts represent the products of evolution processes in volcanic conduits or magma reservoirs could be not
valid for some basaltic lavas.