Magma dynamics at mid-ocean ridges by noble gas kinetic fractionation: Assessment of magmatic ascent rates

Despite its impact in understanding oceanic crust formation and eruptive styles of related volcanism, magma dynamics at midocean
ridges are poorly known. Here, we propose a new method to assess ascent rates of mid-ocean ridge basalt (MORB) magmas,
as well as their pre- and sin-eruptive dynamics. It is based on the idea that a rising magma can reach a variable degree of both CO2
supersaturation in melt and kinetic fractionation among noble gases in vesicles in relation to its ascent rate through the crust. To
quantify the relationship, we have used a model of multicomponent bubble growth in MORB melts, developed by extending the
single-component model of Proussevitch and Sahagian [A.A. Proussevitch, D.L. Sahagian, Dynamics and energetics of bubble
growth in magmas: analytical formulation and numerical modeling, J. Geophys. Res. 103 (1998), 18223–18251.] to CO2–He–Ar
gas mixtures. After proper parameterization, we have applied it to published suites of data having the required features (glasses
from Pito Seamount and mid-Atlantic ridges). Our results highlight that the investigated MORB magmas display very different
ranges of ascent rates: slow rises of popping rock forming-magmas that cross the crust (0.01–0.5 m/s), slightly faster rates of
energetic effusions (0.1–1 m/s), up to rates of 1–10 m/s which fall on the edge between lava effusion and Hawaiian activity. Inside
a single plumbing system, very dissimilar magma dynamics highlight the large differences in compressive stress of the oceanic
crust on a small scale. Constraints on how the systems of ridges work, as well as the characteristics of the magmatic source, can
also be obtained. Our model shows how measurements of both the dissolved gas concentration in melt and the volatile composition
of vesicles in the same sample are crucial in recognizing the kinetic effects and definitively assessing magma dynamics. An effort
should be made to correctly set the studied samples in the sequence of volcanic submarine deposits where they are collected.
Enhanced knowledge of a number of physical properties of gas-bearing MOR magmas is also required, mainly noble gas
diffusivities, to describe multicomponent bubble growth at a higher confidence level.