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The rythm of magmatism under a mid-oceanic ridge in the Pacific

The magma reaching the surface and cooling under mid-oceanic ridges brings new solid material to the oceanic crust where tectonic plates diverge. These deep phenomena occur so slowly that only indirect methods can be used to study them. The objective of this work was to estimate the recharge periodicity of the chamber where magma differentiates and crystallizes before spreading over the seabed as lava. The approach was to apply a mathematical model describing the geochemical evolution of magma through time; according to the conditions of differentiation within the reservoir, the evolution of a same magmatic source can indeed lead to lava of various chemical compositions.

The selected study area was the southern part of the Juan de Fuca ridge, in the north-eastern Pacific. Over about 60 km, this part called Cleft segment is the limit between two plates diverging at a rate of 5.6 cm/yr. Seismic imagery has shown that this part of the ridge is fed by magmatic lenses ~10 km long located about 2 km beneath the seafloor. These lenses pass through successive stages of evolution, within a cycle of magma refilling followed by slow emptying. The southernmost lens is thought to be ~100 m thick and ~900 m wide, and to contain 30% magma not yet crystallized.

 

Cordier_fig2
The study area, between 2000 and 3500 m deep in the NW Pacific. The input parameters of the mathematical model are geochemical data measured on lavas collected along Cleft segment (red dots) and during submersible dives along a fault scarp perpendiculary to it (green lines)

 

Among all the chemical elements present in the melt and the lavas, the group called "rare earths" was selected for the analysis as the properties of these elements prevent them from being incorporated into the crystals that form and grow as the magma cools: their concentration is therefore an indicator of the degree reached by crystallization in the chamber.

A model of the geochemical evolution of the magma was applied to the study site. This model uses a sinusoidal function to describe the fluctuations through time of the amount of magma within the reservoir. The rare earth concentration reaches a minimum just after the injection of new magma in the chamber, then progressively increases between two refilling episodes owing to the fact that these elements are excluded from fractionated crystallization. The model uses rare earth composition of lavas collected during submersible dives and whose relative age could be estimated from their vertical position along the scarp wall. Progressing backwards along the geochemical processes, the model traces back their source to a common source magma whose composition is consistent with results obtained from previous studies.

The parameters deduced from the geometry of the crust and of the magmatic system beneath Cleft segment were then incorporated into the model in order to estimate reservoir evolution. Two opposite scenarios of magmatic chamber functioning were considered, according to the way crystals form and grow: either in a single melt lens where they flow downwards vertically and laterally, or in situ within a network of connected and sheeted sills.

 

Cordier_fig1
Theoretical magma chamber considered in the mathematical modelling (top) with melt (dark grey) refilling from below, crystallizing magma (light dotted grey) and lava erupting at the top. Below, the two extreme models considered for a magmatic chamber located under the oceanic crust.

The results of the model suggest that this magmatic chamber underneath Juan de Fuca ridge is refilled every 1100 years and that the melt resides about 100 years within the reservoir.

 

The paper

Cordier C., Caroff M., Rannou E., 2012. Timescale of open-reservoir evolution beneath the south Cleft segment, Juan de Fuca ridge. Mineralogy and Petrology 104 (1-2): 1-14.

 

The authors

This work is the result of the collaboration of scientists from the laboratory Domaines océaniques (IUEM) and the Mathematics laboratory of Université de Bretagne Occidentale (UBO).

 

The journal

Mineralogy and Petrology was founded in Germany in 1872 and was since published under several different names; it is one of the oldest scientific journals in earth Sciences. This international journal is now published by Springer. Its multidisciplinary coverage includes the various fields of mineralogy, petrology and geochemistry, in an attempt to counteract increasing fragmentation in the earth sciences.

 

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