Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge.

Chao Zhang,Harald Behrens,Niko Götze,Clemens Kirchner,Juergen Koepke

doi:10.1038/srep06342

Abstract

Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion. According to the widely discussed “gabbro glacier” model, the formation of the lower oceanic crust requires efficient cooling of the axial melt lens, leading to partial crystallization and crystal-melt mush subsiding down to lower crust. These processes are believed to be controlled by periodical magma replenishment and hydrothermal circulation above the melt lens. Here we quantify the cooling rate above melt lens using chemical zoning of plagioclase from hornfelsic recrystallized sheeted dikes drilled from the East Pacific at the Integrated Ocean Drilling Program Hole 1256D. We estimate the cooling rate using a forward modelling approach based on CaAl-NaSi interdiffusion in plagioclase. The results show that cooling from the peak thermal overprint at 1000–1050°C to 600°C are yielded within about 10–30 years as a result of hydrothermal circulation above melt lens during magma starvation. The estimated rapid hydrothermal cooling explains how the effective heat extraction from melt lens is achieved at fast-spreading mid-ocean ridges.

Highlights

Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion
In order to examine the validity of the estimated cooling rate above the axial melt lens, we performed a simple heat balance calculation to compare the total heat released from the melt lens and that extracted from the overlying hornfelsic zone during cooling
The calculation assumes (i) that all the heat originated from cooling and partial crystallization of the melt lens is released into the overlying hornfelsic zone which acts as a conductive boundary layer, and (ii) that the timescales of replenishment and starvation of the melt lens is identical to each other

Summary

Introduction

Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion. The ‘‘gabbro glacier’’ model depicts that magma crystallization takes place exclusively in the axial melt lens at the base of sheeted dikes, while the ‘‘sheeted sill’’ model predicts supplementary sill-shaped crystallization sites below the melt lens These two endmember models have contrasting thermal structures[14] and the cooling rates at comparative depths of the MOR crust should be very different[15]. Thermal modelling of fast-spreading ridges for a static melt lens indicates that the off-ridge cooling rate at the roof of the melt lens decreases with distance from the ridge centre and the largest temperature gradient occurs at the vicinity of melt lens[15] This implies, for a dynamic melt lens model, that heat loss within the recrystallized sheeted dike overlying a retreating melt lens should be very intense. The off-ridge cooling proceeds, from about 600uC to 100uC within a relatively short spreading distance from the ridge centre, being about 6 km for a half fast-spreading rate of 100 mm/yr (Fig. 3)

Methods

Results

Conclusion