An integrated kinematic and geochemical model to determine lithospheric extension and mantle temperature from syn-rift volcanic compositions

Abstract

We present an integrated kinematic and geochemical model that determines the composition of melts and their residual source rocks generated by decompression melting of the mantle during continental rifting. Our approach is to construct a unified numerical solution that merges an established lithospheric stretching model which determines the rate and depth at which melting occurs, with several compositional parameterisations of mantle melting to predict the composition of primary melts. We also incorporate a parameterisation for the rare earth elements. Using our approach, we are able to track the composition of the melt fractions and mantle residues as melting progresses. Our unified model shows that primary melt composition is sensitive to rift duration and mantle temperature, with rapid rifting and higher mantle temperatures producing larger melt fractions, at a greater mean pressure of melting, than slower/cooler rifting. Comparison of the model results with primitive basalts recovered from oceanic spreading ridges and rifted margins in the North Atlantic indicates that rift duration and synrift mantle temperature can be inferred independently from the appropriate geochemical data.

Introduction

Melting of the mantle occurs at rifted continental margins in response to decompression as mantle rocks are brought toward the surface by lithospheric extension. This melting is controlled by several parameters including: the initial mantle temperature and source composition; the rate of continental extension; the final degree of lithospheric stretching (β_max); and the initial lithospheric thickness (Bown and White, 1995). Several studies have attempted to estimate some of these parameters from estimates of rift-related magmatic volume (e.g., (Whitmarsh et al., 2001, Hopper et al., 2003, Barton and White, 1997, Kelemen and Holbrook, 1995, Niu and Batiza, 1991)). This volume is usually estimated from wide-angle seismic data. Igneous products, added to the lower crust (i.e., underplated or intruded complexes) are identified as regions of high seismic velocity (White and McKenzie, 1989) relative to normal continental crust (e.g., (Christensen and Mooney, 1995)). Extrusive volcanics (i.e., flood basalts) are identified as high amplitude seaward dipping reflector sequences on the continent-ocean transition. Examples of both intrusive and extrusive magmatic rocks are found on “volcanic” margins such as the East Greenland margin (Korenaga et al., 2000); (Holbrook et al., 2001) the Faeroes margin and Hatton Bank (White et al., 2008). In contrast, such features are absent at “non-volcanic” rifted margins, and the presence at some of these margins of exhumed serpentinized mantle, well-documented by both geophysical studies and by direct sampling in the southern Iberia Abyssal Plain (e.g., (Dean et al., 1999); (Whitmarsh et al., 2001)), shows that melting can be suppressed by different syn-rift conditions.

In both types of rifted continental margin, there are difficulties in measuring accurately the volume of magmatic products using seismic reflection techniques alone. This is due to the high impedance contrast between magmatic intrusions and the surrounding lower continental crustal material, as well as limited spatial resolution, which makes it difficult to image small and discontinuous bodies of intruded material. However, it is possible to put constraints on the total intrusive volume from the average lower-crustal velocity if the end member velocities of the pre-existing crust and the intrusive rocks are known (White et al., 2008). More importantly, the volume of magmatic products is not constrained uniquely by the tectonic process alone; rather it is also subject to variations in mantle temperature, composition and strain rate.

An alternative approach to determining the rifting conditions (encompassing tectonic geometry, strain rate, mantle temperature and composition) is to use the composition of the volcanic products and, where available, upper mantle residues. (Williamson et al., 1995) showed, through modelling, how the rare earth element compositions of syn-rift volcanics on the Labrador margin of eastern Canada and the North Sea Rift are sensitive to mantle temperature, the degree of stretching and the duration of rifting. Using a similar principle, we have developed an integrated kinematic and geochemical model to determine the geochemical compositions of syn-rift melts and their residual mantle source rocks under a variety of rifting conditions. From this model, we demonstrate that the major, minor and rare-earth element geochemistry of the syn-rift melts and their mantle residues are sensitive independently to the extension history and mantle temperature. By inverting the approach, we compare the compositions of syn-rift volcanic products from around the North Atlantic margin with those predicted by our model and hence to infer the most probable rifting conditions prevalent during their genesis.