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U-series Constraints on Intraplate Basaltic Magmatism

Laboratoire de Géochimie et Cosmochimie IPGP-CNRS UMR7579, 4, Place Jussieu, 75252 Paris cedex 05 France

Kenneth W. W. Sims

Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02543, USA

The first 20% of the full text of this article appears below.

	1. INTRODUCTION

 
Intraplate magmatism represents approximately one tenth of the flux of magma to the Earth’s surface (Sleep 1990). This type of magmatism has received considerable attention from petrologists and geochemists as it generally exhibits a wider range of chemical compositions than the more uniform mid-ocean ridge basalts. Hence, it is rather paradoxical that our understanding of intraplate magmatism is rather poor. In this chapter, we review the insights that have been gained from using U-series measurements (combined with other chemical and isotopic constraints) to better understand the sources and processes related to intraplate volcanism.

Several unique constraints can be obtained from measurement of U-series disequilibria in basalts. First, U-series fractionation can tell us about the residual phases present during melting as small differences in partitioning behavior between the nuclides will induce distinct signatures. Second, as has been shown by the earlier work of Allègre and Condomines (1982), Th isotope ratios can be used to infer the Th/U ratio of the mantle source providing another useful probe for mapping mantle heterogeneities. Lastly, as detailed below, the time-dependence of U-series fractionation during melting and melt migration can place constraints on several rate-dependent parameters such as the melt production rate, and melt velocities.

An important feature of hotspot magmatism is that in many cases, the timing of hotspot activity seems to be decoupled from the motion of the lithospheric plate. This observation, which has been the basis for proposing the existence of mantle plumes, suggests that magmas erupted at hotspots should reveal something about the nature of the deeper mantle. Understanding the processes of hotspot magmatism should also tell us about the nature of convective motion responsible for hotspots.

In the following section, we first review some of the outstanding issues that need to be resolved to better understand intraplate magmatism. . . . [Full Text of this Article]

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