Introduction & meeting overview
Department of Earth Science, University of Durham
The present and first meeting of U.K. (non) Plume Skeptics is held in response to a recent upsurge in interest in models for melting anomalies other than the plume model. Two significant resources that are available to those interested are materials relating to the Penrose conference Plume IV: Beyond the Plume Hypothesis, held in Iceland in 2003, and the website www.mantleplumes.org which is open for contributions advocating and presenting both pro-plume and pro-alternative viewpoints. A forthcoming activity that may be of interest is the “Great plume debate” meeting, planned to be held in Scotland, September 2005, convened by Ian Campbell (ANU) and Gillian Foulger (Univ. Durham).
The present meeting will feature presentations and discussion on correlations between magmatism and tectonics with particular focus on the African plate. Such correlations are commonly seen e.g., at Iceland and are heavily dependent on age dating, which always seems to be less complete than desirable. Impact theory for explaining large igenous provinces (LIPs) has been revitalised by new calculations of potential magma volumes by Adrian Jones and David Price. This is a promising development in view of the fact that no theory, plume or otherwise, is presently able to explain the magma volumes and eruption rates observed. The possible correlation of volcanic margins with sutures may be important as volcanic margins are often associated with LIPs and volcanic chains. Continental breakup is also associated with vertical crustal motion, and uplift prior to breakup and the formation of LIPs and volcanic margins is considered by some to be a plume diagnostic. Such motions on the shelf of Great Britain have been studied in detail.
The origin of ocean island basalts (OIB) is a problem for both plume and alternative models. Various sources have been proposed, including an origin in the crustal and/or mantle lithosphere portion of subducted slabs, continental mantle lithosphere, mantle wedge, streaky mantle and separate “reservoirs” in the upper and lower mantles. The depth of origin of OIB is unclear and virtually every depth in the Earth's mantle has been suggested. There is still disparity of opinion regarding the potential temperature of the mantle at “hot spots”. Even at a single hot spot, using fairly similar methods, very different estimates are defended. Some methods that have been widely assumed to give robust results, e.g., seismic tomography and the study of picrites, are ambiguous. At Iceland, for example, estimates of mantle potential temperature anomaly range from zero to ~ 200 K. Models for melting anomalies require knowledge of the amount of anomalous melt, and this also is not well understood. For example, at Iceland, seismology suggests that up to ~ 40 km of melt may be produced by steady state processes, whereas considerations of tectonics and isostasy suggest that only half of this may be melt. A particularly critical question is whether such a melt volume can be produced by an anomalously fertile source, or whether high temperature is required.