Emplacement Timescales and Environmental Impacts of Large Igneous Provinces: Constraints From the Re-Os Isotope System
Abstract
Although widespread evidence indicates that the emplacement of 'large' LIPs (>106 km3) was in some way associated with extensive environmental change, it has proved very difficult to determine the precise nature of the link between the two. The unusually large size and rapidity of LIP emplacement make it hard to quantify their environmental impact by reference to the relatively small impact of present-day volcanic activity. For this reason, much research has focussed on interpreting evidence from the marine sedimentary record, which is the major repository of the history of environmental change. Although this approach is the best available, it is far from straightforward. For example, it is very difficult to correlate sedimentary records accurately with the rocks that represent the remnants of LIPs. Whilst the absolute age of LIP emplacement is often well defined from radiometric dating, a consequence of the relative brevity of their emplacement is that precise emplacement rates and durations often remain imperfectly constrained. New geochemical and isotopic techniques now make it possible to overcome some of these difficulties and enable us to identify unambiguously the timing and some of the environmental consequences of LIP emplacement. The Re-Os isotope system is particularly useful in this regard because the Os-isotope composition of juvenile mantle-derived basalts is distinctively unradiogenic (i.e. low 187Os/188Os ratios); also, the seawater Os-isotope composition responds rapidly because the marine Os residence time is relatively short (c. 40 ka). Chemical weathering of large volumes of LIP basalts, soon after their emplacement, thus imparts a characteristic unradiogenic Os-isotope signature to seawater; weathering rates can be estimated from simple mass balance calculations. The Re-Os system also differentiates very effectively the effects of LIP emplacement from those of a bolide impact because large amounts of Re are also released during LIP weathering, whereas bolides with abundant Os contain relatively little Re. The potential of the Re-Os system to further our understanding of LIP emplacement and its consequences will be illustrated using published and new data. For example, Os-isotope data from marine sedimentary successions that span the Triassic-Jurassic and Cretaceous-Paleogene boundaries show unequivocally that the emplacement of the Central Atlantic Magmatic Province and the Deccan traps, respectively, were implicated with contemporaneous environmental change (Cohen & Coe, Geology 2002; Cohen & Coe, Palaeo-3 2007; Ravizza & Peucker-Ehrenbrink, Science 2003). Importantly, in both these cases the isotope data demonstrate that basalt weathering (and hence LIP emplacement) commenced a relatively short but significant time before these two boundaries. Re-Os data also clearly link LIP emplacement with the development of some of the Mesozoic Oceanic Anoxic Events (OAE), for example during the Toarcian (Cohen et al., Geology 2004) and Cretaceous OAE 2 (Turgeon & Creaser, Nature 2008). However, the interpretation of Re-Os data that pertain to OAEs needs to be made with caution because of the competing effects of high continental weathering rates due to global warming and an enhanced hydrological cycle that often occurred at those times.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V24A..01C
- Keywords:
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- 1039 Alteration and weathering processes (3617);
- 4901 Abrupt/rapid climate change (1605);
- 8415 Intra-plate processes (1033;
- 3615)