Abstract Seafloor and passive margins gradually subside as a result of thermal contraction of the underlying lithosphere. Thermal subsidence is also an attractive mechanism for the Michigan basin. For subsidence to occur within this previously stable continental region, some mechanism is needed to heat the lithosphere and to reduce the buoyancy of the continental crust. Mechanical stretching of the lithosphere along with its crust does both at the same time. The ponding of plume material beneath the crust supplies heat, but does not directly thin the crust. The British Isles and the Congo basin provide analogies to the Michigan basin. Continental stretching before subsidence is evident within the British Isles and the Congo basin. Stretching is not obvious in Michigan, but undetected rifts extending from the Iapetus break-up margin may exist. A closed region of thin lithosphere beneath the Irish Sea may have trapped hot buoyant material from the Iceland plume; the Michigan basin may have trapped material from an Iapetus age plume near Montreal. Cratonic basins provide information on the tail of the subsidence curve, unlike more ephemeral oceanic crust and passive margins. The poorly resolved tails in the Michigan and the Congo basins are consistent with the time subsidence constant, c . 280 myr predicted by stagnant lid convection formalism.

Hotspots are regions of voluminous volcanism, such as Hawaii and Iceland. However, direct tests by mid-mantle tomography and petrology of lavas do not conclusively resolve that mantle plumes and mantle high temperature cause these features. Indirect and model-dependent methods thus provide constraints on the properties of plumes, if they exist. Classical estimates of the excess (above mid-ocean ridge basalt values) potential temperature of the upwelling mantle (before melting commences) are 200–300 K. The estimate for Iceland utilizes the thickness of the oceanic crust. Estimates for Hawaii depend on the melting depth beneath old lithosphere. Flux estimates for Iceland depend on the kinematics of the ridge and those of Hawaii or the kinematics and dynamics of the Hawaiian swell. These techniques let one compute the global volume and heat flux of plumes. The flux is significant—approximately one-third of the mantle has cycled through plumes, extrapolating the current rate over geological time. However, obvious vertical tectonics may not be evident for many hotspots. For example, the relatively weak Icelandic hotspot (one-sixth of the volume flux of Hawaii) would produce only modest volcanism and swell uplift if it impinged on the fast-moving Pacific plate or the fast-spreading East Pacific Rise.

Models for noble gases in the Earth's mantle are evaluated against a number of observational constraints: (1) high 3 He/ 4 He ratios do not correlate with high (initial) 3 He concentrations; (2) the 3 He/ 4 He data for mid-ocean ridge basalts and ocean island basalts do not represent two different distributions (Anderson 2001); (3) globally robust correlations between 3 He/ 4 He ratios and lithophile isotopic systems are not observed; (4) diverse local correlations exist that are broadly linear; (5) large, local geographical 3 He/ 4 He variations are observed, which are inconsistent with a strongly localized (i.e., plume-stem) flux of high- 3 He/ 4 He material; and (6) dramatic temporal 3 He/ 4 He variations are observed on very short time scales (10 2 years). Layered (reservoir) models for noble gases, in which a deep and radially constrained region of the Earth's mantle preserves unradiogenic He and Ne isotopic compositions because of a high noble gas concentration, do not seem consistent with these observations. Heterogeneous (nonlayered) mantle models for noble gases, in which the carrier of unradiogenic He is a relatively noble gas–poor component scattered in the (upper) mantle, appear more consistent with the constraints. We propose that the carrier of unradiogenic noble gases is primarily olivine. Olivine-rich lithologies, produced in previous partial melting events, are a natural part of the statistical upper mantle assemblage (SUMA), a highly heterogeneous assemblage of small- to moderate-scale (∼1–100 km) enriched and depleted lithologies with a wide range of chemical composition, fertility, age, and isotopic signatures (Meibom and Anderson, 2004). The isotopic signatures of oceanic basalts, including noble gases, are obtained by partial melting of the SUMA under slightly different pressure and temperature ( P-T ) conditions, i.e., different degrees of partial melting and different degrees of homogenization prior to eruption (Morgan and Morgan, 1999; Meibom and Anderson, 2004; Rudge et al., 2005; Ito and Mahoney, 2005). Unradiogenic noble gas isotopic compositions are not tracers of deep-mantle components in the source materials of oceanic basalts. Noble gas isotopic compositions may, however, indirectly indicate potential temperature, because the order in which different upper-mantle lithologies melt depends on the P-T conditions.

Abstract The continental cycle of silicate weathering and metamorphism dynamically buffers atmospheric CO 2 and climate. Feedback is provided by the temperature dependence of silicate weathering. Here we argue that hydrothermal alteration of oceanic basalts also dynamically buffers CO 2 . The oceanic cycle is linked to the mantle via subduction of carbonatized basalts and degassing of CO 2 at the mid-ocean ridges. Feedback is provided by the dependence of carbonatization on the amount of dissolved carbonate in sea water. Unlike the continental cycle, the oceanic cycle has no thermostat. Hence surface temperatures can become very low if CO 2 is the only greenhouse gas apart from water. Currently the continental cycle is more important, but early in Earth’s history the oceanic cycle was probably dominant. We argue that CO 2 greenhouses thick enough to defeat the faint early Sun are implausible and that, if no other greenhouse gases are invoked, very cold climates are expected for much of Proterozoic and Archaean time. We echo current fashion and favour biogenic methane as the chief supplement to CO 2 . Fast weathering and probable subduction of abundant impact ejecta would have reduced CO 2 levels still further in Hadean time. Despite its name, the Hadean Eon might have been the coldest era in the history of the Earth.