James Brewster Kellogg

THERMAL CONSTRAINTS ON THE SURVIVAL OF PRIMITIVE BLOBS IN THE LOWER MANTLE

Geochemical models have frequently divided the mantle into depleted upper and undepleted lower mantle reservoirs, usually taken as indication for a layered style of convection. This is difficult to reconcile with seismological and geodynamical evidence for substantial mass flux between lower and upper mantle. Various models have been proposed to jointly interpret the evidence, including that of G.F. Davies [J. Geophys. Res. 89 (1984) 6017‐6040] in which the author suggested that lumps of primitive material may exist in the lower mantle, representing reservoirs for undepleted basalts. Mixing calculations have suggested, however, that such blobs could not survive 4 Ga of convection. Calculations by M. Manga [Geophys. Res. Lett. 23 (1996) 403‐406] on the other hand showed that high-viscosity blobs could persist in convective cells for geologically long times without being substantially deformed and mixed with the surrounding flow. We investigate a blob model of convection based on these ideas and consider dynamical, thermal, geochemical and rheological consequences. The radiogenic heat production in the primitive blobs would lead to higher temperatures. However, these would be modest (1T < 300 K) for sufficiently small blobs (radius<800 km). The resulting thermal buoyancy can be offset by a small intrinsic density excess (<1%) so that blob material is hidden from the ridges but sampled by rising plumes. To satisfy geochemical constraints, blobs would have to fill 30% to 65% of the mantle (less if they are taken to be enriched rather than primitive). Thermal considerations require that they be surrounded by depleted material of lower viscosity that would convectively transport heat to the surface. The thermal decrease in blob viscosity would be about one order of magnitude but constrained to the interior; the stiffer ‘shell’ can then be expected to control the dynamical mixing behavior. On average, the viscosity of the lower mantle would be increased by the presence of the blobs; if they were 100 times more viscous than the surrounding mantle the net effect would be to increase the effective viscosity approximately

Modeling the distribution of isotopic ratios in geochemical reservoirs

Abstract We present an extension of the conventional geochemical reservoir model for the evolution of the Earth’s crust–mantle system in which we calculate not only the mean isotopic ratios, but also the distribution of those ratios within the reservoirs. Owing to low chemical diffusion rates, subreservoirs that are created by mass transport into and out of the mantle effectively exist as distinct geochemical entities for all time. By tracking these subreservoirs, we obtain a model of the full range of isotopic values represented in the mantle. Using results from numerical calculations of mixing, we also track the length scales associated with each subreservoir. Applying simple statistics, we obtain the distribution of expected measurements as a function of the stirring time, effective melt fraction, sampling volume, and mass transport history. We present calculations of isotopic heterogeneity for two simple mantle evolution models and explore the sensitivity of geochemical observables to the variables mentioned. We focus on the Rb–Sr and Sm–Nd systems and are able to reproduce much of the observed complexity of oceanic basalts. We infer that the stirring time of the mantle falls between 250 and 750 Myr, and that the initial length scale of mantle heterogeneity before stirring is of the same order as the length scale of sampling. We also conclude that the differences between isotopic data from mid-ocean ridge basalts and ocean island basalts cannot simply be due to differences in sampling volume, but must also reflect differences in the source reservoirs and/or melting processes. Increasing the size of the ocean island basalt source region by 45% with respect to the mid-ocean ridge basalt source region reproduces the offset between the two distributions, but still fails to explain the more isotopically extreme measurements. Our results show that the argument suggesting that the absence of samples with a primitive isotopic signature indicates that no primitive material remains in the mantle is not valid.

Neoproterozoic sand wedges: crack formation in frozen soils under diurnal forcing during a snowball Earth

Thermal contraction cracking of permafrost produced sand-wedge polygons at sea level on the paleo-equator during late Neoproterozoic glacial episodes. These sand wedges have been used as evidence for high (v 54‡) paleoobliquity of the Earth’s ecliptic, because cracks that form wedges are hypothesized to require deep seasonal cooling so the depth of the stressed layer in the ground reaches v 1 m, similar to the measured depths of cracks that form wedges. To test the counter hypothesis that equatorial cracks opened under a climate characterized by a strong diurnal cycle and low mean annual temperature (snowball Earth conditions), we examine crack formation in frozen ground subject to periodic temperature variations. We derive analytical expressions relating the Newtonian viscosity to the potential crack depth, concluding that cracks will form only in frozen soils with viscosities greater than V10 14 Pa s. We also show numerical calculations of crack growth in frozen soils with stress- and temperature-dependent rheologies and find that fractures may propagate to depths 3^25 times the depth of the thermally stressed layer in equatorial permafrost during a snowball Earth because the mean annual temperature is low enough to keep the ground cold and brittle to relatively great depths. 9 2002 Elsevier Science B.V. All rights reserved.

MYRES: A program to unite young solid earth researchers

The first Meeting of Young Researchers in the Earth Sciences (MYRES-I), held in August of 2004, focused on ?Heat, helium, hotspots, and whole mantle convection.? Biennial meetings, with MYRES-I as the first, are one of the ways the MYRES initiative is building an ?international, interdisciplinary, open and unbiased community of colleagues who interact regularly to informally exchange ideas, data, and tools, and formulate new collaborative research projects? (see Young Solid Earth Researchers of the World Unite! published in Eos, 85(16), 160, 2004). This article reports on our first workshop, discusses what is happening in the community, and calls for proposals to keep MYRES funded. A New Meeting Concept. The MYRES meetings are organized by, and for, junior members of the solid Earth research community. In 2004, funding from the National Science Foundation (NSF), the European Science Foundation (ESF) and The Scripps Institution of Oceanography enabled the initiative to nearly fully fund this meeting with a diverse and international crowd of nearly 100 participants selected from an oversubscribed pool.

Young scientists focus on the dynamics of the lithosphere

Young researchers face a wide choice of scientific approaches and directions that may shape their careers. The Earth sciences, in particular, offer a broad range of topics to study and techniques to use that exceeds what any current scientist was exposed to at the schools they attended. At early stages of their careers, researchers need to gain confidence in their expertise, and they often can benefit by expanding their scientific horizons and forging new collaborations