28 research outputs found
Mantle Pb paradoxes : the sulfide solution
Author Posting. Š Springer, 2006. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 152 (2006): 295-308, doi:10.1007/s00410-006-0108-1.There is growing evidence that the budget of Pb in mantle peridotites is largely
contained in sulfide, and that Pb partitions strongly into sulfide relative to silicate melt. In
addition, there is evidence to suggest that diffusion rates of Pb in sulfide (solid or melt)
are very fast. Given the possibility that sulfide melt âwetsâ sub-solidus mantle silicates,
and has very low viscosity, the implications for Pb behavior during mantle melting are
profound. There is only sparse experimental data relating to Pb partitioning between
sulfide and silicate, and no data on Pb diffusion rates in sulfides. A full understanding of
Pb behavior in sulfide may hold the key to several long-standing and important Pb
paradoxes and enigmas. The classical Pb isotope paradox arises from the fact that all
known mantle reservoirs lie to the right of the Geochron, with no consensus as to the
identity of the âbalancingâ reservoir. We propose that long-term segregation of sulfide
(containing Pb) to the core may resolve this paradox. Another Pb paradox arises from the fact that the Ce/Pb ratio of both OIB and MORB
is greater than bulk earth, and constant at a value of 25. The constancy of this âcanonical
ratioâ implies similar partition coefficients for Ce and Pb during magmatic processes
(Hofmann et al. 1986), whereas most experimental studies show that Pb is more
incompatible in silicates than Ce. Retention of Pb in residual mantle sulfide during
melting has the potential to bring the bulk partitioning of Ce into equality with Pb if the
sulfide melt/silicate melt partition coefficient for Pb has a value of ~ 14. Modeling shows
that the Ce/Pb (or Nd/Pb) of such melts will still accurately reflect that of the source, thus
enforcing the paradox that OIB and MORB mantles have markedly higher Ce/Pb (and
Nd/Pb) than the bulk silicate earth. This implies large deficiencies of Pb in the mantle
sources for these basalts. Sulfide may play other important roles during magmagenesis:
1). advective/diffusive sulfide networks may form potent metasomatic agents (in both
introducing and obliterating Pb isotopic heterogeneities in the mantle); 2). silicate melt
networks may easily exchange Pb with ambient mantle sulfides (by diffusion or
assimilation), thus âsamplingâ Pb in isotopically heterogeneous mantle domains
differently from the silicate-controlled isotope tracer systems (Sr, Nd, Hf), with an
apparent âde-couplingâ of these systems.Our intemperance
should not be blamed on the support we gratefully acknowledge from NSF: EAR-
0125917 to SRH and OCE-0118198 to GAG
The History, Relevance, and Applications of the Periodic System in Geochemistry
Geochemistry is a discipline in the earth sciences concerned with understanding the chemistry of the Earth and what that chemistry tells us about the processes that control the formation and evolution of Earth materials and the planet itself. The periodic table and the periodic system, as developed by Mendeleev and others in the nineteenth century, are as important in geochemistry as in other areas of chemistry. In fact, systemisation of the myriad of observations that geochemists make is perhaps even more important in this branch of chemistry, given the huge variability in the nature of Earth materials â from the Fe-rich core, through the silicate-dominated mantle and crust, to the volatile-rich ocean and atmosphere. This systemisation started in the eighteenth century, when geochemistry did not yet exist as a separate pursuit in itself. Mineralogy, one of the disciplines that eventually became geochemistry, was central to the discovery of the elements, and nineteenth-century mineralogists played a key role in this endeavour. Early âgeochemistsâ continued this systemisation effort into the twentieth century, particularly highlighted in the career of V.M. Goldschmidt. The focus of the modern discipline of geochemistry has moved well beyond classification, in order to invert the information held in the properties of elements across the periodic table and their distribution across Earth and planetary materials, to learn about the physicochemical processes that shaped the Earth and other planets, on all scales. We illustrate this approach with key examples, those rooted in the patterns inherent in the periodic law as well as those that exploit concepts that only became familiar after Mendeleev, such as stable and radiogenic isotopes
Evolution of the continental crust
The continental crust covers nearly a third of the Earth's surface. It is buoyantâbeing less dense than the crust under the surrounding oceansâand is compositionally evolved, dominating the Earth's budget for those elements that preferentially partition into silicate liquid during mantle melting. Models for the differentiation of the continental crust can provide insights into how and when it was formed, and can be used to show that the composition of the basaltic protolith to the continental crust is similar to that of the average lower crust. From the late Archaean to late Proterozoic eras (some 3â1 billion years ago), much of the continental crust appears to have been generated in pulses of relatively rapid growth. Reconciling the sedimentary and igneous records for crustal evolution indicates that it may take up to one billion years for new crust to dominate the sedimentary record. Combining models for the differentiation of the crust and the residence time of elements in the upper crust indicates that the average rate of crust formation is some 2â3 times higher than most previous estimates