Length of the day and evolution of the Earth's core in the geological past     

C. Denis  K.R. Rybicki  A.A. Schreider  S. Tomecka‐Sucho&#x;  P. Varga 《Astronomische Nachrichten》2011,332(1):24-35

In this paper, we study quantitatively the effect of the Earth's core formation on the secular rate of change of the length of day (LOD). We find that for the present epoch, a growth rate of the core comprised between 1 and 10 mm/cy seems to be a plausible guess, leading to a relative de crease of LOD comprised roughly between 10 and 100 μs/cy. Such values do not affect significantly the observed secular in crease of LOD caused by tidal braking, which amounts to about 1.79 ms/cy. However, in the remote geological past, before the Phanerozoic, the effects of core growth may have been much more important, because the total change of LOD associated with core formation has been estimated by Birch in 1965 to be 2.4 hours for an initially undifferentiated cold Earth, and 3.1 hours for an initially undifferentiated hot Earth. We consider a number of scenarios, some of them corresponding to very early and/or very fast core formation, others corresponding to slow and/or late core formation. We show that palaeo‐LOD measurements seem to favour slow core formation during the Proterozoic, contrarily to the now largely prevailing hypothesis based on geochemical arguments that the iron core formed very early in the Earth's history and during a geologically short time interval (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Trace-element heterogeneity in rutile linked to dislocation structures: Implications for Zr-in-rutile geothermometry     

Rick Verberne  Hugo W. van Schrojenstein Lantman  Steven M. Reddy  Matteo Alvaro  David Wallis  Denis Fougerouse  Antonio Langone  David W. Saxey  William D. A. Rickard 《Journal of Metamorphic Geology》2023,41(1):3-24

The trace-element composition of rutile is commonly used to constrain P–T–t conditions for a wide range of metamorphic systems. However, recent studies have demonstrated the redistribution of trace elements in rutile via high-diffusivity pathways and dislocation-impurity associations related to the formation and evolution of microstructures. Here, we investigate trace-element migration in low-angle boundaries formed by dislocation creep in rutile within an omphacite vein of the Lago di Cignana unit (Western Alps, Italy). Zr-in-rutile thermometry and inclusions of quartz in rutile and of coesite in omphacite constrain the conditions of rutile deformation to around the prograde boundary from high pressure to ultra-high pressure (~2.7 GPa) at temperatures of 500–565°C. Crystal-plastic deformation of a large rutile grain results in low-angle boundaries that generate a total misorientation of ~25°. Dislocations constituting one of these low-angle boundaries are enriched in common and uncommon trace elements, including Fe and Ca, providing evidence for the diffusion and trapping of trace elements along the dislocation cores. The role of dislocation microstructures as fast-diffusion pathways must be evaluated when applying high-resolution analytical procedures as compositional disturbances might lead to erroneous interpretations for Ca and Fe. In contrast, our results indicate a trapping mechanism for Zr.  相似文献