HR: 1340h
AN: V43A-1415    [Abstracts]
TI: High Ferric Iron Content of Diopsides From Cape Verdian Lavas and Dykes; Implications for the Oxidation State of the Lower Mantle.
AU: * Duprat, H
EM: heleneduprat@hotmail.com
AF: Geological Institute, Copenhagen University, Oester Voldgade 10, Copenhagen, 2100 Denmark
AU: Holm, P M
EM: paulmh@geol.ku.dk
AF: Geological Institute, Copenhagen University, Oester Voldgade 10, Copenhagen, 2100 Denmark
AU: Madsen, M B
EM: mbmadsen@fys.ku.dk
AF: Center of Planetary Science, Coepnhagen University, Juliane Maries Vej 30, Copenhagen, 2100 Denmark
AB: The oxidation state of bulk rock basalts is not necessarily reflecting the oxidation state of the source regions because pre- and post-eruptive redox modifying processes are likely to have influenced bulk rock chemistry. To circumvent this problem, the ferric iron content of clinopyroxenes separated from 10 Cape Verdian rocks - six lavas and four dykes - were analyzed with $^{57}$Fe moessbauer spectroscopy at 295 K. In addition, four of the pyroxenes were analyzed at 15 K. A fresh well-crystallized dyke rock was selected to represent the oxidation state of the magma. The distribution coefficient Fe$^{3+}$/Fe$^{2+}$ between rock and clinopyroxene was found to be 1, and is thought to be a good approximation. The spectra reveal that 38 - 45% of the iron atoms in the clinopyroxenes are ferric; a ferric iron percentage considered large for mantle rock. The Fe$^{3+}$/Fe$_{tot}$ ratio calculated from micro probe analyses, all overestimate the true Fe$^{3+}$/Fe$_{tot}$ of the clinopyroxenes by up to 20%. The bulk host rock basalts exhibit very large variations in redox state as expressed in a Fe$^{3+}$/Fe$_{tot}$ ratio variations between 0.3 and 0.9, but this is not accompanied by a similar redox variation in the clinopyroxenes; an indication of the stability of the Fe$^{3+}$/Fe$_{tot}$ ratio after its incorporation in the crystal lattice. Thus, the oxidized character of the clinopyroxenes possibly reflects a high-level oxidized state of the lower mantle, which is believed to be the source region of the Cape Verde Mantle Plume. This is in concert with experimental studies in recent years of high pressure Al-rich perovskite having a possible Fe$^{3+}$/Fe$_{tot}$ as large as 0.7, i.e that lower mantle could be a considerable sink for Fe$^{3+}$. The result suggests that lower mantle has been subject to addition of ferric iron since segregation of the core.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3620 Crystal chemistry
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting