Discovery of composite diopside-magnetite lamellae in discrete olivine crytals from Colorado Plateau diatremes: indication of former hydrous ringwoodite
Abstract
We investigate composite diopside (Di) + magnetite (Mt) lamellae in olivine crystals from Oligocene diatremes of serpentinized ultramafic microbreccia located at Buell Park (AZ) and Green Knobs (NM) in the Colorado Plateau, and propose their genesis as breakdown products of precursor hydrous ringwoodite (γ-olivine) lamellae coexisting with α-olivine host. Among a hundred olivines (2-5 mm across, Fo89-93 in mol%) from both localities, the Di + Mt composite lamellae are recognized in only 15 of relatively Fe-rich grains (Fo89-91.5). The olivine host contains minor amounts of Ca (< 0.01 wt% CaO), Mn, Ni, and Co. Lamellar Di (Di95) contains minor amounts of Al, Na, Cr, Mn, and Ni. Lamellar Mt contains Cr (5.0-43.0 wt% Cr2O3) with minor amounts of Si, Ti, Al, Mn, Ni, and Co. The area fractions of olivine host and the lamellae in a typical grain (sample no. BP02-3) were measured at 98.8 % of the host and 1.2 % of the lamellae that are composed of Di:Mt = 85:15 to 53:47, average 66:34. The estimated average CaO content in a lamella reaches 17 wt% and the reintegrated CaO in the host and the lamellae is 0.22 wt%.We propose that Fe3+ in lamellar Mt was produced by dehydration of hydrous precursor phase via the reaction, Fe2+ + OH- = Fe3+ + O2- + 1/2H2. Converting Fe3+ into Fe2+ in the precursor phase based on this reaction, the composition satisfies the stoichiometry of olivine (X2TO4). Thus, the pre-existing phase certainly is of hydrous and contains Ca and other components with olivine stoichiometry. The most likely phase is lamellar hydrous ringwoodite. The precursor phase, hydrous ringwoodite, might have occurred as lamellae with α-olivine host and have probably decomposed by the following reaction, (1+X+Y+Z) hydrous ringwoodite → α-olivine + X Di + Y Mt + Z H2 (where X:Y:Z=2:1:1). The composite Di-Mt lamellae after hydrous ringwoodite lamellae in α-olivine host certainly suggest the materials originated from a deep mantle setting at least 300 km.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMMR33A2672S
- Keywords:
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- 1025 Composition of the mantle;
- GEOCHEMISTRYDE: 1038 Mantle processes;
- GEOCHEMISTRYDE: 3924 High-pressure behavior;
- MINERAL PHYSICSDE: 3630 Experimental mineralogy and petrology;
- MINERALOGY AND PETROLOGY