HR: 1330h
AN: V22B-0591    [PDF]
TI: Navajo Garnetites and Rock-Water Interactions in the Mantle
AU: * Smith, D
EM: doug@geo.utexas.edu
AF: University of Texas at Austin, Dept Geological Sciences, 1 University Station C-1100, Austin, TX 78712-0254 United States
AU: Griffin, W L
EM: wgriffin@els.mq.edu.au
AF: Macquarie University, Dept Earth and Planetary Sciences, Sidney, NSW 2109 Australia
AB: Garnetite xenoliths from ultramafic diatremes in northeastern Arizona provide insights into mantle-water interactions. The garnetites are more than 95% garnet with subequal proportions of Grs, Alm, and Prp. Accessory minerals include rutile, ilmenite, chlorite, clinopyroxene, and zircon. Textures and mineral compositions are consistent with garnetite formation by hydrous metasomatism at about 500$\deg$C near contacts between mafic rocks and peridotite. Trace element abundances and isotope ratios measured by LA-ICP-MS and -MC-MS document xenolith histories. Typical garnets are slightly enriched in MREE relative to HREE, and small positive Eu anomalies are present in garnet of one xenolith. Zircon shapes in one garnetite are diverse. Most U-Pb analyses of these zircons plot on or near concordia in the range 60 to 85 Ma, but analyses of several grains lie off concordia, and chords between these discordant results and the main cluster have Proterozoic upper intercepts. Hf 176/177 ranges from about 0.2818 to 0.2828, yielding depleted mantle model ages from 1.9 to 0.5 Ga that cluster at about 1.8 and 1.1 Ga. More radiogenic Hf is roughly correlated with decreasing U-Pb age in the interval 85 to 60 Ma, and zircons are zoned to more radiogenic Hf, core to rim. The high Hf 176/177 in the young zircons and zircon rims is consistent with the modelled evolution of Hf in the garnet (mean Lu/Hf about 30). These data establish that the garnetite inherited Hf and zircons from a protolith at least 1.8 Ga in age, and suggest that the garnetite itself formed around 85 Ma ago, and some zircon within it crystallized episodically during the interval 85 to 60 Ma. The zircon data eliminate the possibility that the garnetites are fragments of the Farallon plate. The time interval for garnetite crystallization overlaps the 80 to 30 Ma range recorded by zircons in typical Navajo eclogites, consistent with the hypothesis that both eclogites and garnetites record the interaction of water with Proterozoic mantle. These rocks may document movement of water into the overlying continental mantle from the Farallon plate during low-angle subduction and later mobilization of that water.
DE: 1025 Composition of the mantle
DE: 1035 Geochronology
DE: 1065 Trace elements (3670)
DE: 3660 Metamorphic petrology
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting