Lithium isotopes in Guatemalan and Franciscan HP–LT rocks: Insights into the role of sediment-derived fluids during subduction

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• 作者：Kyla K. Simons ; George E. Harlow ; Hannes K. Brueckner ; Steven L. Goldstein ; Sorena S. Sorensen ; N. Gary Hemming ; Charles H. Langmuir
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2010
• 出版时间：15 June 2010
• 年：2010
• 卷：74
• 期：12
• 页码：3621-3641
• 全文大小：1054 K

文摘

High-pressure, low-temperature (HP–LT) rocks from a Cretaceous age subduction complex occur as tectonic blocks in serpentinite mélange along the Motagua Fault (MF) in central Guatemala. Eclogite and jadeitite among these are characterized by trace element patterns with enrichments in fluid mobile elements, similar to arc lavas. Eclogite is recrystallized from MORB-like altered oceanic crust, presumably at the boundary between the down-going plate and overlying mantle wedge. Eclogite geochemistry, mineralogy and petrography suggest a two step petrogenesis of (1) dehydration during prograde metamorphism at low temperatures (<500 °C) followed by (2) partial rehydration/fertilization at even lower T during exhumation. In contrast, Guatemalan jadeitites are crystallized directly from low-T aqueous fluid as veins in serpentinizing mantle during both subduction and exhumation. The overall chemistry and mineralogy of Guatemalan eclogites are similar to those from the Franciscan Complex, California, implying similar P–T–x paths.

Li concentrations (90 ppm) in mineral separates and whole rocks (WR) from Guatemalan and Franciscan HP–LT rocks are significantly higher than MORB (4–6 ppm), but similar to HP–LT rocks globally. Li isotopic compositions range from −5‰ to +5‰ for Guatemalan HP–LT rocks, and −4‰ to +1‰ for Franciscan eclogites, overlapping previous findings for other HP–LT suites. The combination of Li concentrations greater than MORB, and Li isotopic values lighter than MORB are inconsistent with a simple dehydration model. We prefer a model in which Li systematics in Guatemalan and Franciscan eclogites reflect reequilibration with subduction fluids during exhumation. Roughly 5–10 % of the Li in these fluids is derived from sediments.

Model results predict that the dehydrated bulk ocean crust is isotopically lighter (δ⁷Li  +1 ± 3‰) than the depleted mantle (+3.5 ± 0.5‰), while the mantle wedge beneath the arc is the isotopic complement of the bulk crust. A subduction fluid with an AOC–GLOSS composition over the full range of model temperatures (50–600 °C) gives an average fluid δ⁷Li (+7 ± 5‰ 1σ) that is isotopically heavier than the depleted mantle. If the lowest temperature steps are excluded (50–260 °C) as too cold to participate in circulation of the mantle wedge, then the average subduction fluid (δ⁷Li = +4 ± 2.3‰ 1σ, is indistinguishable from depleted mantle. Because of the relatively compatible nature of Li in metamorphic minerals, the most altered part of the crust (uppermost extrusives), may retain a Li isotopic signature (+5 ± 3‰) heavier than the bulk crust. The range of Li isotopic values for OIB, IAB and MORB overlap, making it is difficult to resolve which of these components may contribute to the recycled component in the mantle using δ⁷Li alone.