TY - JOUR A1 - Aulbach, Sonja A1 - Höfer, Heidi E. A1 - Gerdes, Axel A1 - Tinguely, Christel A1 - le Roex, Anton T1 - Mélange Signatures and Low Oxygen Fugacity in Eclogite Xenoliths From the Crust‐Mantle Transition Below a Mesoproterozoic Collision Belt Y1 - 2024-01-30 VL - 129 IS - 2 SP - EP - JF - Journal of Geophysical Research: Solid Earth DO - 10.1029/2023JB027894 PB - N2 -
Mass transfer across the crust‐mantle boundary is a fundamental process governing planetary differentiation, the evolution of geochemical reservoirs and ore formation, controlled by physicochemical conditions at the crust‐mantle interface. In situ trace‐element, clinopyroxene 87Sr/86Sr and garnet Fe3+/ΣFe of kimberlite‐borne eclogite xenoliths from the deep (∼50 km) crust‐mantle transition below the ca. 1.2–1.0 Ga Namaqua‐Natal Fold Belt (southwestern Kaapvaal craton margin) were determined to elucidate their origin and evolution, and to constrain the oxygen fugacity of this pivotal but largely inaccessible environment. Based on a garnet source signature (NMORB‐normalized Er/Lu > 1) in pristine “gabbroic” eclogites with pronounced positive Eu, Sr, and Pb anomalies, the suite is interpreted as originating as plagioclase‐rich cumulates in oceanic crust from melts generated beneath mature oceanic lithosphere, subsequently subducted during the Namaqua‐Natal orogeny. Enriched eclogites have higher measured 87Sr/86Sr in clinopyroxene (up to 0.7054) than gabbroic ones (up to 0.7036), and show increasing bulk‐rock Li, Be and Pb abundances with increasing δ18O in clinopyroxene, and muted Eu‐Sr‐Pb anomalies. These systematics suggest interaction with a siliceous fluid sourced from seawater‐altered oceanic sediment in a subduction mélange setting. Garnet Fe3+/ΣFe in deep crustal eclogites is extremely low (0.01–0.04, ±0.01 1
Eclogite xenoliths sampling deep crust‐mantle transition below Namaqua‐Natal Fold Belt have plagioclase‐rich oceanic protoliths
Enriched xenoliths show signatures of interaction with siliceous, subducted sediment‐derived fluids under shallow fore‐arc conditions
Fe3+‐based eclogite oxybarometry with oxygen fugacities below sulfate stability limits the role of S6+ species in mantle wedge oxidation