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- Title
Osmium Isotope Heterogeneity of the Upper Mantle: Evidence From the Bay of Islands Ophiolite Complex, Newfoundland.
- Authors
Wang, Yujian; Mungall, James E.; Liu, Jingao
- Abstract
Understanding and determining the composition and evolution of the upper mantle is valuable to unravel Earth's evolutionary geodynamics. The compositions of oceanic basalts are inevitably biased by the preferential melting of fusible mantle components and the pooling of melts from multiple sources. Here we present whole rock and mineral chemistry as well as Re‐Os isotopic systematics of the Table Mountain ophiolitic mantle massif from the Bay of Islands ophiolite complex, complemented by thermodynamic models of melt extraction. The Table Mountain lherzolites were formed by moderate degrees (5%–15%) of shallow polybaric melting. We divide the harzburgites into two types. Type I harzburgites represent residues of high‐pressure melting, and Type II harzburgites appear to have been further modified by second‐stage flux melting above a subduction zone induced by slab sediment‐derived aqueous fluids. The Table Mountain peridotites display highly variable "initial" 187Os/188Os485 Ma isotope compositions, including 0.1252–0.1253 for the Type I harzburgites, 0.1217–0.1236 and 0.1192–0.1198 for both lherzolites and Type II harzburgites. These Os isotopic variations cannot be generated by different degrees of partial melting of a homogenous fertile mantle immediately prior to obduction, and hence must have been inherited from ancient melting events. The Table Mountain lherzolites with inherited ancient melting signals are of particular significance because they demonstrate that the record of earlier melting events is not limited to ultra‐refractory harzburgites but is also present even in relatively fertile lherzolites. Our observations underscore the ubiquity of ancient melting signatures in the entire compositional spectrum of the depleted upper mantle. Plain Language Summary: Documentation of the diversity of compositions in Earth's present‐day mantle provides essential clues with which to unravel Earth's evolutionary geodynamics. Oceanic basalts are used as messengers to probe the composition of accessible upper mantle; however, their compositions are strongly biased to the products of those fertile parts of the upper mantle. To circumvent this issue, the compositions of occurrences of upper mantle rocks exposed at the Earth's surface (ophiolitic peridotites, abyssal peridotites and oceanic lithospheric mantle xenoliths) can be used to construct a picture of the nature of the upper mantle. We present whole rock and mineral chemistry, Re‐Os isotopic systematics as well as detailed thermodynamic modeling of the Table Mountain mantle massif from the Bay of Islands ophiolitic mantle massif of Newfoundland. Our results lead us to propose that significant Os isotope heterogeneities are revealed among the Table Mountain peridotites, most of which, even the fertile lherzolites, have been inherited from ancient melting events. Our results show that the convecting upper mantle is highly heterogeneous. Even those rocks that remain closest to their primal compositions are shown to carry signatures of ancient melting events that have been retained over billions of years. Key Points: Geochemical features and thermodynamic modeling reveal a record of melting events under both convergent and divergent plate marginsSignificant Os isotope heterogeneities show that the melting events occurred long before obduction of the massif onto the continentThe convecting upper mantle preserves heterogeneous domains with different fertilities and melting histories for long periods
- Subjects
NEWFOUNDLAND &; Labrador; OSMIUM isotopes; EARTH'S mantle; SUBDUCTION zones; SURFACE of the earth; GEOCHEMICAL modeling; REGOLITH; GEODYNAMICS; HETEROGENEITY
- Publication
Journal of Geophysical Research. Solid Earth, 2023, Vol 128, Issue 11, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2023JB026789