Element and Sr–O isotope redistribution across a plate boundary-scale crustal serpentinite mélange shear zone, and implications for the slab-mantle interface

Scott, James M.; Smith, Steven A.; Tarling, Matthew S.; Roux, Petrus le; Harris, Chris; Hoffmann, J. Elis; Scherzer, Sophie; Tulley, Chris J.

doi:10.1016/j.epsl.2019.06.034

Cited by 13 publications

(4 citation statements)

References 60 publications

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“…With increasing hydration and proximity to metasedimentary rocks at the top of the subducted slab, an aqueous (Sr, Cl, Si)-bearing fluid would have a radiogenic 87 Sr/ 86 Sr signature consistent with our data (Scott et al, 2019; Fig. 7B).…”

Section: Ordovician-silurian Arc-continent Collisionsupporting

confidence: 83%

“…(B) A binary plot of 87 Sr/ 86 Sr vs. d 13 CV-PDB, for dolomite grains and whole-rock samples from drill core. The Sr isotope composition of Bayan Obo group sediment (Yang et al, 2019), subduction-derived fluids (Scott et al, 2019), Silurian-Devonian (S-D) granitoids from the north margin of the North China craton (Zhang et al, 2014), and Archean-Paleoproterozoic (A-P) and Mesozoic (M) subcontinental lithosphere mantle (SCLM) of the North China craton (Zhang et al, 2020;Zou et al, 2020) reworking of the H8 unit (Ling et al, 2013). The northernmost margin of the North China craton is delineated by the Solonker suture, along which the Paleo-Asian Ocean closed to form the southern sections of the Central Asian orogenic belt (Fig.…”

Section: Ordovician-silurian Arc-continent Collisionmentioning

confidence: 99%

“…Degassing reactions normally lower the d 18 OV-SMOW value of reequilibrated carbonate minerals by no more than 3‰ because of the calc-silicate limit (F> 0.6), even at high levels of decarbonation (Valley, 1986). However, 18 O is partitioned preferentially into carbonates relative to their associated silicate phases (Chacko et al, 2001), and fluids derived from subducted metasediment-dominated rocks show enrichment in 18 O (d 18 OV-SMOW ≥9.6‰; Scott et al, 2019). Thus, dolomite recrystallization involving such fluids is not expected to have a significant effect on the primary O isotope signature.…”

Section: Rayleigh Fractionation During Degassingmentioning

confidence: 99%

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Mineralization of the Bayan Obo Rare Earth Element Deposit by Recrystallization and Decarbonation

Wei

Deng

et al. 2022

Economic Geology

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The genesis of the Bayan Obo giant rare earth element (REE) deposit has been debated for several decades. Here, we report the isotopic effects of dynamic recrystallization in the H8 carbonatite, which is the principal ore carrier in the deposit. We studied fresh drill core to a depth of 1.78 km and documented the elemental and C-O-Sr isotope evolution of rock-forming dolomite during its deformation and reaction with fluids. The precursor dolomite and the products of its recrystallization differ in δ13CVienna-PeeDee Belemnite (V-PDB) (–1.09 to 2.37 vs. –3.59 to 0.79‰, respectively) and 87Sr/86Sr (0.70241–0.70394 vs. 0.70288–0.71409, respectively), and show a similar δ18Ovienna-standard mean ocean water (V-SMOW) range (10.3–16.9‰). The strong negative shift in δ13CVPDB indicates that, locally, there was as much as 40% loss of CO2 from the precursor dolomite, although most of the recrystallized dolomite experienced decarbonation on a smaller scale. Clumped monazite grains associated with apatite in paragenetically similar samples yielded variable in situ Th-Pb dates (980–340 Ma), whereas those in monomineralic veinlets give a consistent age of ~400 Ma and consistent initial Nd isotope ratios. This indicates that the wide range of dates may not represent real REE depositional events, and that the primary REE minerals deposited in the Mesoproterozoic underwent isotopic reequilibration and REE remobilization in the mid-Paleozoic. Recrystallization and decarbonation of dolomite in the H8 unit were facilitated by its reaction with subduction-derived silica- and halogen-rich fluid, genetically linked to plate-convergence processes along the northern margin of the North China craton and did not require an influx of REEs from an external source.

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Section: Ordovician-silurian Arc-continent Collisionsupporting

confidence: 83%

Section: Ordovician-silurian Arc-continent Collisionmentioning

confidence: 99%

Section: Rayleigh Fractionation During Degassingmentioning

confidence: 99%

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Mineralization of the Bayan Obo Rare Earth Element Deposit by Recrystallization and Decarbonation

Wei

Deng

et al. 2022

Economic Geology

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“…In metamorphic petrology, Cr-rich spinel is commonly observed in serpentinized peridotites (e.g., Sobolev & Logvinova, 2005;Stewart et al, 2016). As the most resistant phase to alteration from the primary magmatic source, Cr-rich spinel is a useful constituent in understanding the subduction-induced metamorphism of ultramafic complexes (e.g., Kruckenberg et al, 2013;Scott et al, 2019;Titus et al, 2002Titus et al, , 2007Toy et al, 2010;Zadeh et al, 2017). Hence, the physicochemical analysis of Cr-rich spinel bears implications on the degree of low temperature metamorphism (i.e., serpentinization) in a subduction regime.…”

Section: Introductionmentioning

confidence: 99%

Magnetic Cr‐Rich Spinel in Serpentinized Ultramafic Complexes

Tikoff

2020

JGR Solid Earth

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In comparison to the six spinel end-members in the spinel prism, magnetic information on the intermediate spinel compositions is relatively poorly established. To investigate magnetic properties of intermediate compositions of Cr-rich spinel, we collected 12 samples that might contain potentially magnetic Cr-rich spinel. On the basis of chemical composition analysis and physical magnetic behavior, magnetic Group A Fe-rich spinel (i.e., magnetite), magnetic Group B Cr-rich spinel, and paramagnetic Group C Cr-Al-rich spinel were identified. Magnetite is solely responsible for the natural remanent magnetization (NRM) in severely serpentinized samples from intraoceanic island arcs, orogenic exposures of ultramafics, and back-arc continental lithosphere settings. Magnetic Group B Cr-rich spinel shows maximum unblocking temperatures of 200-280°C. Such temperatures permit Cr-rich spinel to contribute to magnetic anomalies up to about 8-12 km in terrestrial lithosphere settings, given a normal geothermal gradient of 25 K/km. The existence of magnetic Cr-rich spinel requires certain compositional conditions including a cation ratio of [Cr]/[Fe 2+ + Fe 3+ ] from 1.33 to 1.56 as well as a low oxide ratio of (Al 2 O 3 + MgO)/ (Cr 2 O 3 + FeO + Fe 2 O 3) less than 10%. It is evident that compositions of spinels are related with the degree of serpentinization as Group A spinel is observed along the fractures between/among olivine grains in heavily serpentinized rocks. Spinels in Groups B and C seem to experience less severe metasomatic or hydrothermal alteration during serpentinization. Distribution of magnetic Cr-rich spinel along the fractures of silicates (mostly olivine) may support a chemical origin of the NRM. Thus, Cr-rich spinel is a potential NRM carrier and a source of magnetic anomalies in ultramafic complexes.

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The role of the subducting slab and melt crystallization in the formation of magnetite-(apatite) systems, Coastal Cordillera of Chile

et al. 2020

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Element and Sr–O isotope redistribution across a plate boundary-scale crustal serpentinite mélange shear zone, and implications for the slab-mantle interface

Cited by 13 publications

References 60 publications

Mineralization of the Bayan Obo Rare Earth Element Deposit by Recrystallization and Decarbonation

Mineralization of the Bayan Obo Rare Earth Element Deposit by Recrystallization and Decarbonation

Magnetic Cr‐Rich Spinel in Serpentinized Ultramafic Complexes

The role of the subducting slab and melt crystallization in the formation of magnetite-(apatite) systems, Coastal Cordillera of Chile

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