Origin of the Meta-Ultramafic Rocks from Massachusetts and Staten Island, New York: a Complex History
Abstract
The upper mantle is often transformed into serpentinite by reaction with water. Serpentinization mainly occurs within the seafloor by interaction with seawater but also in subduction zones where aqueous fluids coming from the dehydration of the slab ascend through the mantle wedge. The main difference between these two environments is, therefore, the source of serpentinizing fluid. In suture zones, serpentinites of different origin, i.e., altered by different fluids, are often found together, so distinguishing between them is difficult. Recently, studies of serpentinites from well-known tectonic settings show that their boron isotopic signatures are distinct based on their tectonic origin. Mantle wedge serpentinites formed from deep metamorphic fluids and have a strongly negative δ11B (as low as -14‰). Serpentinites from the forearc and obducted ophiolites are altered by a mixture of seawater and shallow metamorphic fluids and have a δ11B in the range -5 to +10‰. Serpentinites from subducted slab preserve seawater-derived δ11B (up to +30‰).
Many meta-ultramafic rocks are encountered along the East Coast of America, from Newfoundland (Canada) to Tennessee (USA), but they are so deformed that identifying their tectonic origin is difficult. The literature on meta-ultramafic bodies form the Northeast Coast of the US is limited and often interprets them as one or two belt(s) of dismembered obducted ophiolite. In the present study, we selected samples from Massachusetts and Staten Island, New York to identify the fluids responsible for serpentinization and, therefore, infer their tectonic origin. Preliminary results identify three distinct B-isotopic signatures recorded in these samples from the Northeast Coast. Two antigorite-bearing samples from Massachusetts yield δ11B from +0.1 ± 1.5 to +18.6 ± 2.0‰ and from 7.7 ± 1.0 to 13.4 ± 1.4‰, suggesting they were altered, respectively, by a mixed source of fluids and by deep metamorphic fluids. Two lizardite-bearing samples from Staten Island yield δ11B from +7.4 ± 0.5 to +27.3 ±1.3‰ and from +37.5 ± 1.1 to +45.0 ±1.0‰ and argue for a seawater-derived fluid. Thus, serpentinites from the Northeast Coast record different fluid sources and origins, indicating a more complex tectonic history than was previously interpreted.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43E0133F
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY;
- 3653 Fluid flow;
- MINERALOGY AND PETROLOGY;
- 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY