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Serpentinite Formation Convergent p.b.s (Abyssal serpentinites = where:…
Serpentinite Formation Convergent p.b.s
Tectonic settings
Divergent plate boundaries i.e. slow spreading ridges
Convergent plate boundaries
Serpentinite formation requires:
Mantle rocks
in contact with
water
Convergent p.b. serpentinisation
Abyssal serpentinites
Several stages serpentinisation
Mantle wedge serpentinites
Subducted serpentinites
1. Abyssal Serpentinites
~ Hydrated oceanic peridotites via
seafloor hydrothermal circulation
seawater alteration i.e. weathering oceanic crust
Fractures facilitate greater fluid transfer
Seawater = main driver ocean serpentinisation
Abyssal serpentinites
= where:
Observed mostly @ slow spreading ridges where:
1.
Oceanic mantle peridotites commonly exposed on seafloor
Along normal faults
related to
oceanic crustal extension & thinning
On sea floor-warping oceanic core complexes
Along major scarps / transform faults
Mantle Wedge
= mantle btwn
upper part subducting lithosphere
lower part overriding plate
2. Mantle wedge serpentinites
Fluids released from dehydration of subducted slab
i.e. sediments/altered basalts & gabbros / serpentinites
due to increased pressure & temperature
Water percolates upward by buoyancy
through mantle wedge
Mantle wedge peridotites = hydrated
∴ Serpentinisation induced when T < 700°C
3. Where Subducted Serpentinite
Serpentinisation timing = heterogeneous
Origin: peridotite hydrated
at ridge / trench / in subduction channel / oceanic-continental transition zone
Associated with high T, low P metamorphic rocks
(Mostly @ suture zones)
3. Subducted Serpentinite
Further oceanic crust serpentinisation
-->
associated with subduction depths ~30 - 60 km
Fluids released by slab sediment dehydration
as pressure & temp increase during subduction
Serpentinisation as fluids interact with:
subducted abyssal serpentinites
partially hydrated peridotites
Antigorite Breakdown
--> since serpentine = hydrated phyllosilicate
At higher P/T at depth
Serpentine breakdown releases lotsa water
= significant source of water to mantle