Chlorine isotope evidence for syn-subduction modification of serpentinites by interaction with sediment-derived fluid

Jane Selverstone and Zachary SharpUniversity of New Mexico

NSF grant EAR 0911669

during exhumation

3

1

2

0300 400 500 600°C

Lago di Cignana: dia+coe

GPa

Zermatt-Saas ophiolite, Aosta, Italy

P-T data from Cartwright & Barnicoat 2002, Li et al. 2004, Angiboust et al. 2009, Groppo et al. 2009

Z-S ophiolite

Combin

?

Chlorine Isotope Data

serpentinization via seawater

serpentinization via sedimentary pore fluids

Zermatt-Saas data

A simple story?

Textural Types (Li et al. 2004) Type A

Type B

F.O.V. = 4 mm for all photos

Type B/C

seafloor pseudomorphic replacement

mylonitization ± folding during

subduction recrystallization during initial exhumation

greenschist overprint

Type C

Type D

Ol+TiClh

Type A

Type B

Type C Type D Serpentine compositions correlate with textural types

Texturally youngest serp in each sample has lowest Al and highest Mg#

Serpentine analyses

D

A

B

C

Chlorine Isotopes Revisited

3

1

2

0300 400 500 600°C

LdC

GPa

Serpentinite stages:Li et al. 2004, JMG

A

Chlorine Isotopes Revisited

3

1

2

0300 400 500 600°C

LdC

GPa

Serpentinite stages:Li et al. 2004, JMG

B

A

Chlorine Isotopes Revisited

3

1

2

0300 400 500 600°C

LdC

GPa

Serpentinite stages:Li et al. 2004, JMG

B

C

A

Chlorine Isotopes Revisited

3

1

2

0300 400 500 600°C

LdC

GPa

Serpentinite stages:Li et al. 2004, JMG

B

C

E

DA

1σ error in δ37Cl

Single fluid? Multiple fluids?

Bulk Composition Constraints on Fluid History

High LOI = carbonate-bearing sed-derived?

sed-derived?

???

Chlorine Isotopes Revisited

A B C D

Chlorine Isotopes Revisited

A B C D

Ti-clh

oliv

FIs

Type C&D

Chlorine Isotopes Revisited

Type C&D

A B C D

Calcmica schist with diamond-bearing Mn layers

Chlorine Isotopes Revisited

A B C D

mixing between reservoirs

+

3 1

Faccenda et al. 2009

1) Bending stresses induce mantle serpentinization via seawater

δ37Clserp ≥ 0‰

2) ± Local mantle serpentinization via sedimentary pore fluids

δ37Clserp ≤ 0‰

3) Uppermost sedimentary layer modified by extensive interaction with pore fluids from accretionary prism

δ37Clmetased << 0‰

A Physical Scenario Consistent with the Chemical Data

Seawater-dominatedSedimentary pore fluid

2

A Physical Scenario Consistent with the Chemical Data

Faccenda et al. 2012

4) Unbending stresses create regime of slab-parallel fluid flow in serpentinites during subduction

δ37Clserp≅ constant

5) Onset of exhumation moves rocks into regime of cross-slab fluid flow: fluids from metasediments locally modify serpentinites

δ37Clserp<< 0‰

4

5

Conclusions

3

1

2

0300 400 500 600°C

LdC

GPa

B

C

E

D

A

Serpentinites preserved seafloor δ37Cl values during subduction phase

Fluid-rock interaction during exhumation caused localized shifts to lower δ37Cl values

Fluids likely derived from nearby metasedimentary rocks in subduction channel

Switch from burial to exhumation expanded length scales of fluid-rock interaction and facilitated flow across lithologic boundaries

Cl allows us to see chemical interaction between downgoing slab and accretionary wedge fluids

δ37Cl ≥0‰

≤ -1‰

≤ -2‰

≈ 0‰