fluid-rock interactions recorded in serpentinites subducted to 60...

Fluid-rock Interactions recorded in Serpentinites subducted to 60-80 km DepthD. PETERS1, T. JOHN2, M. SCAMBELLURI3 AND T. PETTKE1

1University of Bern, Institute of Geological Sciences, Bern, Switzerland; 2Freie Universität Berlin, Institute of Geological Sciences, Berlin, Germany; 3University of Genova, DIPTERIS, Genova, Italy - (correspondence: [email protected], [email protected])

Selected Conclusions² Combining the results with halogen [2] and noble gas data [3] suggests that serpentinisation of oceanic lithospheric mantle occurred along bend faults or as

detached slices in the shallow forearc by fluids equilibrated within the accretionary prism²  Initial heterogeneities will govern the spatial extent of serpentinisation and FME enrichment, ultimately amplifying heterogeneities²  FME enrichments from serpentinisation are largely retained and reincorporated into the convecting mantle, providing potential fractionation mechanisms for U-Th

systematics (HIMU) and enriched mantle (EM) sources²  FMEs such as As, Sb, W and Bi, which are rarely quantified, can provide essential information to further discriminate between serpentinisation environments

ZIP: Zooming In between Plates is a Marie Curie Initial Training Network funded from the People of the European Union’s 7th Framework Programme FP7-PEOPLE-2013-ITN under REA grant agreement n° 604713. http://www.zip-itn.eu/

Magmatic Stage

•  Veins have similar PM-normalised trace element patterns as wall rocks but slightly enriched; only Li, P and Ti are highly enriched

•  Li enrichment by dehydration fractionation and Ti is linked to higher modal abundance of Ti-clinohumite; source for P unknown

Ø  No evidence for external fluids; veins are internal dehydration pathways

Ø  Dehydrated rocks retain FME enrichment signature beyond brucite-out (Erro Tobbio) and even antigorite-out (Almirez)

Ø  Collinear dehydration trends (shaded areas) during brucite-out (Erro Tobbio) and even antigorite-out (Almirez) indi-cate depth-independent Alkali-U frac-tionation

Ø  FME fractionation during dehydration produces higher U/Alkali ratios in the residual rocks, i.e., U depletion is less severe

Ø  Imprinted residual signatures are recycled into the mantle inducing, e.g., high U/Th and U/Alkali

•  Despite higher water contents in the mylonites, concentration ranges are similar in mylonitic and granular HPS

•  Larger scatter towards higher concen-trations in granular rocks

Ø  No evidence for additional serpen-tinisation stage in mylonites from FMEs as proposed by [4]

Ø  Indication that hydration and ele-ment enrichment are not necessarily coupled - FME enrichment is accu-mulative

Ø  Post-serpentinisation homogenisa-tion of most hydrated, i.e., mechani-cally weakest, parts by myloniti-sation?

Hydration Stage

•  FME patterns in MOR and FA serpen-tinites from serpentinite compilation allow for discrimination of serpentini-sation environments; patterns are re-tained during progressive metamor-phism (Figs. 3; Peters et al., in prep)

Ø  Erro Tobbio metaperidotites clearly exhibit a FA-like serpenitinite signa-ture, indicating hydration distant to MORs and under sediment influence (cf PaMa serpentinites), i.e., along bend faults in the subducting slab, or serpentinisation of oceanic litho-spheric mantle or mantle wedge along the shallow plate interface

Ø  Distinction between accretionary wedge and plate interface not possible with current FME data set

Ø  Relic mantle clinopyroxenes and olivine cores are preferably pre-served in the granular rocks, hence could account for apparent geo-chemical differences between the mylonitic and granular rocks [4]

2.1 Serpentinisation Environment

During serpentisation, fluid pathways are again highly governed by the distribution of porosity and permeability within the rocks; diffusion is expected to be of subordinate importance. Hydration and enrichment in fluid-mobile elements (FMEs) of the peridotites is expected to imprint a characteristic geochemical signature.

Dehydration Stage

IntroductionThe Erro Tobbio high-pressure metaserpentinites (HPS) exhibit the metamorphic assemblage of antigorite + olivine + magnetite + diopside + Ti-clinohumite formed at 550-600 °C and 2-2.5 GPa, and were one of the first ultrabasic rocks interpreted to represent subducted hydrated lithospheric mantle [1]. HP-LT metamorphism beyond the brucite-out reaction led to the formation of dehydration veins with the same HP assemblage. The origin of the peridotites and location of serpentinisation, i.e., along faults in the bending slab with pore fluid contribution [2,3] or along the slab-mantle interface by slab fluids [4] remains controversial. Here we present constraints on fluid origins and element fractionation during dehydration based on an extensive bulk rock data set and compiled published serpentinite data in order to decipher the serpentinisation environment and characterise fluid cycling in subduction zones.

MethodsBulk rock data were obtained by applying the PPP-LA-ICP-MS protocol described in [5]. The serpentinite compilation compri-ses data from six mid-ocean ridge (MOR) and three forearc (FA) settings. The term forearc is thereby allocated by the location of sampling (Figs. 2.1 and 3).

1

Partial dehydration upon brucite breakdown induces veining. Fluid-mobile elements are expected to be released into the fluids during dehydration reactions and metasomatise the overlying subduction channel and mantle wedge.

2

3

Erro Tobbio peridotites are subject to partial melting and reactive porous flow during decompression [6]:•  Local onset of partial melting is driven by minor differences in bulk chemistry •  Melt percolation is guided by local porosity and permeability•  Mylonitic metaperidotites are slightly more depletedØ  Imprint of first order physicochemical heterogeneity on the peridotites

1 2

3

Mid-ocean Ridge Accretionary Prism

Mantle Wedge

Island Arc

Oceanic Lithosphere Bend faults

0 km

100 km

200 km

PaMa = passive margin; CHLH = chlorite harzburgite; FLINCS = fluid inclusions; VN = Vein; Almirez data from Bretscher and Pettke (in prep);

0.85 0.9 0.95 1 1.05

0.03

0.035

0.04

0.045

0.05

0.055

0.06

0.065

0.07

0.075

Al 2

O3/S

iO2

MgO/SiO2

Erro−Tobbio HPS (mylonitic)Erro−Tobbio HPS (granular)

a) Al-Mg

0.85 0.9 0.95 1 1.05

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

CaO

/SiO

2

MgO/SiO2


b) Ca-Mg

0.85 0.9 0.95 1 1.05

1.8

2

2.2

2.4

2.6

2.8

3

3.2

3.4

x 10−3

MnO

/SiO

2

MgO/SiO2


c) Mn-Mg

[1]Scambellurietal.(1991).JMetamorphGeol9(1),79-91.[2]Johnetal.(2011).EarthPlanetSciLeA,308(1-2),65-76.[3]Kendricketal.(2011).NatGeosci,4,807-812.[4]ScambelluriandTonarini(2012).Geology,40(10),907-910.[5]PetersandPeAke(2016).GGR,DOI:10.1111/ggr.12125.[6]Ramponeetal.(2004).EarthPlanetSciLeA,218(3-4),491-506.[7]Scambellurietal.(2001).JPetrol,42(1),55-67.

5 10 1510

20

30

40

50

60

70

B [µ

g g1 ]

LOI [wt %]

Erro Tobbio HPS (mylonitic; this study)Erro Tobbio HPS (granular; this study)Erro Tobbio HPS (mylonitic; [4])Erro Tobbio HPS (granular; [4])

a) Boron

5 10 15

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

As [

µg g

1]

LOI [wt %]

c) Arsenic


5 10 15

0

5

10

15

20

25

30

Sr

[µ

g g

1]

LOI [wt %]

Erro−Tobbio HPS (mylonitic; [7])Erro−Tobbio HPS (granular; [7])

e) Strontium

Erro−Tobbio HPS (mylonitic, this study)Erro−Tobbio HPS (granular; this study)

5 10 15

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

W [

µg g

1]

LOI [wt %]

b) Tungsten


5 10 15

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

Sb

[µ

g g

1]

LOI [wt %]

d) Antimony


5 10 15

1.5

2

2.5

3

3.5

4

4.5

5

Ba [

µg g

1]

LOI [wt %]

f) Barium


10−2

100

102

104

10−4

10−3

10−2

10−1

100

101

102

103

104

Cs/U

Rb/U

b) Cs−Rb−U

Erro Tobbio HPS (mylonitic)Erro Tobbio HPS (granular)

MOR SerpentinitesFA SerpentinitesPaMa SerpentinitesPrimitive MantleDepleted MantleGLOSS IIOcean water

0.1

1Cs:Rb = 10

Cs:R

b =

0.01

8

0.01

6

0.015

0.0140.012

100

102

104

10−4

10−3

10−2

10−1

100

101

102

103

104

Cs/U

Li/U

a) Cs−Li−U



Cs:Li = 1

0.01

0.1

Cs:Li = 0.0015

0.0018

0.00188

0.00

19

0.00

2

100

101

102

103

104

10−4

10−3

10−2

10−1

100

101

102

103

104

U/Cs

Li/Cs

c) U−Li−Cs



U:Li = 0.01

0.1

1

U:Li = 0.005

0.0065

0.00

675

0.00

8

0.01

10−3

10−2

10−1

100

101

102

103

CsTl

BiRb

BaTh

UB

WNb

TaBe

KLa

CeCd

InSn

PbAs

SbMo

PrSr

PNd

ZrHf

SmEu

TiGd

TbDy

GaLi

YHo

ErTm

YbLu

Measure

d V

alu

e/P

rim

itiv

e M

antle

Erro Tobbio HPS (mylonitic)

Erro Tobbio HPS (granular)

Erro Tobbio Veins

10−2

100

102

104

10−4

10−3

10−2

10−1

100

101

102

103

104

U/Cs

Rb/Cs

d) U−Rb−Cs



U:Rb = 0.1

110

U:Rb = 0.05

0.053

5

0.05

5

0.06

0.1

100

101

102

103

10−4

10−3

10−2

10−1

100

101

102

U/C

s

Li/Cs

a) U−Li−Cs

FA Serpentinites

Primitive Mantle

Depleted Mantle

Almirez HPS

Almirez FLINCS

Almirez CHLH

Erro Tobbio HPS

Erro Tobbio VN

U:Li = 0.005

0.0065

0.0

0675

0.0

08

0.0

1

10−1

100

101

102

10−4

10−3

10−2

10−1

100

101

102

U/C

s

Rb/Cs

b) U−Rb−Cs

FA Serpentinites

Primitive Mantle

Depleted Mantle

Almirez HPS

Almirez FLINCS

Almirez CHLH

Erro Tobbio HPS

Erro Tobbio VN

U:Rb = 0.05

0.053

5

0.0

55

0.0

6

0.1

fluid-rock interactions recorded in serpentinites subducted to 60...

Documents