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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
Erro−Tobbio HPS (mylonitic)Erro−Tobbio HPS (granular)
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
Erro−Tobbio HPS (mylonitic)Erro−Tobbio HPS (granular)
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
Erro−Tobbio HPS (mylonitic)Erro−Tobbio HPS (granular)
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
Erro−Tobbio HPS (mylonitic)Erro−Tobbio HPS (granular)
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
Erro−Tobbio HPS (mylonitic)Erro−Tobbio HPS (granular)
5 10 15
1.5
2
2.5
3
3.5
4
4.5
5
Ba [
µg g
1]
LOI [wt %]
f) Barium
Erro−Tobbio HPS (mylonitic)Erro−Tobbio HPS (granular)
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
Erro Tobbio HPS (mylonitic)Erro Tobbio HPS (granular)
MOR SerpentinitesFA SerpentinitesPaMa SerpentinitesPrimitive MantleDepleted MantleGLOSS IIOcean water
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
Erro Tobbio HPS (mylonitic)Erro Tobbio HPS (granular)
MOR SerpentinitesFA SerpentinitesPaMa SerpentinitesPrimitive MantleDepleted MantleGLOSS IIOcean water
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
Erro Tobbio HPS (mylonitic)Erro Tobbio HPS (granular)
MOR SerpentinitesFA SerpentinitesPaMa SerpentinitesPrimitive MantleDepleted MantleGLOSS IIOcean water
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