scales of preservation and root causes of heterogeneities ... · petrology 98, 13-23. o’driscoll,...
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2. Field observations
Dunites in both ophiolites host podiform chromitite (>60 vol.% Cr-spinel) seams. Chromitite seams are 0.01-5 m thick and at the outcrop scale may appear complexly deformed, forming stock-work veins and patches. The largest chromitites (3-5 m thick) always occur within ~100 m of the petrological Moho
The mantle sections of both ophiolites are compositionally heterogeneous at the cm-to-m scales; noticeably manifested by dunite lenses and layers in dominantly harzburgitic host rock.
Dunite
Harzburgite
Layered harzburgite-dunite at Cliff, Shetlands
Intrusive dunite
Layered dunite/wehrlite
Layered Series, Kvaløya, Leka
'Podiform' chromitite at Hagdale, Shetlands Rhythmic chromitite layering, Kvaløya, Leka
4. Mineral-scale observations: Siting the HSE
4.1 Shetland Ophiolite Complex
4.2 Leka Ophiolite Complex
10 µm
5 µm
Very little primary silicate remains due to serpentinisation. Dunites contain variable sulphide (pentlandite) abundances, whilst chromitites contain minor Ni-sulphide and platinum-group minerals (PGM)
At Cliff, 1-2 vol.% Ni-arsenide is present in chromitites
100 µm
Os Ir Ru Pt Pd Re0.0001
0.001
0.01
0.1
1
10
100
1000
ChromitePentlandite
Viels MineralizedDunite
Cr-spinel
Ni-arsenide
Serpentinite
The Viels dunite
Os Ir Ru Rh Pt Pd Re Au0.001
0.01
0.1
1
10
100
1000
10000
100000
Sam
ple/
CI-C
hond
rite
(Org
ueil)
Chromite (n = 15)
Whole-rock
NiAs grains
Sam
ple
/CI-
Chondrite
Ni-arsenide
Sperrylite
Sperrylite
Cliff chromititeSam
ple
/CI-
Chondrite
LA-ICP-MS analyses show that sulphides, arsenidesand PGM site the highly siderophile elements (HSE). HSE abundances are below detection limits in Cr-spinel, except at Cliff locality
Pt and Ir anomalies indicate the types of HSE-rich alloy present, supported by petrography (above)
In the mantle section, the HSE site in sulphides (and possibly PGM) that concentrate in clinopyroxene-rich veinlets and in cleavage planes in large orthopyroxene(?) pseudomorphs
Olivine
Clinopyroxeneveinlets
Cr-spinel
Ferritchromit
Ferritchromit
Pentlandite
Both the mantle section and Layered Series are remarkably fresh compared to the Shetland rocks,with only <20 vol.% serpentinisation in dunites, harzburgites and pyroxenites in some cases
Above the petrological Moho, chromitite seams and sulphide (pentlandite)-rich dunites become more abundant, and control the HSE budget of the Layered Series portion of the ophiolite
Olivine
Ferritchromit Relict orthopyroxene(?) cleavageswith ferritchromit/sulphide inclusions
Olivine domains
Serpentinedomain
Clinopyroxene
Sulphide andferritchromitinclusions
PGM in ferritchromit rims
References and acknowledgements
Furnes, H., Pedersen, R.B., Stillman, C.J. 1988. Journal of the Geological Society, London 145, 401-412. Dunning, G.R., Pedersen, R.B. 1988. Contributions to Mineralogy and Petrology 98, 13-23. O’Driscoll, B., Day, J. M. D., Walker, R. J., Daly., J. S., McDonough, W. F., Piccoli, P. M. 2012. Earth and Planetary Science Letters 333-334, 226-237 (and references therein). Derbyshire, E.J., O'Driscoll, B., Lenaz, D., Gertisser, R., Kronz, A. 2012. Lithos, in press. Spray, J.G., Dunning, G.R. 1991. Geological Magazine 128, 667-671. Walker, R.J., Prichard, H.M., Ishiwatari, A.,Pimentel, M. 2002. Geochimica et Cosmochimica Acta 66, 329-345.
This work is funded by a NERC New Investigator Grant (NE/J00457X/1) as well as additional support from the Mineralogical Society of Great Britain and Ireland, the GeologicalSociety of London, the Geological Society of Edinburgh and the Royal Society
6. Discussion pointsSome Shetland harzburgites reveal evidence that Neoproterozoic-aged cm-m scale melt depletion domains survived subsequent (492 Ma) supra-subduction zone melt extraction
Leka and Shetland harzburgite HSE concentrations both compare well to the range of Gakkel Ridge abyssal peridotite compositions, suggesting that variable degrees of serpentinisation do not obfuscate primary HSE signatures
Relatively radiogenic γOs in Leka and Shetland dunites (compared to harzburgites), support their formation in upper mantle melt channels. Such channels have been effective at homogenising Os isotope signatures from the surrounding mantle (e.g., γOs ranges in the Shetland chromitites)
Scales of preservation and root causes of heterogeneities in the convecting upper mantle
Corresponding Author: b.o'[email protected]
of the Iapetan Ocean
Paper Number:V53A-2808
Keel e Pet r ol ogy Gr oupKEELE PETROLOGY GROUP
Brian O'Driscoll , Richard J Walker , James MD Day , J Stephen Daly1 2 3 4
School of Physical and Geographical Sciences, Keele University, Keele, United Kingdom, Department of Geology, University of Maryland, College Park, MD, United States, Geosciences Research Division Scripps Institution of Oceanography, La Jolla, CA, United States, UCD School of Geological Sciences, University College Dublin, Dublin, Ireland
1 2 3
4
3. Mantle melting regime and tectonic settingUnaltered ophiolite Cr-spinel compositions, especially those from podiform chromitite, are a useful petrogenetic tool for determining Cr-spinel parental melt compositions and tectonic setting
0.010
10
1.0
0.1
45403530252015105
Cr-
spin
el T
iO
wt.%
2
MORB
OIBLIP
ARC50
Cr-spinel Al O wt.%2 3
Chromitite Cr-spinel compositions are highlighted in red and support field observations (e.g., dunite layering) that suggest mantle melting in a supra-subduction zone (island-arc) setting
Shetland Cr-spinel Leka Cr-spinel
×
Cliff
Cr-spinel localities
Nikka Vord
Hagdale
The Viels
Harolds Grave
Cr-spinel localities
Alteredcompositions(ferritchromit)
Refertilisationtrend?
Kvaløya
Kvaløya-moen
Lauvhatten
Kvaløya
High-T (equilibration with melt) and low-T (ferritchromit) alteration of Cr-spinel is also evident
Cr-spinel Al O wt.%2 3
Cr-
spin
el T
iO
wt.%
2
1. Introduction and Geological Setting
The Shetland (UK; ~492 Ma) and Leka (Norway; ~497 Ma) ophiolites each comprise a portion of early-Paleozoic oceanic lithosphere with well-exposed upper mantle sections
Ophiolites allow an assessment of the timing, causes and extent of processes that operate in the mantle, facilitating the coupling of field-based investigations with geochemical analysis of theseotherwise inaccessible rocks
Siberia
Baltica
Gondwana
Laur
entia
Iape
tus
Oce
an
Tornquist Sea
SOUTHPOLE
Scotland
ShetlandIsles
100 km
1 Wo
60 No
Tornquist Line
Gre
at G
len
Faul
tW
alls B
ound
ary
Fault
Unst
North Sea
Bergen
Trondheim
Leka
Karmøy
Norwegian Sea
Oslo
Norway
Harzburgite (with dunite)
Layered dunite and wehrlite
Layered gabbro/metabasalt dykes
Metabasalt dykes
Pillow basalts and volcaniclastics Cover sequences: Skei Group (Leka) and Muness Phyllites (Shetlands)
Solsemøyene Group (Leka)
Petrological Moho
5 km
55
Balta Sound
65
15
10
05
00
N
The Viels
Hagdale
HaroldsGrave
Unst
Fetlar
Yell
95
90
Aithban
k Fa
ult
Blu
emu
ll Fa
ult
Lamb Hoga Fault
North Sea
MuckleFlugga
Skaw Granite
418m
376m
Raudberget
Solsem Madsøya
Husby
Skei
Lauvhatten
Skeisnesset
Storøya
Leknesøyan
Kvaløya
Steins
Branden
Vattind
Lekafjorden
N
Langdraget
Halin
3 km
Late Cambrian-Early Ordovican ca. 490 Ma
Proterozoic metasediments (Shetlands)
Harzburgite (Upper Nappe, Shetlands)
Skaw Granite (Unst)
Inset maps show regional (geographic) and tectonic setting for the Shetland and Leka Ophiolite Complexes. Corresponding units are coloured the same for each ophiolite in the geological maps. Sampling focused on the mantle section (harzburgite) and around the petrological Moho in both ophiolite complexes.
NikkaVord
Kvaløya-moen
Sampled sitesSampled traverse
Cliff
5. Os isotope systematics and HSE abundances
0 1 10000100010010-25
Os (ppb)
15
5
-5
-15
γOs
Shetland chromititesHagdaleHarolds GraveCliff
Shetland dunitesShetland harzburgites
Leka dunitesLeka harzburgites
0 2 108640.110
0.140
0.120
0.130
0.150
0.160
0.170
Re/ Os187 188
Os/
O
s187
188
γOs calculated at 492 Ma for Shetland samples and 497 Ma for Leka samples
Dunites from both ophiolites have more variable and absolute abundances of the HSE than harzburgites:
Late-stage Re addition
Late-stage Re addition
γOs 3.90-3.95
γOs 0.48-1.00γOs 1.34-1.37
Individual Shetland chromitite localities have very restricted ranges in initial Os isotopic composition (see figure above)
All harzburgites have relatively unradiogenic initial Os isotope compositions (γOs of -5.3 to +2.6) and low Re/ Os (0.02-1.3)187 188
Shetland and Leka dunites range to more radiogenic γOs (-3.3 to +4.4) with higher Re/ Os (0.02-2.8)187 188
The Shetland chromitite data point to significant Os isotope heterogeneity being present at 492 Ma
Harzburgites: 2.6-4.1 ppb Os, 3.5-12.1 ppb Pt, 0.03-0.94 ppb Re, broadly similar to the primitive mantle composition estimate (PM)
Dunites: 0.4-41.7 ppb Os, 0.03-407 ppb Pt, 0.01-14.8 ppb Re
A pyroxenite from below the Leka petrological Moho has 17.8 ppb Os, 1056 ppb Pt and 2.09 ppb Re
Shetland chromitites reveal a significant range in HSE concentrations: 0.09-2.94 ppm Os and 0.02-96.6 ppm Pt
Leka harzburgites
Leka dunites(and pyroxenite)
Shetlanddunites
Shetlandharzburgites
Shetlandchromitites
Range of Shetland harzburgites