Please enable JavaScript.
Coggle requires JavaScript to display documents.
Ophiolites, image, image, W10,1, image - Coggle Diagram
Ophiolites
Types of Ophiolites:
- Non subduction related
MOR
dry Cristallization (higher Mg)
- Fast Spreading
well developed sheeted dyke sequences (penrose model)
Water rich environment
depleted Cpx (abreichering an REE)
- Slow Spreading
no sheeted dykes
Water poor environment
Cpx less depleted
Continental Margin
- similar to slow spreading
- contact to continental crust
- dry Cristallization (higher Mg)
- subduction related
Suprasubduction Zone Type SSZ
- spreading rate coupled by slab roll back
- best preserved and most common
- can form in Back arc & fore arc
- similar to penrose model, although sheeted dykes are not expected to well develop
- special geochemistry
- wet Cristallization
- slab below arc is 80-150km deep, critical pressure for subduction recycling is around25-45KBar
\( \to\) ophiolites show where oceans where, and thus they are tectonic significant (reconstruction!)
Classic model = Penrose model what is important in an ophiolitic sequence?
- mantle; melts get into higher layers
- primary: melting related (higher amount on incompatible elements?)
Peridotite (Lherzolith, Harzburgit und Dunit)
- secondary: metasomatic/hydrothermal altered
Peridotit (Wehrlit)
Pyroxenite
- Al2O3, MgO und CaO geben verarmung/Anreicherung an
\(\to\) viel Al2O3 in gestein, wenig Olivin
higher #Mg = more depletion
#Mg = Mg/(Mg+Fe)
--> If the Peridotite is well equilibrated, olivine, Cpx und Opx haben ähnliche #Mg
\(\to\) olivine steigt mit steigender #Mg, Spinell, Cpx und Opx nehmen ab mit zunahme #Mg
increase of #Cr with more partial melting (due to incompatible Al)
#Cr = Cr/(Cr+Al)
Cranton have highest #Mg, due to age and multiple remelting high pressure under cratons, low pressure under MOR \(\to\) primäre Mineralogie Unterschiede entstehen durch P und T unterschiede
Analysis:
Major, Trace elements and Isotope geochemistry to understand formation setting of ophiolites
- fresh glass represents melt comp. (chilled margins)
- hyaloclasite: volcanic breccia formed by rapid fragmentation and cooling of melt under water
W5, F48?
Subduction Zone
- subduction rate: 10Be in Arc Lavas are indicating "recycle" process
Partial melting of the mantle
- due to spreading (MOR)
- due to adiabatic melt (decompression melting) (max 30% meltage today)
\(\to\) younger earth = hotter earth = higher melting degree (up to 60°?) = thicker oceanic crust \(\to\) influence on plate tectonics?
- fast spreading up to 25% melting. at approx 60km depth melting starts
(Mantle Melting is non-modal)
partial melting goes to higher degrees if fluid is involved
trace elements are much more sensitive to melt changes
\(\to\) increasing mantle depletion - REE decrease in Cpx (LREE are more incompatible than HREE and with water, REE depletion increases
Fractional cristallization
- formation of cumulates: misrepresentation of melt composition (density contrast)
\(\to\) represent the solid minerals of a partially cristallized melt
- eutectic: represents melt composition, since no cumulates are formed
- with H2O added:
- Cr-Spinel cristallizes first - leading to a Cr rich horizon at the Crust-mantle boundary
- Cpx cristallizes before plagio
plagio is more An rich with mo0re H2O
- at lower temp: Amphiboles stabilizes
- Moho is deeper
- depleted mantle
Metasomatic changes in Mantle
- Cryptic: Chemistry change (diffusion etc) --> no change in bulk assemblage
- modal: change in chemistry and texture (new minerals replacing)
- mantle-melt interaction: Cpx crystallizes, when melt passes through (adding Cpx to a cpx-less rock = refertilisation
\(\to\) creates dyke like structures: Cpx dissolves and olivine crystallises
Liquid 1 + opx = Liquid 2 + ol ± cpx ± spinel ± amph, whereas liquid 1 is silica undersaturated
- spinel is more ti-rich and al poor and generally a enrichment in incompatible trace elements near metasomatic reactions
- Harzburgite = metasomatic dunite
- Lherzolite = metasomatic Harzburgite
Mg Metasomatism: removing Na, Ca, add Mg and Chloritize
Na metasomatism: addition of Na
Ca metasomatism: removing Na, add Ca, epidotiside, rodingites
\(\to\) metasomatic changes are visible, even when rock reached high facies
Upper Oceanic Crust
- Pillow Lavas and Sheeted dykes
- Sheeted dykes:
- chilled margins (schock abkühlung am rand) geben relatives Alter der dykes
they ideally show the spreading center (only visible in fast spreading)
- Pillow lava
- fine grained rims and coarse grained cores
Seafloor alteration
- Basalt --> Greenstone
Plagio + Pyroxene + Olivine + glass ---> +H2O + Na+ +Ca2 + --> Epidote + Chlorite + Albite + Quartz
- Peridotite ---> Serpentinite
Pyroxene + olivine + Spinel --> + H2o + CO2 + O2 --> Serpentinite + Chlorite + Magnetite + Carbonate
Ultramafics an mafics hold up more H2O than other parts of the oceanic lithosphere
- hydration at MOR (serpentinisation: up to 12 wt% H2O)
dehydration in subduction: W6, F 34
- Brucite - Ant reaction: Ant + Bru -> ol + Chl + fluid
\(\to\) 1-5wt% H2O up to 70km depth at 20-40°C
- Ant breakdown reaction: ant -> Ol + Opx + Chl + Fluid
\(\to\) Hydrofracturing, leading to earthquakes
- Chlorite brakdown: Chl + Cpx -> Ol + Grt + Fluid
\(\to\) At 750-780°C, 30Kbar
No hydrous Phase left
Greenschist (3wt%) -> Blueschist (1.5wt%) -> Eclogite (0wt%)
- 3 different reaction for each Lherzolite, Hazburgite and Chlorite schists
Serpentinite and Chlorite act as lubricants at the slab - mantle surface (important at the top of slab)
\(\to\ serpentinite orientation can cause seismic anisotropy and they can cause decoupling from mantle wedge
- strong heating after mantle coupling (coupling at 80km leading to 700-900°C)
\(\to\) coupling = friction between slab and mantle
Ant + Bru = Olivine + Chl + Fluidgenerally: Eclogite facies hotter than modeled slab?
how to test actual temp?
- with higher temp, H2O gets lost, but Trace elements remain +- the same(no significant trace element loss)
- Na, Cs most "fluid soluble", HFSE + REE most "fluid insoluble"
- aq. fluid are pretty dilute
- phengite: LILE carrier -> brakdown releases LILE?
- stable at UHP, no breakdown in subduction! But with more water, more Phengite gets dissolved(?)
\(\to\) Elements = temometers! K2O/H2O mainly, bc both are buffered (~10% in arc magma = 1-2% sediment component in mantle source
- continouos water release from mafic rocks at fore arc depth
- discontinuous water release from ultramafic rocks
Case Study: Marianas
- Active subduction with Blueschist in subduction channel
- analog for early life developement
- Seismicity reveals almost vertical slab and different depths (some 500-600km, some only 200-300km)
\(\to\) missing slab parts?
P-Wave velocity: Slab stagnates at 600km discontinuity in some parts
Serpentinite-Mud volcanoes
- returns material directly from subduction channel (slab interface) - surface
- serpentinite, corals, sedimentary and metamafic clasts
- slab depth ~27km, from where the mud can travel across a fault to the ocean floor
\(\to\) H2, CH4 are present (due to serpentinisation), which can be used by chemosynthetic organisms to produce energy (early life evolution?)
- typically up to 8% mafic blueschist , rest: serpentinised dunite and harzburgite clasts. Matrix: fine grained serpentinite "mud"
- oldest: 4Ma, whereas clasts up to ~49Ma (blueschist clast)
- PT conditions: deeper than estimated 27km (~50km) and warmer
Rutile cannot grow under those circumstances: Prograde growth
young subduction still warm, since mantle has not yet been cooled from slab
- how exhumed?
Dating:
- Fe-Ti rich (gabbro): zirkon dating
--> small time range: small ocean or not well preserved
Mantle - Melt interaction
Slab recycling to CC arc:
- HREE depletion
- bigger HFSE enriched
- LREE & LILE are main subduction component
hosted in sediments and altered oc
- pos. Sr & Pb anomaly
- "Nb & Ta" negative anomaly
- H2O, CO2, Cl = Volatiles, hosted in sediments, altered OC and serpentinite
- different MORB types:
- Nb/Yb: tracks evolution of mantle source
- Th/Yb: tracks addition of Slab components
--> normalising to Yb minimizes effects of differentialisation
- Boninites: typical subduction Zone (enriched SiO2 for ~8%MgO)
very low TiO2 and high H2O
\(\to\) typical initiator of subdduction zones
-
-
-
-