Igneous - Subduction Zones Flashcards
What is the Benioff zone?
Benioff zone – plane of earthquakes which define the top of that region of slab
Heat signatures of the moving plates
Downgoing plate
o Top is identified by region of earthquakes
o Fast P-wave velocities in slab = cold
Under volcanic arc
o Slow P-wave velocities = hot
Processes in subduction zones:
- Input: ocean crust sediments, fluids, lithosphere
- Mechanical partitioning
- Metamorphism
- Slab dehydration/melting
- Output (subducted lithosphere): oceanic crust, lithosphere, sediment
- Mantle melting
- Transport, Crystallisation, Assimilation
- Output (Mantle wedge to volcano): Magmatic rock + fluids
Key observations of all subduction zones
Volcanic front typically >100 km above the Benioff zone, independent of subduction angle or velocity.
Subduction zone magmas are wet, often several percent water
( = explosive volcanism).
Geochemical tracers show signature of subducted material (sediment, altered basalt)
within arc lavas.
- Depth of volcanic front is constant – suggestive of pressure-dependent melting or slab dehydration reactions
- Differing subduction angle + distance between trench + Arc front
Arc Magmas:
• Compared to MORBs, arc magmas are:
• Compositionally variable, can be fractionated
basalts –> andesites –> dacites –> rhyolites
- Wet, rich in H2O
- Enriched in many trace elements, such as Rb, that appear to be derived from the subducting slab
• Tholeiitic (MORB, OIB, SZ lavas)
• Alkaline (OIB, SZ)
• Calc-Alkaline (only found in subduction zones)
The effect of water:
- The presence of water in calc- alkaline magmas dramatically lowers the liquidus temperature of plagioclase in basaltic melt
- plagioclase crystallization is inhibited to temperatures well below the dry liquidus.
- residual liquids become enriched in Al and Si and lack of plagioclase
- fractionation prevents strong Fe enrichment
At low pressures the result is abundant crystallisation of plagioclase, resulting in typically plag-phyric subduction zone magmas – in lower pressure zone of rising magma
Oceanic versus Continental Arcs:
- Continental calc-alkaline volcanic suites tend to have higher Si contents (andesites dominate) than those of oceanic suites (where basalts dominate)
- Mafic to intermediate lavas that build strato-volcanoes of continental arcs are typically accompanied by eruption of voluminous felsic rocks
- Granitoid intrusions are also common, typically with intermediate (dacitic) compositions that are uncommon in extrusive rocks
- Continental arcs – intermediate (andesite + dacite)
Composition of typical Arc volcanic rocks:
- Arc tholeiites are dominated by basalt and basaltic andesite. These rocks are common in young island arcs (isolated islands built directly on oceanic crust)
- Calc-alkaline series is dominated by andesite, although the whole sequence from basalt to rhyolite is usually found. This suite dominates the building of older island arcs (Japan, Indonesia), where islands have joined together and built a continuous arc of proto-continent above sea-level.
- Calc-alkaline series also dominates continental margin arcs like the Andes and Cascades.
Island arc volcanism:
Variations in major element chemistry result from:
(1) Primary variations in the source composition, amount of subduction input, or degree of melting of mantle material
(2) Progressive variations due to fractional crystallization
- high water content
- assimilation of older overlying crust
H2O content of Arc Magmas:
- Arc magmas typically contain significantly more H2O than MORBs, OIBs. Measured directly using “melt inclusions”. Many subduction zone magmas have at least 4-6 wt% H2O
- H2O is dissolved in the melt until it becomes saturated at low pressures. Then, continued magma ascent results in degassing and bubble nucleation, leading to explosive eruptions
- Hornblende (amphibole) is common phenocryst in hydrous magmas (may also see biotite)
Potential source materials for subduction magmas
- The crustal portion of the subjecting slab
- Altered oceanic crust (hydrated by circulated seawater and metamorphosed in large part to green schist facies
- Subducted oceanic and forearm sediments
- Seawater trapped in pore spaces - The mantle wedge between the slab and the arc crust
- The arc crust
- The lithospheric mantle of the subducting plate
- The asthenosphere beneath the slab
H20 melting
Lowers melting T of rocks - mantle solidus reduced
- Dehydration: hydrous minerals e.g. chlorite and amphibole releases water in older arcs (no slab melting)
- Slab melting: occurs in arcs subjecting young lithosphere (debated)
Hydrothermal circulation at mid-ocean ridges:
- Adds H2O, CO2 to crust
- Basalt, gabbro à minerals stable at high temperature, low pressure minerals (amphibole, chlorite, and epidote)
- Could go deeper than oceanic crust (into lithosphere) at slow-spreading ridges/where fractures exist
Sea-floor weathering - Low T alteration
During deposition, water is incorporated into sediments.
Carbonate sediments provide CO2
Trace element characteristics of arc rocks:
Arc basalts, as compared to MORBs, are
- Enriched in large-ion lithophile elements (LILE)
K, Rb, Cs, Sr, Ba, and Pb, U, B, Be
- Depleted in high field strength elements (HFSE)
Y, Zr, Hf, HREE, Nb, Ta
H2O-rich fluids will preferentially contain more mobile LILE
derived from ‘slab contribution’
Be and Th are relatively enriched in arc basalts, but believed to be relatively insoluble in H2O-rich fluids
derived from sediments
Alkali basalts are enriched in incompatible trace elements (relative to MORB) as a result of small degrees of partial melting; typically more enriched mantle sources. High pressure generation results in lower HREE contents (retained in garnet)
Relative to MORB, arc lavas are enriched in fluid-mobile, incompatible elements (Ba, Rb, Pb) and depleted in fluid-immobile, incompatible elements (Nb, Ta) and HREE
Summary
- Diverse magma compositions due to varying source inputs (ocean crust, sediments, mantle wedge) and magmatic processes (fractional crystallisation, mixing)
- High water content. Hydrous minerals are common (e.g. amphibole) as is abundant low-pressure crystallisation of plagioclase
- Trace elements indicate enrichment in fluid-mobile elements. Retention of Nb, Ta
- Isotopes point to mantle peridotite, sediment and altered oceanic crust as sources
- Dehydration of AOC and sediments releases water and fluid-mobile elements, transferred into to mantle wedge
- Hydrated mantle wedge undergoes partial melting at relatively low temperatures
