L20- Island Arcs

Draw a cross section of an island arc with isotherms and identify all elements

*Describe each of these elements

Elements in order away from trench:

• deep trench

  • marks the plate boundary at the earth’s surface before being subducted

  • up to 11km

• forearc

  • accretionary prism dependent on subduction steepness 150-300km

  • mix of flows, pyroclastic flows, deformed/imbricated/thrusted sediments

• volcanic arc

  • where volcanic activity begin until reducing towards the back arc 10km

  • can have a second arc at 50 km

• zone of earthquake foci/benioff zone

  • constant 110km distance due to minerals dehydration controlled by pressure

• back arc basin

  • MORB like volcanism = extensional = thin ocean-type crust

*Why plagioclase comp in arc magmas are more calcic than MORBS/OIBS

since raising H2O content favors the crystallization of An over Ab

• H2O disrupt SiO2 polymers

• Anorthite has 50% less Si-O-Si bonds

*How and why does the subduction dip angle changes

vary from 30 to 90 degrees due to plate’s age and thickness

• younger plate = hotter = smaller dip angle

• thicker crust = harder to subduct = smaller dip angle

*What is the average subduction rates

0.9 to 10.8 cm per year

*Give key general elements used to differentiate IA magmatism to other settings

volcanic island chains along subduction zones

• comp of volcanic suites = more diverse & silicic = basalt is subordinate

• stratovolcanoes = main landform = Mont Fuji

*Mont Fuji

3 composite stratacone of alternating lavaflows/pyroclastics

Komitake:

• middle pleistocene

• andesite lava flows

Older Fuji:

• 80ka to 11ka

• pyroclastic falls

• tephra changed the chemistry from dacitic to basaltic (no andesitic)

Younger Fuji:

• high and famous due to position in the triple junction of subducting plates = earthquakes trigger more magma and eruptions

Characterize magmas with respect to petrography, mineralogy, major, and trace element chemistry, and the origin

Petrography:

• dominant: Andesites and Basalts (thoeliitic) + lost of pyroclastic rocks

• subordinate: Dacites (calc-alk)

• rare: rhyolites

• never: alkaline series rocks

Mineralogy:

• Phenocrysts and GM: Plag, Augite, Hb, Phl

• Phenocryst opx only when there’s no Ol, Hb, Bi, Phl

• Fe-Mg solidsolution minerals = Mg Rich

• Plag SS = Ca-Rich since high h20 content

• evolved high K calc alk series

  • Bi + hypersthene

  • Ksp

  • highly porphyritic

• thoeliitic

  • pigeonite

  • quartz most common felsic

  • usually less porphyritic

Major:

Trace:

• high (fractionated much), med, low (flat) K series : LREE height varies due to removal of ol, fsp, px

• no deep garnet = melt not from the subducted crust

• Nb depression (subduction magma hallmark) because of residual rutile, illmenite, sphene

• 2 different behaviors due to decoupling of LIL and HFS:

  • LIL: hydrophilic (readily fractionated into a hydrous fluid phase)

  • HFS: non-soluble in aqueous pore fluid

Origin:

• heterogeneous depleted mantle source which is not the subducted crust

• some also have an origin of the enriched region in Sr/Nd (detrial sediments super rich in Sr)

*Why are LIL more soluble in aqueous pore fluids

because of their large ionic radii and low valence

*Def shards

bubble walls with curved shapes id in thin sections

*Name one type of pyroclastic rock expected in this setting

welded tuff/ignimbrite

• explosive eruptions

*What information does strongly pleochroic oxy hornblende provide

info on: water content + depth of magma segregation

• magnetite/anhydrous phases opatic rim = decomposition and reaction with O2 (Fe oxidation) of hydrous phenocrysts (Hb, Bi) due to instability in hornblende after lost of H2O pressure upon eruption = dehydration of magmas

• common in volcanic rocks which was in conditions: > 3wt% H2O at > 2Kb (Hb stability field)

define (sub-alkaline categories) the calc-alkaline and tholeiitic series

Calc-alkaline:

• convergent boundaries

• oxidizing conditions Fe3+ = Fe early crystallized as Fe-Ti oxides = small oxide pieces in ppl

• increase followed by decrease of FeO vs Sio2 diagram

• only found in subduction settins

Tholeiitic:

• Fe stays in melt

• black late-crystallizing glass

identify the source of the h2o in island arcs and explain its effect on the composition, evolution, and mineralogy of magma

done without the slides:

• from the dehydration of minerals present in the subducted crust at 110km water is released.

• makes hydrous minerals like Bi and Hb which later oxide as they loose their environment

• also contributes to calc-alk early oxide crystallization

• cause a different trace element trend

NOTE

• mainly in the pacific (2 in the atlantic = south sandwich and antilles)