Generation/Evolution of Magma

• Earth Structure:

  • earth has elastic properties of solid (except liquid outer core; evidenced by S-waves not transmitted through liquids); change from solid-liquid from change in temperature, pressure and composition at depth
  • 'low velocity zone'; in upper mantle where velocity of seismic waves slow down (mantle rock weaker/capable of flowing) + this is asthenosphere; flows plastically enabling plates on lithopshere to move
• Magma:

  • mantle melting dependant on temperature/pressure (geotherm increases with depth/pressure; melting curve for peridotite changes with increases in pressure/temperature)
  • solids near melting temperature 'weaken'; boundary between asthenosphere/lithopshere where mantle close to 1300$^o$C (loss of strength is gradual) - asthenosphere base; pressure/temperature mean melting curve temperature greater than geotherm
• Partial Melting:

  • no single melting point for whole of rock as aggregate of various minerals; as temperature/pressure rise, rocks partially melt when individual minerals exceed their melting points
  • minerals with lowest melting points melt first along crystal edges; solid crystals squeezed into melt pockets + liquid flows outwards and collects in sphererical regions; absorb melt from surrounding zone/rise quickly towards surface - mantle source rock stays solid
• • partial melting of mantle begins when earth's temperature or geotherm exceeds melting point temperature for mantle rock
  • in areas of no volcanism, mantle isn't melted as geotherm below temperature required to melt peridotite
  • partial melting mainly occurs at plate boundaries (dependant on changes to geotherm/local melting temperature of mantle)
• Hotspot Melting:

  • geotherm locally raised/exceeds melting point of local peridotite (induces melting); linked to mantle plumes with higher local temperatures from uprising of hot rock, eg. mid-plate volcanic activity in Hawaii
• Subduction Zone Melting:

  • at convergent boundaries, partial melting achieved by release of water from hydrous minerals in subduction zone
  • locally lowers melting temperature of overlying mantle wedge (melting curve below geotherm as water induces partial melting)
  • eg. Aleutian Island Arc
• MOR Melting:

  • at divergent boundary, partial melting results from rapid decompression of mantle; mantle flows towards surface in solid state leading to reduction in pressure/rapid decompression
  • mantle rock rises rapidly/retains heat as poor conductor; magma collects below crust (extruded during eruptions), eg. Iceland
• Partial Melting at Plate Boundaries:

  • divergent - basaltic magma (mafic)
  • convergent - andesitic magma (intermediate)
  • mantle plumes/hotspots - basaltic magma (mafic)
  • orogeny - granitic magma (silicic/felsic)
• Discontinuous (Bowen Reaction) Series:

  • crystallisation of minerals rich in iron/magnesium with silica (mafic minerals); mafic magma, olivine first to form (>1500$^o$C) then pyroxene, amphibole and biotite (as temperature lowers)
  • if cooling slow, early formed/high temperature minerals reacts with magma to form next mineral down series (olivine reacts to form pyroxene if enough silica present)
  • if cooling quick, reaction doesn't have time to occur/olivine is preserved; reaction is incomplete (form reaction rim around edge)
• Continuous (Bowen Reaction) Series:

  • crystallisation of plagioclase feldspar (albite-anorthite); anorthite is Ca-rich forming at high temperatures + albite is Na-rich forming at low temperatures - intermediate composition as temp. drops
  • plagioclase continuously reacting with melt to form Na-rich crystals as temp. decreases + crystals may show zoning; centre of crystal Ca-rich/towards edge more Na-rich plagioclase
  • Ca-rich in ultramafic/mafic rocks + Na-rich in silicic/intermediate rocks
• Chemical Equilibrium:

  • given sufficient time/continued contact between crystals and melt, all of olivine converts into pyroxene
  • if crystallisation occured in chemical equilibrium, rocks formed will have identical composition to original magma
• Magma Differentiation (MD) - processes that cause a parent magma to evolve into magmas of different compositions (different rocks produced from single parent magma).
• Fractional Crystallisation (MD):

  • olivine/pyroxenes form at high temperatures using Fe/Mg from magma in their crystal lattices + Ca-rich, high temperature plagioclase crystals form
  • magma depleted in Fe, Mg and Ca - remaining liquid enriched in silica, potassium, sodium and water (early formed minerals poor in these) - composition of magma changes over time
• Gravity Settling (MD):

  • magma 10% less dense than equivalent composition in solid rock; crystals denser than liquid so settle out
  • early formed minerals (olivine/pyroxene) have greater % of iron so are denser; sink to form cumulate layer at intrusion/magma chamber base + crystals suspended in magma react with magma
  • gravity settling removes crystals from remaining liquid so no longer react with remaining magma changing the composition
• Filter Pressing (MD):

  • during magma crystallisation, point where crystals/liquid exist together; due to weight of overlying crystals, liquid squeezed out forming separate layer above
  • liquid depleted in elements incorporated into early formed crystals and enriched in elements forming felsic minerals
• Magma Mixing:

  • magmas of two different compositions are immisicle (incapable of being mixed); density/thermal contrasts act to keep them seperate
  • keeping then seperated requires stirring mechanism (convection within chamber) to produce a magma intermediate in composition between two parent magma.
• Magma Contamination:

  • stoping is a process where blocks of country rock (from conduits or walls/roof of magma chamber) are broken off by rising magma and incorporated into magma as xenoliths
  • if these xenoliths melt and become assimilated into the magma, they can contaminate it/change the bulk composition of the magma - some xenoliths are preserved (not melted)
• Granite Formation:

  • found at destructive margins; from fractional crystallisation of andesitic magma/partial melting of andesitic crust
  • overriding plate cold/brittle; early formed andesitic magma gather together into diapirs which rise into upper crust (initially, don't rise far until they solidify) - process is continuous
  • as more diapirs rise, overriding plate becomes hotter; partial melting of early formed rocks produces silicic magma (granite)
• Production of Granites:

  • silicic magma rises to higher levels in crust before solidifying into plutonic rocks - granite melts highly viscous + crystals plutonic and interlocking
  • granitic magma less dense than surrounding rocks; density difference enables magma to rise; if rising granite body enter region of lower density rocks, it stops rising (cools/solidifies)
  • stoping occurs where blocks of existing country rock fall into magma allowing it to rise (blocks may be assimilated)
• • Xenoliths - angular/not assimilated.
  • Enclave - rounded 'blob'/not assimilated; may have incorporated minerals from main magma mix.
• • in deeply buried lower continental crust during orogeny, melting of crustal material generates granitic magma
  • continental geotherm crosses melting point curve for wet/hydrous granite at 24km (partial melting of wet granitic rocks begin)
  • continental geotherm crosses melting point curve for dry granite at 40km (partial melting of dry granitic rocks begin)
• Palisades Sill, New Jersey

  • upper/lower edges cooled rapidly (contact with cold country rock); chilled margins (basalt) have fine crystals/same composition as original magma as cooled before differentiation
  • main part of intrusion crystallised/early formed olivine crystals began to sink (gravity settling; olivine 0.8$gcm^{-3}$ denser) + olivine crystals form 10m thick layer at base of intrusion

  
• Palisades Sill, New Jersey

  • crystallisation from top/bottom of sill as crystals grew in cooler areas; dolerite (medium crystals) forms intrusion + last part of magma to crystallise 200m above intrusion base (plutonic gabbro)
  • fractionation caused composition to be lower in mafic mineral than original composition (magma depleted in Fe/Mg + richer in plagioclase/silica)
• Skaergaard Intrusion, Greenland

  • intruded during tertiary/55Ma (North Atlantic was opening) as a magma chamber for basalt volcanoes; magma intruded in a single injection into a huge conical intrusion
  • layered intrusion with rhythmic layering of olivine, pyroxene and feldspar + plutonic crystal size (>5mm) + bulk composition is mafic (basaltic)

  
• Skaergaard Intrusion, Greenland

  • Marginal Border Series; chilled margin with fine crystal size (cooled rapidly) + no longer same composition as original magma (contaminated by country rock) + crystals grew inwards.
  • Upper Border Series; thinner + mirrors 2500m Layered Series (layers crystallised by top down).
• Skaergaard Intrusion, Greenland

  • Layered Series; rhythmic layering (crystal setting interrupted by periodic large-scale convection) + sequence of denser crystals (olivine/pyroxene) beneath light plagioclase deposited by gravity settling - filter pressing causes explusion of differentiated liquid then convection mixes the magma; each cycle means magma more evolved from removal of crystals (cumulates form on floor).
• Solid Solution Phase Diagram - the behaviour of chemical solid solution series, such as the transition from high temperature, Ca-rich plagioclase to low temperature Na-rich plagioclase, or the transition from high temperature Mg-rich to low temperature Fe-rich crystals in ferromagnesium minerals.
• Continuous Series (Plagioclase):

  • crystallisation of plagioclase feldspar (Albite-Anorthite); anorthite is Ca-rich forming at high temperatures + albite is Na-rich forming at low temperatures - intermediate composition as temp. drops
  • plagioclase continuously reacting with melt to form more Na-rich crystals as temperature decreases - individual crystals show zoning; centre Ca-rich/edges Na-rich
  • Ca-rich in mafic/ultramafic rocks + Na-rich in silicic/intermediate rocks
• Albite-Anorthite Phase Diagram:

  • melt reaches liquidus line (seperates liquid from liquid-crystal phase) and crystals begin to form; first formed crystals richer in Ca than original composition
  • as crystals remove Ca from melt, it becomes richer in Na (composition migrates down liquidus)
  • point on solidus horizontally opposite liquidus indicates crystal composition/most stable form of plagioclase at that temp. crystallises

  
• Albite-Anorthite Phase Diagram:

  • plagioclase crystals continuously react with melt; allows for free substitution of Ca for Na within crystal lattice + forms more Na rich crystals as temp. decreases (composition in equilibrium with temperature)
  • if system remains in equilibrium, then last drop of melt must form crystal with same composition as original melt
• Solid Solution Series: Olivine

  Forsterite $Mg_2SiO_4$ - Fayalite $Fe_2SiO_4$

  • discontinuous series; high temperature, Mg-rich olivine (forsterite) to low temperature Fe-rich olivine (fayalite) where magnesium is substituted for iron as temperature decreases

  
• • when two reaction series converge at low temperatures, minerals remain that will not react with remaining liquid (felsic minerals, rich in silica; K Feldspar, muscovite mica and quartz at 700$^o$C)
• Magma at MORs:

  • slow spreading (Mid-Atlantic Ridge) - insuffienct partial melting to maintain magma chamber (may be mush zones where small volumes of melt exist within softened mantle) + magma chambers short lived/discontinuous + each eruption is distinct
  • fast spreading (East Pacific Rise) - magma rises quickly/more heat passes into plate + rock poor thermal conductor so heat can't escape quickly/lithosphere becomes hotter/ductile + ridge crest can't subside due to rising magma
• Decompression Melting - rising magma experiences decrease in pressure; expansion causes a reduction in temperature with loss of heat as molecules use energy to move further apart - melting occurs as melting point decreases as pressure decreases.
• Magma Generation:

  • depths 100-200km, mantle at 1300$^o$C (close to peridotite melting point); mantle material rises below MOR, pressure drops/mantle material expands (cooling effect from increase in volume)
  • if rising mantle hot/rises far enough, starts to melt by decompression melting; 3% of rocks melts each 10km rock rises once melting starts
  • amount magma produced depends on latent heat of fusion/melting (takes lot of heat to change silicates from solid to liquid state)
• Magma Generation:

  • melt forms as thin films inbetween crystals; liquid moves upwards joining other melts, accumulating in shallow magma chamber cooled by ocean above (sea water infiltrates)
  • melt percolates upwards, pressure inside chamber rises until top of shallow chamber splits apart creating narrow crack
  • pressurised melt flows up cracks/erupts onto seafloor as basalt lava; melt solidifies into dolerite dykes + magma chamber solidifes to form gabbro of lower ocean crust
• Zoned Crystals:

  • crystallisation of melt takes place too rapidly and early formed crystals don't have time to react back fully with the melt
  • early Ca-rich crystals only partly react back with melt/leave crystals remnents behind + crystal grows around this surrounding the core as temperatures fall
  • zone crystal of plagioclase; centre core of Ca-rich anorthite surrounded by layers of increasingly Na-rich albite