Differentiation Chemical Evolution

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GEO1003Spring2008Lecture 8 (Differentiation of
Magmas)
Ch.11 in Winter
Differentiation (Chemical Evolution) of Magmas
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Differentiation of Magmas Introduction

• We learned in last class that wide range of
  primary basaltic magmas can be
• generated by partial melting of mantle lherzolite
• We also learned that many of these same magmas
  may also be generated by
• fractional crystallization of ultramafic (eg
  lherzolite) melts
• Fractional crystallization of basaltic melts can
  also lead to an even wider
• variety of magma compositions
• Fractional crystallization (FC) is just one of
  several mechanisms by which basaltic
• magmas can generate magmas of different
  compositions
• Other mechanisms of diversification
  (differentiation/evolution) include
  assimilation,
• filter pressing (compaction), flow segregation,
  various mechanisms of volatile
• release, liquid immiscibility, magma mixing,
  thermal diffusion, compositional
• convection
• All these processes involve SEPARATION of
  crystals from liquids, vapors
• from liquids, liquids from liquids etc, or some
  form of CONTAMINATION

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Processes of Crystal-Liquid Separation

• Three main processes FC, filter pressing and
  flow segregation
• FC often considered dominant mechanism by which
  magmas differentiate
• (but important to note that it is only one of
  many such mechanisms)
• Process of crystal settling (gravity settling)
  where crystals are denser than
• magma usually invoked
• Olivine and pyroxene commonly settle out from
  mafic magmas
• Concentrations of settled crystals are called
  cumulates
• Crystal accumulation at top of magma chambers
  also possible if crystals are less
• denser than magma (feldspar can do this). These
  are called flotation cumulates
• Crystal settling is extremely slow (but possible)
  in most intermediate and felsic magmas
• (because they are so viscous)
• FC often involves settling of more than one phase

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Analyses of products (glass) from 1959 eruption
of Kilauea. Parental magma most primitive
glass other glass analyses plotted to show that
range of chemistry can be explained by either
olivine accumulation or extraction
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Filter Pressing and Flow Segregation

• Two other mechanisms of crystal-liquid separation
• Filter pressing is the expulsion of liquid from
  around crystals due to compaction
• (for example in a cumulate layer) or constriction
  (filtering) or crystals through
• a narrowing or obstruction in magma flow path
• Flow of magma against walls, other areas of
  cooler magma, lithics, crystals
• creates differential flow, and provides a
  mechanism for segregating crystals
• and liquids (flow segregation)

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Flow segregation of olivine crystals towards
centers of dikes is fairly common (these examples
are from Skye, Scotland)
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Liquid-Vapor Separation

• Mechanisms of liquid-vapor separation also lead
  to magma differentiation
• Almost all magmas release vapor phase(s) as they
  rise to lower pressures and the
• solubility of the volatile is lowered. Water
  vapor is by far the most important
• Vapor has a lower density than melt so rises (and
  separates)
• Water-rich fluids escaping from magmas are
  responsible for wide variety of
• hydrothermal alteration/metasomatism
• Water may also be release by retrograde (or
  second) boiling following
• crystallization of anhydrous phases and the
  concentration of water in the remaining
• melt. The solubility of water in the melt is
  exceeded (but for a different reason
• this time)
• Vapor separation is significant for magma
  differentiation because certain elements
• partition into the vapor also. These include K,
  Na, S, Cl, F, B and many other large
• or highly charged ions that dont fit easily into
  crystal lattices

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Liquid-Liquid Separation (Liquid Immiscibility)

• Liquid immiscibility (two liquids that dont mix)
  is not uncommon with magmas
• Liquid immiscibility provides a mechanism for
  liquid-liquid separation
• Liquid immsicibilty textures can be observed in
  micro and macroscales
• (droplet or blebby textures)
• Liquid immiscibility is most common in three
  magmatic systems
• 1. A granitic melt may separate from Fe-rich
  tholeiitic basalt magmas
• that are gt70 crystallized. Very small volumes
  of granitic melt
• 2. Sulfide droplets often separate from basaltic
  liquids. May be very large
• Volumes (some massive sulfide deposits are
  economically viable)
• 3. Carbonatite magma can separate from some
  highly alkaline magmas
• (eg phonolites, nephelinites)

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Liquid immiscibility texture thin-section from
Suswa volcano, Kenya, illustrating immiscibilty
between trachyte magma (now brown glass globs)
and carbonate magma (grey matrix)
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Magma Mixing (Liquid-Liquid Mixing)

• Magma mixing is a common process of magma
  differentiation
• It involves the initial mingling and variable
  degrees of (homogenous) mixing
• of two magmas
• Whether two (any two) liquids mix depends on
  their densities, viscosities,
• velocities etc
• Liquids (eg magmas) mix most easily when their
  densities and viscosities are
• similar
• Abundant field and microscale evidence for magma
  mixing (eg swirls etc)
• Replenishment of a magma chamber with more mafic
  magma is a common
• Process and provides a likely mechanism for much
  mixing.
• Magma mixing may also be due to convection
  currents

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Basalt blobsin granite, formed by magma mixing
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Thermal Boundary Layers and (Independent) Magma
Differentiation

• A thermal boundary layer is a boundary between
  two areas of a fluid, across which at least one
• form of heat is not transferred in the case of
  magmas, this is convection
• Many magma chambers now thought to have stagnant
  caps separated from a convecting interior
• by a thermal boundary layer
• Formation of this upper layer is complex (and
  controversial) but can involve (as one example)
• input of H20 into magma from wall rocks and
  formation of a less dense (water-rich) cap
• (density stratification)
• Boundary layer effectively separates the cap from
  the rest of the chamber and allows some
• INDEPENDENT DIFFERENTIATION

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Example of Thermal Boundary Layer Compositional
Convection
Here crystallization against the cooler walls of
minerals denser than melt, allow melt to rise and
possibly spread out as a less dense stagnant top
(or it may sink if it cools significantly (ie if
it becomes more dense than underlying magma
due to lower temperature). This process is
called compositional convection. Note cap and
wall boundary layers, and note that this process
can be isothermal (in theory), hence thermal
boundary
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Assimilation

• Assimilation is the chemical contamination of
  magma by interaction and incorporation
• of wall rock
• Evidence from geochemistry and partially resorbed
  xenoliths
• Most important when melting point of wall rock is
  much less than magma, and volume
• of magma is large (eg large mafic intrusions into
  continental crust)
• Higher surface area of contact (eg from
  stoping) favors assimilation
• Best way to detect if assimilation has occurred
  is by using isotopes (esp of Sr, Nd, U and Pb)
• that we will discuss later in the course

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Combined Differentiation Mechanisms

• Two or more differentiation processes can operate
  at same time, one common pairing is..
• AFCassimilation plus fractional crystallization
• FC recharge of mafic magma is also common
• Can be difficult to unravel relative
  contributions
• Sometimes several models of differentiation
  mechanisms fit the data for a cogenetic suite of
  rocks