Trace element characteristics

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Trace element characteristics of lavas from
destructive plate boundaries.
Shane Turner
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Trace Elements and Subduction Zones

• A trace element is a chemical element which has a
  concentration of less than 1000 parts per million
  (ppm) or in other words, makes up 0.1 of a
  rock's composition.
• Subduction zones are zones where cool
  lithospheric plates are dragged down back into
  the mantle.

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Mid Ocean Ridge Subduction Zone System
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Destructive Plate Boundaries
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Magma Composition

• Have a basaltic composition and form the
  Volcanic Arc Basalts.
• They can be sub-divided into Island Arc
  Tholeiites, Calc-Alkaline Basalts and Shoshonites
  according to the K2O-SiO2 discriminant of Taylor
  and Peccerillo (1976).

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Magma Composition

• The most convenient way of comparing the analyzed
  data for these various magma types is to plot the
  data as geochronological patterns.

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Figures 1(a) 1(b).

• 1(a) Mid Ocean Ridge Basalt variations
• 1(b) Within Plate variations

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Figures 1(a) 1(b).

• 1(a) Mid Ocean Ridge Basalt variations
• 1(b) Within Plate variations

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Volcanic Arc Basalt variations
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Discrimination and Modeling.

• Discriminant diagrams based on Ti (immobile
  incompatible element) and Cr (fractionation
  index), are used to successfully discriminate
  between IAT and MORB magma types. Y is often used
  instead of Ti.

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Discrimination diagram for characterizing
island-arc tholeiites
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Figure 3 Possible explanations for the lower Y
contents of IA Tholeiites relative to MOR
Tholeiites.
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Fig 3a. (Degree of partial melting).
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Fig 3b. (Presence of residual minor phases)
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Fig 3c.(The source composition).
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Fig 3d.(Mixing with melt derived from the
subducted oceanic crust)
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Mobile - immobile element co-variations

• It is also necessary to filter out the effects of
  other types of enrichment processes.
• Easiest Way Choose a pair of elements which are
  enriched to a similar extent in within plate and
  MOR settings, yet behave differently in the
  volcanic arc environment, because of their
  different mobilitys in aqueous fluids.
• Examples of such element pairs (fig 1a and 1b),
  are Sr, Ce and Nb or Ta and Ba.

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Fig 5a.(Discrimination diagram on the basis of
Ce/Sr ratios)
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Final words

• By using a careful selection of elements,
  discrimination diagrams can be plotted which
  highlight and indicate the various
  characteristics of lavas. These diagrams can
  therefore enable us to recognize volcanic arc
  basalts, where geological evidence is
  insufficient, due to the factors of deformation,
  erosion, metamorphism etc. These plots can be
  modeled in terms of vectors, which represent
  different petrogenetic processes.
• A convenient way of comparing analyses for these
  various magma types is to plot the data as
  geochronological patterns.
• Many geologists rely heavily on these volcanic
  rocks for providing the evidence by which past
  destructive plate boundaries can be located. It
  is therefore important to be able to detect
  whether or not a lava sequence was interrupted in
  a volcanic arc. This is particularly true in many
  orogenic belts, where allochtonous fragments of
  island arc and back-arc basins are thought to
  exist.

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References

• Armstrong, R.L. (1971). Isotopic and chemical
  constraints on models of magma genesis in
  volcanic arcs, 137 142.
• Jakes, P. and Gill, J. (1970). Rare earth
  elements and the island arc tholeiite series, 17
  28.
• Jakes, P. and White A.J.R (1972). Major trace
  element abundances in volcanic rocks of orogenic
  areas, 29 40.
• Pearce, J.A (1982). Trace element characteristics
  of lavas from subduction plate boundaries, 525
  548.
• Skinner, B.J. Porter, S.C. and Botkin, D.B
  (1999). The Blue Planet (2nd edition). Page 152.