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Trace Elements and Subduction
Zones
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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.
Mid Ocean Ridge &
Subduction Zone System
Destructive Plate Boundaries
www.platetectonics.com/.../Subduction1.gif
http://www.earthquakes.bgs.ac.uk/latest/plate_boundaries.jpg
Magma Composition:
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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).
Magma Composition
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The most convenient way of comparing
the analyzed data for these various
magma types is to plot the data as
geochronological patterns.
Figures 1(a) & 1(b).
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1(a): Mid Ocean Ridge Basalt variations
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1(b): Within Plate variations
Figures 1(a) & 1(b).
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1(a): Mid Ocean Ridge Basalt variations
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1(b): Within Plate variations
Volcanic Arc Basalt variations
Discrimination and Modeling.
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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.
Discrimination diagram for
characterizing island-arc tholeiites
Figure 3: Possible explanations for
the lower Y contents of IA Tholeiites
relative to MOR Tholeiites.
Fig 3a.
(Degree of partial melting).
Fig 3b.
(Presence of residual minor phases)
Fig 3c.
(The source composition).
Fig 3d.
(Mixing with melt derived from
the subducted oceanic crust)
Mobile - immobile element
co-variations
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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 mobility’s in
aqueous fluids.
Examples of such element pairs (fig 1a and 1b), are
Sr, Ce and Nb or Ta and Ba.
Fig 5a.
(Discrimination diagram on the
basis of Ce/Sr ratios)
Final words:
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By using a careful selection of elements, discrimination diagrams can
be plotted which highlight and indicate the various characteristics of
lava’s. 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.
References:
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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 lava’s
from subduction plate boundaries, 525 – 548.
Skinner, B.J. Porter, S.C. and Botkin, D.B (1999). The Blue
Planet (2nd edition). Page 152.