Chapter 5. Geochemical Zoning in Metamorphic Minerals

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Chapter 5. Geochemical Zoning in Metamorphic
Minerals

• Introduction
• Major element zoning e.g. Garnet
• (a) growth zoning (b) diffusion zoning.
• 3. Trace element zoning e.g. Garnet
• (a) growth zoning (b) exception case.
• Isotope zoning
• (a) Oxygen isotope (b) Radiogenic isotope
• Summary

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Introduction

• Table 1 Examples of metamorphic minerals
• that display chemical zoning (Spear, 1993)

? Most common mineral garnet ?
Metamorphic P-T history 1 Progressive P-T
path 2 Retrograde P-T path
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Major element zoning Garnet

• Common end members
• ? Pyrope Mg3Al2Si3O12
• ? Almandine Fe3Al2Si3O12
• ? Spessartine Mn3Al2Si3O12
• ? Andradite Ca3Fe2Si3O12

Fig. 1. Prograde growth zoning in a garnet from a
lower-grade part of High Himalaya, Ref Waters
webpage.

• Typical growth zoning
• ? Mn/-Ca-rich core
• ? Mg increases towards rim
• ? Fractionation process
• ? Temperature lt 650 C

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(b) Typical diffusion zoning ? Pre-existing
garnet changes composition via diffusion ? Mg
decreases and Mn enriches towards rim ? More
extensive in high-grade rocks ? Temperature gt
600 C
Fig. 2. Retrograde diffusion zoning in a garnet
from a high-grade part of High Himalaya, Ref
Waters webpage.
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Fig. 3a. X-ray maps showing the distribution of
elements in a garnet from SW New Hampshire, USA.
Dark areas are low and light areas are high
concentrations.
Fig. 3b. Line traverse along line shown in the
Fig. 3a, showing the variation of elements in a
1-dimentional traverse. Ref Spear, 1993.
Metamorphic phase equilibria and P-T-t path.
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Retrograde diffusive exchange and reaction
e.g. garnet.
Fig. 4. Diagrams illustrating the change in
Fe/(FeMg) for garnet and biotite during
retrograde reactions (Spear, 1993 Kohn Spear,
2000). G1-B1 shows peak metamorphic
compositions, while G2-B2 and G2-B3 are
retrograde compositions. T0 is metamorphic peak,
t8 is final zoning profile.
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• Two types of reactions related to diffusion
  zoning
• Exchange reactions (ERs) only involve the
  exchange of two elements
• between two minerals and do
  not affect the mineral modes,
• e.g. Fe-Mg exchange between
  garnet and biotite
• almandinephlogopite
  annitepyrope.
• Net transfer reactions (NTRs) involve production
  and consumption of
• minerals, which affect
  modal proportions, e.g.
• garnetK-feldsparH2Osil
  limanitebiotitequartz.

Diffusion to the interpretation of geothermometry
in high-grade rocks
_at_ Equilibrium compositions are meaningful in
thermometry calculation and may obtain real
metamorphic peak P-T conditions in high-grade
rocks. _at_ Disequilibrium compositions resulting
from chemical zoning may produce apparent or
lower temperatures than real peak values.
e.g. in Fig. 4, G1-B1 garnet-biotite composition
pairs normally yield peak metamorphic
conditions, whereas G1-B2 composition pairs are
not in equilibrium and usually produce
lower values.
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Fig. 5. X-ray element maps of Darondi section
garnets with plagioclases. GHS, Great Himalayan
Sequence, LHS, Lesser Himalayan Sequence. GHS
unzoned garnet core high-T difussive
homogenization. rimward Mn increase
retrograde diffusion during cooling. LHS
general Mn decrease growth zoning with
increasing T, some Mn
sharp increase at rim means
back diffusion
after maximum T. Ref Kohn etc,
2001. Geology, 29, 571-574.
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Monazite age 10-22 Ma
Monazite age 8-9 Ma
Fig. 6. Pressure vs. temperature plots for rocks
along Darondi River traverse. A. Main Central
thrust (MCT) zone, P-T conditions increase
toward GHS. B. Structurally higher rocks show
P-T paths with T increase and P decrease. C.
Structurally lower rocks show P-T paths with
both T and P increases. D. P-T path from LHS
along MCT. B heating with exhumation, C
heating with loading. Why? Thermal relaxation
along MCT or in part thrust reactivation at
footwall. Ref Kohn etc, 2001. Geology, 29,
571-574.
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Garnet porphyroblasts in paragneiss around Zhong
Shan station.
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Partially molten cordierite-bearing pelitic
gneiss around Zhong Shan station.
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Broken-up garnet-bearing mafic granulite around
Zhong Shan station.
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Folded banded gneiss from Zhong Shan station in
east Antarctica.
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Deformed mafic granulite in east Antarctica
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Fig. 7. Two types of P-T paths for post-peak P-T
history for most granulites over the world
(Harley, 1989) (a) near isothermal decompression
(ITD) P-T paths (b) near isobaric cooling (IBC)
paths.
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Fig. 8a. Garnet porphyroblast and the symplectite
asemblage in a felsic granulite from Dabie Shan,
China.
Fig. 8b. Zoning profiles of the garnet in Fig.
8a. ? I Xsps decreases, Xpyr increases,
growth zoning. ? II Xsps and Xalm increases,
Xpyr and Xgrs decreases, retrograde
diffusive zoning.
Ref Chen etc, 1998, J Metamorph Geol, 16,
213-222.
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Fig. 9. Backscattered eclectronic image of the
garnet porphyroblast (a) in Fig. 8a and its
corresponding X-ray map of Mg element for the
same garnet (b).
Ref Chen etc, 1998, J Metamorph Geol, 16,
213-222.
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Fig. 10a. Peak P-T estimates via
(1) geothermometry and (2) geobaro- metry. ?
peak P-T conditions P13.5 kb,
T850 C. ? post-peak P-T conditions
P6.0 kb, T700 C.
Fig. 10b. P-T path derived from the garnet
growth zoning and the symplectite assemblages
coupled with the retrograde garnet zoning.
Ref Chen etc, 1998, J Metamorph Geol, 16,
213-222.
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Tectonic implications
Garnet growth zoning formed during prograde P-T
stage, prior to peak metamorphism. Clockwise
P-T path with prograde heating and post-peak near
isobaric cooling reflects a typical collisional
tectonics in Dabie Shan orogeny. Garnet growth
zoning suggests a short residence time for the
granulite at peak metamorphism, whereas
retrograde diffusive zoning indicates a rapid
tectonic uplift history. The rapid tectonic
uplift may be correlated with unroofing of ultra-
high pressure eclogites in the area.
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Other representative examples

• Plagioclase zoning
• (a) Normal zoning Na increases from core to
  rim in metamorphic
• plagioclase.
• (b) Reverse zoning Ca increases from core to
  rim in metamorphic
• plagioclase.
  This is more common, and often
• arises as a
  prograde growth zoning.
• Orthopyroxene zoning
• Al zoning In high-T metamorphic rocks, as Al
  has lower diffusion
• than Fe and Mg elements, Al
  increases from core to rim
• in metamorphic
  orthopyroxene, indicating a prograde
• growth zoning.

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Trace element zoning e.g. garnet
(e.g. Y, Yb, P, Ti, Sc, Zr, Hf, Sr, etc)
Growth zoning
High-T may generally result in homogenization of
the major elements (Fe, Mg, Mn Ca). Trace
element has different charge to impede
diffusion, e.g. P-Si, Na-Mg, thus permit
preservation of trace element zonation in
minerals. e.g. Fig. 11 shows dramatic yttrium
zoning in one garnet is related to garnet growth
in a prograde metamorphic series, this is
correlated with rimward disappearance of xenotime
and garnet growth consumes it.
Fig. 11. (a) X-ray map and (b) composition
profile of yttrium across a garnet, showing a
slight outward increase in Y, and a quick drop
halfway to rim, then Y remains low to to the rim
(Pyle and Spear, 2003).
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Y in garnet is termed as YAG.
Fig. 12. P-T pseudosections for (a) moles of
monazite and xenotime and (b) XYAG in garnet, in
pelitic assemblages. Xenotime is only stable at
relatively low P, and monazite abundance
decreases at higher P relative to apatite. XYAG
contours are strongly dependent on the major
mineral assemblages (Spear etc, 2002 Pyle
Spear, 2003).
e.g. the increase in monazite abundance at
expense of apatite with decreasing P accords
with observations in ultra-high pressure (UHP)
metamorphic terranes that monazite exsolves from
apatite during exhumation (Liou etc, 1998).
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Exception case
Trace element zoning as a record of chemical
disequi- librium during garnet growth. Trace
element zoning in a garnet in metapelites from
New Mexico is ascribed to transitory
participation of different trace
element- enriched phases in garnet forming
reaction, rather than the result of any event
(e.g. changes of P-T or fluid conditions).
Fig. 13. X-ray maps of trace elements and Ca from
garnet-bearing quartzite, showing spatially
obvious spikes (Chernoff Carlson, 1999). The
coincidence of spikes in trace elements and Ca
is interpreted to reflect modal changes in a
mineral like apatite or allanite.
Ref Chernoff Carlson, 1999. Geology, 27,
555-558.
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Isotope zoning
Oxygen isotope
Garnet growth zoning Kohn etc (1993) described
the first isotope zoning profiles that accords
with independent predictions of growth
models. The increase in d18O from core to rim in
garnet is compatible with prograde growth
inferred from major element zoning.
Fig. 14. Oxygen isotope profiles across a garnet
from Tierra del Fuego, Chile, showing general
0.5 increase in d18O from core to rim,
consistent with independent calculations of
oxygen growth zoning in a closed chemical or
isotope system (Kohn etc, 1993).
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Radiogenic isotope
Rb-Sr, Sm-Nd, U-Pb and Lu-Hf in garnet, U-Th-Pb
in monazite. They have slow diffusivities. Core
vs rim isotopic variability is rarely studied due
to sample size requirements. However, Christensen
etc (1994) suggest that 87Sr/86Sr ratio increases
from core to rim in garnet, and is consistent
with progressive growth of the garnet. Grove
and Harrison (1999) showed that diffusional
zoning of 208Pb in monazite could be measured
via ion microprobe, resolving cooling histories,
e.g. monazite from Great Himalayan sequence shows
better cooling history than that from major
elements. Williams etc (1999) dated zoned
monazites via utilizing electron microprobe,
showing a better future use by this technique for
studying multiple tectonothermal histories.
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Summary

• Major element zoning in metamorphic minerals
  (e.g.
• garnet) can be used to determine prograde or
  retrograde
• P-T history via growth or diffusive zoning,
  and so tectonic
• process can be inferred.
• Trace element zoning sometimes may provide
  important
• information on metamorphic process and
  history due to its
• low diffusivity.
• Isotope zoning (particularly Radiogenic) may
  constrain the
• timing of P-T history and tectonic process,
  and may be
• more useful in studying multiple P-T
  histories.