Chapter 5 geochemical zoning in metamorphic minerals

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 >

Chapter 5 Geochemical Zoning

in Metamorphic Minerals

1 Introduction

2 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

4 Isotope zoning:

(a) Oxygen isotope; (b) Radiogenic isotope

5 Summary

Major element zoning: Garnet

Common end members:

(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 >

~600 °C

Fig 2 Retrograde diffusion zoning in a

garnet from a high-grade part of High

Himalaya,

Ref: Waters webpage

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

Fig 4 Diagrams illustrating the change in Fe/(Fe+Mg) 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,

t∞ is final zoning profile

Retrograde diffusive exchange and reaction: e.g garnet

Two types of reactions related to diffusion zoning:

1 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:

almandine+phlogopite=annite+pyrope

2 Net transfer reactions (NTRs): involve production and consumption of minerals, which affect modal proportions, e.g

garnet+K-feldspar+H2O=sillimanite+biotite+quartz

Diffusion to the interpretation of geothermometry in high-grade rocks:

@ Equilibrium compositions are meaningful in thermometry calculation and may obtain real metamorphic peak P-T conditions in high-grade rocks

@ 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

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.

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

Monazite age: 10-22 Ma

Monazite age: 8-9 Ma

Garnet porphyroblasts in paragneiss around Zhong Shan station.

Partially molten cordierite-bearing pelitic gneiss around Zhong Shan station.

Broken-up garnet-bearing mafic granulite around Zhong Shan station

Folded banded gneiss from Zhong Shan station in east Antarctica.

Deformed mafic granulite in east Antarctica

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

Ref: Chen etc, 1998, J Metamorph Geol,

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

Fig 10a Peak P-T estimates via (1)

geothermometry and (2)

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

Other representative examples

Trace element zoning: e.g garnet

to the rim (Pyle and Spear, 2003)

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

(e.g Y, Yb, P, Ti, Sc, Zr, Hf, Sr, etc)

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)

Y in garnet

is termed

as “YAG”

e.g the increase in monazite abundance at expense of apatite with decreasing

P accords with observations in ultra-high pressure (UHP) metamorphic terranesthat monazite exsolves from apatite during exhumation (Liou etc, 1998)

Exception case:

Fig 13 X-ray maps of trace elements and Ca from garnet-bearing quartzite, showing spatially obviousspikes (Chernoff & Carlson, 1999) The

coincidence

of spikes in trace elements and Ca is interpreted toreflect modal changes in a mineral like apatite or allanite

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)

Ref: Chernoff & Carlson, 1999

Geology, 27, 555-558.

Fig 14 Oxygen isotope profiles across a garnet from Tierra del Fuego, Chile, showing general

~0.5‰ increase in δ 18 O from core to rim, consistent with independent calculations of oxygen growth zoning in a closed chemical or isotope system (Kohn etc, 1993)

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 δ 18 O from

core to rim in garnet is

compatible with prograde

growth inferred from major

element zoning.

Isotope zoning

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

garnet, and is consistent with progressive growth of the garnet

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

• 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.