Chapter 5. Geochemical Zoning in Metamorphic Minerals

1. Introduction2. 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 isotope5. Summary

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

Major element zoning: GarnetCommon end members:

● Pyrope Mg3Al2Si3O12

● Almandine Fe3Al2Si3O12

● Spessartine Mn3Al2Si3O12

● Andradite Ca3Fe2Si3O12

(a)Typical growth zoning:

● Mn+/-Ca-rich core

● Mg increases towards rim

● Fractionation process

● Temperature < ~650 °C

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

(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 °CFig. 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, 16, 213-222.

Fig. 8a. Garnet porphyroblast and the symplectite asemblage in afelsic 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.

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) geobaro-metry. ● peak P-T conditions: P=13.5 kb, T=850 °C. ● post-peak P-T conditions: P=6.0 kb, T=700 °C.

Fig. 10b. P-T path derived from the garnet growth zoning and the symplectite assemblages coupledwith the retrograde garnet zoning.

Ref: Chen etc, 1998, J Metamorph Geol, 16, 213-222.

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.

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.

Trace element zoning: e.g. garnet

Growth zoning:

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 toto 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 elementzonation in minerals.

e.g. Fig. 11 shows dramatic yttrium zoning in one garnet isrelated to garnet growth in aprograde metamorphic series,this is correlated with rimwarddisappearance of xenotime andgarnet 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 stableat 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 garnetis 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 garnetforming reaction, rather thanthe 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 δ18O from core to rim, consistentwith 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 δ18O 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 (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.

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.