Chapter 5 geochemical zoning in metamorphic minerals

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Chapter Geochemical Zoning in Metamorphic Minerals Introduction Major element zoning: e.g Garnet (a) growth zoning; (b) diffusion zoning Trace element zoning: e.g Garnet (a) growth zoning; (b) exception case Isotope zoning: (a) Oxygen isotope; (b) Radiogenic isotope Summary Introduction ● Most common mineral: garnet ● Metamorphic P-T history: Progressive P-T path Retrograde P-T path Table Examples of metamorphic minerals that display chemical zoning† (Spear, 1993) Major element zoning: Garnet Common 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 Prograde growth zoning in a garnet from a lower-grade part of High Himalaya, Ref: Waters webpage (b) Typical diffusion zoning: Fig Retrograde diffusion zoning in a garnet from a high-grade part of High Himalaya, Ref: Waters webpage ● Pre-existing garnet changes composition via diffusion ● Mg decreases and Mn enriches towards rim ● More extensive in high-grade rocks ● Temperature > ~600 °C 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 Retrograde diffusive exchange and reaction: e.g garnet Fig 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 Two types of reactions related to diffusion zoning: Exchange reactions (ERs): only involve the exchange of two elements between two minerals and not affect the mineral modes, e.g Fe-Mg exchange between garnet and biotite: almandine+phlogopite=annite+pyrope 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 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 Monazite age: 10-22 Ma Monazite age: 8-9 Ma Fig 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 Broken-up garnet-bearing mafic gran around Zhong Shan station Folded banded gneiss from Zhong Shan station in east Antarctica Deformed mafic granulite in east Antarctica Fig 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 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 Fig 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) geobarometry ● 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 coupled with 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 ultrahigh 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 (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) 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) Exception case: Trace element zoning as a record of chemical disequilibrium during garnet growth Trace element zoning in a garnet in metapelites from New Mexico is ascribed to transitory participation of different trace elementenriched 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 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 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 δ18O from core to rim in garnet is compatible with prograde Fig 14 Oxygen isotope profiles across a garnet growth inferred from major from Tierra del Fuego, Chile, showing general ~0.5‰ increase in δ18O from core to rim, consistent element zoning with independent calculations of oxygen growth zoning in a closed chemical or isotope system (Kohn etc, 1993) 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
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