Chapter 9: Trace Elements Note magnitude of major element changes Element Distribution...as a tool in the interpretation of the history of igneous rocks Different elements have diff. affinities for environments to reside: Si later melts - Mg early xls. Remember from Chapter 1: A. Goldschmidt: some elements metals "Siderophile" Fe, Pt, Mo some elements sulfides "Chalcophile" S,Cu,Zn some elements silicates "Lithophile" Si,K,Ca,REE SIMPLISTIC Xl Field Theory best results. Study of elec. envir. in lattice & melt... Trace Elements: very low conc. TE's don't govern the appearance of a phase (as K req. Ksp or Bi), but enter various phases by substitution. Compare Harkers Major E usually vary by < 101 Figure 8.2. Harker variation diagram for 310 analyzed volcanic rocks from Crater Lake (Mt. Mazama), Oregon Cascades. Data compiled by Rick Conrey (personal communication). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Chapter 9: Trace Elements Now note magnitude of trace element changes TE often vary by > 103 very useful since so sensitive to distr. & fractionation Figure 9.1. Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Element Distribution Goldschmidt’s rules (simplistic, but useful) 1. 2 ions with the same valence and radius should exchange easily and enter a solid solution in amounts equal to their overall proportions How does Rb behave? Ni? therefore some TE's will follow similar major E Periodic Table is next slide

Rb follows K & conc. in Ksp, mica, & late melt Ni follows Mg & conc in olivine

Goldschmidt’s rules 2. If 2 ions have a similar radius and the same valence: the smaller ion is preferentially incorporated into the solid over the liquid Fig. 6.10. Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, 177-213. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. smaller ion preferentially -> solid (Mg is smaller than Fe so more Mg in Ol than in melt)

3. If 2 ions have a similar radius, but different valence: the ion with the higher charge is preferentially incorporated into the solid over the liquid (Cr+3 and Ti+4 are always preferred in solids/liquids)

Chemical Fractionation The uneven distribution of an ion between two competing (equilibrium) phases

K = equilibrium constant Exchange equilibrium of a component i between two phases (solid and liquid) i (liquid) = i (solid) eq. 9.2 K = = K = equilibrium constant a solid a liquid  X solid  X liquid i i i i i i

Thus if XNi in the system doubles the XNi in all phases will double Trace element concentrations are in the Henry’s Law region of concentration, so their activity varies in direct relation to their concentration in the system Thus if XNi in the system doubles the XNi in all phases will double This does not mean that XNi in all phases is the same, since trace elements do fractionate. Rather the XNi within each phase will vary in proportion to the system concentration For example: suppose C(Ni) = 20 ppm in a system C(Ni) in olivine may be 100 ppm C(Ni) in plagioclase may be 1 ppm C(Ni) in liquid may be 10 ppm Double C(Ni) in system to 40 ppm: Ol -> 200 ppm, Plag -> 2 ppm and liquid -> 20 ppm

incompatible elements are concentrated in the melt (KD or D) « 1 compatible elements are concentrated in the solid KD or D » 1

For dilute solutions can substitute D for KD: D = Where CS = the concentration of some element in the solid phase CS CL Since the concentration of a trace element (unlike major elements--note Fo-Fa) in any phase is proportional to the coverall concentration of that element, a more convenient constant is commonly used for them. It is commonly referred to as "D" for TE's (although some authors still use KD) and is called a partition coefficient:   D = C(S)/C(L) Where CS and CL are the concentration of a trace element in the solid and liquid phases, respectively (the wt% or ppm value directly). As long as the element occurs in very dilute concentrations, D is a constant.

Incompatible elements commonly  two subgroups Smaller, highly charged high field strength (HFS) elements (REE, Th, U, Ce, Pb4+, Zr, Hf, Ti, Nb, Ta) Low field strength large ion lithophile (LIL) elements (K, Rb, Cs, Ba, Pb2+, Sr, Eu2+) are more mobile, particularly if a fluid phase is involved Depends on the minerals involved! Sr -> melt as ol & px separate -> plag (Ca) & not melt if plag is phenocryst phase Commonly standardized to mantle compositions (olivine, pyroxenes, and perhaps garnet) Thus the major elements Mg and Fe would usually be referred to as compatible, while K and Na as incompatible

Which are incompatible? Why? Compatibility depends on minerals and melts involved. Which are incompatible? Why? Not exact, since D varies with the composition of mins & melt

For a rock, determine the bulk distribution coefficient D for an element by calculating the contribution for each mineral eq. 9.4: Di =  WA Di WA = weight % of mineral A in the rock Di = partition coefficient of element i in mineral A A

Note that 85% Ol + Opx, but 5% Grt raises bulk D to 0.366 Example: hypothetical garnet lherzolite = 60% olivine, 25% orthopyroxene, 10% clinopyroxene, and 5% garnet (all by weight), using the data in Table 9.1, is: DEr = (0.6 · 0.026) + (0.25 · 0.23) + (0.10 · 0.583) + (0.05 · 4.7) = 0.366

Trace elements strongly partitioned into a single mineral Ni - olivine in Table 9.1 = 14 Figure 9.1a. Ni Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Incompatible trace elements concentrate  liquid Reflect the proportion of liquid at a given state of crystallization or melting Figure 9.1b. Zr Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Trace Element Behavior The concentration of a major element in a phase is usually buffered by the system, so that it varies little in a phase as the system composition changes At a given T we could vary Xbulk from 35  70 % Mg/Fe without changing the composition of the melt or the olivine

Trace element concentrations are in the Henry’s Law region of concentration, so their activity varies in direct relation to their concentration in the system

Trace element concentrations are in the Henry’s Law region of concentration, so their activity varies in direct relation to their concentration in the system Thus if XNi in the system doubles the XNi in all phases will double

Trace element concentrations are in the Henry’s Law region of concentration, so their activity varies in direct relation to their concentration in the system Thus if XNi in the system doubles the XNi in all phases will double Because of this, the ratios of trace elements are often superior to the concentration of a single element in identifying the role of a specific mineral

K/Rb often used  the importance of amphibole in a source rock K & Rb behave very similarly, so K/Rb should be ~ constant If amphibole, almost all K and Rb reside in it Amphibole has a D of about 1.0 for K and 0.3 for Rb K/Rb for amphibole: Usually behave similarly, so ~ constant ratio Unless amphibole which has a D of about 1.0 for K and 0.3 for Rb almost all K and Rb reside in it Melt of amphibole-bearing rock will -> decrease K/Rb in the partial melt Other factors being equal, a magma produced by partial melting of an amphibole-bearing source rock would have a lower K/Rb than one derived from amphibole-free source High absolute K or Rb could also  an amphibole-bearing source, but may result from other causes (high phlogopite, or an alkali-enriched fluid) The ratio is more indicative of amphibole due to the different D values Fractional crystallization of amphibole would also -> low K/Rb ratio in the evolved liquid

Sr and Ba (also incompatible elements) Sr is excluded from most common minerals except plagioclase Ba similarly excluded except in alkali feldspar Ba/Sr will help identify Kspar vs Plag Both are incompatible -> first partial melts (or residual liquids of FX) Effect depends on the mineral phases involved Sr is excluded from most common minerals except plagioclase Ba similarly excluded except in alkali feldspar Thus the ratio Ba/Sr increases with crystallization of plagioclase, but may decrease when orthoclase begins to crystallize

Compatible example: Ni strongly fractionated  olivine > pyroxene Cr and Sc  pyroxenes » olivine Ni/Cr or Ni/Sc can distinguish the effects of olivine and augite in a partial melt or a suite of rocks produced by fractional crystallization In all of the above cases using ratios, the idea is to find a mineral with a unique pair of elements for which it alone has a relatively high value of D for one element and a relatively low value of D for the other. The ratio of these elements is then sensitive only to liquid/crystal fractionation associated with that particular mineral

Models of Magma Evolution Batch Melting The melt remains resident until at some point it is released and moves upward Equilibrium melting process with variable % melting

Models of Magma Evolution Batch Melting eq. 9.5 CL = trace element concentration in the liquid CO = trace element concentration in the original rock before melting began F = wt fraction of melt produced = melt/(melt + rock) C 1 Di (1 F) F L O = - +

A plot of CL/CO vs. F for various values of Di using eq. 9.5 Batch Melting A plot of CL/CO vs. F for various values of Di using eq. 9.5 Di = 1.0 D = 1.0 No fractionation so CL/CO = 1 for all values of F Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Di » 1.0 (compatible element) Very low concentration in melt Especially for low % melting (low F) Values of F > 0.4 unlikely for batch melting since greater amounts should separate and rise Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Highly incompatible elements Greatly concentrated in the initial small fraction of melt produced by partial melting Subsequently diluted as F increases Highly incompatible elements are greatly concentrated in the initial small fraction of melt that is produced by partial melting, and subsequently get diluted as F increases Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

As F  1 the concentration of every trace element in the liquid = the source rock (CL/CO  1) Di (1 F) F L O = - + All -> 1.0 because all of the source is melted Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

C 1 Di (1 F) F L O = - + As F  0 CL/CO  1/Di If we know CL of a magma derived by a small degree of batch melting, and we know Di we can estimate the concentration of that element in the source region (CO) If know (CL) for magma derived by a small degree of batch melting, and we know D, we can estimate the concentration of that element in the source region (CO). This can provide very valuable information in constraining and characterizing the source region. Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

For very incompatible elements as Di  0 equation 9.5 reduces to: 1 Di (1 F) F L O = - + C 1 F L O = If we know the concentration of a very incompatible element in both a magma and the source rock, we can determine the fraction of partial melt produced

Worked Example of Batch Melting: Rb and Sr Basalt with the mode: 1. Convert to weight % minerals (Wol Wcpx etc.) Table 9.2 . Conversion from mode to weight percent Mineral Mode Density Wt prop Wt% ol 15 3.6 54 0.18 cpx 33 3.4 112.2 0.37 plag 51 2.7 137.7 0.45 Sum 303.9 1.00 Suppose that a source rock with a mode of 51% plagioclase, 33% clinopyroxene, and 18% olivine undergoes batch melting

Worked Example of Batch Melting: Rb and Sr Basalt with the mode: 1. Convert to weight % minerals (Wol Wcpx etc.) 2. Use equation eq. 9.4: Di =  WA Di and the table of D values for Rb and Sr in each mineral to calculate the bulk distribution coefficients: DRb = 0.045 and DSr = 0.848 Table 9.2 . Conversion from mode to weight percent Mineral Mode Density Wt prop Wt% ol 15 3.6 54 0.18 cpx 33 3.4 112.2 0.37 plag 51 2.7 137.7 0.45 Sum 303.9 1.00 Rb is incompatible and Sr only slightly so, but near unity

3. Use the batch melting equation (9.5) to calculate CL/CO for various values of F C 1 Di (1 F) F L O = - + Table 9.3 . Batch Fractionation Model for Rb and Sr C L /C O = 1/(D(1-F)+F) D Rb Sr F 0.045 0.848 Rb/Sr 0.05 9.35 1.14 8.19 0.1 6.49 1.13 5.73 0.15 4.98 1.12 4.43 0.2 4.03 3.61 0.3 2.92 1.10 2.66 0.4 2.29 1.08 2.11 0.5 1.89 1.07 1.76 0.6 1.60 1.05 1.52 0.7 1.39 1.04 1.34 0.8 1.23 1.03 1.20 0.9 1.01 1.09 From Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

4. Plot CL/CO vs. F for each element Figure 9.3. Change in the concentration of Rb and Sr in the melt derived by progressive batch melting of a basaltic rock consisting of plagioclase, augite, and olivine. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Results: Incompatible element Rb (no K minerals) strongly concentrated in the early small melt proportions (low F) Thus  a sensitive measure of the progress of fractional crystallization (at least until rock half melted) As melting proceeds, the incompatible element is gradually diluted by more compatible ones Since D(Sr) is close to 1.0, the ratio Rb/Sr vs. F is nearly the same as Rb alone Any ratio of incompatible to compatible element should then be sensitive to the degree of partial melting (at least in the initial stages). Important for Rb/Sr isotopic systems Note that can create a series of melts from a single source each with diff Rb/Sr

Incremental Batch Melting Calculate batch melting for successive batches (same equation) Must recalculate Di as solids change as minerals are selectively melted (computer)

Fractional Crystallization 1. Crystals remain in equilibrium with each melt increment Incremental batch melting in reverse Equation 9.5 would still apply Batch FX: F = proportion of liquid remaining

Rayleigh fractionation The other extreme: separation of each crystal as it formed = perfectly continuous fractional crystallization in a magma chamber Rayleigh: Crystals form and accumulate -> isolated from reaction with the remaining liquid

Rayleigh fractionation The other extreme: separation of each crystal as it formed = perfectly continuous fractional crystallization in a magma chamber Concentration of some element in the residual liquid, CL is modeled by the Rayleigh equation: eq. 9.8 CL/CO = F (D -1) Rayleigh Fractionation Can also apply the Rayleigh equation to Rayleigh fractional melting

Other models are used to analyze Mixing of magmas Wall-rock assimilation Zone refining Combinations of processes

The Rare Earth Elements (REE) Members of Group IIIA of the Periodic Table La -> to Lutetium (Z = 57  71) All have very similar chemical and physical properties -> behave as a coherent series All have a 3+ oxidation state Ionic radius decreases steadily with increasing atomic number (lanthanide contraction)

Contrasts and similarities in the D values: All are incompatible Also Note: HREE are less incompatible Especially in garnet Eu can  2+ which conc. in plagioclase

La Ce Nd Sm Eu Tb Er Dy Yb Lu REE Diagrams Plots of concentration as the ordinate (y-axis) against increasing atomic number Degree of compatibility increases from left to right across the diagram (“lanthanide contraction”) Concentration La Ce Nd Sm Eu Tb Er Dy Yb Lu

estimates of primordial mantle REE chondrite meteorite concentrations Chondrite normalization: Normalize to chondrite meteorites to: 1) avoid Oddo-Harkins (even Z more common) 2) think chondrite = primitive earth, so can compare to initial distribution Eliminate Oddo-Harkins effect and make y-scale more functional by normalizing to a standard estimates of primordial mantle REE chondrite meteorite concentrations

What would an REE diagram look like for an analysis of a chondrite meteorite? 0.00 2.00 4.00 6.00 8.00 10.00 56 58 60 62 64 66 68 70 72 sample/chondrite L La Ce Nd Sm Eu Tb Er Yb Lu ?

Divide each element in analysis by the concentration in a chondrite standard 0.00 2.00 4.00 6.00 8.00 10.00 56 58 60 62 64 66 68 70 72 sample/chondrite L La Ce Nd Sm Eu Tb Er Yb Lu

REE diagrams using batch melting model of a garnet lherzolite for various values of F: Figure 9.4. Rare Earth concentrations (normalized to chondrite) for melts produced at various values of F via melting of a hypothetical garnet lherzolite using the batch melting model (equation 9.5). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Diagram created in similar way to Rb-Sr diagram in Example Problem Calculate D(La) for lherzolite and model conc. in melt at F = 0.1 -> point on this diagram for La … LREE are less compatible than HREE, so melts enriched in LREE -> (-) slopes Slope is steeper for low values of F (low % partial melting) As F  1 slope  0 at S/C = 1.0 since all of mantle sample is melted Note again the use of RATIOS to -> slope on REE La/Lu ratio -> REE slope Tb/Lu for HREE only (garnet) La/Sm -> LREE only

Europium anomaly when plagioclase is a fractionating phenocryst or a residual solid in source Figure 9.5. REE diagram for 10% batch melting of a hypothetical lherzolite with 20% plagioclase, resulting in a pronounced negative Europium anomaly. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Eu* is the value Eu “should” have if Eu+2 did not -> plagioclase Another example of how RATIOS can help Eu alone is inconclusive (low REE of low Eu) Sm/Eu is slope or Eu anomaly trough (Use Eu*/Eu anyway)

Normalized Multielement (Spider) Diagrams An extension of the normalized REE technique to a broader spectrum of elements Chondrite-normalized spider diagrams are commonly organized by (the author’s estimate) of increasing incompatibility L  R Different estimates  different ordering (poor standardization) Order of elements based on estimates of increasing incompatibility from right to left in a "typical" mantle undergoing partial melting Elements are all incompatible (D<1) during most partial melting and fractional crystallization processes. The main exceptions are Sr, which may be compatible if plagioclase is involved, Y and Yb with garnet Ti with magnetite Troughs at these elements would indicate respective mineral involvement Oceanic basalts = large degrees of PM, their spider diagrams should reflect the trace element patterns of their source Less incompatible elements on the right-hand side should be less enriched during PM (particularly for small degrees of it), tilting the curve up on the left -> (-) slope Additionally, FX subsequent to magma segregation from the source should tip the pattern even further Fig. 9.6. Spider diagram for an alkaline basalt from Gough Island, southern Atlantic. After Sun and MacDonough (1989). In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345.

MORB-normalized Spider Separates LIL and HFS Figure 9.7. Ocean island basalt plotted on a mid-ocean ridge basalt (MORB) normalized spider diagram of the type used by Pearce (1983). Data from Sun and McDonough (1989). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. MORB-normalization LIL on left and HFS on right. Incr. incompatibility toward center. Coherent magma with enrichment has hump shape

Application of Trace Elements to Igneous Systems 1. Use like major elements on variation diagrams to document FX, assimilation, etc. in a suite of rocks More sensitive  larger variations as process continues Figure 9.1a. Ni Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

2. Identification of the source rock or a particular mineral involved in either partial melting or fractional crystallization processes Example: can use REE to distinguish between high pressure and low pressure sources of a mantle-derived magma In the deep continental crust, and at depths over about 100 km in the mantle, garnet and clinopyroxene are important phases, which remain as residual solids during the generation of up to 15-20% partial melting

Garnet concentrates the HREE and fractionates among them Thus if garnet is in equilibrium with the partial melt (a residual phase in the source left behind) expect a steep (-) slope in REE and HREE Shallow (< 40 km) partial melting of the mantle will have plagioclase in the resuduum and a Eu anomaly will result

Garnet and Plagioclase effect on HREE 0.00 2.00 4.00 6.00 8.00 10.00 56 58 60 62 64 66 68 70 72 sample/chondrite La Ce Nd Sm Eu Tb Er Yb Lu 67% Ol 17% Opx 17% Cpx Garnet and Plagioclase effect on HREE 0.00 2.00 4.00 6.00 8.00 10.00 sample/chondrite 60% Ol 15% Opx 15% Cpx 10%Plag La Ce Nd Sm Eu Tb Er Yb Lu 0.00 2.00 4.00 6.00 8.00 10.00 56 58 60 62 64 66 68 70 72 sample/chondrite La Ce Nd Sm Eu Tb Er Yb Lu 57% Ol 14% Opx 14% Cpx 14% Grt

Figure 9.3. Change in the concentration of Rb and Sr in the melt derived by progressive batch melting of a basaltic rock consisting of plagioclase, augite, and olivine. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Trace elements have much broader general application in petrology than indicating individual mineral involvement They have been used to indicate mama mixing, assimilation, the degree of partial melting of fractional crystallization, etc. We will see numerous examples in forthcoming chapters

Use as a Petrogenetic Indicator Table 9.6  A Brief Summary of Some Particularly Useful Trace Elements in Igneous Petrology Element Use as a Petrogenetic Indicator Ni, Co, Cr Highly compatible elements. Ni and Co are concentrated in olivine, and Cr in spinel and clinopyroxene. High concentrations indicate a mantle source, limited fractionation, or crystal accumulation. Zr, Hf Very incompatible elements that do not substitute into major silicate phases (although they may replace Ti in titanite or rutile). High concentrations imply an enriched source or extensive liquid evolution. Nb, Ta High field-strength elements that partition into Ti-rich phases (titanite, Ti-amphibole, Fe-Ti oxides. Typically low concentrations in subduction-related melts. Ru, Rh, Pd, Re, Os, Ir, Pd Platinum group elements (PGEs) are siderophile and used mostly to study melting and crystallization in mafic-ultramafic systems in which PGEs are typically hosted by sulfides. The Re/Os isotopic system is controlled by initial PGE differentiation and is applied to mantle evolution and mafic melt processes. Sc Concentrates in pyroxenes and may be used as an indicator of pyroxene fractionation. Sr Substitutes for Ca in plagioclase (but not in pyroxene), and, to a lesser extent, for K in K-feldspar. Behaves as a compatible element at low pressure where plagioclase forms early, but as an incompatible element at higher pressure where plagioclase is no longer stable. REE Myriad uses in modeling source characteristics and liquid evolution. Garnet accommodates the HREE more than the LREE, and orthopyroxene and hornblende do so to a lesser degree. Titanite and plagioclase accommodates more LREE. Eu2+ is strongly partitioned into plagioclase. Y Commonly incompatible. Strongly partitioned into garnet and amphibole. Titanite and apatite also concentrate Y, so the presence of these as accessories could have a significant effect.

Trace elements as a tool to determine paleotectonic environment Useful for rocks in mobile belts that are no longer recognizably in their original setting Can trace elements be discriminators of igneous environment? Approach is empirical on modern occurrences Concentrate on elements that are immobile during low/medium grade metamorphism A bit ambiguous, since many variables (country rx, %PM, %FX)   Need immobile TE's, so met has no effect. Ti, Zr, Y, Hf & apply to basic volc's to eliminate high %FX

Figure 9.8  Examples of discrimination diagrams used to infer tectonic setting of ancient (meta)volcanics. (a) after Pearce and Cann (1973), (b) after Pearce (1982), Coish et al. (1986). Reprinted by permission of the American Journal of Science, (c) after Mullen (1983) Copyright © with permission from Elsevier Science, (d) and (e) after Vermeesch (2005) © AGU with permission.

Isotopes C Same Z, different A (variable # of neutrons) General notation for a nuclide: 6 14 C 6 = Z = # of protons  element identity 14 = A = p + n

Isotopes C 12C 13C 14C Same Z, different A (variable # of neutrons) General notation for a nuclide: 6 14 C As n varies  different isotopes of an element 12C 13C 14C May drop the subscript (redundant with symbol)

Stable Isotopes Stable: last ~ forever Chemical fractionation is impossible Mass fractionation is the only type possible Mass differences « chemical differences, so mass fractionation is always small, but some fract may occur during reactions If mass fractionation takes place, the light isotope -> vapor > liquid > solid The efficiency of mass fractionation = f(mass difference/total mass) 204Pb - 205Pb will not fractionate 1H - 3H will fractionate appreciably Stable isotope studies are usually limited to sulfur and lighter elements Stable isotopes can give us information on: -eqm T -source indicators -history of evolution

Example: Oxygen Isotopes 16O 99.756% of natural oxygen 17O 0.039% “ 18O 0.205% “ Concentrations expressed by reference to a standard International standard for O isotopes = standard mean ocean water (SMOW)

result expressed in per mille (‰) 18O and 16O are the commonly used isotopes and their ratio is expressed as d: d (18O/16O) = eq 9.10 result expressed in per mille (‰) ( O/ O) x 1000 18 16 sample SMOW - What is d of SMOW?? What is d for meteoric water?

What is d for meteoric water? Evaporation seawater  water vapor (clouds) Light isotope enriched in vapor > liquid Pretty efficient, since D mass = 1/8 total mass

What is d for meteoric water? Evaporation seawater  water vapor (clouds) Light isotope enriched in vapor > liquid Pretty efficient, since D mass = 1/8 total mass d = therefore < thus dclouds is (-) ( O/ O) x 1000 18 16 vapor SMOW - ( O/ O) 18 16 Vapor ( O/ O) 18 16 SMOW

the value of d can vary from - 40 to almost 0 True, but: the value of d can vary from - 40 to almost 0 d appears to be a function of temperature Why? Hi T low % condensation, so fract vapor liquid is high and gets closer to SMOW. Where T is lower and a higher % liq. the fract is less efficient and d is lower. Paleothermometry of snow accumulation, carbonates Greenland ice sheet cores paleoclimates? Figure 9.9. Relationship between d(18O/16O) and mean annual temperature for meteoric precipitation, after Dansgaard (1964). Tellus, 16, 436-468.

Stable isotopes useful in assessing relative contribution of various reservoirs, each with a distinctive isotopic signature O and H isotopes - juvenile vs. meteoric vs. brine water d18O for mantle rocks  surface-reworked sediments: evaluate contamination of mantle-derived magmas by crustal sediments O and H isotopes used to evaluate juvenile vs. meteoric vs. brine characteristics of water and the type of water responsible for rock alteration d18O is quite different for mantle rocks and surface-reworked sediments, so it may be used to evaluate the extent to which mantle-derived magmas are contaminated by crustal sediments Thermometry of minerals? Beware of re-equilibration and alteration!

Radioactive Isotopes Unstable isotopes decay to other nuclides The rate of decay is constant, and not affected by P, T, X… Parent nuclide = radioactive nuclide that decays Daughter nuclide(s) are the radiogenic atomic products Other products = subatomic products and energy

Isotopic variations between rocks, etc. due to: 1. Mass fractionation (as for stable isotopes) Only effective for light isotopes: H He C O S

Isotopic variations between rocks, etc. due to: 1. Mass fractionation (as for stable isotopes) 2. Daughters produced in varying proportions resulting from previous event of chemical fractionation 40K  40Ar by radioactive decay Basalt  rhyolite by FX (a chemical fractionation process) Rhyolite has more K than basalt 40K  more 40Ar over time in rhyolite than in basalt 40Ar/39Ar ratio will be different in each Last line: 39Ar is neither radiogenic nor radioactive - why use it??

Isotopic variations between rocks, etc. due to: 1. Mass fractionation (as for stable isotopes) 2. Daughters produced in varying proportions resulting from previous event of chemical fractionation 3. Time The longer 40K  40Ar decay takes place, the greater the difference between the basalt and rhyolite will be

Radioactive Decay The Law of Radioactive Decay eq. 9.11 - µ dN dt N or = N l 1 ½ ¼ Note half-life is a constant (as is decay constant) # parent atoms time 

D = Nelt - N = N(elt -1) eq 9.14  age of a sample (t) if we know: D the amount of the daughter nuclide produced N the amount of the original parent nuclide remaining l the decay constant for the system in question Practical limitations on age range to which apply: Very young rocks: cannot measure tiny amount of daughter accurately Very old rocks: cannot measure tiny amounts of parent left accurately Range depends on lambda How can we distinguish radiogenic daughter isotopes from identical stable isotopes that were in a rock at its formation?

The K-Ar System 40K  either 40Ca or 40Ar 40Ca is common. Cannot distinguish radiogenic 40Ca from non-radiogenic 40Ca 40Ar is an inert gas which can be trapped in many solid phases as it forms in them When a rock is hot all of the Ar escapes, “resetting the radiometric clock” - all of the daughter is removed When a volcanic rock forms, the clock is reset, and 40Ar that accum. after must be daughter from 40K

The appropriate decay equation is: eq 9.16 40Ar = 40Aro + 40K(e-lt -1) Where le = 0.581 x 10-10 a-1 (proton capture) and l = 5.543 x 10-10 a-1 (whole process) l e æ è ç ö ø ÷ Normally 40Aro = 0 for igneous rocks Thus can get an age from a single rock by measuring 40K and 40Ar in it (only unknowns)

Blocking temperatures for various minerals differ 40Ar-39Ar technique grew from this discovery A recent discovery- blocking temperatures differ Stepwise heating of sample releases Ar from different minerals -> several ages reflecting thermal history of rock We can use it to -> uplift rates in eroded orogenic belts

Sr-Rb System 87Rb  87Sr + a beta particle (l = 1.42 x 10-11 a-1) Rb behaves like K  micas and alkali feldspar Sr behaves like Ca  plagioclase and apatite (but not clinopyroxene) 88Sr : 87Sr : 86Sr : 84Sr ave. sample = 10 : 0.7 : 1 : 0.07 86Sr is a stable isotope, and not created by breakdown of any other parent Add to end: 87Sr is stable & non-radiogenic Sr is present in nearly any rock The amount of 87Sr in any rock = original 87Sr + radiogenic 87Sr from 87Rb over time Thus a rock cannot  an unambiguous age if you don’t know what % or 87Sr is radiogenic Isochron technique use to solve this problem

Requires 3 or more cogenetic samples with a range of Rb/Sr Isochron Technique Requires 3 or more cogenetic samples with a range of Rb/Sr Could be: 3 cogenetic rocks derived from a single source by partial melting, FX, etc. Figure 9.3. Change in the concentration of Rb and Sr in the melt derived by progressive batch melting of a basaltic rock consisting of plagioclase, augite, and olivine. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Requires 3 or more cogenetic samples with a range of Rb/Sr Isochron Technique Requires 3 or more cogenetic samples with a range of Rb/Sr Could be: 3 cogenetic rocks derived from a single source by partial melting, FX, etc. 3 coexisting minerals with different K/Ca ratios in a single rock Bt - Ms - Kfs

Recast age equation by dividing through by stable 86Sr 87Sr/86Sr = (87Sr/86Sr)o + (87Rb/86Sr)(elt -1) eq 9.17 l = 1.4 x 10-11 a-1 For values of lt less than 0.1: elt-1  lt Thus eq. 9.15 for t < 70 Ga (!!) reduces to: eq 9.18 87Sr/86Sr = (87Sr/86Sr)o + (87Rb/86Sr)lt y = b + x m = equation for a line in 87Sr/86Sr vs. 87Rb/86Sr plot

( ) Begin with 3 rocks plotting at a b c at time to 86Sr 87Sr 86Sr 87Sr o ( ) Why have same value of 87Sr/86Sr? (87Sr/86Sr)o = value at time to to a b c 86Sr 87Rb

After some time increment (t0 t1) each sample loses some 87Rb and gains an equivalent amount of 87Sr a b c a1 b1 c1 t1 to 86Sr 87Sr 87Rb o ( ) Result: a1 b1 and c1 are still colinear Slope = f(t) Intercept at zero 87Rb = (87Sr/86Sr)o

( ) At time t2 each rock system has evolved  new line Again still linear and steeper line a b c a1 b1 c1 a2 b2 c2 t1 to t2 86Sr 87Sr o ( ) 86Sr 87Rb

Isochron technique produces 2 valuable things: 1. The age of the rocks (from the slope = lt) 2. (87Sr/86Sr)o = the initial value of 87Sr/86Sr Isochron technique produces 2 valuable things: The age of the rocks (from the slope = lambda t) t = slope/lambda = 0.00127/1.4 E-11 = 90.7 Ma (87Sr/86Sr)o = the initial value of 87Sr/86Sr at the time of crystallization WHY?? Figure 9.12. Rb-Sr isochron for the Eagle Peak Pluton, central Sierra Nevada Batholith, California, USA. Filled circles are whole-rock analyses, open circles are hornblende separates. The regression equation for the data is also given. After Hill et al. (1988). Amer. J. Sci., 288-A, 213-241.

If a modern melt is derived from partial melting of the mantle: Primordial mantle ~ chondrite Major melting event about 3 Ga ago  continental crust (hypothetical simplification) Crust has high Rb/Sr so evolution curve is steep Mantle is depleted in Rb, so lower Rb/Sr and shallower evolution curve If a modern melt is derived from partial melting of the mantle: (87Sr/86Sr)o < 0.706 If derived from old crust: (87Sr/86Sr)o > 0.706 Figure 9.13. Estimated Rb and Sr isotopic evolution of the Earth’s upper mantle, assuming a large-scale melting event producing granitic-type continental rocks at 3.0 Ga b.p After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

The Sm-Nd System Both Sm and Nd are LREE Incompatible elements fractionate  melts Nd has lower Z  larger  liquids > does Sm Sm/Nd lower in partial melts than source, and also lower in late FX liquids

Decay equation derived by reference to the non-radiogenic 144Nd 147Sm  143Nd by alpha decay l = 6.54 x 10-13 a-1 (half life 106 Ga) Decay equation derived by reference to the non-radiogenic 144Nd 143Nd/144Nd = (143Nd/144Nd)o + (147Sm/144Nd)lt Also requires the isochron technique to solve

Evolution curve is opposite to Rb - Sr Partial melting selectively depletes the mantle in the daughter (Nd) so Sm  Nd and radiogenic swamps original Nd Basalt derived from the mantle will have a higher initial 143Nd/144Nd than a magma derived from the crust Figure 9.15. Estimated Nd isotopic evolution of the Earth’s upper mantle, assuming a large-scale melting or enrichment event at 3.0 Ga b.p. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

The U-Pb-Th System Very complex system. 3 radioactive isotopes of U: 234U, 235U, 238U 3 radiogenic isotopes of Pb: 206Pb, 207Pb, and 208Pb Only 204Pb is strictly non-radiogenic U, Th, and Pb are incompatible elements, & concentrate in early melts Isotopic composition of Pb in rocks = function of 238U  234U  206Pb (l = 1.5512 x 10-10 a-1) 235U  207Pb (l = 9.8485 x 10-10 a-1) 232Th  208Pb (l = 4.9475 x 10-11 a-1)

The U-Pb-Th System Concordia = Simultaneous co- evolution of 206Pb and 207Pb via: 238U  234U  206Pb 235U  207Pb 206Pb* means radiogenic 206Pb The example shows 2.5 Ga of development of an isotopic system Figure 9.16a. Concordia diagram illustrating the Pb isotopic development of a 3.5 Ga old rock with a single episode of Pb loss. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York.

The U-Pb-Th System Discordia = loss of both 206Pb and 207Pb Discordia: loss of both 206Pb and 207Pb due to some thermal event (metamorphism) All -> origin but not same amount Suppose this occurs 2.5 Ga after original crystallization Figure 9.16a. Concordia diagram illustrating the Pb isotopic development of a 3.5 Ga old rock with a single episode of Pb loss. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York.

The U-Pb-Th System Concordia diagram after 3.5 Ga total evolution 3.5 Ga = age of igneous crystallization 1.0 Ga = age of metamorphism We will leave the details of this system until Chapter 14 when we will use it to distinguish crustal contamination of mantle magmas Figure 9.16b. Concordia diagram illustrating the Pb isotopic development of a 3.5 Ga old rock with a single episode of Pb loss. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York.

The U-Pb-Th System 3.5 Ga = age of igneous crystallization 1.0 Ga = age of metamorphism We will leave the details of this system until Chapter 14 when we will use it to distinguish crustal contamination of mantle magmas Figure 9.17. Concordia diagram for three discordant zircons separated from an Archean gneiss at Morton and Granite Falls, Minnesota. The discordia intersects the concordia at 3.55 Ga, yielding the U-Pb age of the gneiss, and at 1.85 Ga, yielding the U-Pb age of the depletion event. From Faure (1986). Copyright © reprinted by permission of John Wiley & Sons, Inc.