Radiogenic Isotope Geochemistry – Applications To Igneous .

Lecture 39Radiogenic Isotope Geochemistry –applications to igneous systemsReading – - White Chapter Ch 8.3 and Ch11.5-11.6(this is the last assigned reading for the semester; you’ve already read parts ofthese chapters earlier).Today1. Overview.GG325 L39, F2013Radiogenic Isotopes in Igneous SystemsLike stable isotopes, radiogenic isotopes play many roles indifferent branches of Earth science. They are among the mostpowerful tools geochemists have.We’ve already discussed some of the isotopic evidence for theages and timing of various events in Earth and early solarsystem history. But we haven’t yet examined exactly how thisevidence works in detail.quick review of radioactive decay again:Recall the decay equation: Nt N0e-λtwhere N is the radioactive parent, t is the time of decay, and “o”is the initial condition (at t 0).GG325 L39, F2013

Radiogenic Isotopes in Igneous Systems(1) Nt N0e-λtOne can determine an age by comparing Nt and N0.We can also determine ages if we don’t know N0, because Ndecays to a daughter isotope, D, such thatN0 Nt Dt , assuming that D is also not radioactive (we willdiscuss that special case later)Substituting into eqn (1) Nt Dt Nteλt yields:(2)Dt Nteλt – NtDt Nt(eλt – 1) In many cases, D0 0, so we need to include it:Dt D0 Nt(eλt-1)-or-Dt D0 Nt(eλt-1)dddWhere d a non-radiogenic isotope of the daughter element;it is called the normalizing isotope.GG325 L39, F2013Radiogenic Isotopes in Igneous Systems(D/d)t (D/d)0 (N/d)t(eλt-1)Commonly, (D/d) and (N/d) are symbolized as R and P, respectively.We usually measure ratios rather than absolute amounts of isotopes becausewe can measure isotope ratios far more precisely.Two uses of the above equation:1. it serves as the basis for several powerful radiometric dating methods; i.e.,where the goal is to measure t, the age of geological samples. This branch ofgeochemistry is called geochronology. Your reading describes some of thedetails and gives several good examples.2. An equally important application involves using radiogenic isotope ratios astracers of the composition and history of sources and geochemical processes.This latter application follows directly from our discussions of trace elementsand planetary differentiation, so we’ll start with it first.GG325 L39, F2013

Radiogenic Isotopes in Igneous SystemsIn the past few weeks, we’ve learned that:1. Igneous rock compositions reflect the processes thatformed the rocks (“petrogenesis”), and we can investigatethese processes with a range of geochemical tools.2. Planetary-scale differentiation has produced crustal andmantle reservoirs within the silicate Earth; these reservoirshave distinctly different compositions.GG325 L39, F2013Radiogenic Isotopes in Igneous SystemsCompositional variability in igneous rocks can result from botha. effects of past planetary differentiation on mantleb. crustal compositions the magmatic activity that forms therocks and the ongoing.We use radiogenic isotopes of heavy elements to disentangleand illuminate these different processes, and to tell us aboutthe time scales over which they operate.GG325 L39, F2013

Radiogenic Isotopes in Igneous SystemsWe use radiogenic isotopes that are heavy, and produced fromradioactive parents with very long half lives, because they can distinguish ancient from recent/modern processesand ratios of two isotopes of a heavy element are generally notfractionated during melting or crystallization (or we can correctfor any minor fractionation); i.e., isotope ratios allow us to “seethrough” petrogenesis.The second point is a fundamental difference between heavy-elementisotopes and light-element isotopes, such as those of H, C, and O. As wesaw earlier this semester, the light isotopes can be fractionated significantlyby natural processes. For the light isotopes, it is the fractionation itself thatmakes them so useful.GG325 L39, F2013Radiogenic Isotopes in Igneous SystemsConsider how the mantle operates to see why time is animportant aspect of this process:Each new mantle melting makes event the mantle a little bit moredepleted in lithophile and atmophile elements, so differentiation ofEarth is ongoing.However, subduction of oceanic lithosphere also returns somelithophile and atmophile material back into the mantle.Together, they form a compositional cycle within the silicateEarth that operates very much like the hydrologic cycle(although at rates that are far slower).GG325 L39, F2013

Radiogenic Isotopes in Igneous SystemsOver time, the mantle has developed compositionalvariability through this cycle. We see the evidence for thisvariability in igneous rocks from around the world.Using trace elements, it’s usually very difficult or impossible totell whether compositional differences were produced by pastmelting and crystallization events or very recently, possiblyeven in the same melting event that formed a particularsequence of magmas.In contrast, ratios of radiogenic isotopes to non-radiogenicisotopes provide a record of ancient events that is not erasedor overprinted by recent melting or crystallization events,including the chemical fractionation that accompanies theformation of an igneous rock itself.GG325 L39, F2013Radiogenic Isotopes in Igneous SystemsWe usually use isotope ratios, R, where one isotope is radiogenic (produced from the decay ofanother) the other is a stable, non radiogenic isotope.R (radiogenic daughter)/(non-radiogenic isotope)The parent and daughter element always have somewhat differentincompatibility, so the ratio of radiogenic to non-radiogenic isotopes ina rock or mineral reflects the composition of the source but in a time-integrated way because the half lives of the parentisotopes are very long. For example, high R records a high timeintegrated parent-daughter ratio, and thus preserves information onthe time-integrated chemical composition and history of a mantlesource.GG325 L39, F2013

Radiogenic Isotopes in Igneous SystemsIn a material that has remained a chemically closed system fora long time the daughter isotope will closely reflect theconcentration of the parent87RbLet’s use the Rb-Sr system as an example. 87SrGiven enough time, 87Sr/86Sr will reflect 87Rb/86Sr via 87Rbdecay and 87Sr ingrowth.The evolution of the system over time is described by theradioactive decay equation we derived a few minutes ago,which can be written here as:87Srtoday 87Srinitial 87Rbtoday(eλt-1) -or-87Srtoday86Sr 87Srinitial 87Rbtoday(eλt-1)86Sr86Sr86Sris the normalizing isotope used in Rb-Sr work. It is stable and nonradiogenic.GG325 L39, F2013Radiogenic Isotopes in Mantle EvolutionAn isochron diagram illustrates how an isotope ratio changesin a closed system after an initial fractionation event.Example: let’s say our closed system is a rock composed ofseveral minerals that formed from a melt of the mantle at sometime t in the past.For the Rb-Sr system:87Sr86Srtoday 87Sr initial 87Rb(eλt-1)86Sr86SrFor the bulk rock and for each phase in it, we can measure thepresent-day Sr isotope ratio and parent/daughter ratio anddetermine the age and the initial Sr isotope ratio.GG325 L39, F2013

Radiogenic Isotopes in Mantle EvolutionWe do this with an isochron diagram, based on compositional relationshipsbetween different phases in a sample having the same age.(87Sr/86Sr)today (87Sr/86Sr)initial (87Rb/86Sr)today(eλt – 1)y b x·mThis is an equation for a straight line with m (eλt – 1) and b (87Sr/86Sr)initial.We obtain the age, t, from the slope ofthe line, since m (eλt – 1) and the initialratio from the intercept.Each dot in the isochron plot at the leftrepresents values for a given mineral or thebulk rock at a given time. The heavy linesconnecting the dots are called isochrons.As a rock ages, the slope of the isochronincreases. Each dot evolves as indicatedby the arrows.GG325 L39, F2013Radiogenic Isotopes in Igneous Systems87Srtoday86Sr 87Srinitial 87Rb(eλt-1)86Sr86SrOpen systems: material from which Rb and/or Sr were recentlyextracted will have Rb/Sr (and therefore 87Rb/86Sr) differentfrom the value we would predict from the present-day 87Sr/86Srof the material.In the mantle, there’s a wide diversity of 87Rb/86Sr and 87Sr/86Srvalues, reflecting how and when material has been added to orremoved from different parts of rGG325 L39, F2013

Radiogenic Isotopes in Igneous Systems.Commonly used isotopes include those of Sr, Nd, Hf .Isotope pair 87Sr/86SrParent87RbUses87Srdecays to(t1/2 4.967 x 1010 yr)note:is a stable, nonradiogenic isotope 143Nd/144Nd176Hf/177Hfin a rock, this tells usabout Rb/Sr of thesource86SrRb is typically moreincompatible than Sr.147Smdecays to 143Nd(t1/2 1.06 x 1011 yr).tells us about Sm/Nd ofthe sourcenote: 144Nd is a stable,non-radiogenic isotope.Sm is typically lessincompatible than Nd.176Ludecays to 176Hf(t1/2 3.5 x 1010 yr)tells us about Lu/Hf ofthe sourcenote: 177Hf is a stable,non-radiogenic isotope.Lu is typically lessincompatible than Hf.GG325 L39, F2013Radiogenic Isotopes in Igneous Systems. and these isotopes of Pb 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb238Udecays to 206Pb(t1/2 4.47 x 109 yr)in a rock tells us aboutU/Pb of the sourcenote: 204Pb is a stable,non-radiogenic isotopePb is typically lessincompatible than U.235Udecays to 207Pb(t1/2 7.04 x 108 yr)same, but with adifferent time scalefor changessamesame232Thdecays to 208Pb(t1/2 1.40 x 1010 yr).in a rock tells us aboutTh/Pb of the sourcesamePb is typically lessincompatible than Th.GG325 L39, F2013

Radiogenic Isotope (and other) MixturesBesides age and source information, isotope ratios areparticularly useful for identifying and understanding mixtures.In igneous rocks, mixtures are produced from melting of mantlewith small-scale heterogeneity and from addition of crustal andmantle components to a magma (contamination, assimilation).The amount of component A in the mix is:In such cases, magmas areAmix B mix (A2-A1) A1B2 - A2B1mixtures of melts drawn from(B2-B1)B2 - B1more than one source, or arefrom a single source but2experienced different additionalXinputs during ascent and storage.XXFor a mixture of two endmembers (1 and 2), mixtures aredescribed by a hyperbola.1Component AGG325 L39, F2013Radiogenic Isotope MixturesMixing of materials with different isotopic ratios and elementalconcentrations produces predictable hyperbolic arrays.1Component AComponent BXXX2Component BThese mixingpatterns holdfor componentsA, B, C, D beingchemical elementsor isotopesThese general types of mixingpatterns hold for both elementconcentrations and isotope andtrace element ratios.(You can think of a concentrationas a ratio with a denominator of 1;e.g., Sr Sr/1).2Component AComponent CXXX1Component BComponent C1Component AComponent CXXXxXXyyfor conc A conc BXx2for conc A conc BComponent BComponent DGG325 L39, F2013