Isotope geochemistry of non-redox active elements

Transcription

1 Isotope geochemistry of non-redox active elements Reading: White s last chapter has short sections on B and Li. - Resources: See Chapters in Johnson, Beard and Albaréde, 2004, Geochemistry of Non- Traditional Stable Isotopes Reviews in Mineralogy and Geochemistry, Vol. 55. Includes Li, Mg, Ca, Zn, Cl - Also see excellent chapter on B isotopes in Reviews in Mineralogy and Geochemistry Vol. 33. Guide Questions: How does diffusion fractionate isotopes? What happens to this effect with increasing T? What is the major driver of Li isotope fractionation at low T? What is the major driver of B isotope fractionation at low T? How does it gives us information about the ocean s ph in the past? How can Ca isotopes be used as a trophic level indicator? How do Si isotopes in marine diatoms change as nutrient levels change in a given area? General issues: Elements with only one valence in nature tend to have smaller isotopic variations. Example: Calcium (Ca): We measure 44/40; 10% mass difference. But Ca is almost always bonded ionically to oxygens, both in solution and in minerals. Little contrast in the bonding environment of Ca between dissolved Ca and Ca in minerals. Observed fractionation is only a few per mil (see DePaolo Chapter in RiMG volume 55) Compare to Chromium (Cr): 53/52. This is only a 2% mass difference, BUT we see 3-4 fractionation for reduction of Cr(VI) to Cr(III). Exceptions: Some elements have major bonding changes without any redox change. o B and Li switch between tetrahedral and octahedral coordination When an element is bonded to organic molecules, the local bonding environment may be altered relative to that in the free aqueous phase Diffusion-related fractionation of isotopes In general, we assume that both equilibrium and kinetic fractionation decrease with increasing T. There is one major exception to this: Diffusion. Recall our very simple equation that describes why, in a gas phase, light isotopes diffuse faster than heavier isotopes: 1 2 m 1v 1 2 = 1 2 m 2v 2 2 This says that at a given T, the kinetic energies of light and heavy isotopes are the same. It demands that the lighter isotopes have greater velocities to compensate for their lesser masses. Note that this is NOT T-dependent, and the isotopic fractionation related to this type of diffusion does not disappear even at very high T. Diffusion in liquids is more complex and the above equation does not directly apply. However, experiments have revealed that diffusion of Mg, Ca, and Li between a basaltic liquid and a rhyolitic liquid caused up to 7 shift in 26 Mg/ 24 Mg, 6 shift in 44 Ca/ 40 Ca, and 40 shift in 7 Li/ 6 Li at temperatures of roughly Lighter isotopes diffuse faster. See series of papers in Geochimica et Cosmochimica Acta by Frank Richter and Creaig Lundstrom s group. Oddly, diffusion in water at room T leads to less isotopic fractionation! In a 2006 paper, Richter and others found no measurable isotopic fractionation when Mg diffuses through water. The Li isotope

2 fractionation was surprisingly small, whereas the 35 Cl diffused about 1.4 faster than 37 Cl. These results are explained somewhat by the fact that small cations like Mg 2+ are bound to a larger sphere of perhaps dozens of polar H 2 O s, the solvation sphere. This makes the effective mass of the diffusing species quite large, and the mass difference between, say, 26 Mg and 24 Mg in the center matters little compared to the entire mass. Equilibrium fractionation of isotopes in a thermal gradient When hot molten silicate materials (i.e., in magmas) come into contact with cooler material, the strong temperature gradients cause isotopic fractionation (and elemental fractionation too). Lighter isotopes migrate toward the hot side; heavy isotopes toward the cool side Eventually, a stable equilibrium fractionation is attained Elements migrate too; mafic elements toward the hot end Temperature contrasts of a few hundred degrees are needed; this effect is probably very weak in aqueous environments Important: This mechanism and rapid diffusion are the ways to get isotopic fractionation at magmatic temperatures. Use of various isotope ratios in igneous systems We expect that equilibrium isotope fractionation between mineral phases, or between minerals and molten silicate material, are very small at magmatic T s (recall our general rules about how isotopic fractionation changes with T). But, there are two types of fractionation that persist to higher T: 1) Diffusion, as described above. So if a magmatic system has strong gradients that move elements rapidly via diffusion, we expect to see isotopic shifts resulting from this. a. Craig Lundstrom has observed shifts in Li and other isotope ratios close to rock structures that look like cracks where magma rose toward a mid-ocean ridge 2) Thermal gradient fractionation. For reasons that are not very well understood so far, when a silicate liquid is subjected to a temperature gradient, lighter isotopes tend to migrate toward the hotter region. This seems to be a thermodynamic effect (i.e., putting the lighter isotopes in hot region and heavier isotopes in cooler region leads to an overall lower free energy). The Lundstrom group is currently exploring isotope ratio difference in igneous systems, and finding trends that suggest elements are being moved around in thermal gradients. Non-redox fractionation of light elements (Li through Cl) Lithium ( 7 Li/ 6 Li): See Tomascak chapter in RiMG Geochemistry of Non-Traditional Stable Isotopes book. Standard is NIST SRM-8545, also known as L-SVEC. Probably close to bulk earth? Bonding contrasts: - 4-fold coordination (tetrahedral) in aqueous solutions - 4- or 6-fold (or irregular, > 4) in minerals In general, lower coordination number means more strength for each bond; stronger bonds means a tendency to take heavier isotopes. So a simple coordination change is enough to induce an equilibrium isotope fractionation; valence change is not needed. We thus expect that minerals with 6-fold coordination of Li + will tend to take the lighter isotope preferentially. This is observed in both experiments and natural settings. In rivers, dissolved Li + is isotopically heavy by about 30 relative to clay minerals in suspension (which are close to 0 ). This presumably reflects partial or complete equilibration of waters and clays in soil environments, with possible exchange later.

3 Ocean water is roughly +30 relative to bulk earth, carbonate rock inherits this somewhat. Clayrich marine sediments tend to be close to 0. Considerable variation has been found in igneous rocks. Subducted crust is heavier than normal mantle, and it is thought that isotopically heavy Li that shows up in subduction-related magmas may be from the subducted slab. Granites have a relatively wide range, positive and negative. Perhaps this reflects diffusive fractionation. Craig Lundstrom found that Li isotopes are fractionated near fissures through which magma migrates upward at mid-ocean ridges. This suggests strong diffusion of Li from the magma into or out of the host rock, which further implies that magmas evolve continuously as they ascend- contrary to traditional models. Boron (B) ( 11 B/ 10 B) -- See Palmer and Swihart chapter in Reviews in Mineralogy, Volume 33, Boron: Mineralogy, Petrology and Geochemistry (Grew and Anovitz, eds.) Standard is SRM 951- synthetic, not related to bulk earth Bonding environment contrast: B(OH) 3 is trigonal, 3-fold coordination B(OH) 4 - is tetrahedral, 4-fold Equilbrium between these two is very quickly attained B(OH) 3 is enriched in the heavier isotope Equilibrium fractionation at ocean temperatures: α 1.020, Δ 20 Application: Possible paleo-ph indicator: Very important, because oceanic ph is critical in determining the CO 2 in the atmosphere via exchange with oceans!!! Speciation (B(OH) 3 vs. B(OH) 4 - ) in waters changes with ph (pk = 8.8) o Total B in seawater is +40 o B(OH) 3 trigonal complex takes heavy isotopes o B(OH) 4- tetrahedral complex takes lighter isotopes! At ph < 7, B(OH) 3 dominates, B(OH) 4 - is +20! At ph > 10.2, B(OH) 4 - dominates, B(OH) 4 - is +40! At ph between 7 and 10.2: δ 11 B in B(OH) 4- is sensitive to ph! This is another linear mixing model: f 4 δ 4 + f 3 δ 3 = δ mix o Carbonate-producing organisms take in B(OH) 4 - preferentially, so! Lower δ 11 B in forams means lower ph in the past! Higher δ 11 B in forams means higher ph Seawater B and B from human use (borax) are very different isotopically Calcium (Ca) Measure: 44/40 via TIMS (best option) or 44/42 for plasma-based instruments (difficult) Low-Temperature environments: Little contrast in bonding environments, but still enough to cause isotopic shifts of a few per mil- much greater than analytical precision. General relationship: Ca put into mineral matter (bone or shell) by organisms is isotopically light relative to the fluids in the organism. Counterintuitive; could be a kinetic effect. Bone formation causes fractionation. Bone is about 1.5 lower in 44/40 relative to blood and/or soft tissues So, if Ca in the diet is well in excess of that needed to make bone, then the soft tissue is little affected by bone formation (preferential removal of light Ca) and is close to the avg. diet. So in this case (common) bone should be 1.5 less than diet.

4 BUT, if Ca in diet is mostly taken up by bone growth, then the soft tissue and blood will become isotopically heavy and the bone will be closer to the avg. diet isotope ratio. FURTHERMORE, if a person has bones that are 1.5 less than the avg. diet 44 Ca/ 40 Ca, AND they begin to lose calcium from their bones (astronauts do this- weightless) this bone loss can be detected via 44 Ca/ 40 Ca measurements of their blood. Trophic level effects. Consistent trends seen in animals. Herbivores are 1 lighter than plants they eat Carnivores are 1 lighter than their diet, roughly Increase in 44/40 observed with increasing trophic level- terrestrial and oceans Paleotemperatures? People would like to use this as a paleotemperature indicator. Studies show the fractionation factor is T-dependent, but I think there are important complications. High-T: Diffusion effects observed in lab experiments with magmatic diffusion. Possible application as a diffusion/disequilibrium indicator in igneous rocks Strontium ( 88 Sr/ 86 Sr mass dependent shifts) Similar to Ca, but Sr is heavier, so less fractionation. Has been used in several studies. Magnesium (Mg) ( 26 Mg/ 24 Mg; plasma mass specs only) Little bonding contrast, like Ca Possible paleoceanographic and other applications Responds to diffusion in high-t studies. Silicon (Si) ( 30 Si/ 28 Si; gas source MS or plasma source MS) see articles by De La Rocha et al. Diatom tests are1.0 to 1.3 LIGHT relative to ocean water the diatoms grew in Surface layer of ocean shifter to greater values This effect more intense when diatoms are more Si-limited- high levels of other nutrients, and high productivity Germanium (Ge) Plasma mass specs. Measureable to high precisision. May tell us about weathering? See articles by Rouxel et al. Chlorine (Cl) ( 37 Cl/ 35 Cl; gas source MS or PIMMS) Standard is SMOC- ocean chloride See Michael Stewart s chapter: RiMG Non-traditional book We dealt with redox-related effect with perchlorate reduction earlier But perchlorate is rare; almost all Cl is found as Cl - Natural δ 37 Cl does vary, probably because of diffusion effects and/or bonding contrasts between Cl - in various matrices (aqueous vs. clays vs. amphiboles, etc.) o Igneous rocks: -1 to +8! Mantle seems heterogeneous! Evidence for recycling of subducted crust in e.g. Hawaii?? o Evaporites and brines- small contrasts used to learn about Cl sources and behavior Important environmental contaminants (TCE, DCE, PCE, etc.)

5 - Chlorinated organics compounds can be very toxic and are a major current challenge - δ 37 Cl varies strongly between different batches of the chemicals- can be used to identify source of contamination - δ 37 Cl is also fractionated during biodegradation of these compounds Zinc (Zn 2+ ) ( 66 Zn/ 64 Zn; Plasma mass specs) preferential uptake of lighter isotopes by algal production in oceans 0.2 changes observed (measurement precision claimed was 0.04 ) Use as an indicator of paleoproductivity in the oceans (maybe lakes)? Used to distinguish different sources of particulate pollution (tires?) Cadmium (Cd 2+ ) Important contaminant Small differences observed in contaminated areas suggest contaminant Cd may be isotopically distinct from the background- little work done yet, see papers by Bullen et al. Summary Although redox-active elements tend to have larger isotopic fractionations than other elements, sometimes large isotopic variations are observed in elements without valence changes. - Contrasts in bonding (e.g., coordination number changes or binding to organic molecules) can cause large equilibrium isotope fractionation - Diffusion or thermal gradients can cause fractionation as well, and this fractionation does not disappear at high T - Even without much bonding contrast, small fractionations can occur and provide ways to track transport and chemical processes