Cavity ringdown spectroscopy, instrument

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Cavity Ringdown Spectroscopy, instrument cost ~$90K vs ~$400K for mass spec from Gupta, P 2009, Chapman Conf poster Cavity Ringdown Spectroscopy, principles from - idea is to compare empty chamber and full-chamber ring-down across several absorption lines - must determine unknowns against a calibration of ring-downs of known standa Geothermometry & paleoclimate proxies 10/10/12 The JOIDES Resolution drillship Temperature-dependent fractionation - recap Equilibrium fractionation is temperature-dependent, always - we’ve discussed the liquid-vapor fractionation for precipitation - today: carbonate-liquid fractionation any solid phase red=warm blue=cold; arrows track movement of 18O through phase changes Carbonate δ18O – introduction Minerals (e.g carbonate, quartz, barite, etc) form from super-saturated solution δ18O of these minerals is a fxn of δ18O of solution and temperature of solution Remember: the δ18O of a solid phase is usually reported in PDB (heavy standard) while δ18O of liquid phase is usually reported in SMOW (light standard) interconversion equation: δ 18OSMOW = 1.03086(δ 18OPDB ) + 30.86 Friedman and O’Neil (1977) Important: You need to know the δ18O of the solution to derive temperature from δ18Osolid The ocean δ18O is defined as 0‰ (Standard Mean Ocean Water), and it’s a big volume, so how you change δ18O of seawater? Carbonate δ18O – temperature relationships The relationship between water-δ18O, temperature, and the equilibrium δ18O of calcite was determined empirically by Sam Epstein et al., (1953) and later modified by Craig (1965): T °C = 16.9 − 4.2(δ − δ ) + 0.13(δ − δ ) c w c w O’Neil et al (1969) determined an experimental relationship for the temperature-dependence of α for the calcite-water system: 103 ln α = 2.78(106 T −2 ) − 2.89 NOTE δ c must be wrt PDB, δ w must be wrt SMOW good for low T, paleoceanography T in Kelvin good for high T Aragonite δ18O – temperature relationships Why is the δ18Oarag-water α different than the δ18Ocal-water α? Is the α larger for aragonite or calcite? T-dependence of α (T in Kelvin): Zhou & Zheng, GCA, 2003 LGM Glacial-Interglacial foraminifera δ18O, revisited Data from deep-sea (benthic) foraminifera show +1.5‰ δ18O shift during LGM The million-dollar question in paleoceanography: How much of this shift was due to ice volume (sea level change) and how much was due to temperature change? Schrag modelled the glacial-interglacial shift in porewater δ18O (~1.0‰), so we have 0.5‰ left over for temperature change How much did bottom water temperatures change during the LGM? (problem set) Or you could measure temperature (trace metal concentrations in carbonates), and obtain a “residual” δ18O that gives you the δ18OSW change Complications: Kinetic effects, vital effects and carbonate δ18O Fact: very few organisms precipitate carbonate in isotopic equilibrium with the surrounding water (the vital effect) environments”, Kinetic isotope effects underlie vital effect One problem: skeletons are precipitated in super-saturated “microwith sources from surrounding water & metabolic products Another problem: isotopic exchange may be rate-limited in biological reactions Can track kinetic effects with isotope-isotope plot (δ13C vs δ18O), check for slope = values covary with a positive d O/d C slope of 0.29 The foraminiferal d1 O/d1 C relationships obtained herearevirtually identical, with slopes ranging between 0.29 and 0.33 (Fig 3) Other ahermatypic corals, calcareous algae and invertebrates such as cidaroid 18 15,16 urchins show oxygen and carbon isotope covariance , although the slopes can differ from the experimental range In symbiont- probably indicate higher calcification rates Chamber m for G bulloides areinconclusive Although we cannot offer a definitive mechanism isotope:[CO 23 − ] relationships, mass balance calculations sh the results cannot be explained by effect, a simple redistribu * Not a very big isotopes between the dissolved carbon species19 MConna Carbonate ion effect on foram δ O Results from culturing living forams: increase CO32-, δ18O foram decreases Constant alkalinity 2− Constant SCO a 4.00 but casts further doubt on inferring G-I T changes from forams Figure Effect of [CO ] on the d C and d HL Dark 4.00 13 18 O Orbulina universa shell calcite under c HL Dark constant alkalinity (a and b) and constant SC 13 Shell d C (‰) d) conditions Specimens grown under high li the dark are shown by open and close 2.00 2.00 the Southern California Bight Data are gro 0.00 -2.00 100 200 300 400 500 600 700 800 -0.50 b HL Dark -1.50 -2.50 -2.50 2– 200 300 2– 400 500 [CO ] (mmol kg–1) 600 200 300 400 500 600 d the data 700 800 HL Dark Spero et al., Nature 1997 d18O =1.60 — 0.002 [CO3 ] R =0.92 2– d18O =1.44 — 0.002 [CO3 ] R =0.97 100 100 individual shells Lines are linear regression 2– d18O =1.56 — 0.002 [CO3 ] R =0.89 2– d13C =4.29 — 0.006 [CO3 ] R =0.90 2– d13C =3.30 — 0.006 [CO3 ] R =0.92 -2.00 -1.50 -3.50 values 1s.d.; most groups are compose 0.00 2– d13C =4.74 — 0.006 [CO3 ] R =0.95 2– d13C =3.56 — 0.006 [CO3 ] R =0.93 -0.50 18 Shell d O (‰) respectively Arrowheads identify ambient 2– 700 800 -3.50 d18O =1.31 — 0.002 [CO3 ] R =0.96 100 200 300 400 500 600 700 800 2– [CO ] (mmol kg–1) Figure Effect of [CO23 − ] on the d1 C and d1 O Globigerina bulloides chamber calcite under Glacial-Interglacial climate reconstruction In order to reconstruct surface temperatures from carbonate δ18O formed during the LGM, you need to 1) remove the ice-volume effect 2) constrain the δ18O of your local water mass 3) apply the paleo-temperature equation However, people can use other proxies to get at temperature: 1) foraminifera assemblage data (CLIMAP) 2) tree lines and snow lines will be lower during cold times 3) trace metals in carbonates (Mg/Ca in forams or Sr/Ca in corals) 4) alkenones (saturation index of long-chained alkanes in coccolithophores) δ18O as tracer of igneous processes Applications of oxygen isotopes in igneous rocks: 1) 2) determine temp of formation (water-mineral or mineral-mineral pairs) quantify “water-rock” ratios of altered rocks spectrometer light intake A “black smoker” from the East Pacific Rise Oxygen Isotopic compositions of geological materials Why does eclogite have a heavier δ18O than MORB? Lunar rocks MORB basic lavas mantle nodules eclogites andesites Why metamorphic rocks ophiolites exhibit such a range of δ18O? rhyolites & tuffs granitic rocks altered igneous rocks If carbonates precipitate metamorphic rocks from a “light” ocean, clastic sediments why are they so heavy? marine limestones composition of lunar rocks, carbonaceous chondrites, and MORB Why is the ocean so light compared to MORB? What scale is this? Principle of Geothermometry in igneous applications The fractionation of oxygen or hydrogen in different minerals of a rock can be used as a geothermometer, provided that: minerals deposited at same time, at equilibrium no subsequent alteration fractionation factors and T-dependence known experimentally NOTE: Using multiple mineral pairs will increase confidence in the calculated temperature, if the mineral pair temperatures agree – i.e they are concordant For phases m and n: 1000 ln α m − n = a *10 +b T General form of geothermometry fractionation equations Handy conversions: 1000 ln α m − n = ∆ m − n = δ m − δ n T in Kelvin 1000 ln α calcite − water 2.78*106 = − 2.89 T Remember from last lecture we talked about the high-T water-calcite equation? T-dependent fractionation in various mineral pairs Where must these lines converge? NOTE: These slopes are different – so all you need to determine T is ∆m-n The highest fractionation is between quartz and magnetite In general, 18O is increasingly favored in higher-quartz minerals, and less favored in hydrous minerals (magnetite, amphibole, chlorite) How could we determine the slope of the Quartz-Muscovite fractionation? Today’s Handouts: Tables of T-dependent Fractionation Example Quartz, calcite, and chlorite were all precipitated in a hydrothermal vent setting Measured δ18O’s: Quartz: 5.1‰ SMOW Calcite: 3.8‰ SMOW Chlorite: -1.5‰ SMOW Why does quartz have the heaviest δ18O, and chlorite the lightest? And why is the quartz only 5.1‰ heavier than SMOW? Did these minerals precipitate at the same temperature? How would you begin to solve this problem? Metamorphism: Water-Rock interactions Fact: In several places it is possible to measure igneous rocks with δ18O values of -5‰! These are places were fluid has interacted with the rock (usually at high T) to change the isotopic composition of the rock We can use a mass balance approach to calculate the amount of water that has reacted with a host rock (or “water/rock ratio”) over time (assuming equilibrium): for a closed system What does a “closed system” mean? and mass balance equation: and combining these: is the equilibrium values for water and mineral, (need to know temperature independently) ∆ = δw − δr cwW δ + cr Rδ = cwW δ + cr Rδ r i w i r f w  cr  W δ rf − δ ri = i * ÷ f R δ w − δ r − ∆  cw  f cw = conc of O in water cr = conc of O in rock W = mass water R= mass rock superscript I = initial superscript f = final Water-Rock interactions II for an open system Now we only have a small parcel of water (dW) interacting at any given time, but new water parcels are injected continuously in time, causing dδr Rcr d δ r = ( δ wi − [ ∆ + δ r ] ) cw dW in this scenario we need to integrate to calculate W/R ratios  δ rf − δ ri   cr  W = ln  i + 1÷ ÷ f R  δ w − δ r − ∆   cw  Probably much more realistic, because water flows through the rock In order to solve for W/R interactions, you need to know: the temperature of the interaction (hopefully you can get that by a mineral-mineral pair) the mineral phases that experienced fluid alteration the isotopic composition of the water before it interacted with the rock (δD of rock… why?) the isotopic composition of the rock before it interacted with the water (unaltered samples) A real-world example A characteristic signature of hydrothermal activity is a “bulls-eye” pattern of δ18O values, with low values in the middle Alteration occurs along an established conduit of weakened structures Most gems are the product of low-T, high-fluid metamorphism – $1M worth of gold mined in the Bohemia complex between 1870 and 1940 happy hunting! A cool early Earth? Idea: measure U-Pb dates and δ18O of old zircons - if you find low δ18O relative to today’s ‘primitive’ mantle, then that implies interaction with meteoric waters at low temperatures This work is done using a laser flourination line plumbed to a dual inlet mass spec A Cool Early Earth, (2002) Geology 30: 351-354 So what’s causing the relatively high zircon δ18O values at 4.2Ga? time-line along bottom indicates: (1) accretion of the Earth, (2) formation of the Moon and the Earth’s core, (3) minimum age of liquid water based on high δ18O zircon, (4) Acasta gneiss, and (5) Isua metasedimentary rocks likely interaction with meteoric waters which implies a period of few impacts
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