GEOS 425 Whole Earth Geochem BSU Final Review

What defines a trace element?
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Terms in this set (42)
What defines a trace element?
Elements that are not stoichiometric components of phases which do not affect the physical and chemical properties of the system.
LILE- what are they?
Large Ion Lithophile Elements - K, Rb, Cs, Sr, Ba (mono-divalent and soluble). Don't fit in most minerals (excl. micas and feldspars). Ex: Sr -> Ca (in calcite)
HFSE- what are they?
High Field Strength Elements - Zr, Hf, Nb, Ta, Th, U (immobile, insoluble excl. Uranium). Incompatible due to charge (+4,+5)
REE - what are they?
Rare Earth Elements (Lanthanide series: La->Lu), trivalence (+3) makes them incompatible.
Eu anomaly
Spike due to +2 charge being more stable ground state config, enriched in low oxygen fugacity (magmas) and doesn't like crystalline mineral phases
Chondritic Normalization
REEs normalized to chondrites because they are assumed to homogeneous in composition, good for referencing deviations
Lanthanide Contraction
Ionic radii of REEs decrease with increasing Z (to the right)
Ce anomaly
Enrichment due to +4 charge in highly oxidized environments so it prefers high oxygen fugacity (like seawater)
Partition coefficients (D)
D(part. coeff) = C(mineral)/C(melt), D<1, incompatible. D>=1, compatible.
Onuma diagrams
Shows the relationship between D and ionic radius. Diagram shows an upside down parabolic curve where maxima is ideal fit in a given site
Blundy and Wood ModelD(initial)*exp(4pi*(young's)*(atomsinmole)*(optimal_radius)/2*(exchange radius - optimal)^2*1/3(exchange - optimal)^3))/(R*T)Equilibrium (batch) meltingMelt always in eq with solids. C(l)/C(s) = 1/(D(1-F) + F)Fractional meltingMelt and solids not in eq: C(l)/C(i) = (1/D)*(1-F)^(1/D-1)Accumulated fractional meltaccumC(l)/C(i) = (1-(1-F)^(1/D))/F (EQUALS batch melt!)Eq (batch) crystallizationSolids in eq with const. melt. C(l)/C(i) = 1/(DX+1(1-X)) and C(s) = DC(l)Fractional crystallization (instantaneous melt)Solids not in eq. with const. melt C(l)/C(i) = (1-X)^(D-1)Fractional crystallization (instantaneous solid)C(s)/C(i) = D(1-X)^(D-1)Aggregate frac. crystalaggC(s)/C(i) = (X^D)/X (EQUALS batch crystal)Valley of stabilitystable nuclides have nearly even numbers of proton and neutrons; however, electrostatic repulsion of protons is buffered by excess neutrons, giving the valley of stability its curvatureIstopes and isobarsIsotopes=same element (Z), different # of N. Isobars = isotopes of roughly same mass, can throw off a mass specGamma decayemission of high energy (and frequency) photonSpontaneous Fissionsplitting of U-235,238 Th-232 into two or more mod. heavy daughters (which decay by B-minus)Alpha decayEmission of helium nucleus (alpha particle)Beta minusemission of an alectron and associated conversion of a neutron to a protonPositron Decay (Beta Plus)emission of a positron and assoicated onversion of a proton to a neutronElectron captureAddition of an electron to a nucleus, with accompanying conversion of a proton to a neutronDecay constantProportionality constant of probability nuclide will decay, Costant = ln(2)/(t-1/2)Half life (t-1/2)Time it takes for half of the parent nuclides to undergo decay t-1/2 = ln(2)/decay constantEquation for parent decayN = N(i)*exp(-decay*t) to back track to inital N(i) = N*exp(+decay*t)Equation for daughter generationD*= N(i)(1-exp(-decay*t)Age equationt = 1/(decay)*ln((D-D(i))/N + 1) = 1/(decay)*ln(D*/N + 1)1st tenet of geochron1). Rock or mineral system has neither gained nor lost either parent or daughter atoms so that the D*/N has only been changed due to rad. decay2nd tenet of geochron2). It must be possible to assign a realistic value to D(i) (easier when D*>>D(i))3rd tenet of geochron3). The value of decay constant must be known accurately and should be appropriate to the timescales being considered4th tenet of geochronThe measurements of D and N must be accurate and representative of the rock or mineral to be datedModel agesIf D(i) << D*, D* ~ D, age is insensitive to D(i), so can ignore D(i) or assume DK-Ar model datingUses 40K-40Ar ratios, since Ar is a gas which is easily lost from high temp minerals and magmasZircon U-PbU-238/Pb-206 and U-235/Pb-207 in zircons exclude D(i), (Pb) while incoprorating N, thus allowing for D(0) << D*Isochrons: example Rb-SrIf D(0-initial) is high? Isochron plot with 87Sr/86Sr vs. 87Rb/86SrLinear equation of Rb-Sr87/86Sr(t) = (87/86Sr)(i) + (87Rb/86Sr)(exp(decay*t)-1)87Sr/86Sr of Earth systemsBABI (initial) ->over time -> high Rb/Sr in crust created at 3.8 Ga and low Rb/Sr depleted at 3.8 GaSr in hydrosphereSeawater Sr is a mixture of three sources: 1) continental-derived Sr (relatively high 87Sr/86Sr); 2) mantle-derived Sr from the mid-ocean ridge system (relatively low 87Sr/86Sr); 3) recycled Sr from Phanerozoic marine carbonates (relatively low 87Sr/86Sr)