Major and Trace Element Geochemistry. Major and Trace Element Geochemistry

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1 Major and Trace Element Geochemistry Just as seismology is an important tool to image the earth s interior, so too are chemical and isotopic compositions of igneous rocks that originate at great depths with the upper mantle and lower crust. Importance of chemical compositions of igneous rocks Petrogenesis of primary magmas these reflect mineralogy and chemistry of the source rock Differentiation of magmas need to decipher shallow processes to infer deep source Radiogenic isotopes allow a time-integrated view of changing composition Major and Trace Element Geochemistry Major elements Comprise most of the rock Expressed as weight (wt.) % oxides, each >0.1% Analyzed by XRF, ICP-MS Trace elements Present in concentrations <0.1% Expressed in ppm or ppb Analyzed by XRF, ICP-MS, INAA Volatile elements H 2 O, CO 2, SO 4 Rare gases: He, Ar, Ne, etc. Analyzed by spectroscopy or mass spectrometry Radiogenic isotopes Ratios of radiogenic to nonradiogenic isotopes of an element recall isotopes of an element have same atomic no., but variable # of neutrons Variations in ratios reflect differences produced over time by radioactive decay in source Variations are extremely small: analyzed by magnetic sector mass spectrometry Stable Isotopes Lighter masses fractionated by geological processes Analyzed by magnetic sector mass spectrometry

2 E. Major and Trace Element Geochemistry Variation Diagrams Plot chemical differences and trends among related rocks (lavas = magmas?) Only true for liquids (aphyric lavas and tephras) Can define and help model products of partial melting and crystallization Plot ME, TE or both Major elements, Harker diagrams Cogenetic lavas = well-defined trends Lever-rule can quantify fractionating mineral assemblage Inflected trends = changes in crystallizing mineral assemblage Simple, yet powerful way to compare/distinguish suites of rocks (magmas) A 36o S 70 o 35' W Qot d ice Contour interval = 250 m km Qot 3000 San Pedro Estero Qsp ice Qoh Qot Volcán San Pedro 3621 m H73 H H72 ice QH Qoh 3000 Quebrada H 2750 QH1-2 RIO DE LA PUENTE o 2500 ndo 1 PED12 4 Qoh 70 o 50' W Qm Qcf 2 H20 H12 H11i H14 H16 Estero El Tgh Qda Pellado 3 H23 Qcf Qpv Tgh H Cerro Pellado Tgh 3213 Qcg Geologic Map Volcán San Pedro Surficial Deposits Alluvium - Colluvium Qm Neoglacial Moraines Qda Debris avalanche deposit Volcán San Pedro Qsp Younger Holocene Summit Lavas Qcf Younger Holocene Composite Flow Qoh Older Holocene Lavas Volcán Tatara ka Mainly Basaltic Andesite Lavas d Tatara Dacite 68 ka pre-volcán Tatara Qpv Qcg Qot Tgh Vent Volcan Pellado Guadal Lavas Flow direction ka ka Older TSPC lavas ka Basement Rocks Huelmul Granite Ma Volcanics metamorphosed 7-9 Ma H23 Sample locality Costa and Singer (2002) Journal of Petrology Volc<n San Pedro Southern Volcanic Zone Chilean Andes B Older Holocene H73 QH1-2 Moraine Volcan San Pedro H72 H m QH2-1 Volcan Tatara C Older Holocene Volcan Tatara H23 Volcan San Pedro 3621 m H72 H70 Older 2 Holocene PED12 Moraine 1 4 H20 2 H14 H12 1 H16 3 Guadal Lavas H8 4 Estero Pellado Volcan Pellado

3 Costa and Singer (2002) Harker Diagrams, Volc<n San Pedro Lavas

4 Major and Trace Element Geochemistry Trace elements Partitioning between crystalline and liquid phases Partition coefficient: D liq xtal D << 1, incompatible elements Large Ion Lithophile Elements (LILE)» K, Rb, Sr, Ba,» Zr, U, Th, REE, etc. = concentration in mineral concentration in liquid D > 1, compatible elements» Ni, Cr, Co, etc. Major and Trace Element Geochemistry Rare Earth Elements (REE) 15 elements from mass 57 to 71 (14 occur naturally) La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu Useful because similar in geochemical behavior Trivalent except Eu can be Eu 3+ or Eu 2+, depending on f O2 To eliminate Oddo-Harkins effect, normalize to chondritic meteorites Basalt, garnet-bearing source Basalt, plag fractionation or plag in source Basalt, garnet-free source

5 Major and Trace Element Geochemistry Rare Earth Elements (REE) Particular minerals influence shape of chondrite-normalized REE pattern by virtue of D values: Feldspar: 2 + negative Eu anomaly Garnet: high D for Heavy REE (HREE) Olivine: D < 0.1 for all REE; uniform effects on magma Hornblende: D > 1.0 for middle REE Zircon, Sphene, Apatite: strong affinity, high D for REE Mantle REE: originally flat pattern, 2-3x chondritic Partial melting leaves upper mantle depleted in LREE Degree of enrichment of REE in melts Abundances and mineralogy in source Degree (percentage) of melting Extent of fractional crystallization See Wilson Fig. 2.3 from previous panel Major and Trace Element Geochemistry Rare Earth Elements (REE) Extend normalization approach to several other elements = Spiderdiagrams Plot in order of increasing D Normalization is arbitrary: to primordial mantle, chondrites, MORB Peaks, troughs, slopes, anomalies indicative of specific crystal-liquid equilibria processes

6 Major and Trace Element Geochemistry Primary Magmas Formed by partial melting of upper mantle in equilibrium with olivine+pyroxene unmodified by fractional xtlln, assimilation/contamination, magma mixing, etc. Truly primary magmas are rare to nonexistant most basaltic magmas fractionated olivine and assimilated some lithosphere on way up Criteria not firm but: K d = (Fe 2+ / Mg) olivine /(Fe 2+ /Mg) melt K d = 0.3 so that: Mg = Mg/(Mg+ Fe 2+ ) of basalt in equilibrium with Fo 91 is Typically: Ni > ppm Cr > 1000 ppm SiO 2 < 50% Metasomatism (addition of fluids + new minerals) of mantle may change possible primary magma composition Radiogenic Isotopes Rutherford and Soddy (1902) [Nobel Prize in Physics] Experiments indicated that thorium decay to radium is exponential over time. Radioactivity is an atomic property. Atoms in radioactive elements are unstable. Within a given amount of time, a fixed proportion of atoms disintegrate to form new atoms. Disintegration accompanied by emission of alpha or beta particles. Activity, or intensity, of radioactivity is proportional to number of atoms that disintegrate per unit time. Thus activity is directly proportional to number of atoms of substance present: dn = λn dt where 8 is the decay constant, i.e., probability that atom will decay in unit time. N No dn N t = λt to ln N/N o = -8t N = N o e -8t basic radioactive decay formula. N o is initial number of atoms N is number of atoms at time t.

7 The age equation N = N o e -8t Radiogenic Isotopes need to realize that daughter atoms D can be expressed as D = N o -N N o = D + N from above N = (D + N) e -8t D = N (e -8t -1) ln(1+d/n) = 8t t=1/8 ln(1+d/n) need to measure D, daughter atoms present, N parent atoms left. Half-life used to determine decay constants t = ln2/8 = 0.693/8 If some daughter isotope was incorporated into mineral at t o, this must be subtracted from the amount measured today: 1 D D t = 1 + λ ln N o

8 Radiogenic Isotopes The K-Ar system 40 K undergoes branched decay to 40 Ar half-life of 1.25 x 10 9 yr 8 = 5.81 x yr Ar o is small or can be corrected for System used to date rocks from historical time, 2 ka, to 4.5 Ga (meteorites) λec + λb Ar Aro t = ln λec + λb λec K The 40 Ar/ 39 Ar variant of K-Ar dating: 40 1 Ar t = 1+ J 39 λ ln Ar K J is a constant including a factor for fraction of 39 K atoms converted to 39 Ar in the neutron flux of a nuclear reactor More powerful than K-Ar dating: more precise; all measurements in single mass spectrometer smaller samples -- down to single phenocrysts incremental-heating; many ages from gas released over range of T in single sample Thermally disturbed samples yield discordant release spectrum of ages 40 Ar/ 39 Ar age spectra and isochrons