Rock suites

1. Rock suites Rock types that share a common chemical attribute, kindred groups: Silica undersaturated: phonolite, tephrite, basanite, nephelinite, melilitite Metaluminous: rhyolite, dacite, andesite Rock suites occur in specific geographic areas: Calc-alkaline volcanism in island arcs Sub-alkaline to alkaline basalts on Hawaii Alkaline rocks: have excess Na and K, silica undersaturated, most have Ne and Lc in the norm. Can be split in sodic and potassic series. Mineralogy analcime, alkali-feldspar, alkali-rich amphiboles, Na-Ti-Al-rich cpx, biotite-phlogopite, olivine, no quartz or opx Sub alkaline rocks: silica saturated and silica oversaturated: feldspars, hornblende, augite, opx, biotite, olivine (low Si), quartz (high Si)

2. Rocksuites cont’d Subalkaline to alkaline suites Tholeiitic and calc-alkaline series. - Tholeiitic series more Fe-enrichment. Calc-alkaline series Magnetite crystallizes earlier

3. Calc-alkaline suite Rock suites cont’d Low, medium and high K series Difference in conditions? Basalt classification: Quartz-hyperstene normative: quartz tholeiite Olivine- hyperstene normative: olivine tholeiite Nepheline normative: alkaline basalt

4. Trace elements Compatible trace elements prefer the solid phase Incompatible trace elements prefer the melt phase cxx is concentration in solid phase, cm is concentration in melt Important for partitioning: Size of the ion with respect to size of lattice site Charge of the ion with respect to the one that is replaced Difference between the crystal and melt structure Substitutions based on size/charge Ge and P for Si V for Ti Ga for Al Cr, Co and Ni for Fe and Mg Sr, Eu and REE for Ca Eu for Na Rb, Ba, Sr and Eu for K

5. Melts are often in equilibrium with crystalline phases: Bulk Partition Coefficient: Dbulk=X1D1+X2D2+X3D3…….. Trace elements cont’d Rare Earth Elements: Generally insoluble in aqueous fluids Lanthanide contraction. Concentration normalized to chondrites

6. Trace element cont’d Trace element patterns: Normalized abundance patterns to: Chondrite, BSE, N-MORB, ….. Coryell-Masudo diagrams, spider diagrams Concept of primitive mantle: also BSE: initial condition, but normalizes away the effects of nucleosynthesis. For most incompatible elements concentrations in BSE are 3.5 times those in chondrite Enriched: incompatible element enriched Depleted: incompatible element depleted Related to melting

7. Isotopes • Stable and radiogenic isotopes • Stable isotopes fractionate during igneous processes indicators of both source and process • Radiogenic isotopes do not fractionate during geological process. Variation due to decay and P/D ratios, indicator of source and age Light stable isotopes most important for igneous petrology: H, C, O and S Oxygen isotopes: 16O (99.76%), 17O (0.04%), 18O (0.2%) Reported as fractional deviation from standard: 18O=[(Rsample-Rstandard)/Rstandard)] Standard is Vienna Standard Mean Ocean Water Positive -values, heavy isotope enriched Fractionation occurs during crystallization and is T dependent O-isotopes can be used as a geothermometer Isotope exchange reaction: 2Si16O2+Fe318O4 = 2Si18O2+Fe316O4 Examples of thermometers: Quartz-magnetite Plagioclase magnetite Fractionation factors are Plagioclase-pyroxene experimentall determined Quartz-plagioclase as a function of temperature Quartz muscovite

8. Stable isotopes cont’d In systems that include a fluid or melt and solid the heavier isotopes is most times concentrated in the solid Fractionations decrease as a function of temperature At igneous temperatures fractionations are small: source indicator At metamorphic temperatures fractionations are larger: geothermometer

9. Radiogenic isotopes Age determination dates the time of closure Rb-Sr U, Th-Pb Sm-Nd Lu-Hf K-Ar The line on which the data lies on a plot that included both normalized parent and normalized daughter concentration is called an isochron

10. Radiogenic isotopes cont’d