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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

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Rock suites

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)


Rocksuites cont d
Rocksuites cont’d

Subalkaline to alkaline suites

Tholeiitic and calc-alkaline series.

- Tholeiitic series more Fe-enrichment. Calc-alkaline series Magnetite crystallizes earlier


Rock suites cont d

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


Trace elements
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


Trace elements cont d

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


Trace element cont d
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


Isotopes
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


Stable isotopes cont d
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


Radiogenic isotopes
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



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