12.109 Lecture Notes December 1, 2005 How does fluid composition vary during metamorphism? Hit reaction curve 22dol qtz H Ocalc tr CO++++U Fluid composition becomes more CO2 rich Will reach invariant point, and then travel along 22calc tr dol di H O CO+++ +Ucurve (univariant). When reaction is complete, the composition leaves the curve. The fluid composition remains constant as T increases.Fluid composition changes during metamorphism, depends on bulk composition of rock. Fluid can also sometimes buffer reactions – this is controlled by the fluid to rock ratio. Metamorphism of Mafic RocksEx. basalts See Spear, Ch. 11 Chemically complex—system not easy to look at graphically SiO2 – Al2O3 – FeO – MgO – CaO – K2O – Na2O – H2O 8 components  10 phases! ACF triangle diagram – Al2O3, FeO + MgO, CaO Facies for metamorphic rocks – mineral assemblage characteristic of a set of P-T conditions for a particular bulk composition At low T, metamorphic minerals only appear in small amounts Zeolite facies – rich diversity of minerals, often form in holes in igneous rocks Analcine 26 2()NaAlSi O H O• Analcine + qtz U albite + H2O Lamontite 2412 24( )CaAl Si O H O• Lamontite + qtz U lawsonite 227 2 2()CaAl Si O OH H O• Warikite lawsonite + qtz 2412 22( )CaAl Si O H O• UPrehnite 22310 2()Ca Al Si O OHPumpellyite 322 310 2()Ca Fe Si O OH+Breaks down to epidote + chlorite + albite + qtz  lower greenschist facies Amphibolite facies – marked by the appearance of amphibole and plagioclase (albite-anorthite solid solution). Plagioclase is not appearing for the first time in this facies, but as T goes up, the variety changes from pure albite to about 20% anorthite Zoisite (like epidote, but Al rather than Fe3+) + qtz  actinolite + H2O Zoisite + chlorite + qtz  actinolite + anorthite + H2O @ higher T, make hornblende Granulite facies – appearance of orthopyroxene Hornblende + qtz  cpx + opx + plag + H2O Blueschist – albite + chlorite  glaucophane + H232822()Na Mg Al Si O OH22O 2 jadeite + 1 talc Classic blueschist assemblage: glaucophane + lawsonite (10% H2O!) + chlorite Eclogite – garnet, cpx (Ca and Na, apple green colored), NO PLAGIOCLASE Omphacite Diopside +