SPRING 2003GEOLOGY 285:INTRO. PETROLOGYDr. Helen LangDept. of Geology & GeographyWest Virginia UniversityMetamorphism of Ultramafic (UM) RocksMost ultramafic rocks come from Earth’s mantleOriginal Mineralogy of UM Rocks:by definition >90% mafic minerals, • Dunite >90% Olivine• Harzburgite Olivine + Opx• Lherzolite Olivine + Opx + Cpxmostly Olivine Ol (Mg,Fe)2SiO4Orthopyroxene Opx (Mg,Fe)SiO3Clinopyroxene Cpx Ca(Mg,Fe)Si2O6Many UM rocks are part of Ophiolites• Special sequence of rocks thought to represent a cross section of oceanic crust and upper mantle formed near mid-ocean ridges• The Ophiolite Sequence Deep sea pelagic sediments Pillow basalts Basaltic sheeted dike complex Layered gabbros Deformed ultramafic rocks (upper mantle)bottomtopThe Ophiolite SequencePelagic SedimentsPillow BasaltsSheeted DikesLayered GabbrosDeformed UM Mantle RocksHow do Ultramafic Rocks get incorporated in the Crust?• Obduction = overthrusting of big blocks, uncommon• Oceanic crust and upper mantle (ophiolites) get caught between colliding continental blocks; Alps, Himalayas, Appalachians• Oceanic crust and upper mantle get scraped off in the subduction zone; “dismembered” ophiolitesHow do Ultramafic Rocks get incorporated in the Crust?Most UM Rocks get Serpentinized as they are incorporated into continental crust• Serpentinization Olivine + H2O = Serpentine Mg2SiO4+ H2O = Mg3Si2O5(OH)4• Mantle UM rocks are high grade metamorphic rocks; serpentinization is a retrograde (changing from higher grade to lower grade) metamorphic process• Retrogression usually involves hydrationSerpentine in Thin SectionPPL-low relief, relict Ol and Px shapes visibleXPL-low birefringence, mesh texture apparentWe will look at changes in a serpentinite caused by increasing metamorphic Temperature•A serpentinite is a rock composed mostly of serpentine• Serpentine is a sheet silicate (2 main kinds)– chrysotile: What do you know about this?– antigorite: The main metamorphic serpentine• Metamorphism caused by increasing T is called prograde metamorphismMetamorphosed UM Rocks are not especially common; why discuss them now?• They can be represented by a simple chemical system: CaO + MgO + SiO2+ H2O• Changes mostly reflect increasing Temperature• Metamorphic chemical reactions in UM rocks are mostly dehydrations• Dehydration (loss of H2O) is the main effect of most prograde metamorphism!!We need a new Triangle!Starting mineralsOl (Mg,Fe)2SiO4Opx (Mg,Fe)SiO3Cpx Ca(Mg,Fe)Si2O6All are very Mg-rich, so we can ignore FeOThree remaining chemical components:CaO, MgO, SiO2We’ll let H2O (or OH) be present in some mineralsCaO, SiO2, MgO triangle for UM RocksDi (Cpx) CaMgSi2O6En (Opx) MgSiO3Fo (Ol) Mg2SiO4CaO MgOSiO2FoEnDiCompositions of most UM rocksOther Minerals that can occur in UM RocksAntigorite (Atg) ~Mg3Si2O6(OH)4Tremolite (Tr) Ca2Mg5Si8O22(OH)2Talc (Tlc) Mg3Si4O10(OH)2Quartz (Qtz) SiO2Anthophyllite (Ath)Mg7Si8O22(OH)2CaO MgOSiO2FoEnDiBrucite (Brc)Mg(OH)2Periclase (Per)MgOAtgAthTlcTrQtzBrc,PerChanges in UM assemblages happen in this triangleAppalachian UM bodies = suture zoneMinerals of a low-grade serpentiniteAtgTlcQtzBrcDiTrBP-T Phase Diagram for UM RocksTemperature (oC)Pressure (H2O) (kbars)12345200 400 600 800Chrysotile**Atg+ Brc**Atg+BrcFo+H2OAtg+DiFo+Tr+H2OAtgFo+Tlc+H2OFo+TlcAth+H2OAth+FoEn+H2OTr+FoDi+En+H2OABCEGHp. 441 in textDFfrom B to C (rxn Atg+Brc=Fo+H2O)AtgTlcQtzBrcAtg+BrcFo+H2OBCFoAtgTlcQtzTrDiMineral appearance:new mineral (Fo) and tie-lines to it appear BrcDiTrfrom C to D (rxn Atg+Di=Fo+Tr+H2O)AtgTlcQtzBrcAtg+DiFo+Tr+H2OCDFoAtgTlcQtzTrDiCrossing tie-lines: old tie-line disappears, new tie-line forms BrcFoTrDifrom D to E (rxn Atg=Fo+Tlc+H2O)AtgTlcQtzBrcAtgFo+Tlc+H2ODEFoTlcQtzTrDiMineral disappearance:mineral (Atg) and tie-lines to it disappearBrcPerFoTrDiNotice limited range of Talc+Fo(Ol)Temperature (oC)Pressure (H2O) (kbars)12345200 400 600 800Chrysotile**Atg+ Brc**Atg+BrcFo+H2OAtg+DiFo+Tr+H2OAtgFo+Tlc+H2OFo+TlcAth+H2OAth+FoEn+H2OTr+FoDi+En+H2OABCEGHp. 441 in textDFOlivine-Talc Rock with “jackstraw” textureOutcropThin section in XPLAlmost all reactions are Dehydrations: H2O always on hi T side; curved as shownT (oC)Pressure (H2O) (kbars)12345200 400 600 800Chrysotile**Atg+ Brc**Atg+BrcFo+H2OAtg+DiFo+Tr+H2OAtgFo+Tlc+H2OFo+TlcAth+H2OAth+FoEn+H2OTr+FoDi+En+H2OA BCE GHD FSerpentineGrnschistAmphib.GranuliteAbove about 900oC all UM rocks have completely anhydrous mantle assemblageFoEnQtzPerDiRetrograde hydration of this assemblage produced serpentinite. Progrademetamorphism (dehydration) of serpentinite eventually produces the same high grade assemblage (Di+En+Fo).Detail of UM Phase DiagramTemperature (oC)Pressure (H2O) (kbars)Atg+BrcFo+H2OAtg+DiFo+Tr+H2OAtgFo+Tlc+H2OBCEDQtzDiTr TlcAtgBrcFoDetail of UM Phase DiagramTemperature (oC)Pressure (H2O) (kbars)Atg+BrcFo+H2OAtg+DiFo+Tr+H2OAtgFo+Tlc+H2OBCEDQtzDiTr