Small-fraction volatile-rich melts

Small-fraction volatile-rich melts

1. Kimberlites

   • Composition
     • Volatile-rich: H2O and CO2
     • Ultrabasic
       • High K2O/Na2O
     • Ultrapotassic
     • High concentrations of highly incompatible trace elements
     • Complex hybrid: not a magma or a melt
       • Fragments of HT mantle xenoliths
       • Macrocrysts - mainly Ol
       • Small euhedral Ol crystals - not phenocrysts
       • Wall-rock xenoliths
   • Occurance
     • Cratons and stable platforms
       • Thick lithosphere
     • Diamondiferous: Archean/Protorzoic cratons
     • Exact location: faults
     • E.g. Canada, Greenland, South Africa
     • Diatremes & pipes
       • Facies
         • Crater
           • pyroclastic and epiclastic rocks
             • Tuffs and tuffs breccia
             • Well-stratified tuffs and epiclastic lucustrine deposites
               • No depositional feature so probably airfall
           • Rarely produce lava flows
           • Small-volume
         • Diatreme
           • Carrot-shaped
             • Circular/elipitical cross-section
             • Vertical axes
             • Steeply dipping margins
           • Converge/terminate at depth
           • Tuffistic breccia
             • Ol-cored pelletal lapilli
             • Country rock
             • Macrocrysts in fine-grained matrix
         • Hybassal
           • Macrocryst-rich
           • Dykes (& sills)
             • Swarms of parallel features
           • Igneous textures
           • Country rock xenoliths
           • Root zones
       • Emplacement
         • Fast enough that diamond don't alter
         • Transport loads of mantle xenoliths - dense
         • Pyroclastic material at depth - normally too deep to have fragmentation by volatile
         • Loads of fracturing to host rocks but little contact metamosphism
         • Complex shallow strucutre
         • So rich in CO2 it exploded?
         • Hydrovolcanic interaction: magma + groundwater?
         • Volatile content of magma causes fragmentation/explosive eruption?
           • No metamorphism though
           • Plinian eruption
           • don't see wide zone of tephra - eroded?
         • No evidence of big magma chamber
   • Source
     • CO2-rich metasomatised harzburgite
     • >150km

2. Lamprophyres

   • Enter text here