Lithogeochem_II - Earth Sciences 4502/6510...

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Earth Sciences 4502/6510 Lithogeochemistry II: Introduction to the Lithogeochemistry of Sedimentary Rocks Stephen J. Piercey, PhD, PGeo* Memorial University of Newfoundland St.John’s,NL, Canada A1B 1N3 [email protected] Monday, 13 August, 12
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Outline Basics of sediment geochemistry. Major elements - weathering, sedimentary maturity, CIW. Provenance of clastic sedimentary rocks. Geochemistry of shales. Hydrothermal sedimentary rocks (Jan Peter will cover this next week). Case studies. Monday, 13 August, 12
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Sedimentation Weathering - physical, chemical, and biological Erosion, transport and deposition Diagenesis Lithification All processes influence the geochemical signature of a sedimentary rock. Monday, 13 August, 12
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Major Elements and Sedimentary Rocks Major elements, especially alkali elements (Na, Ca, K) are very mobile during weathering and erosion. Upper crust weathering can be considered largely a function of weathering of granite and the feldspar system, thus, the elements Al 2 O 3 , Na 2 O, CaO, and K 2 O can explain many surface weathering reactions. For basaltic crust we are weathering plagioclase, pyroxenes, olivine, and hornblende, hence, we must also consider FeO* and MgO as well. As sedimentary rocks become more mature we must also consider the SiO 2 contents in these rocks and SiO 2 /Al 2 O 3 ratios (i.e., quartz to clay ratios). Monday, 13 August, 12
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Typical Surface Weathering Reactions 2KAlSi 3 O 8 + 9H 2 O + 2H + = Al 2 Si 2 O 5 (OH) 4 + 2K + + 4H 4 SiO 4 (Kfsp) (kaolinite) (silicic acid) 2NAlSi 3 O 8 + 9H 2 O + 2H + = Al 2 Si 2 O 5 (OH) 4 + 2Na + + 4H 4 SiO 4 (Ab) (kaolinite) (silicic acid) CaAl 2 Si 2 O 8 + H 2 O + 2H + = Al 2 Si 2 O 5 (OH) 4 + Ca 2+ (An) (kaolinite) For all reactions there is a loss of alkalis (±silica) and a residual gain in aluminum Monday, 13 August, 12
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Chemical index of weathering (Nesbitt and Young, 1984): where: Al 2 O 3 , Na 2 O, and K 2 O = molar abundances of elements; CaO* = molar abundance of calcium from silicates (must remove CaO from carbonates and apatite). Must convert data to molar form (i.e., divide by molecular weight of oxide). Removing carbonate/apatite CaO can be done following method of Fedo et al. (1995) . Chemical Index of Alteration CIA = 100 # Al 2 O 3 + CaO *+ Na 2 O + K 2 O Al 2 O 3 ; E Monday, 13 August, 12
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Chemical index of weathering (Nesbitt and Young, 1984): CIA < 50 for unweathered rocks (generally). ~100 for rocks that are pure clay minerals. 50-100 for variably chemically weathered. Does not account for silica rich rocks - we then must use SiO 2 /Al 2 O 3 ratios. Chemical Index of Alteration CIA = 100 # Al 2 O 3 + CaO *+ Na 2 O + K 2 O Al 2 O 3 ; E Monday, 13 August, 12
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CaO*+Na 2 O K 2 O Al 2 O 3 Kaolinite, Gibbsite, Chlorite Illite Muscovite Biotite K-feldspar Granodiorite Andesite Basalt Hornblende Clinopyroxene Natural Waters Smectite Plagioclase 0 10 CIA 20 30 40 50 60 70 80 90 From Nesbitt (2003) based on initial work of Nesbitt and Young (1984) Felsic Diagram Monday, 13 August, 12
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Mafic Diagram Kaolinite, Gibbsite Illite Musc Fsp Smec Cal Hbl Bio Chl Oxide OPX FeO*+MgO Diopside Mafic Rocks Felsic Rocks CaO*+Na 2 O+K 2 O Al 2 O 3 From Nesbitt (2003) based on initial work of Nesbitt and Young (1984) Monday, 13 August, 12
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