Localities and Ages of Paleoproterozoic Granitic Paleosols In the Lake Superior Region (map courtesy of Mark Jirsa) Schematic Paleoweathering Profile protolith saprolite regolith The A-C*N-K Plot (molar) Chemical Index of Alteration (CIA) CIA = 100 molar Al 2 O 3 /(Al 2 O 3 +CaO*+Na 2 O+K 2 O) Chemical Index of Alteration (CIA) CIA = 100 molar Al 2 O 3 /(Al 2 O 3 +CaO*+Na 2 O+K 2 O) modern tropical weathering profile protolith A-C*N-K Plots for Six Precambrian Paleosols all saprolites are displaced from the weathering vectors 1740 Ma1460 Ma Potassium Metasomatism !! (11) Compositional Changes in Saprolite Relative to Protolith % change in saprolite = 100 [(c j,s /c j,p )/(c i,s /c i,p ) 1] where c j,s = concentration of oxide j in saprolite where c j,s = concentration of oxide j in saprolite c j,p = concentration of oxide j in protolith c j,p = concentration of oxide j in protolith c i,s = concentration of immobile oxide in saprolite c i,s = concentration of immobile oxide in saprolite c i,p = concentration of immobile oxide in protolith c i,p = concentration of immobile oxide in protolith !! A caveat: the protolith must be homogeneous !! Immobile candidates are Al, Ti, and Zr Al is chosen because it is a major constituent, and it has the lowest variation among the three elements Compositional variation with depth % change relative to Al 2 O 3 in protolith Depth, cm Baraboo Pronto Denison Ville Marie McGrath Lauzon Bay Reconstruction of CIA and K 2 O in Metasomatized Samples Pronto 67.5 K 2 O:Al 2 O 3 22:78 from K from C*N Compositional variation with depth note K MEAS & K CALC % change relative to Al 2 O 3 in protolith Depth, cm Baraboo Pronto Denison Ville Marie McGrath Lauzon Bay McGrath M j = p c j,p j,s ( z ) d z Mass Balance Equation where: M = mass of oxide removed or added = density of protolith c = concentration of oxide in protolith = translocation of oxide in saaprolite z = depth where: M = mass of oxide removed or added = density of protolith c = concentration of oxide in protolith = translocation of oxide in saaprolite z = depth Depths of Weathering and Mass Fluxes (Removal & Addition) Magnitude of Weathering = Total Mass Removed removal of: SiO 2 CaO Na 2 O K 2 O CALC MW and depth are a function of: rate of weathering rate of erosion duration of weathering MW and depth are a function of: rate of weathering rate of erosion duration of weathering Baraboo Ville Marie Pronto Baraboo Ville Marie Pronto R 2 = 0.97 R 2 = 0.53 Magnitude of Weathering vs. Intensity of Weathering Plagioclase Index of Weathering PIW = 100 CN CALC / CN TOTAL where CN CALC is calculated mass flux of CaO and Na 2 O CN TOTAL is total removal of CaO and Na 2 O Feldspar Index of Weathering FIW = 100 CNK CALC / CNK TOTAL where CNK CALC is calculated mass flux of CaO, Na 2 O, and K 2 O CNK TOTAL is total removal of CaO, Na 2 O, and K 2 O Feldspar Index of Weathering FIW = 100 CNK CALC / CNK TOTAL where CNK CALC is calculated mass flux of CaO, Na 2 O, and K 2 O CNK TOTAL is total removal of CaO, Na 2 O, and K 2 O Bboo calculated Bboo inferred VM M/t p CO 2 [(K CO 2 r)/ ( D CO 2 )/L] Calculation of Atmospheric CO 2 (Sheldon 2006) M/t (mol CO 2 /cm 2 year): time-averaged flux of CO 2 required for observed weathering for observed weathering XO + 2CO 2 + H 2 O = X HCO 3 - pCO 2 : partial pressure of atmospheric CO 2 K CO 2 : Henrys Law constant for CO 2 r : rainfall rate : a constant involving time and gas volume D CO 2 : diffusion constant for CO 2 in air : diffusion constant CO 2 (soil)/diffusion constant CO 2 (air) L : depth to the water table K CO 2 : Henrys Law constant for CO 2 r : rainfall rate : a constant involving time and gas volume D CO 2 : diffusion constant for CO 2 in air : diffusion constant CO 2 (soil)/diffusion constant CO 2 (air) L : depth to the water table Calculated pCO 2 Relative to Pre-Industrial Atmospheric Level (280 ppm) for a weathering duration of 100,000 years Calculated pCO 2 Relative to Pre-Industrial Atmospheric Level (280 ppm) the effect of weathering duration Conclusions K-metasomatism is a common phenomenon in Precambrian paleosols Despite such metasomatism, application of the A-C*N-K plot allows for calculation of minimum values of mass flux in paleoweathering profiles and minimum estimates of atmospheric pCO 2 The Feldspar Index of Weathering (FIW) is an effective parameter for evaluating the proportion of total feldspar removed from an entire weathering profile, i.e. the intensity of weathering, and is useful in comparing the intensity of weathering among paleosols Among the investigated localities, the highest FIW value is found in the Baraboo paleosol, from which all feldspar was removed by weathering, accounting for the absence of feldspar in the overlying supermature Baraboo Quartzite