The major ion, δ44/40Ca, δ44/42Ca, and δ26/24Mg geochemistry of granite weathering at pH = 1 and T = 25 °C: power-law processes and the relative reactivity of minerals

作者：Jong-Sik Ryu; Andrew D. Jacobson; Chris Holmden; Craig Lundstrom; Zhaofeng Zhang

来源：Geochimica et Cosmochimica Acta, 2011, 75(20): 6004-6026.

We dissolved Boulder Creek Granodiorite in a plug flow reactor for 5794 h at pH = 1 and T = 25 °C. The primary purpose of the experiment was to identify controls on dissolved δ44/40Ca, δ44/42Ca, and δ26/24Mg values during granite weathering. Herein, we also examine the origin of Ca and Mg isotopic variability among minerals composing the Boulder Creek Granodiorite, and we constrain fundamental characteristics of granite weathering important for quantifying the elemental and isotopic geochemistry of the reactor output. Nine Ca-bearing minerals display an 8.80‰ range of δ44/40Ca values and a 0.51‰ range of δ44/42Ca values. Three Mg-bearing minerals display a 1.53‰ range of δ26/24Mg values. These ranges expressed at the mineralogical scale are higher than the ranges thus far reported for bulk igneous rocks. Most of the δ44/40Ca variability reflects 40Ca enrichment in K-feldspar, and to a lesser extent, biotite, due to the radioactive decay of 40K over the 1.7 Ga age of the rock, whereas the entire range of δ44/42Ca values reflects mass-dependent isotope fractionation during igneous differentiation and crystallization. The range of δ26/24Mg values may represent either fractionation during the chloritization of biotite or interaction of the Boulder Creek Granodiorite with Mg-rich metamorphic fluids having low δ26/24Mg values.@#
@The elemental and isotopic composition of the reactor output varied substantially during the experiment. We synthesize the mineralogical and fluid data using coupled mass-conservation equations solved at non-steady-state. Model calculations reveal an intricate balance between increasing specific surface area and decreasing mineral concentrations. While surface area normalized dissolution rate constants were time-invariant, specific surface area increased as a power-law function of time through positive feedbacks between mechanical disaggregation, chemical dissolution, and mineral depletion. Variations in dissolved δ44/40Ca, δ44/42Ca, and δ26/24Mg values reflect conservative mixing rather than fractionation. Apatite and calcite initially control δ44/40Ca and δ44/42Ca values, followed by biotite, titanite, epidote, hornblende, and plagioclase. The release of radiogenic 40Ca clearly defines the period where biotite dissolution dominates. The brucite layer of chlorite initially controls δ26/24Mg values, followed by biotite, the TOT layer of chlorite, and hornblende. Through direct isotopic tracking, these results demonstrate that trace minerals, such as apatite and calcite in the case of Ca and brucite in the case of Mg, dominate elemental release during the incipient stages of granite weathering. The results further show that biotite dissolution dominates the middle stages of granite weathering and that plagioclase dissolution only becomes important during relatively late stages. The Ca and Mg isotope variations associated with these stages are distinct and potentially resolvable in soil mineral weathering studies.