Geochimica et Cosmochimica Acta, Dec 15, 2012, Vol.99, p.159(20)
To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2012.09.019 Byline: J. Donald Rimstidt (a), Susan L. Brantley (b), Amanda A. Olsen (c) Abstract: This paper demonstrates a method for systematic analysis of published mineral dissolution rate data using forsterite dissolution as an example. The steps of the method are: (1) identify the data sources, (2) select the data, (3) tabulate the data, (4) analyze the data to produce a model, and (5) report the results. This method allows for a combination of critical selection of data, based on expert knowledge of theoretical expectations and experimental pitfalls, and meta-analysis of the data using statistical methods. Application of this method to all currently available forsterite dissolution rates (0〈pH〈14, and 0〈 T 〈150[degrees]C) normalized to geometric surface area produced the following rate equations: For pH〈5.6 and 0[degrees]〈 T 〈150[degrees]C, based on 519 data logr.sub.geo=6.05(0.22)-0.46(0.02)pH-3683.0(63.6)1/T(R.sup.2=0.88) For pH5.6 and 0[degrees]〈 T 〈150[degrees]C, based on 125 data logr.sub.geo=4.07(0.38)-0.256(0.023)pH-3465(139)1/T(R.sup.2=0.92) The R.sup.2 values show that [approximately equal to]10% of the variance in r is not explained by variation in 1/T and pH. Although the experimental error for rate measurements should be[+ or -][approximately equal to]30%, the observed error associated with the log r values is [approximately equal to]0.5logunits ([+ or -]300% relative error). The unexplained variance and the large error associated with the reported rates likely arises from the assumption that the rates are directly proportional to the mineral surface area (geometric or BET) when the rate is actually controlled by the concentration and relative reactivity of surface sites, which may be a function of duration of reaction. Related to these surface area terms are other likely sources of error that include composition and preparation of mineral starting material. Similar rate equations were produced from BET surface area normalized rates. Comparison of rate models based on geometric and BET normalized rates offers no support for choosing one normalization method over the other. However, practical considerations support the use of geometric surface area normalization. Comparison of Mg and Si release rates showed that they produced statistically indistinguishable dissolution rates because dissolution was stoichiometric in the experiments over the entire pH range even though the surface concentrations of Mg and Si are known to change with pH. Comparison of rates from experiments with added carbonate, either from CO.sub.2 partial pressures greater than atmospheric or added carbonate salts, showed that the existing data set is not sufficient to quantify any effect of dissolved carbonate species on forsterite dissolution rates. Author Affiliation: (a) Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, United States (b) Earth and Environmental Systems Institute, 2217 EES Building, Pennsylvania State University, Univ Pk, PA 16802, USA (c) School of Earth and Climate Sciences, 5790 Bryand Global Sciences, Center, University of Maine, Orono, ME 04469, United States Article History: Received 14 July 2011; Accepted 12 September 2012 Article Note: (miscellaneous) Associate editor: Chen Zhu
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