Accuracy of stable Mg and Ca isotope data obtained by MC-ICP-MS using the standard addition method

The standard addition method is evaluated to verify the accuracy and precision of Mg and Ca isotope data with complex matrices, using the standard-sample bracketing technique and analysis by MC-ICP-MS. The Ca-44/Ca-42 ratio of seawater (expressed as delta Ca-44/42 relative to SRM915a) was determined as 0 93 +/- 0.093 parts per thousand (95% confidence), in agreement with estimates obtained by the double spike method. Using standard addition, the seawater Mg-26/Mg-24 ratio (expressed as delta Mg-26 relative to the DSM3 standard) was determined as -0.80 +/- 0.06 parts per thousand (95% confidence) in agreement with previous estimates. Four terrestrial silicate rocks (MORB, flood basalt. glacial flour, and granodiorite) and olivine mineral separates from an island basalt are shown to exhibit no scatter within the error of the method, averaging a delta Mg-26 of -0.20 +/- 0.05 parts per thousand (95% confidence). Although a number of silicate rock data for Mg isotope ratios have already been reported this is, the first detailed effort to validate the accuracy of such data and test for residual analytical artifact after chemical purification of samples. Data regressions were evaluated statistically using the mean square weighted deviate (MSWD), demonstrating that the uncertainty on individual data points are generally over estimated. The external two standard deviation uncertainty on individual data points is estimated by Monte Carlo simulation as &lt;0.075 parts per thousand (about a factor of two improvement on early publications of Mg isotope data). The consistency of the standard addition estimates of delta Mg-26 in silicate rocks imply that if any residual matrix effects are present, then they must be less than the spread of the data (0.11 parts per thousand) given the diverse range of matrices in each of the samples. The delta Mg-26 values of the silicate rocks suggest that Mg isotope ratios in silicate material may only have a very restricted range. The delta Mg-26 values of silicate material in the present study falls between the average values reported by Teng et al. [Teng, F.Z., Wadhwa, M., Helz, R.f., 2007. Investigation of magnesium isotope fractionation during basalt differentiation: implications for a chondritic composition of the terrestrial mantle. Earth and Planetary Science Letters 261, 84-92. doi:10.1016/j.epsl.2007.06.004] and Wiechert and Halliday [Wiechert, U., Halliday, A.N., 2006. Non-chondritic magnesium and the origins of the inner terrestrial planers. Earth and Planetary Science Letters 256, 360-371. doi:10.1016/j.epsl.2007.01.007] and given the spread of published delta Mg-26 values for chondritic material, a chondritic composition for terrestrial Mg cannot be ruled out. We suggest that some of the small discrepancies between our data and analysis of the same samples in earlier studies, may have arisen because the chemical purification of Mg prior to analysis can easily induce analytical artifact. This method could be expanded to the isotope ratios of other elements, which also rely on correcting for mass bias using the standard-sample bracketing method, where similar analytical discrepancies may also exist. (C) 2008 Elsevier B.V. All rights reserved.</p

Accuracy of stable Mg and Ca isotope data obtained by MC-ICP-MS using the standard addition method

The standard addition method is evaluated to verify the accuracy and precision of Mg and Ca isotope data\ud with complex matrices, using the standard-sample bracketing technique and analysis by MC-ICP-MS. The\ud 44Ca/42Ca ratio of seawater (expressed as δ44\ud 42Ca relative to SRM915a) was determined as 0.93±0.03‰\ud (95% confidence), in agreement with estimates obtained by the double spike method. Using standard\ud addition, the seawater 26Mg/24Mg ratio (expressed as δ26Mg relative to the DSM3 standard) was determined\ud as −0.80±0.06‰ (95% confidence) in agreement with previous estimates. Four terrestrial silicate rocks\ud (MORB, flood basalt, glacial flour, and granodiorite) and olivine mineral separates from an island basalt are\ud shown to exhibit no scatter within the error of the method, averaging a δ26Mg of −0.20±0.05‰ (95%\ud confidence). Although a number of silicate rock data for Mg isotope ratios have already been reported, this is\ud the first detailed effort to validate the accuracy of such data and test for residual analytical artifact after\ud chemical purification of samples. Data regressions were evaluated statistically using the mean square\ud weighted deviate (MSWD), demonstrating that the uncertainty on individual data points are generally over\ud estimated. The external two standard deviation uncertainty on individual data points is estimated by Monte\ud Carlo simulation as b0.075‰ (about a factor of two improvement on early publications of Mg isotope data).\ud The consistency of the standard addition estimates of δ26Mg in silicate rocks imply that if any residual matrix\ud effects are present, then they must be less than the spread of the data (0.11‰) given the diverse range of\ud matrices in each of the samples. The δ26Mg values of the silicate rocks suggest that Mg isotope ratios in\ud silicate material may only have a very restricted range. The δ26Mg values of silicate material in the present\ud study falls between the average values reported by Teng et al. [Teng, F.Z., Wadhwa, M., Helz, R.T., 2007.\ud Investigation of magnesium isotope fractionation during basalt differentiation: implications for a chondritic\ud composition of the terrestrial mantle. Earth and Planetary Science Letters 261, 84–92. doi:10.1016/j.\ud epsl.2007.06.004] and Wiechert and Halliday [Wiechert, U., Halliday, A.N., 2006. Non-chondritic magnesium\ud and the origins of the inner terrestrial planets. Earth and Planetary Science Letters 256, 360–371. doi:10.1016/\ud j.epsl.2007.01.007] and given the spread of published δ26Mg values for chondritic material, a chondritic\ud composition for terrestrial Mg cannot be ruled out.We suggest that some of the small discrepancies between\ud our data and analysis of the same samples in earlier studies, may have arisen because the chemical\ud purification of Mg prior to analysis can easily induce analytical artifact. This method could be expanded to the\ud isotope ratios of other elements, which also rely on correcting for mass bias using the standard-sample\ud bracketing method, where similar analytical discrepancies may also exist

Mg isotope constraints on soil pore-fluid chemistry: Evidence from Santa Cruz, California

Mg isotope ratios (Mg-26/Mg-24) are reported in soil pore-fluids, rain and seawater, grass and smectite from a 90 kyr old soil, developed on an uplifted marine terrace from Santa Cruz, California. Rain water has an invariant Mg-26/Mg-24 m ratio (expressed as delta Mg-26) at -0.79 +/- 0.05 parts per thousand, identical to seawater delta Mg-26. Detrital smectite (from the base of the soil profile, and therefore unweathered) has a delta Mg-26 value of 0.11 parts per thousand, potentially enriched in Mg-26 by up to 0.3 parts per thousand to the bulk silicate Earth Mg isotope composition (although within the range of all terrestrial silicates). The soil pore-waters show a continuous profile with depth for delta Mg-26, ranging from -0.99 parts per thousand near the surface to -0.43 parts per thousand at the base of the profile. Shallow pore-waters (&lt;1 m) have delta Mg-26 values that are similar to, or slightly lower than the rain waters. This implies that the degree of biological cycling of Mg in the pore-waters is relatively small and is quantified as &lt;32%, calculated using the average Mg isotope enrichment factor between grass and rain (delta Mg-26(grass) - delta Mg-26(rain)) of 0.21 parts per thousand. The deep pore-waters (1-15 m deep) have delta Mg-26 values that are intermediate between the smectite and rain, ranging from -0.76 parts per thousand to -0.43 parts per thousand, and show a similar trend with depth compared to Sr isotope ratios. The similarity between Sr and Mg isotope ratios confirms that the Mg in the pore-waters can be explained by a mixture between rain and smectite derived Mg, despite the fact that Mg and Sr concentrations may be buffered by the exchangeable reservoir. However, whilst Sr isotope ratios in the pore-waters span almost the complete range between mineral and rain inputs, Mg isotopes compositions are much closer to the rain inputs. If Mg and Sr isotope ratios are controlled uniquely by a mixture, the data can be used to estimate the mineral weathering inputs to the pore-waters, by correcting for the rain inputs. This isotopic correction is compared to the commonly used chloride correction for precipitation inputs. A consistent interpretation is only possible if Mg isotope ratios are fractionated either by the precipitation of a secondary Mg bearing phase, not detected by conventional methods, or selective leaching of Mg-24 from smectite. There is therefore dual control on the Mg isotopic composition of the pore-waters, mixing of two inputs with distinct isotopic compositions, modified by fractionation. The data provide (1) further evidence for Mg isotope fractionation at the surface of the Earth and (2) the first field evidence of Mg isotope fractionation during uptake by natural plants. The coherent behaviour of Mg isotope ratios in soil environments is encouraging for the development of Mg isotope ratios as a quantitative tracer of both weathering inputs of Mg to waters, and the physicochemical processes that cycle Mg, a major cation linked to the carbon cycle, during continental weathering. (C) 2010 Elsevier Ltd. All rights reserved.</p