Silicates dissolution during interaction with sea-water and its potential contribution to the isotopic composition of Sr in the oceans Daniel Winkler (1), Yehudit Harlavan (2), Jiwchar Ganor (1) 1. Department of Geological and Environmental Science, Ben Gurion University of the Negev, Beer Sheva 84105, Israel 2. Geological Survey of Israel, 30 Malkhe Israel, Jerusalem 95501, Israel Introduction The isotopic composition of Sr ( 87 Sr/ 86 Sr) in the oceans is known to fluctuate since the beginning of the phanerozoic eon, reflecting the mixture between riverine input of weathered rocks and hydrothermal exchange in mid-ocean ridges. Sr [mg/L] 87 Sr/ 86 Sr [Bulk sample] Size fraction [µm] Sample Albite 0.2N/M K-Feldspar Amram Rhyolite N/M Yehoshafat Granite Roded Quartz-Diorite N/M Eilat Granite 87 Sr/ 86 SrSr 2+ [μmol/L] Al 3+ [μmol/L] Si [μmol/L] K + [mg/L] Ca 2+ [mg/L] SO 4 2- [mg/L] Mg 2+ [mg/L] Na + [mg/L] Cl - [mg/L] Ionic Strength Density (gr/cm 3 ) pHSolution Sea Water Borax+HCl Solution BGU GSI Sediment’s properties Chemical composition of experiments solutions Eilat Granite dissolution: Comparison between Sea-Water and Borax solutions Observations and interpretations: 1.The dissolution rate in Borax solution is ~2 times faster than that in SW. 2.Dissolution rate decreases with time due to the extinction of the more reactive particles. 3.Long term experiment shows a catalytic/inhibitory effect where the dissolution rate of the Eilat Granite in the Borax solution is higher than that in SW. 4.While Si concentration increases with time, Al and pH remain constant. 5.The 87 Sr/ 86 Sr ratio decrease exponentially with time, suggesting exhausting of radiogenic mineral (biotite or others). K-Feldspar dissolution in Sea-Water Observations and interpretations: 1.Si and Al concentrations increase as a result of K- feldspar dissolution in Sea Water. 2.Si release rate is ~3 times faster than the Al release rate (i.e. stoichiometric dissolution). 3. The dissolution rate decreases by a factor of 3 in the later stage of the experiment. 4.No significant release of Sr, due to low Sr concentration in Bulk sample (~0.9ppm). SEM image of the Albite sample (a) with trace amounts of Apatite (b). Motivation Theoretically, there could be additional sources of Sr resulting from interaction of seawater with sediments at the ocean floor, hence altering its Sr isotopic composition. Though numerous laboratory based experiments used many different types of solution to study the dissolution of various rocks and minerals, only one has used sea water as the interaction solution. In the present study, the dissolution of alluvial sediment of silicate origin during interaction with synthetic seawater is studied by tracking the changes in water chemistry and in 87 Sr/ 86 Sr ratio. Methodology Alluvial Sediments from streams that drain single homogenous silicate-rock unit exposed in Eilat Region were sampled. The samples, along with Albite and K-Feldspar, were sieved to a desired size fraction and subsequently were ultrasonically rinsed and were used in single-point dissolution experiments (SPB), held in thermostatic shaking-baths at a constant temperature of 25°C. Two types of solutions were used: 1) Synthetic seawater was used in order to avoid high Sr background. 2) Borax-HCl solution with a similar pH value (8.2) but with a much lower ionic strength. In all the experiments the sediment/solution ratio is 0.4gr sediment in 40gr solution. Conclusions 1.Albite dissolves incongruently in the SW Experiment. The change with time in Sr isotopic composition is explained by dissolution of albite together with trace amounts of Apatite. 2.The K-feldspar dissolves congruently in Sea-Water. The K-feldspar does not contribute Sr, and thus does not have a significant effect on the 87 Sr/ 86 Sr ratio. 3.The dissolution rate in the Borax-HCl solution is faster than that in SW, even though the pH is similar. Observations and interpretations: 1.In two independent experiments of the Amram Rhyolite, an apparent decrease in Si concentration after 80days is observed. 2.The Rhyolite contributes relatively large amount of Sr compared to the other samples. 3.The Roded Quartz-Diorite, Yehoshafat granite and Amram Rhyolite experiments exhibit Large variations in the 87 Sr/ 86 Sr ratio. Samples Solutions Rate1 Rate2 Time Variation in Si, Al and pH withh time. During the initial stage [0-20days] the release rate of Si is ~8 times higher than during the subsequent stage [20-172days]. Isotopic composition of Sr vs. 1/Sr. Albite dissolution in Sea-Water Variation in 87 Sr/ 86 Sr ratio with time. The synthetic SW composition is more radiogenic than that of the bulk and the experiment solutions. b a Observations and interpretations: 1.The initial high Si release rate is probably due to dissolution of fine particles (higher reactivity). 2.A hyperbolic shape suggests a mixture of two components, having different 87 Sr/ 86 Sr ratios. 3.The two components are probably albite and trace amounts of apatite. The two components varies in their Sr concentrations, 87 Sr/ 86 Sr ratio and dissolution rate. 4.Similar experiment conducted using Borax solution indicates higher release rate and similar two-component mixing behavior. SW b b Amram Rhyolite Dissolution of other silicate rocks in Sea-Water The variation in 87 Sr/ 86 Sr of the oceans since the Phanerozoic eon (McArthur et al., 2001). Input from sources having high ratio (i.e radiogenic) tend to increase the ratio, and vice-versa. Simplified model of the sea water Sr budget of the oceans (Dickin 2005). Note that most of the present day sea water Sr comes via Riverine influx as a result of weathering. Roded Quartz-Diorite Eilat Granite Yehoshafat Granite