Silicate Sediments Dissolution during interaction with Seawater Ben Gurion University, Israel Geological and Environmental Studies Department Silicate Sediments Dissolution during interaction with Seawater Daniel Winkler Under supervision of Prof. Jiwchar Ganor (Ben Gurion University) and Yehudit Harlavan (Geological Survey of Israel)

Background 1. Silicate minerals contain Si. In addition they contain: Al, Mg, Ca, Fe, etc. They may also contain Sr in trace amount. 2. During contact with seawater, these minerals may dissolve, given that the solution is under-saturated with respect to these minerals. 3. With dissolution, the ions are released to the solution. 4. We may track the changes the solution is undergoing in order to compute the dissolution-rate.

Sr Strontium (atomic number 38) is an alkaline-earth metal which has four naturally occurring stable isotopes: 88Sr which is the most abundant (82.58%), 87Sr (7.0%), 86Sr (9.86%), and 84Sr (0.56%). Among these isotopes, only 87Sr is radiogenic (a product of first-order 87Rb beta-decay).

SrCO3 SrSO4 Strontianite Celestite Other Minerals Strontium appears in the following minerals: Strontianite SrCO3 in hydrothermal veins in evaporitic rocks Celestite SrSO4 Other Minerals Sr2+ 1.13Å  Ca2+ 0.99Å  Cationic exchange Accessory Cation +14% Common minerals Anortite CaAl2Si2O8 Apatite Ca5(PO4)3(OH,F,Cl) Calcite CaCO3 K-feldspar KAlSi3O8 (Sr2+ ↔K+)

87Sr/86Sr Rb-Sr decay: spontaneous process Decay parameter Father Daughter Sr n +p [beta particle = electron] spontaneous process is independent of environmental conditions. Decay constant is known to us: λ=1.42*10-11years-1 Isotopic composition 87Sr/86Sr

Sr budget of the Ocean: ? Interaction with sea-floor sediments Basic assumptions Sr is homogenized in the oceans (long residence time in comparison with mixing rate) There are differences in the isotopic composition between the various donors. Interaction with sea-floor sediments (Unknown) ?

87Sr/86Sr variations through time: 0.7092 Today 0.70685 Jurassic today ערך נוכחי 0.7082 triassic 0.7070 perm 0.7076 carbon 0.7077 devon Jurassic 0.7087 silur 0.7077 ordovican Via Marine carbonates Wickmann (1947) ↓ Burke (1982) 0.7091 Cambrium

87Sr/86Sr in the oceans is affected by the following: Faure 1986 Old crust rocks young volcanics carbonates fractions Typical Values

carbonates young volcanics Old crust volcanics carbonates today today Jurassic

One homogenous river-bed Dry sieving to fractions of Roded Quartz-Diorite Eilat Granite Yehoshafat Granite Amram Rhyolite Sampling One homogenous river-bed Dry sieving to fractions of 60-90µm Ultra-sonic shaking in Ethanol and drying in 60c for 24hr (removal of Clay particles) Leaching in acetic acid 0.5M for 1hr and in DDW washing x3 (removal of dust) Dissolution experiments

Batch experiments Closed system (Batch). Each tube represents one single point. All experiment tubes have the same conditions (pH, Temp, Solution/Sediment ratio) After some time the solution is separated from the sediment for chemical analyses. t1 t2 t3 t4 t5 t6 t7 t8 Solution 40gr Sediment 0.4gr

Filtering via 0.22μm mesh t0+t Temp=25C Analyses of Si,Al: Spectrophotometer Perkin Elmer Lambda 2S precision 5%± Analyses of Sr, Mg, Ca, Fe : ICP-MS [at G.S.I] Perkin Elmer SCIEX ELAN precision 10%± Sr separation 87Sr/86Sr Analysis Nu Plasma MC-ICP-MS precision 0.002% ±

Comparison of Seawater and borax solutions: Ionic strength Conc. Elec. charge Synthetic Seawater Solution: NaCl, KCl, MgCl2x6H2O, KBr, MgSO4x7H2O ,CaCl2x2H2O Borax Solution: Na2B4O7x10H2O+ HCl

Dissolution experiments Sample size [μm] Experiment Duration [days] Albite 67-75 Seawater[without washing] Borax solution [with washing] 172 152 K-feldspar 60-90 Seawater[with washing] Amram Rhyolite 0-63 Yehoshfat Granite 140 Eilat Granite 162 Roded Quartz-Diorite Minerals Silicate sediments (Aluvium) Washing = replacement of solution after one week of dissolution

Albite sample under the SEM: After ultra-sonic cleansing Before treatment

Seawater Albite [NaAlSi3O8 ] dissolution experiment in Seawater solution Mixing-line Isotopic-composition variations Al, Si release Mixing between 2 components What are the 2 components?

Albite sample under the SEM: Apatite Ca5(PO4)3(F,Cl,OH) Biotite K(Mg,Fe)3AlSi3O10(F,OH)2 Albite sample under the SEM:

Dissolution experiment: הידרוניום Dissolution experiment: אלביט Solutes → Mineral Debye–Hückel term Degree of saturation Albite is under-saturated

PHREEQC - A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations Under-saturation Over-saturation

K-feldspar dissolution in seawater

Dissolution Rate Eilat Granite : SW and Bx Both had the same washing x2 process

Eilat granite: Seawater Borax solution

Roded Quartz-Diorite in seawater: effect of washing

Various silicates dissolution in Seawater: (without washing) 1.Plagioclase dissolves faster than K-feldspar. 2.Plag/KSP in Roded quartz diorite=50 (Bogoch et al.2002) in Yehoshafat granite only 0.3 (Steinitz et al.2009). 3. Amram rhyolite size fraction :0-63μm (opposed to 60-90 μm in the others) Among the 3 silicates in sea-water experiments; the Roded quartz diorite has the highest Si concentration towards the end of the experiment, followed by Amram rhyolite and Yehoshafat granite (116, 82 and 46μM, respectively. See Figure 21a, page 60). This may be explained both by the mineralogy and by the size fraction of the samples. The ratio between plagioclase and K-feldspar in the samples is important since plagioclase dissolution rate is faster than that of K-feldspar (Langmuir, 1997). The ratio of plagioclase to K-feldspar is highest in Roded quartz-diorite (~50; Bogoch et al.2002) and indeed it has the highest Si concentration at steady-state (Figure 21a). The plagioclase to K-feldspar ratio in Yehoshafat granite is much lower (~0.3; Steinitz et al.2009) which may explain the lower Si release rate (compared to Roded quartz-diorite). As for Amram Rhyolite, this ratio is probably very low (<~0.5;Mushkin et al.2002), but not as in the other experiments, Amram rhyolite consists of the most fine particles (0-63μm, compared to the others, 60-90μm), and therefore probably dissolves faster than Yehoshafat granite.

Various silicate sediments in seawater: (without washing)

Albite in seawater and borax solution: 87Sr/86Sr Similar pattern in Roded-quartz-diorite experiment אפטיט/ביוטיט.

Conclusions: The albite sample dissolves incongruently (non-stoichometric) in seawater due to precipitation of secondary phases. The isotopic composition of Sr decreases in a hyperbolic manner. This means that there are two components in the sample with two different distinct isotopic composition of 87Sr/86Sr: albite with trace amounts of apatite/biotite. 2. The K-feldspar sample dissolves congruently in seawater (Si:Al ratio of 3:1). The sample does not contain Sr and therefore does not contribute to the 87Sr/86Sr ratio in the solution. 3. The dissolution rate in the Borax-HCl solution is faster than in the seawater solution, even though the pH is the same (pH~8.2). 4. In the beginning of the experiment (until day 20) the dissolution rate is fast and is decreasing with time. The reason is, the presence of fine reactive particles (large surface area in comparison with their volume). 5. In the albite experiements (and also in Roded-quartz-diorite) a shift in the 87Sr/86Sr isotopic composition was observed. This may result from the fact that both solutions (seawater and borax-HCl) does not have the same distance from equilibrium.

Thank you for listening..

Albite in borax solution appendix Albite in borax solution

K-feldspar in borax solution appendix K-feldspar in borax solution

Eilat Granite in borax solution appendix Eilat Granite in borax solution