To assess the degree of Si isotope fractionation that is associated with the bulk-rock serpentinisation of mafic and ultramafic rocks, a suite of variably serpentinised abyssal peridotites have been analysed for their whole-rock Si isotope compositions. The samples were taken from cores drilled on the Ocean Drilling Program Leg 209 cruise40 , whose main aim was to drill mantle peridotites and associated mafic rocks from fracture zones along the Mid-Atlantic Ridge41 The samples were taken from two cores, Holes 1274 A and 1268 A. Briefly, Hole 1274 A is composed of harburgites, dunites, and gabbroic rocks, all of which had undergone 60–100% serpentinisation at low temperatures (100–200 °C). Major element analyses suggests that the serpentinisation was isochemical, i.e., the rocks from Hole 1274 A were hydrated, but no other metasomatism took place. Hole 1268 A also returned a mixture of harzburgites, dunites and gabbros; in contrast to Hole 1274 A, the rocks from 1268 A were 100% serpentinised before undergoing a late-stage, high temperature (>200 °C) metasomatic event (associated with a gabbroic intrusion) which resulted in the variable desilicification of some rocks to form talc (steatisation).
To specifically study the potential effect of serpentisation (both isochemical and metasomatic) on the Si isotope composition of ultramafic rocks, only variably altered, serpentinised harzburgites were chosen from both cores for analysis. Six samples were taken from Hole 1274 A and eleven samples were taken from Hole 1268 A. Samples were provided in powder form, and prepared for Si isotope analysis via solution MC-ICP-MS following the methods first described in42 (link) and further detailed in43 (link).
Sample powders were dissolved using alkali fusion, whereby ~10 mg of sample powder was weighed into silver crucibles along with ~200 mg of NaOH flux (semiconductor grade, Merck). The crucibles were placed into a muffle furnace heated to 720 °C for 15 min to perform the fusion. Subsequently, the crucibles and fusion cake were placed into individual PFA vials filled with 20 ml of MQ-e water and left overnight to equilibrate. The fusion cake was then transferred via pipette to precleaned PP bottles, diluted with enough MQ-e to reach a final Si concentration of between 10 and 25ppm, and acidified with enough conc. HNO3 to bring the pH of the solution to ~2. Final Si concentrations of the sample solutions were ascertained via the Heteropoly Blue method using a photospectrometer.
Samples were purified for Si isotopes using a single stage cation exchange procedure. Samples were loaded into BioRad Polyprep columns filled with 1.8 ml of AG50W X12 cation exchange resin (200–400 mesh, BioRad; the resin was cleaned in the column before samples were loaded—see43 (link) for the cleaning method). Silicon is in either anionic or neutral forms in solution at low pH and so can be directly eluted using 5 ml of MQ-e water. Post-column, the samples were acidified to ~0.22 M HNO3. Total procedural blanks of the whole chemical preparation procedure were measured at less than 100 ng of Si, which is approximately 0.35% of the total measured Si signal and considered negligible.
Silicon isotopes were measured on a Neptune Plus MC-ICM-MS instrument (ThermoFischer Scientific, Bremen, Germany), running at medium resolution (M/∆M ~7500) to avoid significant molecular interferences on the 29Si and 30Si beams. Samples were introduced into the instrument using a 100 μl min-1 PFA ESI (Elemental Scientific, Omaha, USA) microflow nebulizer running into the SIS spray chamber. Silicon isotopes were measured in the L3 (28Si), C (29Si), and H3 (30Si) Faraday cups, and depending on instrumental conditions, a 2ppm Si beams typically gave a total analyte signal of ~7 V. Isotope ratios were measured in static mode with each measurement consisting of 25 cycles with a ~8 sec integration time.
Silicon isotope measurements were calculated using the standard-sample bracketing protocol relative to the NBS28 standard. Variations in Si isotopes are defined using the delta notation (δ30Si NBS28) as described before. Each measurement session consisted of 10 samples, two of which were always the external standards BHVO-2 and Diatomite. Long-term δ30Si error on the NBS28 bracketing standard over the analytical sessions is 0.00 ± 0.10‰ (2 s.d.).
The δ18O values used in this study are those from different serpentinite samples but of the same ODP holes measured by ref. 43 (link).
To specifically study the potential effect of serpentisation (both isochemical and metasomatic) on the Si isotope composition of ultramafic rocks, only variably altered, serpentinised harzburgites were chosen from both cores for analysis. Six samples were taken from Hole 1274 A and eleven samples were taken from Hole 1268 A. Samples were provided in powder form, and prepared for Si isotope analysis via solution MC-ICP-MS following the methods first described in42 (link) and further detailed in43 (link).
Sample powders were dissolved using alkali fusion, whereby ~10 mg of sample powder was weighed into silver crucibles along with ~200 mg of NaOH flux (semiconductor grade, Merck). The crucibles were placed into a muffle furnace heated to 720 °C for 15 min to perform the fusion. Subsequently, the crucibles and fusion cake were placed into individual PFA vials filled with 20 ml of MQ-e water and left overnight to equilibrate. The fusion cake was then transferred via pipette to precleaned PP bottles, diluted with enough MQ-e to reach a final Si concentration of between 10 and 25ppm, and acidified with enough conc. HNO3 to bring the pH of the solution to ~2. Final Si concentrations of the sample solutions were ascertained via the Heteropoly Blue method using a photospectrometer.
Samples were purified for Si isotopes using a single stage cation exchange procedure. Samples were loaded into BioRad Polyprep columns filled with 1.8 ml of AG50W X12 cation exchange resin (200–400 mesh, BioRad; the resin was cleaned in the column before samples were loaded—see43 (link) for the cleaning method). Silicon is in either anionic or neutral forms in solution at low pH and so can be directly eluted using 5 ml of MQ-e water. Post-column, the samples were acidified to ~0.22 M HNO3. Total procedural blanks of the whole chemical preparation procedure were measured at less than 100 ng of Si, which is approximately 0.35% of the total measured Si signal and considered negligible.
Silicon isotopes were measured on a Neptune Plus MC-ICM-MS instrument (ThermoFischer Scientific, Bremen, Germany), running at medium resolution (M/∆M ~7500) to avoid significant molecular interferences on the 29Si and 30Si beams. Samples were introduced into the instrument using a 100 μl min-1 PFA ESI (Elemental Scientific, Omaha, USA) microflow nebulizer running into the SIS spray chamber. Silicon isotopes were measured in the L3 (28Si), C (29Si), and H3 (30Si) Faraday cups, and depending on instrumental conditions, a 2ppm Si beams typically gave a total analyte signal of ~7 V. Isotope ratios were measured in static mode with each measurement consisting of 25 cycles with a ~8 sec integration time.
Silicon isotope measurements were calculated using the standard-sample bracketing protocol relative to the NBS28 standard. Variations in Si isotopes are defined using the delta notation (δ30Si NBS28) as described before. Each measurement session consisted of 10 samples, two of which were always the external standards BHVO-2 and Diatomite. Long-term δ30Si error on the NBS28 bracketing standard over the analytical sessions is 0.00 ± 0.10‰ (2 s.d.).
The δ18O values used in this study are those from different serpentinite samples but of the same ODP holes measured by ref. 43 (link).
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