Person: Chiara Borrelli

X-ray diffraction (XRD)

Powdered samples were examined for mineral presence using a Rigaku Ultima III X-ray Diffraction (XRD) System at the Institute of Imaging and Analytical Technologies (i2AT) at Mississippi State University. These analyses were conducted at 40 kV and 40 mA.

Elemental analysis of experimental aragonite crystals and experimental fluids with ICP-MS

Crystals were dissolved and solutions were analyzed by ICP-MS. Experimental fluids were also analyzed with ICP-MS, as well. Analyses were conducted on an Agilent 7900 quadrupole ICP-MS equipped with a glass nebulizer at the University of Rochester (Rochester, NY). Prior to analysis, fluid samples were diluted with 2% trace metal grade HNO3 (Fisher) in the volume proportion of 1:100. Aragonite crystals were dissolved in HNO3 in the mass proportion of 1:1000. Experimental fluid samples were analyzed using a 5-point calibration curve (one blank and four standards). Seawater standards were prepared by diluting a seawater certificate reference material (CRM-SW; High-Purity Standards) to a final concentration of ~300, 200, 150, and 80 ppm. Experimental crystals were analyzed using a 5-point calibration curve (one blank and four standards). Calcium carbonate standards were prepared by dissolving and diluting the original MACS-3 powder to a final concentration of ~111, 55, 27, and 11 ppm. Because S concentration was not reported in the certificate of analysis provided by the USGS, we added a small amount of sulfur single element standard (Inorganic Ventures) to a final concentration of ~1.5, 0.75, 0.375, and 0.150 ppm, in each MACS-3 standard we prepared. Samples, blanks, and standards were also spiked with 2 ppb indium, which was used as the internal standard to check for possible instrumental drift during each analytical session.

The instrument was tuned at the beginning of each analytical session (two analytical sessions in two consecutive days). 24Mg, 34S, 43Ca, 88Sr, 138Ba, and 238U were analyzed.  Integration times were the following: 0.3 s for 24Mg; 0.12 s for 43Ca, 88Sr, and 138Ba; 0.99 s for 238U. The calibration curve was run before analyzing each set of samples. For each element analyzed, the correlation among standards was 0.9923 or better. Elements were identified using three peaks. Each analysis included three replicates, with 100 sweeps/replicate. The carrier gas (argon) flow was set at 1.15 L/min. In addition, we used helium flow of 4.2 milliliters per minute in the collision reaction cell in the ICP-MS to minimize oxide interference. At the end of each tuning, oxides and doubly-charged ion interferences were below 0.8% and 1.4%, respectively. Data were computed automatically during the run using the Agilent Mass Hunter 4.1 workstation software v. C.01.01.

Results

XRD spectra identified the presence of aragonite and monohydrocalcite (Table 2). Only aragonite crystallized at intermediate pressure (110 bars). Calculations of fluid carbonate chemistry were conducted with a CO2SYS spreadsheet (Lewis and Wallace, 1998)  and presented in Table 1. There, measured pH and total alkalinity (TA) were used to calculate the concentration of CO32-, which together with Ca2+ is necessary to calculate fluid saturation states with respect to aragonite (Omega-Ar). Precipitation started at saturation state, which exceeds that of artificial seawater (ASW) by the factor of ~25. Over the course of the experiments, Omega-Ar decreased back to 2, i.e. Omega-Ar value of ASW prior to the addition of Na2CO3. Table 2 contains E/Ca of experimental products (solids and liquids) as well as the Doener-Hoskins apparent partition coefficients between solid and fluid (KE). Fluid composition changed during individual experiments, and therefore, values of KE were calculated using the Doener-Hoskins relationship:

       KE= log(1+ mE^aragonite /mE^fluid) / log(1+ mCa^aragonite /mCa^fluid)    (1)

where m^aragonite is the total number of moles of element (i.e., Mg, S, Sr, Ba, or U) or Ca in the final precipitate, and m^fluid is the total number of moles of element or Ca in the final fluid (Doerner and Hoskins, 1925).

Software products used:
XRD: Jade and Microsoft Excel
ICP-MS: Agilent Mass Hunter 4.1 workstation software v. C.01.01 and Microsoft Excel

Crystallization experiment
In order to simulate the high-pressure conditions at the seafloor, three laboratory experiments involving aragonite precipitation were conducted.  Two experiments were conducted using a high-pressure apparatus (Figure 1). The control experiment was performed at atmospheric pressure and 8°C in a container immersed in a constant temperature bath. The experimental fluid (growth medium) was prepared by mixing artificial seawater (ASW) and small aliquots of Na2CO3. The addition of Na2CO3 allowed the saturation state of the fluid with respect to aragonite to be increased, in order to promote nucleation and crystallization on carbonate minerals. Artificial seawater was prepared by dissolving 32 g of Instant Ocean salt (Instant Ocean Spectrum Brands) mix in 1 kg of distilled water and filtering it through a 0.2 µm Nalgene filter. A small amount of 0.1M Na2CO3 was then introduced into the continuously stirred artificial seawater using a syringe pump set at a rate of ~1 ml/min, to reach a final volume ratio of Na2CO3 to artificial seawater of ~1:13. During the addition of Na2CO3, the solution was in contact with the atmosphere. This procedure homogenized the fluid and also precluded aragonite nucleation prior to the beginning of the experiment. The absence of crystal nucleation was confirmed visually by monitoring the solution pH, which remained stable for the 1-2 hours needed for the experiment setup. The observation that aragonite crystallization did not start prior to the beginning of the experiment is consistent with previously published data, which show that 6 hours elapsed before onset of crystallization in the solution similar to the one we used (7.4 mmol of Na2CO3 :1 liter of seawater) (Pytkowicz, 1965).

Petrographic glass slides frosted with silicon carbide (SiC) abrasive paper were used to promote aragonite crystallization along scratches. For each experiment, one slide was placed into the high-pressure vessel prior to the transfer of the experimental fluid. The experimental fluid was then placed into the vessel after it was precooled to 8°C. After that, the air space above the solution (<200 cm3) was purged with nitrogen three times before the pressure was increased to a desired value (110 or 345 bars). The experimental fluid was sampled daily using a valve on the pressure vessel (Figure 1) and pH was measured to estimate time of nucleation and crystallization of CaCO3. For pH measurements, electrode calibration and measurements were conducted at temperatures similar to those of the experiment. NBS buffer solutions with pH of 4, 7, and 10 were used for electrode calibration. The pH initially decreased due to carbonate precipitation and reached near-constant values less than 170 hrs from the beginning of the experiments, suggesting that most of the CaCO3 precipitated in the first few days of each experiment. At the end of each run, the experimental products were extracted from the pressure vessel. Crystals (<10 µm in size) were rinsed with distilled water and dried, whereas the experimental fluid was filtered and refrigerated for subsequent elemental analyses. The alkalinity of the final fluid was measured the day after the end of each experiment. Experimental parameters are presented in Table -1.