Analyses of the sulfur isotopic composition (δ34S) of pyrite from methane-bearing sediments have typically focused on bulk samples. Here, we applied secondary ion mass spectroscopy to analyze the δ34S values of various pyrite generations to understand the diagenetic history of pyritization.
The overall goal of this procedure is to analyze the In Situ sulfur isotopic compositions of various pyrite generations by secondary ion mass spectroscopy in order to understand the diagenetic history of pyritization in methane bearing sediments. This method can help identify the biogeochemical processes that impact pyritazation, like organiclastic sulfur reduction and sulfur-driven anaerobic oxidation of methane which can now be resolved by a box of isotopic analyzers. The main advantage of this technique is high-resolution and precision, which allowed us to analyze the sulfur isotopic composition of a pyrite on a micro-scale.
Demonstrating the SIMS analyzed procedure will be Qing Yang, a technician from the SIMs laboratory at the Guangzhou Institute of Geochemistry. To begin, clean the surface of a sediment core and collect a set of samples across the entire length using a knife. Pack the wet samples individually in zippered plastic bags, and label them using a marker.
When ready to continue, transfer the wet sediment samples into pre-cleaned beakers and place them in an oven at 40 degrees Celsius for 24 hours to dry them out. After drying, separate the sediments into two aliquots, one for the collection of pyrite aggregates and the other for bulk sulfur extraction. Add 400 milliliters of distilled water to soften one aliquot of the sediment for two hours.
Transfer the slurry into a 063 millimeter civ and sift the sediment with distilled water so that all fine grains are washed through the civ. Collect the coarse fraction in beakers and dry them in an oven at 40 degrees Celsius for 24 hours. Once dry, place some of the coarse segment fractions on a glass slide and set the slide under a binocular microscope.
Turn to the 20x objective and identify the pyrite aggregates. Handpick the identified pyrite aggregates using a needle and pack them individually into zippered plastic bags. Pulverize a second aliquot of dry sediment sample into a fine powder using an agate mortar for further bulk sulfur extraction.
Store the powder in zippered plastic bags. Select some representative pyrite tubes from the selected aggregates under a binocular microscope to examine the morphological and textural features of the pyrite aggregates. Stick double-sided tape on a slide, and place the selected pyrite tubes on the tape.
Then, place a 25 millimeter diameter mounting tube on the slide to cover all of the pyrite aggregates. Mix 10 milliliters of epoxy resin with 1.3 milliliters of hardener at room temperature and pour the mixing liquid into the mounting tube. Then, place the slide and the mounting tube into a vacuum chamber.
Pump the air out of the chamber until the pressure in the chamber is below 0.2 bar so that all the pore spaces of the samples are filled with epoxy. Next, move the slide and the mounting tube out of the chamber and let the epoxy cure at room temperature for 12 hours. Once the epoxy has cured, hand grind the pyrite tubes on a fixed nine micrometer diamond mesh pad until the pyrite grains are exposed.
Then, hand polish the pyrite grains to produce a smooth and flat surface. Observe the morphology and the texture of the pyrite under a reflected light microscope at 200x magnification using a three millimeter working distance. Select representative pyrite aggregates with characterized crystal habits from different sediment samples after the petrographic study.
And then stick them to double-sided tape. Then, position the samples within five millimeters of the center of a 25 millimeter mount, and add epoxy. After the epoxy has cured, hand grind the disc on a fixed nine micrometer diamond mesh pad to the desired level so that the pyrite grains are exposed.
As before, hand polish the epoxy discs to produce a smooth, flat surface successively using five, three, and one micrometer diamond pads. Next, clean the surface of the epoxy disc with deionized water, followed by ethanol. Observe the sample under a reflective light microscope at 200x magnification, and a three millimeter working distance.
For greater magnification, gold coat the sample and image using an electron microscope. For SIMS analysis, use a cesium primary ion beam to measure the sulfur isotope ratios of pyrite. Focus the cesium primary ion beam onto a 15 micrometer by 10 micrometer spot at an energy of 10 kilobolts, with a 2.5 nanoamp current.
The identification of pyrite aggregates should be large enough to avoid missing a optimum pyrogenetic faces. Besides, it's better to analyze a sufficient number of spots to ensure the obtained isotopes in nature are representative. Use three off-axis faraday cups for the simultaneous measurement of sulfur-32, sulfur-33, and sulfur-34 in multi-collector mode with an entrance slit width of 60 micrometers and an exit slip width of 500 micrometers on each the three faraday cup detectors.
Carry out the sulfur isotope analyses and automated sequences with each analysis consisting of 30 seconds of pre-sputtering, 60 seconds of secondary ion automated centering, and 160 seconds of data acquisition and sulfur isotope signal integration. Analyze Sonora pyrite as a standard at regular intervals between every five to six samples. Most pyrite aggregates handpicked from the sediment are black in color and tubular in shape varying from three to eight millimeters in length, and 0.2 to 0.6 millimeters in diameter.
These longitudinal cross-sections of the pyrite tubes show typical hollow interiors and different wall thicknesses. When analyzed by secondary ion mass spectrometry, the pyrite found in shallow zones is mainly framboidal and depleted in sulfur-34. Meanwhile, overgrowths and euhedral crystals enriched in sulfure-34 are abundant in deeper zones.
The amount of chromium reducible sulfur content ranges from zero weight percent to 0.98 weight percent in the sample shown here. Below 50 centimeters below sea-floor, the sample shows minor fluctuations around the main value of 0.44 weight percent. And two distinct peaks of 0.98 weight percent at 490 centimeters below sea-floor and 0.78 weight percent that 590 centimeters below sea-floor are present.
The sulfur isotopic composition of three types of pyrite in the delta sulfur-34 values of chromium reducible sulfur content and handpicked pyrite aggregates are shown here. The shaded portion of the graph refers to the area affected by the sulfate-driven anaerobic oxidation of methane. The dashed line separates a zone to the left, suggested to be dominated by organiclastic sulfate reduction and the zone to the right suggested to be dominated by the sulfate driven anaerobic oxidation of methane.
After watching this video, you may have a good understanding of how to prepare orthogenic pyrites from a thin bearing sediments for institute sulfur isotope analysis using SIMS. Following this procedure, only a few pyrite material samples are needed for sulfur isotope analysis, and you can obtain the results in a few minutes, which is much more effective than the traditional bulk sulfur isotope analysis. This approach can serve as a sensitive tool for reconstructing the pyrite elation sequences that develop during diagenesis in modern marine sediments.
What's more, it should also target ancient sedimentary sequences aiming to result of different biogeochemical processes on mineral formation when is the data is lacking.
