Preparation of Authigenic Pyrite from Methane-bearing Sediments for In Situ Sulfur Isotope Analysis Using SIMS

Summary

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.

Abstract

Different sulfur isotope compositions of authigenic pyrite typically result from the sulfate-driven anaerobic oxidation of methane (SO₄-AOM) and organiclastic sulfate reduction (OSR) in marine sediments. However, unravelling the complex pyritization sequence is a challenge because of the coexistence of different sequentially formed pyrite phases. This manuscript describes a sample preparation procedure that enables the use of secondary ion mass spectroscopy (SIMS) to obtain in situ δ³⁴S values of various pyrite generations. This allows researchers to constrain how SO₄-AOM affects pyritization in methane-bearing sediments. SIMS analysis revealed an extreme range in δ³⁴S values, spanning from -41.6 to +114.8‰, which is much wider than the range of δ³⁴S values obtained by the traditional bulk sulfur isotope analysis of the same samples. Pyrite in the shallow sediment mainly consists of ³⁴S-depleted framboids, suggesting early diagenetic formation by OSR. Deeper in the sediment, more pyrite occurs as overgrowths and euhedral crystals, which display much higher SIMS δ³⁴S values than the framboids. Such ³⁴S-enriched pyrite is related to enhanced SO₄-AOM at the sulfate-methane transition zone, postdating OSR. High-resolution in situ SIMS sulfur isotope analyses allow for the reconstruction of the pyritization processes, which cannot be resolved by bulk sulfur isotope analysis.

Introduction

Methane emissions from sediments are common along continental margins^1,2. However, most of the methane in areas of diffusive seepage is oxidized at the expense of sulfate within the sediments, a process known as SO₄-AOM (Equation 1)^3,4. The production of sulfide during this process commonly results in the precipitation of pyrite. Also, OSR also drives the formation of pyrite by releasing sulfide (Equation 2)⁵.

CH₄ + SO₄^2– → HS^– + HCO₃^– + H₂O (1)

2CH₂O + SO₄^2– → H₂S + 2HCO₃^– (2)

It has been found that authigenic sulfide in the sulfate-methane transition zone (SMTZ) reveals high δ³⁴S values, which was suggested to be caused by enhanced SO₄-AOM in areas of seepage^6,7,8. In contrast, pyrite induced by OSR commonly displays lower δ³⁴S values⁹. However, it is challenging to identify different pyrite generations induced by these processes (i.e., OSR and SO₄-AOM) if only a bulk sulfur isotope measurement is used, since the successively formed interfingering pyrite generations are characterized by different isotopic compositions. Therefore, microscale in situ sulfur isotope analysis is required to improve our understanding of the actual mineralizing processes^10,11,12. As a versatile technique for in situ isotope analysis, SIMS requires only a few nanograms of sample, which sparked its designation as a nondestructive technique. A primary ion beam sputters the target, causing the emission of secondary ions that are subsequently transported to a mass spectrometer for measuring¹³. In an early in situ sulfur isotope analysis application of SIMS, Pimminger et al. successfully analyzed the δ³⁴S values in galena by using a 10 - 30 µm-diameter beam¹⁴. This approach has been increasingly applied to the microanalysis of sulfur isotopic compositions in sulfides, with significant improvements in both measurement precision and resolution^{11,12,13,14,15,16,17,18,19,20}. Pyrite with various morphologic attributes and distinct sulfur stable isotope patterns has been reported from seep and non-seep environments^21,22,23,24. However, to the best of our knowledge, prior to our recent SIMS study⁶, only one study used the in situ sulfur isotope analysis of pyrite from seep environments and revealed large sulfur isotope variability in biogenic pyrite²⁵.

In this study, we applied SIMS to analyze the δ³⁴S values of different generations of authigenic pyrite from a seepage site in the South China Sea, which allowed for microscale discrimination of OSR- and SO₄-AOM-derived pyrite.

Protocol

1. Collection of Samples from a Sediment Core

Note: The core HS148 was obtained from a site near the gas hydrate drilling zone in the Shenhu area, South China Sea, during a cruise of the R/V Haiyang Sihao in 2006.

1. Cut the piston core (here, HS148) into sections at intervals of 0.7 m from the top to the bottom (onboard the vessel) and transfer the sections to a cold room (4 °C) for storage after retrieval.
2. Transfer the core sections to a cold room (4 °C) in the land-based lab for storage after the cruise. Take the sections out of the cold room and use a saw to cut them into halves lengthwise.
3. Clean the surface of the sediment and collect a set of sediment samples (15 cm in length; 1/4 of the sediment core) across the entire length using a knife. Pack the wet samples individually in zippered plastic bags and label them using a marker.
4. Place the wet sediment samples (~30 g) in pre-cleaned beakers and dry them at 40 °C in a drying oven for 24 h. After drying, separate the sediments into two aliquots: one for the collection of pyrite aggregates (i.e., authigenic pyrite), and the other for bulk sulfur extraction (see step 3).
5. Put one aliquot of dry sediment into beakers and add distilled water to soften the sediment for 2 h. Transfer the slurry (including the sediments and water in the beaker) to a 0.063-mm sieve washed with distilled water.
   1. Sift the sediment with distilled water so that all fine grains (<0.063 mm) are washed through. Collect the coarse fraction (e.g., quartz grains, fossil shells, and authigenic minerals) in beakers and dry them at 40 °C in a drying oven for 24 h.
6. Place some of the coarse segment fractions on a glass slide under a binocular microscope (20X magnification). Identify the pyrite aggregates from the coarse fraction. Handpick such pyrite aggregates using a needle and pack them individually into zippered plastic bags.
   NOTE: Most of the pyrite aggregates are black in color and tubular in shape.
7. Pulverize a second aliquot of dry sediment sample into a fine powder (<0.074 mm) using an agate mortar for further bulk sulfur extraction (see step 3).

2. Observation of Variable Morphologies

1. Select some representative pyrite tubes from the handpicked pyrite aggregates under a binocular microscope (20X magnification) for thick section preparation to examine the morphological and textural features of the pyrite aggregates.
2. Stick double-sided tape on a slide and place the selected pyrite tubes on the tape. Put a mounting tube (25 mm in diameter) on the slide to cover all pyrite aggregates. Mix 10 mL of epoxy resin with 1.3 mL of hardener at room temperature and pour the mixing liquid into the mounting tube.
   1. 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. Move the slide and the mounting tube out of the chamber and let the epoxy cure at room temperature for 12 h.
   2. After the epoxy has cured, hand-grind the pyrite tubes on a fixed, 9-µm diamond mesh pad until the pyrite grains are exposed. Hand-polish the pyrite grains to produce a smooth and flat surface, using 5-, 3-, and 1-µm diamonds successively.
3. Observe the morphology and the texture of the pyrite under a reflected light microscope at 200X magnification, with a ~3-mm working distance.
4. Perform petrographic observation under a reflected light microscope⁶ and then coat the thick sections with a 25-nm layer of carbon. Examine their morphological and textural features using a thermal field emission scanning electron microscope with secondary electron imaging and backscattered electron modes^6,19.
   NOTE: This step was performed at the School of Earth Science and Geological Engineering, Sun Yat-sen University.

3. Bulk Sulfur Isotope Analyses

Note: The total sulfur (as sulfide) was extracted as hydrogen sulfide via wet chemical sequential extraction^26,27 at the Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster.

1. Place 4 g of the dried sample powder or 10 mg of pyrite aggregates per flask into round-bottom flasks and add 10 mL of ethanol into each flask as a catalyst.
   1. Prepare zinc acetate (3%) acetic acid solution in a 500-mL glass flask to trap hydrogen sulfide. Connect the zinc acetate-containing flask to the sample-containing flask. Check the connections of the flasks and flush nitrogen into the flasks to remove the air.
2. Inject 20 mL of HCl solution (25%) into the round-bottom flasks using a syringe to liberate the acid-volatile (mono)sulfides (AVS) from the sample; allow the samples to react for 1 h at room temperature.
   NOTE: Here, the analysis revealed that no AVS was present in the studied samples.
3. Inject 30 mL of 1 M CrCl₂ solution into the round-bottom flasks when the above reaction is completed; allow the samples to react for 2 h at 85 °C.
   NOTE: Chromium reducible sulfur (CRS, pyrite) reduces to hydrogen sulfide (H₂S) after the reaction and precipitates as zinc sulfide in the zinc acetate trap.
4. Transfer all the solutions containing zinc sulfide precipitates to beakers and convert the zinc sulfide precipitates to silver sulfide (Ag₂S) by adding a 0.1 M AgNO₃ solution to the beakers. Place the beakers on the heating plate and heat them to 90 °C so that the finely disseminated Ag₂S better coagulates.
   1. Collect the Ag₂S precipitates by filtration (<0.45 µm) after the solution has cooled down to room temperature and dry the filtrate overnight at 40 °C.
5. Weigh 200 µg of Ag₂S precipitate and mix it with an equal amount of V₂O₅ in tin cups. Have the sulfur composition analyzed on SO₂ molecules via combustion using a mass spectrometer connected to an elemental analyzer (EA-IRMS)⁶.
   NOTE: The above step was performed at the Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster.

4. In Situ SIMS Analysis

1. Select representative pyrite aggregates with characterized crystal habits (e.g., framboids, overgrows, and euhedral crystals) from different sediment samples after petrographic study. Stick the selected pyrite aggregates and pieces of Sonora pyrite standard to double-sided tape. Mold them within 5 mm of the center of a 25-mm epoxy mount.
   NOTE: The epoxy disc production process is the same as in step 2.2.
   1. After the epoxy has cured, hand-grind the disc on a fixed 9-µm diamond mesh pad to the desired level so that pyrite grains are exposed. Hand-polish the epoxy discs to produce a smooth, flat surface, successively using 5-, 3-, and 1-µm diamonds, which is required for high-precision isotope ratio analysis by SIMS²⁸.
2. Clean the surface of the epoxy disc with deionized water and ethanol. Place the epoxy disc in a gold-coating machine and coat the dry surface sections with a 25-nm layer of gold.
   1. Observe the sample again under a scanning electron microscope at 1,000X magnification, with a 9-mm working distance, to select spots characterized with different crystal habits (e.g., framboids, overgrows, and euhedral crystals) for SIMS analysis.
      NOTE: High-spatial resolution SIMS sulfur isotope analysis was applied to reveal the sulfur isotope variability of the different pyrite types.
3. Perform SIMS analysis^15,16.
   NOTE: Performed at the SIMS lab of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.
   1. Use a Cs⁺ primary ion beam to measure the sulfur isotope ratios (³⁴S/³²S) of pyrite. Focus the Cs⁺ primary ion beam onto a 15 µm × 10 µm spot at an energy of 10 kV, with 2.5-nA current. Use three off-axis Faraday cups for the simultaneous measurement of ³²S, ³³S, and ³⁴S in multi-collector mode, with an entrance slit width of 60 µm and an exit slit width of 500 µm on each of the three Faraday cup detectors.
4. Carry out sulfur isotope analyses in automated sequences, with each analysis consisting of 30 s of pre-sputtering, 60 s of secondary ion automated centering, and 160 s of data acquisition and sulfur isotope signal integration (40 cycles × 4 s).
   1. Analyze Sonora pyrite as a standard at regular intervals, every 5 - 6 sample analyses.
      NOTE: See Chen et al.¹⁹ for more detailed analytical methods and instrument parameters.

Results

Data Expression - Bulk Sulfur Isotopes:

The bulk sulfur isotope ratio is expressed in relation to the Vienna Canyon Diablo Troilite (V-CDT) standard, and the analytical precision is better than ±0.3‰. The sulfur isotope measurements were calibrated with international reference materials: IAEA-S1 (δ³⁴S = -0.30‰), IAEA-S2 (δ³⁴S = -21.55‰), IAEA-S3 (δ³⁴S = -31.4‰), and NBS 127 (δ³⁴S = 20.30‰...

Discussion

The sulfur isotope analysis of pyrite is a useful approach and can help in identifying the biogeochemical processes that impact pyritization. However, if bulk sulfur isotope analysis is applied, the obtained sulfur isotope signatures commonly represent mixed signals, as sedimentary pyrite aggregates typically consist of multiple, closely interfingering generations. Here, we present a method (i.e., SIMS analysis) for analyzing the in situ sulfur isotopic compositions of various pyrite generations on the ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
secondary ion mass spectroscopy	Cameca	IMS-1280
thermal field emission scanning electron microscopy	Quanta	Quanta 400F
elemental analyser - isotope ratio mass spectrometry	ThermoFinnigan	ThermoFinnigan Delta Plus
binocular microscope	any	NA
reflected light microscope	Carl Zeiss	3519001617
polishing machicine	Struers	60210535
cutting machicine	Struers	50110202
carbon/gold coating machicine	any	NA
ethanol	any	NA
acetic acid	any	NA
zinc acetate solution (3%)	any	NA
HCl solution (25%)	any	NA
1 M CrCl2 solution	any	NA
0.1 M AgNO3 solution	any	NA
V2O5 powder	any	NA
pure nitrogen	any	NA
syringe	any	NA
filter(<0.45 µm)	any	NA
tin cups	any	NA
round bottom flasks	any	NA
epoxy	Struers	41000004