Geochemical and mineralogic analysis of authigenic carbonates collected offshore the U.S. Mid- and South Atlantic

Authigenic carbonate samples were collected during HOV and ROV dives. Samples were rinsed with freshwater, allowed to dry, photographed, and stored at room temperature.

Authigenic carbonate samples were cut using a diamond saw to expose fresh surfaces. Subsamples were isolated using trim saws, diamond drills and hand scribes.

Unprocessed subsamples were submitted to the USGS MRP Analytical Chemistry project where they were prepared according to standard operating procedures and then analyzed by: 1. major and trace elements by ICP-OES or ICP-MS following a sodium peroxide sinter fusion; 2.essential water (loss on ignition) by gravimetric methods; 3. major element oxides by wavelength dispersive X-ray fluorescence. Results undergo two levels of data validation: (1) the contract laboratory conducts quality control with every batch of samples sent to measure accuracy and precision and (2) all sample batches sent to the laboratory contain USGS in-house reference materials submitted at a rate of 10 percent, the results returned for the reference materials are compared to expected values for each element. Values must fall within the range of acceptance and relative standard deviation (RSD) at five times the lower limits of detection for the analytical methods. The acceptable accuracy and precision (RSD) is +/- 15 percent and 15 percent respectively for the major and trace elements by ICP-OES and ICP-MS. The acceptable accuracy and precision (RSD) is +/- 5 percent and 5 percent, respectively, for the wavelength dispersive X-ray fluorescence method. Quantitative mineralogy via XRD analysis was performed using a Rigaku Miniflex 600 benchtop x-ray diffractometer utilizing a copper anode tube to generate x-rays operated at 40 kV and 15 mA. Data processing and analysis were performed using the Rigaku PDXL2 software and the Crystallography Open Database, NIST Inorganic Crystal Structure Database, and the International Centre for Diffraction Data PDF-4/Minerals database to search and match diffraction patterns, identify component mineral phases and perform whole-powder pattern fitting of high-probability candidate phases and Rietveld structure refinement.