Advances in carbon isotope analysis of trapped methane and volatile hydrocarbons in crystalline rock cores.

RATIONALE The isotopic composition of hydrocarbons trapped in rocks on the microscale (fluid inclusions, mineral grain boundaries, microfractures) can provide powerful information on geological and biological processes but are an analytical challenge due to low concentrations. We present a new approach for the extraction and carbon isotopic analysis of methane (CH4 ) and hydrocarbons in trapped volatiles in crystalline rocks. METHODS An off-line crusher with cryogenic trapping and a custom-made silica glass U-trap were attached to an external injector port on a continuous flow gas chromatograph/combustion/isotope ratio mass spectrometer to demonstrate the accuracy, reproducibility, and sensitivity of δ13 C measurements for CH4 . RESULTS The method can isotopically characterize CH4 in crushed rock samples with concentrations as low as 3.5 × 10-9 mol/g of rock, and both sample and isotopic standards are analyzed with an accuracy and reproducibility of ±0.5‰. High H2 O/CH4 ratios of 98 to 500 have no effect on measured δ13 CCH4 values. The method is successfully applied to natural samples from the north range of Sudbury Basin, Ontario, Canada. The δ13 C isotopic signatures of CH4 trapped microscopically in rock from the north range overlap significantly with that of CH4 contained in larger scale flowing fracture fluids from the same part of the Sudbury Basin, indicating a potential genetic link. CONCLUSIONS A novel method for δ13 CCH4 analysis was developed for the extraction of nanomole quantities of CH4 trapped microscopically in rocks. The technique has an accuracy and reproducibility comparable to that of on-line crushing techniques but importantly provides the capability of crushing larger rock quantities (up to 100 g). The benefit is improved detection levels for trace hydrocarbon species. Such a capability will be important for future extension of such crushing techniques for measurement of 2 H/1 H for CH4 , clumped isotopologues of CH4 and other trapped volatiles species, such as C2 H6 , C3 H8 , C4 H10 , CO2 and N2 .