Age and Composition of Source Rocks: New Steps toward Tracking Hydrocarbon Origin

摘要

A variety of chemical fingerprints link migrated hydrocarbons to their source rocks, defining the ends of migration pathways. Rhenium (Re) and osmium (Os), redox sensitive elements concentrated in organic material, add unique information–time. Decay of ~(187) Re to ~(187) Os provides a radiometric clock measuring time since chemical closure of the organic material. Here we show that Re-Os geochemistry of source rocks defines the age of deposition and tracks environmental changes through time. This geochronometer also reduces ambiguity with a fingerprint for migrated hydrocarbons: evolving ~(187) Os/~(188) Os in migrated hydrocarbons, dependent on the ~(187) Re/~(188) Os ratio and age of both source rock and hydrocarbons, constrains models for the timing of migration. Black shales from the lower Streppenosa Formation, deposited in a deep euxinic intraplatform basin, yield a Re-Os age of 200. 3 Ma and initial ~(187) Os/~(188) Os of 0. 87. This Hettangian age aligns perfectly with the known biostratigraphic age, is nominally younger than the 201. 3 Ma Triassic-Jurassic boundary, and postdates major magmatic pulses of the Central Atlantic Magmatic Province (CAMP, 201. 6 and 200. 9 Ma). Primitive CAMP magmatism produced sharp decreases in both ~(87) Sr/~(86) Sr and ~(187) Os/~(188) Os ratios of seawater, as both track relative inputs from continental versus chondritic sources. Osmium, however, has a much shorter seawater residence time than Sr; our data reveal that ~(187) Os/~(188) Os returns to high ratios within 1 m. y. of cessation of CAMP magmatism. High ~(187) Os/~(188) Os at 200. 3 Ma documents reduced contribution of Os from CAMP, and may also reflect enhanced continental runoff from uplift along newly rifted margins. Osmium isotope variations in seawater, archived in organic-rich shales, provide a sensitive record of tectonic and environmental changes during source rock deposition.