U-PB AGES AND HF-ISOTOPES OF ZIRCONS IN ASH BEDS AND RE-OS ISOTOPES OF THE EAGLE FORD FORMATION OF SOUTH TEXAS
Although the major element and trace elements of the ash beds suggest their origin as a result of arc-derived volcanism, the εHf(T) of most of the ash bed zircons yield values between 0 to +8 averaging around +3, clearly of a mantle component in the host magmas of the zircons.
6 Lower Eagle Ford and 4 Middle Eagle Ford kerogen rich rocks show characteristic high Re-Os ratios; Re concentrations range from 7560 to 17701 ppt and Os concentrations vary from 37.5 to 932.1 ppt, yielding 187Re/188Os of 155.5 to 1537.4. The 187Os/188Os ratio in these rocks range from 2.7233 to 5.3529. In a Re-Os isochron diagram only three whole rock samples yield a reasonable isochron of 76±13 Ma with an initial 187Os/188Os ratio of 2.52±0.22. Other seven samples show scattering and do not define reasonable isochrons. All together, the Re-Os data suggest mobility of the kerogen-rich layers within the analyzed 100ft range of this core.
The initial 187Os/188Os at 93 Ma, the time of deposition of the Eagle Ford of the ten whole rock samples, range between 2.466 to 5.111 with a majority of the samples lying between 2.46 and 3.62 indicating a continental crustal origin of the Os without taking into account the sea water Os-isotopic composition at 93 Ma. The contrasting inference from Hf and Os-isotopes need to be resolved by future work.