STRONTIUM ISOTOPIC ANALYSIS OF TEPHRA: A CASE STUDY FROM THE WESTERN INTERIOR SEAWAY
The strontium isotope signature of 87Sr/86Sr was determined for 11 individual bentonite layers collected near Hot Springs, SD and dated to the Cenomanian (approx. 97.9 +-1.4 Ma). In addition, a series of calcitic fossil material (bivalves and ammonoids) were collected to test for evidence of meteoric diagenesis and to determine possible similarities between inorganic and biogenic strontium. XRD analysis confirmed the bentonite clay content to be nearly 95-100% dioctahedral montmorillonite. The isotopic analysis of carbonate material fell within a range of .707389 to .707539, with an average of .707444. The 87Sr/86Sr signature of the fossil material precisely matches the well-documented global strontium ratio during the Cenomanian, approximately .7074, demonstrating an absence of any significant diagenetic alteration.
After applying a rubidium correction for post-depositional radiogenic Sr, the 87Sr/86Sr values of sampled bentonites range from .70762 to .70782, the average being .70769. The average 87Sr/86Sr of the bentonites differ from the oceanic average 87Sr/86Sr by .0002 during the Cenomanian, a significant variation in terms of strontium content. Timing of the Cenomanian ash deposits coincide with the accretion and assimilation of exotic island arc terranes to the western border of Laurentia. It appears that the strontium values preserved in the bentonites are recording the radiogenic signatures of these magmatic events rather than ocean water, and therefore, could be used to reconstruct the timing of terrane accretion and associated magmatism along convergent margins.