Dany Savard

TCF selenium preconcentration in geological materials for determination at sub-μg g−1 with INAA (Se/TCF-INAA)

In geological samples, Se concentration ranges from 1x10(-9)gg(-1) up to 1x10(-3)gg(-1). The analytical difficulty at low concentration (<1mugg(-1)), is one of the main reasons why the geological cycle of Se is poorly known. The analytical method that consisted of preconcentration of Se with thiol cotton fiber (TCF) followed by graphite furnace atomic absorption spectrometry (GFAAS) has been modified by finishing with instrumental neutron activation analysis (INAA). The modified technique involves sample dissolution (HF-HNO(3)-H(2)O(2)) and evaporation to dryness at low temperature (55-60 degrees C) to avoid selenium volatilization. Se(VI) is converted to Se(IV) by adding 6M HCl to the dry residuum and the solution is then heated in a covered boiling bath (95-100 degrees C). The solution is diluted to obtain 0.6M HCl and then collected on TCF. The TCF is placed in a polyethylene vial for irradiation in the SLOWPOKE II reactor (Montréal) for 30s at a neutron flux of 10(15)m(-2)s(-1). The 162keV peak of (77m)Se (half-life 17.36s) is read for 20s after a decay of 7s. The amount of sample to be dissolved is controlled by two competing effects. To obtain low detection limits, a larger amount of sample should be dissolved. On the other hand, the TCF could become saturated with chalcophile elements when large sample is used. Sulfur is a good indicator of the amount of Se and chalcophile elements present. In S poor sample (<100mugg(-1)) 3.0g of sample was used and the L(D) was approximately 2ngg(-1). In S high samples (>1.5% S) 0.05g of sample was used and the L(D) was approximately 120ngg(-1). The present work also includes suggested Se concentration for eight international geological reference materials (IGRM) that compare favorably with literature values.

Forced subduction initiation recorded in the sole and crust of the Semail Ophiolite of Oman

Subduction zones are unique to Earth and fundamental in its evolution, yet we still know little about the causes and mechanisms of their initiation. Numerical models show that far-field forcing may cause subduction initiation at weak pre-existing structures, while inferences from modern subduction zones suggest initiation through spontaneous lithospheric gravitational collapse. For both endmembers, the timing of subduction inception corresponds with initial lower plate burial, whereas coeval or delayed extension in the upper plate are diagnostic of spontaneous or forced subduction initiation, respectively. In modern systems, the earliest extension-related upper plate rocks are found in forearcs, but lower plate rocks that recorded initial burial have been subducted and are inaccessible. Here, we investigate a fossil system, the archetypal Semail Ophiolite of Oman, which exposes both lower and upper plate relics of incipient subduction stages. We show with Lu–Hf and U–Pb geochronology of the lower and upper plate material that initial burial of the lower plate occurred before 104 million years ago, predating upper plate extension and the formation of Semail oceanic crust by at least 8 Myr. Such a time lag reveals far-field forced subduction initiation and provides unequivocal, direct evidence for a subduction initiation mechanism in the geological record.The subduction system recorded by the Semail Ophiolite of Oman was initiated by far-field events, according to a comparison of the ages of the upper and lower plate material.