H160-07
When chlorine isotopes challenge the pore fluid paradigm

Tuesday, 15 December 2020: 04:30
Virtual
Pierre Agrinier1, Magali Bonifacie2, Gerard Bardoux1, Francis Lucazeau3, Thomas Giunta4 and Magali Ader1, (1)Institut de Physique du Globe de Paris, Paris, France, (2)Université de Paris, Institut de Physique du Globe de Paris - OVSG, Gourbeyre, Guadeloupe, (3)Institut de Physique du Globe, Paris, France, (4)IFREMER, Plouzané, France
Recorded Presentation
Abstract:
To study the seawater-oceanic crust interactions that establish the chemical composition of the seawater, we examine low temperature pore fluids of clay-rich sedimentary piles. The chloride δ37Cl values systematically decrease with depth from 0 ‰, the seawater value, down to -8.5‰, while their water δ18O values systematically decrease with depth from the seawater values ≈ 0 ‰ down to -5.7 ‰. Unexpectedly, δ37Cl of chlorides and δ18O of water are highly correlated. The chlorides 37Cl-depletion cannot be explained by water-rock exchanges because low T alteration minerals are not able to sequester significant amounts of chlorides. Fluid-specific processes are thus responsible for the 37Cl-depletions.

Reviewing the fluid-specific processes that are known to change δ37Cl of chlorides leads us to exclude influx of 37Cl-depleted fluid, diffusion and gravitational isotope fractionations. Rather, compaction-induced ion filtration of chlorides through clay membrane can produce the observed 37Cl-depletion in chlorides by expelling 37Cl-enriched chlorides out of the sediments, into the ocean, with a fractionation factor a37/35Cl(expelled fluid-retentate) comprised between 1 and 1.008. High values are obtained for smectite-rich sediments, medium values for Illite/chlorite-rich sediments and low values for clay-poor sediments, showing that the fractionation might be controlled by the mineralogy of the clay. Most importantly, compaction induced filtration can also produce the observed 18O-depletion for a18/16O(expelled fluid-retentate) between 1 and 1.005, in agreement with the experimental data of Haydon (1986) but in opposition to those of Demir (1988). Oxygen and chlorine isotope fractionation factors are positively correlated.

The correlation between δ37Cl and δ18O shows that either the same process is the cause for both the 18O- and 37Cl-depletions or two strongly coupled processes are operating, one causing the 18O-depletions and the other the 37Cl-depletions. We propose here that the ion filtration might be this common cause. This suggestion challenges the widely accepted pore fluid paradigm that explains 18O-depletions by water-rock exchange, that has a lot of geochemical merits but cannot explain the 37Cl-depletions of pore fluid chlorides.

Ref

Haydon, 1986, GCA 50, 115

Demir, 1988, GCA 52, 727

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