Deep Crustal Fluid Advection and Exchange of Oxygen-18: Application to Carpathians

Pawel M. Lesniak Institute of Geological Sciences, Polish Academy of Science,

Zwirki i Wigury 93, 02 089 Warsaw, Poland


In the several papers concerning transport and oxygen isotope exchange (McKibbin and Abasar, 1989; Bowman et al., 1994), two features were essentially presented: rock oxygen-18 evolution on present local scale, i.e., past flow in fossil systems and in present geothermal systems. In both cases fluids are usually considered as lost.

On the contrary, the system is described where meteoric water pushing the original fluid, exchange oxygen isotope with rocks during advection. The final isotopic composition of resulting waters is known, but isotopic imprint on rock is unknown.

Carpathians waters high in d18O (up to 6.7”) occur in a large front in the West Carpathians in Magura nappe. Though, at present cold, they have other peculiar composition as high concentration of boron, though chloride is essentially lower than in the sea waters. As results from crossection (Jankowski and Lefeld, 1983) flow direction imposed by topography and tectonic setting is from south to north at the depth 12 to 16 km. Indeed large front of geoelectric anomaly with resistivity as low as 4 Wm occur at this depth. Heat flux is about 50mW/m2.


The flow and isotope exchange profile were obtained by solving numerically 1-D advection-exchange equations. The results were checked against analytical formulas (Lassey and Blattner, 1988; Blattner and Lassey; Lassey, 1982). The question addressed is: what is a quantitative evolution of d18Ow and d18Or, along 50 km horizontal profile?

The lower boundary condition was d18Ow=6.7 ” and initial condition of water was assumed to be 0.0 ” as a representative for seawater trapped in sediments. The oxygen isotopic compositions for average rocks were assumed to be 10 ” (silicate) and 20 ” (carbonate). Unfortunately, temperature (though not exceeding 300°C (Jankowski and Lefeld, 1983)), as well as rock compositions are uncertain, therefore fractionation factor between silicate as well as carbonate rocks and water was assumed as equal 5 ”. With advection speed 1m/yr., porosity f=0.05, and assuming that matrix porosity > fissure porosity (Zuber and Motyka, 1994), integrated fluid flux is about 5kg of H2O /m2/yr.


The minimum time for reflushing of only 2500 yrs., is so low in comparison to the time passed from the last tectonic event that most of the fossil waters must have been replaced. Therefore, high oxygen-18 waters occurring in the large front in the Outer Carpathians have to be regarded as waters which exchanged oxygen-18 with rocks or were dehydrated from metasediments. Although, in the case of exchange with carbonate d18Ow could even surpass the present value 6.7 ”, after 15*t steady state profile d18Ow emerges. Other scenario is tried as pulse recharge in upper boundary condition which could correspond to certain tectonic event.


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