Statistics – Computation
Scientific paper
May 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.2037w&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 61, Issue 10, p. 2037-2051.
Statistics
Computation
2
Scientific paper
High-resolution Scanning Proton Microprobe (SPM) analyses of fissure filling calcite (from the Mendip Hills, UK) show fine-scale chemical zonation patterns in Fe and Mn concentrations from fissure centre to wall. These fluctuations are sharp and steep (peaks of 0.01 0.1 mm width), with concentrations changing by as much as 8000 ppm. Zonation in Pb and Zn is similar to that obtained for Mn and Fe, and maxima in the concentrations of these two elements are correlated with maxima in Mn concentration. Longer wavelength fluctuations (1.0 3.0 mm) are evident in data obtained for Sr, for which changes in concentration are gradual and fluctuate by at most 500 ppm. Auto-correlation calculations show that oscillations in the Mn and Fe data have a wavelength of 0.9 1.3 mm in contrast to auto-correlations and cross-correlations with Zn and Pb which indicate wavelengths of 0.4 mm. Strontium correlations indicate a peak-to-trough distance of 4.2 mm, which implies a much longer wavelength fluctuation in the Sr data. Cross-correlations also clearly show that Pb and Zn concentrations co-vary strongly (cross-correlation function value of 0.7 at lag 0) but all other cross-correlations are weak. These computations demonstrate that intrinsic controls on trace element partitioning cannot alone account for the observed patterns. Long period fluctuations are interpreted to result from a sustained change in the starting composition of the aqueous phase, while the high frequency oscillations are interpreted to be caused by mixing reactions among fluids that have reacted with different portions of country rock prior to calcite precipitation. Analyses of the carbon, oxygen, and strontium isotopes from these vein calcites have also been completed. δ 18O plotted as a function of δ 13C results in an inverted triangular array. We interpret this, along with the trace element data, as resulting from three distinct fluids participating in the formation of these fissure fills: a low MnFe, high Sr marine fluid (apex at δ 18O = 0, δ 13C = +4; both relative to PDB); a high MnFe, low Sr hydrothermal fluid (+2, -9.5); and a low MnFe, low Sr meteoric fluid (-12, -5). The trace element and isotopic compositions of this zoned calcite thus document the following processes in detail: (1) the formation of a meteoric fissure system (Triassic), (2) reactions with hotter and more reduced circulating fluids, and (3) the subsequent flooding of the system with seawater during transgression of the area (Rhaetian-Early Jurassic; Strontium isotopic data from a belemnite located within one of the fissures gives an Early Jurassic age of approximately 198 Ma). Hot, reduced fluids with high metal concentrations were probably forced up into these fissures by seismic pumping during movement on associated regional faults.
Fraser Donald G.
Grime Geoff W.
Wall Gavin R. T.
Wogelius Roy A.
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