The stable isotope signature of kilometre‐scale fracturedominated metamorphic fluid pathways, Mary Kathleen, Australia

Abstract

Large calcite veins and pods in the Proterozoic Corella Formation of the Mount Isa Inlier provide evidence for kilometre-scale fluid transport during amphibolite facies metamorphism. These 11% to lWm-scale podiform veins and their surrounding alteration zones have similar oxygen and carbon isotopic ratios throughout the 200 x 10-km Mary Kathleen Fold Belt, despite the isotopic heterogeneity of the surrounding wallrocks. The fluids that formed the pods and veins were not in isotopic equilibrium with the immediately adjacent rocks. The pods have S13Cakirc values of -2 to -7%. and 6180ulfilc values of 10.5 to 12.5%. Away from the pods, metadolerite wallrocks have 6180whorc-mck values of 3.5 to 7%., and unaltered banded calc-silicate and marble wallrocks have S"C,,,, of -1.6 to -0.6% and ,5180w,e of 18 to 21%. In the alteration zones adjacent to the pods, the 6l8O values of both metadolerite and calc-silicate rocks approach those of the pods. Large calcite pods hosted entirely in calc-silicates show little difference in isotopic composition from pods hosted entirely in metadolerite. Thus, 100-to 500-m-scale isotopic exchange with the surrounding metadolerites and calc-silicates does not explain the observation that the 6"O values of the pods are intermediate between these two rock types. Pods hosted in felsic metavolcanics and metasiltstones are also isotopically indistinguishable from those hosted in the dominant metadolerites and calc-silicates. These data suggest the veins are the product of infiltration of isotopically homogeneous fluids that were not derived from within the Corella Formation at the presently exposed crustal level, although some of the spread in the data may be due to a relatively small contribution from devolatilization reactions in the calc-silicates, or thermal fluctuations attending deformation and metamorphism. The overall L-shaped trend of the data on plots of 6I3C vs. 6I80 is most consistent with mixing of large volumes of externally derived fluids with small volumes of locally derived fluid produced by devolatilization of calc-silicate rocks. Localization of the vein systems in dilatant sites around metadolerite/calc-silicate boundaries indicates a strong structural control on fluid flow, and the stable isotope data suggest fluid migration must have occurred at scales greater than at least 1 km. The ultimate source for the external fluid is uncertain, but is probably fluid released from crystallizing melts derived from the lower crust or upper mantle. Intrusion of magmas below the exposed crustal level would also explain the high geothermal gradient calculated for the regional metamorphism.

The noble gas systematics of late-orogenic H2O–CO2 fluids, Mt Isa, Australia

“…1; Oliver et al, 2008). The results build on previous work investigating the halogen and 40 Ar/ 36 Ar systematics of Mt Isa's ore deposits (Kendrick et al, 2006a(Kendrick et al, ,b,c, 2007(Kendrick et al, , 2008a and earlier fluid inclusion and stable isotope investigations Heinrich et al, 1989;Oliver et al, 1993Oliver et al, , 2008Oliver, 1995;Mark et al, 2004Mark et al, , 2006a. Our aim was to develop a fuller understanding of processes affecting noble gases in high-temperature magmato-metamorphic fluids and thereby provide a more robust framework for interpretation of noble gases in crustal fluids.…”

Diffuse fluid flux through orogenic belts: Implications for the world ocean

“…Mantle-derived fluids are known in a variety of metamorphic belts based on He, C, and Sr isotopes (27)(28)(29)(30). The mantle contribution to the total fluid flux in these localities is evidently quite variable.…”

Fluids in the Continental Crust

“…Jamie Connolly has developed ideas about the way in which this happens on a large scale (summarised in Connolly, 2010) and described how, if metamorphic rocks are able to compact as reaction proceeds, fluid will escape in the form of migrating "porosity waves". An important challenge today is integrating this large-scale view with field scale studies (Oliver et al, 1990(Oliver et al, , 1993Oliver, 1996;Yardley et al, 1991;McCaig, 1997;Peniston-Dorland and Ferry, 2008). …”