Contact aureole

Fig. 1. Regional geology surrounding the Morila Au Mine, southern Mali. The deposit is hosted by immature Birimian metasediments and is proximal to a large composite post-Eburnean D2 pluton (Doubalkoro) and within a localized low-P contact aureole.

Georg RB, Reynolds BC, Frank M, Halhday AN (2006) Mechanisms controlhng the silicon isotopic compositions of river water. Earth Planetary Sci Lett 249 290-306 Gerdes ML, Baumgartner LP, Person M, Rumble D (1995) One- and two-dimensional models of fluid flow and stable isotope exchange at an outcrop in the Adamello contact aureole, Southern Alps, Italy. Am Miner 80 1004-1019... 

Teng FZ, McDonough WF, Rudnick RL, Wing BA (2007) Limited lithium isotopic fractionation during progressive metamorphic dehydration in metapelites a case study from the Onawa contact aureole, Maine, Chem Geol 239 1-12... 

Giere, R. Williams, C. T. 1992. REE-bearing minerals in a Ti-rich vein from the Adamello contact aureole (Italy). Contributions to Mineralogy and Petrology, 112, 83-100. 

Reactions of this nature are very important for the return of CO2, originally sequestered in CaCC>3 deposition on the sea floor, to the atmosphere-hydrosphere system (see Chapter 10). The chemical reaction of equation 8.7 can also occur in the contact aureoles associated with the intrusion of igneous bodies into limestone. 

Barboza S. A. and Bergantz G. W. (2000) Metamorphism and anatexis in the mafic complex contact aureole, Ivrea zone, northern Italy. J. Petrol. 41, 1307-1327. 

Giere R. (1990) Hydrothermal mobility of Ti, Zr, and REE examples from the Bergell and AdameUo contact aureoles (Italy). Terra Nova 2, 60—67. 

Jenkins 2000). No zones of higher temperature were found at exposed igneous contacts, though the locally low values of A C(Cc-Gr) 5 km W of Canton may be an unrecognized contact aureole. The Cc-Gr RAM thermometer provides the best estimates for metamorphic temperature in this terrane. 

Examples of double porosity models applied to stable isotope transport in regional and contact metamorphism are given by Bowman et al. (1994) (see discussion of Alta contact aureole below), though this is not explicitly stated. Curves calculated for onedimensional transport using a first order rate law were fitted to observed profiles with a... 

In general, the effects of volatilization, fluid infiltration, and temperature are all in the same direction calc-silicates and marbles tend toward lower values of 5 0 and 5 C at higher grades of metamorphism. Thus, these processes mimic one another on a 5 0-5 C diagram. Detailed analysis shows that (1) volatilization is always a factor in heavy isotope depletion, but is not the dominant cause of large shifts (2) polythermal or disequilibrium effects can be identified and may be important and (3) most contact aureoles studied (Table 1) have been infiltrated by fluids and 0-C-H isotopic ratios frequently enable identification of fluid sources (see reviews by Valley 1986 Bowman 1998). 

The infiltration of surface derived fluids into a contact aureole requires that fluid pressures be close to hydrostatic. Thus, if stable isotope ratios indicate exchange with large amounts of meteoric water, Ph20 must have been much less than Piithostatic at some time during the metamorphic history. The deepest known instances of meteoric water exchange represent the transition towards lithostatic fluid pressure. All known zones of meteoric water infiltration are shallower than 15 km and most are less than 6 km (Valley and O Neil 1982). Likewise, penetration of seawater to depths of 6-7 km in oceanic crust is well documented (Gregory and Taylor 1981). Possibly the deepest known penetration of surface-derived fluids is in veins and in faults of extensional terranes (Fiicke et al. 1992 Nesbitt and Muehlenbachs 1995 Morrison and Anderson 1998). 

Ferry JM, Dipple GM (1991) Fittid flow, mineral reactions and metasomatism. Geology 19 211-214 Ferry JM, Dipple GM (1992) Models for coupled fittid flow, mineral reaction, and isotopic alteration during contact metamorphism The Notch Peak Aureole, Utah Am Mineral 77 571-577 Ferry JM, Rumble D in (1997) Formation and destraction of periclase by fluid flow in two contact aureoles. Contrib Mineral Petrol 128 313-334... 

Romer RL, Heinrich W (1998) Transport of Pb and Sr in leaky aquifers of the Bufa del Diente contact metamorphic aureole, northeast Mexico. Contrib Mineral Petrol 131 155-170 Roselle GT (1997) Integrated petrologic, stable isotopic, and statistical study of fluid-flow in carbonates of the Ubehebe Peak contact aureole, Death Valley National Park, California. PhD Dissertation, University Wisconsin-Madison, 279 p... 

Roselle GT, Baumgartner LP, Valley JW (1999) Stable isotope evidence of heterogeneous fluid infiltration at the Ubehebe Peak contact aureole, Death Valley National Park, California. Am J Sci 299 93-138 Rumble D III (1982) Stable isotope fractionation during metamorphic volatilization reactions. Rev Mineral 10 327-353... 

The crystal size of monazite and xenotime tends to increase with increasing metamorphic grade (e.g., Franz et al. 1996, Rubatto et al. 2001). Ferry (2000) and Wing et al. (2002) report xenoblastic monazite at lower grades in a contact aureole, a low-P... 

The encircling zone of metamorphic rocks around an intrusion is referred to as the contact aureole (Fig. 1.14). The size of an aureole depends on the size and temperature of the intrusion when emplaced, the quantity of hot gases and hydrothermal solutions that emanated from it,... 

A sketch map of Skiddaw Granite and Its contact aureole, Cumbria, England. 20... 

Such buffered assemblages are quite common in nature and make contact aureoles of impure carbonate rocks one of the best areas to study the evolution of metamorphic fluids, because fluid compositions are essentially controlled by the mineral reactions. Progressive metamorphism initially leads to a rapid enrichment in CO2/ because the talc and tremolite reactions consume H2O and release CO2. At higher temperatures, water enrichment can take place by the decomposition of tremolite to diopside and/or forsterite. Changes in the compositions of the solids will affect the fluid ratios, and in fig. 6 the compositional changes of coexisting calcite and dolomite as a function of temperature have been taken into account. 

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