Optimized petrology of impure marbles

With this project, an improvement of the usefulness of "impure" marbles for petrological evaluation of the metamorphic history of an area is intended. This will be accomplished by two complimentary approaches. On the one hand, thermodynamic calculations of a full petrogenetic grid of calcite-dolomite marbles in the system (CaO-MgO-Al2 O3 -SiO2 -CO 2 -H2 O (CMASCH) will be done and presented by using the softwares THERMOCALC and PERPLEX. This grid will then be expanded by each of the components K2 O, TiO2 or ZrO2 in order to know P-T locations of additional important reactions among accessory minerals that can be used to constrain the metamorphic history. The use of univariants and grids was not common for carbonate-bearing rocks in the past but is demonstrably very fruitful (univariants are mapable isograds in the field) and effective (univariants are responsible for almost all reaction textures observed in thin section). On the other hand, these results and their applicability will be tested against field evidence from occurrences of calcite-dolomite marbles that are known to contain the above mentioned impurities in the form of minerals like muscovite, phlogopite, K-feldspar, rutile, titanite, geikielite, Ti-clinohumite, zircon, baddelyite, zirconolite and others. Hot cathode cathodoluminescence color imaging will be used as main technique additional to routine optical micrsocopy and electron beam techniques for an optimized detection and description of mineral compositions and textures (mineral generations, zoning patterns, inclusion relationships, reaction textures). The field focus will be on marbles from high- and ultrahigh-pressure areas because a) this is still an actively expanding field of modern research with many open questions of general importance, b) HP and UHP marbles have received relatively little attention yet c) the chances of preservation of a high-pressure record during exhumation of a marble can be investigated and typical traces of a (U)HP history determined and highlighted for fellow researchers, and d) the applicant has long-term experience with this high-pressure research. Hence, very focused sampling will occur in key known outcrop areas in the Eastern and Western Alps (Austria, Italy), he Rhodope mountains (Greece) and China (Dabie/Sulu orogen, Tianshan).

The investigations performed in this project have demonstrated that marble is not only a useful material for construction and arts but may also contain infomation useful for understanding the geological history of a region. Earlier underestimated in its usefulness, this rock type can also contain silicate and oxide minerals which separated by a matrix of the carbonate minerals calcite and dolomite may retain information about early parts of the geological history that has long been overprinted by later events in other types of rocks. Observations on such marbles from the Chinese Dabie-Sulu mountains showed that sediments which formed originally at the Earths surface were transported by tectonic processes to such extreme depths (150 to 200 kilometres), that even diamond would have become stable under such conditions. (And extremely rare microdiamonds were actually found by other research groups in this mountain belt.)A comparison of thermodynamic calculations with measurements on rock samples in the laboratory established the fact that the conditions of formation of marbles deep within the Earth can be determined in more detail by use of titanium, zirconium and potassium-bearing minerals which occasionally occur as impurities in marbles. Hence the spectrum of rock types that does contain useful information about the geological past is now including impure marbles and could thus be expanded significantly.