Titel: Thermobarometry at extreme conditions - what can possibly go wrong? An example
Thorsten Joachim Nagel1, Kathrin Fassmer2
1Aarhus University, Denmark; 2Innsbruck University, Austria
Veranstaltung: GeoKarlsruhe 2021
We present eclogites and garnet pyroxenites from Danmarkshavn (Greenlandic Caledonides). So far, one ultra-high pressure (UHP) location has been described from NE Greenland. There, thermobarometry yielded conditions of 972 ºC/36 kbar (Gilotti and Ravna, 2002).
Eclogites from Danmarkshavn show spectacular exsolution of Qtz from Cpx, which is known from UHP assemblages. The sample most suitable for unraveling precise conditions, however, is a garnet pyroxenite containing abundant Cpx and Opx, some Grt, minor accessory minerals as well as little retrograde Am and Pl. Opx and Cpx preserve high-pressure compositions in cores of large crystals and extremely low Al-content in Opx clearly indicate UHP conditions. A considerable portion of these minerals, however, reequilibrated during exhumation with especially Cpx showing complex retrograde zoning. We infer that Grt grains completely reequilibrated during net-transfer reactions producing Am and Pl.
Precise conditions in such rocks are often achieved by intersection of isopleths, e.g. Al-in-Opx with Grt-Cpx-Mg-Fe thermometry. Both isopleth sets have positive slopes in pressure-temperature space. If exhumation occurs along a trajectory steeper than the thermometer isopleth, equilibration of Mg-Fe-exchange during exhumation leads to possibly dramatic overestimation of peak conditions. In our sample, this yields up to 1000 ºC/>40 kbar. Based on Cpx and Opx core compositions alone, however, we infer considerably lower peak conditions of 800-830 ºC/30-32 kbar.
We find that micro-xrf scans of whole thin sections yield powerful data on mineral zoning, reaction progress and the degree of reequlibration. Such maps allow better defining targets for high-resolution mapping and high-precision microprobe work.
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