High-grade fluid metasomatism in the lower crust and upper mantle
The chemical and physical evolution and stability of the mid to lower crust and upper mantle can be strongly affected by fluids such as H2O, CO2, and KCl/NaCl/CaCl2 brines. Studies of high-grade fluid metasomatism in the lower crust and upper mantle include:
- Changes in the mineral chemistry across traverses of both regional and localised dehydration zones. In these studies, solid-state dehydration by low H2O activity fluids has been and is currently being utilized to demonstrate how hornblende and/or biotite react with quartz to form orthopyroxene +/- clinopyroxene, feldspar and a fluid phase during granulite-facies metamorphism. Low H2O activity fluids include those with a significant CO2 and/or (Na,K)Cl-CaCl2 brine component. Such fluids have been proposed to play a significant role during the granulite-facies metamorphism of basaltic and granitoid rocks in the lower crust.
- SIMS analysis of 29 zircon separates across a regional (100 km) traverse of late Archean, lower crust, Shevaroy Block, Tamil Nadu, South India to study how zircon HREE and actinide chemistry, along with U-Th-Pb dating, change during metamorphism. One of the purposes of this study is to understand how potential fluids, streaming upwards from the crust-mantle boundary, could influence gradations in mineral chemistry and REE distribution along the traverse.
- LA-ICPMS study of fluorapatite, garnet, amphibole, and clinopyroxene (Y+REE) chemistry across a lower crustal, fluid-activated, localised orthopyroxene-bearing dehydration zone, Söndrum stone quarry, SW Sweden.
- A regional study investigating the role of CO2-rich and H2O-rich fluids during the genesis and evolution of a co-genetic granite and magmatic charnockite association as well as their influence on the surrounding amphibolite-facies gneiss, country rock, Varberg-Torpa charnockite-granite association, SW Sweden.
- The role of CO2-rich fluids in the formation of charnockite patches in a granitic magma during emplacement of the Weinberg granite, north central Austria.
- Comprehensive study of the influence of fluids on apatite mineral chemistry (focusing on Cl, F, OH, CO3 chemistry) in the lower crust and upper mantle.
- Experimental dehydration of granitoid rocks, under granulite-facies conditions, utilizing both partial melts and low H2O activity fluids including both CO2 and supercritical NaCl-KCl brines.
- Experimental formation of simple symplectites of K-feldspar and albitic plagioclase at the quartz-plagioclase interface utilizing both partial melts and low H2O activity fluids.
© GFZ Potsdam
- Plot of 29 zircon ages as a function of distance (and depth) along a traverse of lower, late Archean crust, Shevaroy block, Tamil Nadu, south India. Zircon ages are lined up with sample location Dotted lines designate the approximate boundaries between the higher-grade southern granulite-facies zone (SGF), the lower-grade central granulite-facies zone (CGF) and the northern amphibolite-facies zone (NAF). Darkened region straddling the orthopyroxene-out boundary between the NAF and CGF is very rich in clinopyroxene.
© GFZ Potsdam
- Plot of biotite compositions for TiO2, FeO, and MnO as a function of distance along a traverse of lower late Archean crust, Shevaroy block, Tamil Nadu, south India (see Figure 1) going southwards from the northernmost sample. Dotted lines designate the approximate boundaries between the higher-grade southern granulite-facies zone (SGF), the lower-grade central granulite-facies zone (CGF) and the northern amphibolite-facies zone (NAF). Note the correlation between metamorphic grade and the Ti content. The increase in Fe going towards the NAF is a result of decreasing oxygen fugacty with decreasing metamorphic grade. The increase in Mn is the result of increasing ilmenite breakdown, again as a function of decreasing metamorphic grade. See Hansen and Harlov (2007) J Petrol 48, 1641–1680 for further details.
© GFZ Potsdam
- Histograms showing the microthermometry data of fluid inclusions from the magmatic Varberg charnockite in the vicinity of the Varberg Fortress, Varberg, southwest Sweden. Homogenization temperatures (Th CO2) measured for the carbonic and aqueous-carbonic inclusions are shown above the X-axis. The colours distinguish between Th measured for inclusions in quartz, plagioclase, fluorapatite, garnet, clinopyroxene, and zircon. The partial CO2 homogenizations of aqueous-carbonic inclusions are shown with different symbols (see legend). Melting temperatures of ice, clathrate, and salt hydrate for the aqueous and aqueous-carbonic inclusions are shown below the X-axis. Pie charts show the relative abundance of carbonic, aqueous, and aqueous-carbonic inclusions. Note the high mean CO2 content of the fluid inclusions from the magmatic charnockite. These contrasts with the co-genetic, associated Torpa granite in which the fluid inclusions are dominated by H2O. See Harlov et al. (2012) J Petrol (in press) for further details.
Contact: Daniel Harlov