Geochemistry and evolution of MORB-type eclogites from the Münchberg Massif, southern Germany

Stosch, H.-G.; Lugmair, G. W.

doi:10.1016/0012-821x(90)90113-c

Cited by 130 publications

(57 citation statements)

References 32 publications

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“…The age of high-pressure metamorphism ranges between 380 and 395 Ma, as deduced from concordant Sm/Nd and Rb/Sr mineral isochron ages for the Weissenstein eclogite (Stosch & Lugmair, 1990). These dates coincide with Ar-Ar plateau ages of about 390 Ma on coarse 3T-phengites from the light Oberkotzau eclogite .…”

Section: Geological Settingmentioning

confidence: 84%

Reaction textures and fluid behaviour in very high pressure calc‐silicate rocks of the Münchberg gneiss complex, Bavaria, Germany

Klemd

Matthes

Schüssler

1994

Journal Metamorphic Geology

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Reaction textures and fluid behaviour in very highABSTRACT Calc-silicate rocks occur as elliptical bands and boudins intimately interlayered with eclogites and high-pressure gneisses in the Munchberg gneiss complex of NE Bavaria. Core assemblages of the boudins consist of grossular-rich garnet, diopside, quartz, zoisite, clinozoisite, calcite, rutile and titanite. The polygonal granoblastic texture commonly displays mineral relics and reaction textures such as postkinematic grossular-rich garnet coronas. Reactions between these mineral phases have been modelled in the CaO-Al 2 0 3 -Si02-C02-H 2 0 system with an internally consistent thermodynamic data base. High-pressure metamorphism in the calc-silicate rocks has been estimated at a minimum pressure of 31 kbar at a temperature of 630°C with X^oSQ.Gi. Small volumes of a C0 2 -N 2 -rich fluid whose composition was buffered on a local scale were present at peak-metamorphic conditions. The P-T conditions for the onset of the amphibolite facies overprint are about 10 kbar at the same temperature. A'co., of the H 2 0-rich fluid phase is regarded to have been <0.03 during amphibolite facies conditions. These P-T estimates are interpreted as representing different stages of recrystallization during isothermal decompression. The presence of multiple generations of mineral phases and the preservation of very high-pressure relics in single thin sections preclude pervasive post-peak metamorphic fluid flow as a cause of a re-equilibration within the calc-silicates. The preservation of eclogite facies, very high-pressure relics as well as amphibolite facies reactions textures in the presence of a fluid phase is in agreement with fast, tectonically driven unroofing of these rocks.

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Section: Geological Settingmentioning

confidence: 84%

Reaction textures and fluid behaviour in very high pressure calc‐silicate rocks of the Münchberg gneiss complex, Bavaria, Germany

Klemd

Matthes

Schüssler

1994

Journal Metamorphic Geology

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“…Pure graphite and 3 wt% of iridium (or platinum) metal were then added to control and measure the fO 2 , respectively (Woodland and O'Neill 1997;Stagno and Frost 2010). The garnet and cpx glass compositions are given in Table 1 and were derived from mineral analyses reported for a Bavarian eclogite sample W6.80 (Stosch and Lugmair 1990). These glasses were synthesized from oxides and carbonates and then glassed by melting at 1,650 °C in an iron-saturated platinum crucible followed by rapid quenching.…”

Section: Starting Compositionsmentioning

confidence: 99%

“…COMP3 is a glass starting composition (see text). Grt (COMP1) and Cpx (COMP2) are synthetic glasses used for sandwiched experiments with compositions similar to natural garnet and cpx from an eclogite (WM 6.80) reported by Stosch and Lugmair (1990) a Natural kyanite from Minas Gerais (Brazil). The analytical standard deviation (1σ) is reported in terms of least units cited in parentheses.…”

Section: Starting Compositionsmentioning

confidence: 99%

The oxygen fugacity at which graphite or diamond forms from carbonate-bearing melts in eclogitic rocks

Stagno

Frost

McCammon

et al. 2015

Contrib Mineral Petrol

110

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eclogites. The raised prevalence of diamonds in eclogites compared to peridotites may, therefore, reflect more effective scavenging of carbon by melts in these rocks. The ferric iron contents of monomineralic layers of clinopyroxene and garnet contained in the same experiments were also measured using Mössbauer spectroscopy. A preliminary model was derived for determining the fO 2 of eclogitic rocks from the compositions of garnet and clinopyroxene, including the Fe 3+ /ΣFe ratio of garnet, using the equilibrium,The model, which reproduces the independently determined fO 2 of the experimental data to within 0.5 log units, can be used to estimate the fO 2 of ultrahigh-pressure metamorphic eclogites and cratonic eclogitic xenoliths. Although there are very few analyses of garnet Fe 3+ /ΣFe ratios from eclogite samples, the range in fO 2 recorded by available eclogitic xenoliths is similar to that reported for peridotitic xenoliths and generally within the graphite/diamond stability field. Estimates for the average bulk Fe 3+ / ΣFe ratio of modern basaltic oceanic crust, however, are higher than the values for most of these xenoliths, and upon subduction, crustal carbon is likely to remain in the carbonate stability field to depths of at least 250 km. If eclogite xenoliths originated from subducted oceanic crust, then their generally lower fO 2 most likely reflects either lower initial basaltic Fe 3+ /ΣFe ratios, loss of Fe 2 O 3 through partial melting or the initial presence of organic carbon. AbstractThe oxygen fugacity (fO 2 ) at which carbonatebearing melts are reduced to either graphite or diamond in synthetic eclogite compositions has been measured in multi-anvil experiments performed at pressures between 3 and 7 GPa and temperatures between 800 and 1,300 °C using iron-iridium and iron-platinum alloys as sliding redox sensors. The determined oxygen fugacities buffered by the coexistence of elemental carbon and carbonatebearing melt are approximately 1 log unit below thermodynamic calculations for a similar redox buffering equilibrium involving only solid phases. The measured oxygen fugacities normalized to the fayalite-magnetite-quartz oxygen buffer decrease with temperature from ~−0.8 to ~−1.7 log units at 3 GPa, most likely as a result of increasing dilution of the carbonate liquid with silicate. The normalized fO 2 values also decrease with pressure and show a similar decrease with temperature at 6 GPa from ~−1.5 log units at 1,100 °C to ~−2.4 log units at 1,300 °C. In contrast to previous arguments, the stability field of the carbonatebearing melt extends to lower oxygen fugacity in eclogite rocks than in peridotite rocks, which implies a wider range of conditions over which carbon remains mobile in natural Communicated by Chris Ballhaus. Electronic supplementary materialThe online version of this article (

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“…Gebauer and Grunenfelder (1979) obtained 380 Ma on zircons from the eclogites (but see the comments in Ducrot et al 1983! ), and Stosch and Lugmair (1987) have reported Sm/Nd and Rb/Sr mineral isochrons of approx. 395 Ma.…”

Section: Devonian and In A Normalmentioning

confidence: 97%

The Geological Framework of the KTB Drill Site, Oberpfalz

Franke

1989

Exploration of the Deep Continental Crust

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Geochemistry and evolution of MORB-type eclogites from the Münchberg Massif, southern Germany

Cited by 130 publications

References 32 publications

Reaction textures and fluid behaviour in very high pressure calc‐silicate rocks of the Münchberg gneiss complex, Bavaria, Germany

Reaction textures and fluid behaviour in very high pressure calc‐silicate rocks of the Münchberg gneiss complex, Bavaria, Germany

The oxygen fugacity at which graphite or diamond forms from carbonate-bearing melts in eclogitic rocks

The Geological Framework of the KTB Drill Site, Oberpfalz

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