Raman-based geobarometry of ultrahigh-pressure metamorphic rocks: applications, problems, and perspectives

Raman-based geobarometry has recently become increasingly popular because it is an elegant way to obtain information on peak metamorphic conditions or the entire pressure-temperature-time (P-T-t) path of metamorphic rocks, especially those formed under ultrahigh-pressure (UHP) conditions. However, several problems need to be solved to get reliable estimates of metamorphic conditions. In this paper we present some examples of difficulties which can arise during the Raman spectroscopy study of solid inclusions from ultrahigh-pressure metamorphic rocks

Silicate and carbonate melt inclusions associated with diamonds in deeply subducted carbonate rocks

Deeply subducted carbonate rocks from the Kokchetav massif (Northern Kazakhstan) recrystallised within the diamond stability field (P = 4.5-6.0 GPa; T ≈ 1000°C) and preserve evidence for ultra high-pressure carbonate and silicate melts. The carbonate

The age of metamorphism of diamondiferous rocks determined with SHRIMP dating of zircon

In situ dating of zircon domains with a Sensitive High Resolution Ion Microprobe (SHRIMP) has been proved to be the best way to determine the age of peak metamorphic conditions in the diamondiferous, deeply subducted Kokchetav rocks. Three independent studies obtained within error-consistent results, providing evidence for peak metamorphic conditions at about 530 Ma. These studies demonstrated through mineral inclusions that the dated zircon crystals formed under peak metamorphic conditions, within the diamond stability field. Mineral inclusions indicate that zircon formed not only at the metamorphic peak but also during the retrograde evolution at granulite- to amphibolite-facies conditions. The ages of the retrograde zircon domains are within error not distinguishable from the age of peak metamorphism. This provides evidence for very fast exhumation rates of minimum 1.8 cm/yr from diamond to granulite-facies metamorphism and high cooling rates of > 40 °C/Ma

Applications of Raman Spectroscopy in Metamorphic Petrology and Tectonics

Raman spectroscopy is widely applied in metamorphic petrology and offers many opportunities for geological and tectonic research. Minimal sample preparation preserves sample integrity and microtextural information, while use with confocal microscopes allows spatial resolution down to the micrometer level. Raman spectroscopy clearly distinguishes mineral polymorphs, providing crucial constraints on metamorphic conditions, particularly ultrahigh-pressure conditions. Raman spectroscopy can also be used to monitor the structure of carbonaceous material in metamorphic rocks. Changes in structure are temperature-sensitive, so Raman spectroscopy of carbonaceous material is widely used for thermometry. Raman spectroscopy can also detect and quantify strain in micro-inclusions, offering new barometers that can be applied to understand metamorphic and tectonic processes without any assumptions about chemical equilibrium

Multiple zircon growth during fast exhumation of diamondiferous, deeply subducted continental crust (Kokchetav Massif, Kazakhstan)

Diamondiferous rocks from the Kokchetav Massif. Kazakhstan, represent deeply subducted continental crust. In order to constrain the age of ultra high pressure (UHP) metamorphism and subsequent retrogression during exhumation, zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study. Four different zircon domains were distinguished on the basis of transmitted light microscopy, cathodo-luminescence, trace element contents and mineral inclusions. Mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions. Domain 1 consists of rounded cores and lacks evidence of UHP metamorphism. Domain 2 contains diamond, coesite, omphacite and titanian phengite inclusions providing evidence that it formed at UHP metamorphic conditions (P > 43 kbar; T ~ 950 °C). Domain 3 is characterised by low-pressure mineral inclusions such as garnet, biotite and plagioclase, which are common minerals in the granulite-facies overprint of the gneisses (P ~ 10 kbar; T ~ 800 °C). This multi-stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from Zr-saturated partial melts present in the gneisses. Domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet, most probably at amphibolite-facies conditions (P ~ 5 kbar; T ~ 600 °C). The metamorphic zircon domains were dated by SHRIMP ion microprobe and yielded ages of 527 ± 5, 528 ± 8 and 526 ± 5 Ma for domains 2, 3 and 4 respectively. These indistinguishable ages provide evidence for a fast exhumation beyond the resolution of SHRIMP dating. The mean age of all zircons formed between UHP metamorphic conditions and granulite-facies metamorphism is 528 ± 3 Ma, indicating that decompression took place in less than 6 Ma. Hence, the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1.8 cm/year. We propose a two-stage exhumation model to explain the obtained P-T-t path. Fast exhumation on top of the subducted slab from depth > 140 to ~ 35 km was driven by buoyancy and facilitated by the presence of partial melts. A period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision

Aqueous fluids and hydrous melts in high-pressure and\ud ultra-high pressure rocks: implications for element transfer in subduction zones

High-pressure (HP) and ultra-high pressure (UHP) terranes are excellent natural laboratories to study subduction-zone processes. In this paper we give a brief theoretical background and we review experimental data and observations in natural rocks that constrain the nature and composition of the fluid phase present in HP and UHP rocks. We argue that a fluid buffered by a solid residue is compositionally well defined and is either an aqueous fluid (total amount of dissolved solids<30 wt.%) or a hydrous melt (H2O<35 wt.%). There is only a small temperature range of approximately 50–100 °C, where transitional solute-rich fluids exist. A review of available experimental data suggest that in felsic rocks the second critical endpoint is situated at 25–35 kbar and ∼700 °C and hence must be considered in the study of UHP rocks. Despite this, the nature of the fluid phase can be constrained by relating the peak metamorphic conditions of rocks to the position of the wet solidus even if the peak pressure exceeds the pressure where the wet solidus terminates at the second critical endpoint. Transitional solute-rich fluids are expected in UHP terrains (P>30 kbar) with peak temperatures of about 700±50 °C. At higher temperatures, hydrous\ud granitic melts occur whereas at lower temperatures aqueous fluids coexists with eclogite-facies minerals. This argument is complemented by evidence on the nature of the fluid phase from high-pressure terrains. We show that in the diamond-bearing, high-temperature UHP rocks from the Kokchetav Massif there are not only hydrous felsic melts, but probably also carbonate and sulfide melts present.\ud Hydrous quartzo-feldspathic melts are mainly produced in high temperature UHP rocks and their composition is relatively well constrained from experiments and natural rocks. In contrast, constraining the composition of aqueous fluids is more problematic. The combined evidence from experiments and natural rocks indicates that aqueous fluids liberated at the blueschist to eclogite facies transition are dilute. They contain only moderate amounts of LILE, Sr and Pb and do not transport significant amounts of key\ud trace elements such as LREE, U and Th. This indicates that there is a decoupling of water and trace element release in subducted oceanic crust and that aqueous fluids are unable to enrich the mantle wedge significantly. Instead we propose that fluid-present melting in the sediments on top of the slab is required to transfer significant amounts of trace elements from the slab to the mantle wedge. For such a process to be efficient, top slab temperature must be at least 700–750 °C at sub-arc depth. Slab melting is likely\ud to be triggered by fluids that derive from dehydration of mafic and ultramafic rocks in colder (deeper) portions of the slab

Aqueous fluids and hydrous melts in high-pressure and ultra-high pressure rocks: Implications for element transfer in subduction zones

High-pressure (HP) and ultra-high pressure (UHP) terranes are excellent natural laboratories to study subduction-zone processes. In this paper we give a brief theoretical background and we review experimental data and observations in natural rocks that constrain the nature and composition of the fluid phase present in HP and UHP rocks. We argue that a fluid buffered by a solid residue is compositionally well defined and is either an aqueous fluid (total amount of dissolved solids 30 kbar) with peak temperatures of about 700 ± 50 °C. At higher temperatures, hydrous granitic melts occur whereas at lower temperatures aqueous fluids coexists with eclogite-facies minerals. This argument is complemented by evidence on the nature of the fluid phase from high-pressure terrains. We show that in the diamond-bearing, high-temperature UHP rocks from the Kokchetav Massif there are not only hydrous felsic melts, but probably also carbonate and sulfide melts present. Hydrous quartzo-feldspathic melts are mainly produced in high temperature UHP rocks and their composition is relatively well constrained from experiments and natural rocks. In contrast, constraining the composition of aqueous fluids is more problematic. The combined evidence from experiments and natural rocks indicates that aqueous fluids liberated at the blueschist to eclogite facies transition are dilute. They contain only moderate amounts of LILE, Sr and Pb and do not transport significant amounts of key trace elements such as LREE, U and Th. This indicates that there is a decoupling of water and trace element release in subducted oceanic crust and that aqueous fluids are unable to enrich the mantle wedge significantly. Instead we propose that fluid-present melting in the sediments on top of the slab is required to transfer significant amounts of trace elements from the slab to the mantle wedge. For such a process to be efficient, top slab temperature must be at least 700-750 °C at sub-arc depth. Slab melting is likely to be triggered by fluids that derive from dehydration of mafic and ultramafic rocks in colder (deeper) portions of the slab