Journal articles by Ralf Halama
Journal of Petrology, 2020
Magma-carbonate interaction is an increasingly recognized process occurring at active volcanoes w... more Magma-carbonate interaction is an increasingly recognized process occurring at active volcanoes worldwide, with implications for the magmatic evolution of the host volcanic systems, their eruptive behaviour, volcanic CO2 budgets, and economic mineralization. Abundant calc-silicate skarn xenoliths are found at Merapi volcano, Indonesia. We identify two distinct xenolith types: magmatic skarn xenoliths, which contain evidence of formation within the magma; and exoskarn xenoliths, which more likely represent fragments of crystalline metamorphosed wall rocks. The magmatic skarn xenoliths comprise distinct compositional and mineralogical zones with abundant Ca-enriched glass (up to 10 wt % relative to lava groundmass), mineralogically dominated by clinopyroxene (En 15-43 Fs 14-36 Wo 41-51) + plagioclase (An 37-100) +/- magnetite in the outer zones towards the lava contact, and by wollastonite +/- clinopyroxene (En 17-38 Fs 8-34 Wo 49-59) +/- plagioclase (An 46-100) +/- garnet (Grs 0-65 Adr 24-75 Sch 0-76) +/- quartz in the xenolith cores. These zones are controlled by Ca transfer from the limestone protolith to the magma and by the transfer of magma-derived elements in the opposite direction. In contrast, the exoskarn xenoliths are unzoned and essentially glassfree, representing equilibration at sub-solidus conditions. The major mineral assemblage in the exoskarn xenoliths is wollastonite þ garnet (Grs 73-97 Adr 3-24) + Ca-Al-rich clinopyroxene (CaTs 0-38) + anorthite +/- quartz, with variable amounts of either quartz or melilite (Geh 42-91) + spinel. Thermobarometric calculations, fluid-inclusion microthermometry and newly calibrated oxybarometry based on Fe3+ /TotalFe in clinopyroxene indicate magmatic skarn xenolith formation conditions of 850 +/- 45 C, < 100 MPa and at an oxygen fugacity between the NNO (nickel-nickel oxide) and HM (hematite-magnetite) buffer. The exoskarn xenoliths, in turn, formed at 510-910 C under oxygen-fugacity conditions between NNO and air. These high oxygen fugacities are likely imposed by the large volumes of CO2 liberated from the carbonate. Halogen-and sulphur-rich mineral phases in the xenoliths testify to infiltration by a magmatic brine. In some xenoliths, this is associated with the precipitation of copper-bearing mineral phases by sulphur dissociation into sulphide and sulphate, indicating potential mineralization in the skarn system below Merapi. The compositions of many xenolith clinopyroxene and plagioclase crystals overlap with that of magmatic minerals, suggesting that the crystal cargo in Merapi magmas may contain a larger proportion of skarn-derived xenocrysts than previously recognized. Assessment of xenolith formation timescales demonstrates that magma-carbonate interaction and associated CO2 release could affect eruption intensity, as recently suggested for Merapi and similar carbonate-hosted volcanoes elsewhere.
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Geology Today, 2024
The beauty of an eclogite is something to behold: any petrologist marvels in the combination of r... more The beauty of an eclogite is something to behold: any petrologist marvels in the combination of red garnet and green omphacite that are the main mineral constituents of the rock. But besides their stunning appearance, there is much more to eclogites: fundamental concepts in metamorphic petrology and geodynamics were developed based on scientific investigations of eclogites. it is well established that they derive from precursor rocks of basaltic composition and form under high-pressure conditions at more than c. 45 km depth, but other aspects of their occurrences and geological significance remain debated. The relative scarcity of eclogites among crustal rocks renders them largely unknown to the layperson, so following the 200th anniversary of the term eclogite in 2022, there is an opportunity to take a closer look at this fascinating rock.
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Space Science Reviews, 2021
Understanding the Earth's geological nitrogen (N) and carbon (C) cycles is fundamental for assess... more Understanding the Earth's geological nitrogen (N) and carbon (C) cycles is fundamental for assessing the distribution of these volatiles between solid Earth (core, mantle and crust), oceans and atmosphere. This Special Communication about the Earth's N and C cycles contains material that is relevant for researchers who are interested in the Topical Collection on planetary evolution "Reading Terrestrial Planet Evolution in Isotopes and Element Measurements". Variations in the fluxes of N and C between these major reservoirs through geological time influenced the evolution and determined the unique composition of the Earth's atmosphere. Here we review several key geological aspects of the N and C cycles of which our understanding has significantly advanced during the last decade through field-based, experimental and theoretical studies. Subduction zones are the most important pathway of both N and C from the Earth's surface into the deep Earth. A key question in the flux quantification is how much of the volatile elements is stored in the downgoing slab and introduced into the mantle and how much is returned back to the surface and the atmosphere through arc magmatism. For N, the retention of N as NH+4 in minerals has a major influence on fluxes between reservoirs. The temperature-dependent stability of NH+4-bearing minerals determines whether N is predominantly retained in the slab to mantle depths (in subduction zones with a low geothermal gradient) or devolatilized (in subduction zones with a high geothermal gradient). Several lines of evidence suggest that the mantle is regassing with respect to N due to a net influx of subducted N over time, but this issue is highly debated and evidence to the contrary also exists. Nevertheless, there is consensus that the majority of the planetary N budget is stored in the Earth's mantle, with the continental crust also constituting a significant N reservoir. For C, release from the subducting slab occurs through decarbonation reactions, dissolution and formation of carbonatitic liquids, but reprecipitation of C in the slab or the forearc mantle wedge may limit the effectiveness of direct return of C into the atmosphere. Carbon release through regional metamorphism in collision zone orogens also has potentially profound effects on C release into the atmosphere and consensus has emerged that such orogens are sources rather than sinks of atmospheric CO2. On shorter timescales, contact metamorphism through interaction of mantle-derived magmas with Cbearing country rocks, and the resulting release of large quantities of CH4 and/or CO2 , has been linked to global warming events.
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Lithos, 2015
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Contributions to Mineralogy and Petrology, 2020
This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pres... more This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subduction-related metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluid–rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 μg/g B) occur in peak metamorphic phengite from a garnet–phengite quartzite. The B isotopic composition is variable (δ11B = − 10.3 to − 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fail...
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Space Science Reviews, 2021
Understanding the Earth’s geological nitrogen (N) and carbon (C) cycles is fundamental for assess... more Understanding the Earth’s geological nitrogen (N) and carbon (C) cycles is fundamental for assessing the distribution of these volatiles between solid Earth (core, mantle and crust), oceans and atmosphere. This Special Communication about the Earth’s N and C cycles contains material that is relevant for researchers who are interested in the Topical Collection on planetary evolution “Reading Terrestrial Planet Evolution in Isotopes and Element Measurements”. Variations in the fluxes of N and C between these major reservoirs through geological time influenced the evolution and determined the unique composition of the Earth’s atmosphere. Here we review several key geological aspects of the N and C cycles of which our understanding has significantly advanced during the last decade through field-based, experimental and theoretical studies. Subduction zones are the most important pathway of both N and C from the Earth’s surface into the deep Earth. A key question in the flux quantificatio...
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Scientific Reports, 2019
Interaction between magma and crustal carbonate at active arc volcanoes has recently been propose... more Interaction between magma and crustal carbonate at active arc volcanoes has recently been proposed as a source of atmospheric CO2, in addition to CO2 released from the mantle and subducted oceanic crust. However, quantitative constraints on efficiency and timing of these processes are poorly established. Here, we present the first in situ carbon and oxygen isotope data of texturally distinct calcite in calc-silicate xenoliths from arc volcanics in a case study from Merapi volcano (Indonesia). Textures and C-O isotopic data provide unique evidence for decarbonation, magma-fluid interaction, and the generation of carbonate melts. We report extremely light d13CPDB values down to −29.3‰, which are among the lowest reported in magmatic systems so far. Combined with the general paucity of relict calcite, these extremely low values demonstrate highly efficient remobilisation of crustal CO2 over geologically short timescales of thousands of years or less. This rapid release of large volumes of crustal
CO2 may impact global carbon cycling.
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Lithos, 2018
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Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reac... more Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth's internal dynamics. Element exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Here we show in high-pressure/low temperature rocks that element transfer during mineral dissolution and reprecipitation can occur in an alkali-Al–Si-rich amorphous material that forms directly by depolymerization of the crystal lattice and is thermodynamically decoupled from aqueous solutions. Depolymerization starts along grain boundaries and crystal lattice defects that serve as element exchange pathways and are sites of porosity formation. The resulting amorphous material occupies large volumes in an interconnected porosity network. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface. This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth.
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Lithos, 2014
ABSTRACT Quantitative geochemical modeling is today applied in a variety of geological environmen... more ABSTRACT Quantitative geochemical modeling is today applied in a variety of geological environments from the petrogenesis of igneous rocks to radioactive waste disposal. In addition, the development of thermodynamic databases and computer programs to calculate equilibrium phase diagrams has greatly advanced our ability to model geodynamic processes. Combined with experimental data on elemental partitioning and isotopic fractionation, thermodynamic forward modeling unfolds enormous capacities that are far from exhausted.
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International Journal of Earth Sciences
Sonderforschungsbereiche (SFBs) are a successful funding model in use by the German Science Found... more Sonderforschungsbereiche (SFBs) are a successful funding model in use by the German Science Foundation (DFG) for over 30 years to strengthen basic research first locally at universities and later also supra-regionally by including academic institutions at different cities and states. Literally translated, SFB means “special research area” that comprises research that complements but does not duplicate research at participating institutions and departments. The English terminology used by the DFG is “Collaborative Research Centre,” which better describes the expected approach by emphasizing collaboration and interdisciplinary efforts in such a way that the overall result is better than the sum of individual results. The SFB 574 had united more than 70 scientists with expertise in structural geology, geophysics, sedimentology, geochemistry, empirical and experimental petrology, volcanology, and biology for 11 years (2001–2012). The overarching theme addressed the role of volatiles in ...
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Nature Geoscience, 2012
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European Journal of Mineralogy, 2000
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Earth and Planetary Science Letters, 2007
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Chemical Geology, 2004
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Chemical Geology, 2012
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Earth and Planetary Science Letters, 2008
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Contributions to Mineralogy and Petrology, 2009
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In order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic
sy... more In order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic
systematics in high-pressure metamorphic rocks, we investigated three different profiles representing
three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan
(ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced
blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations
from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an
eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show
only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent
with a batch devolatilization process. A second profile from the Tianshan represents a retrograde
eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the
eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to
+5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern
Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed
eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses
have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations
become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for
the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N
isotopic compositions (δ15N = + 2.1 to +3.6‰).
Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N
in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances
and isotopic composition of N are controlled by the stability and presence of white mica. Phengite
breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid.
Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace
the extent of N transport during metasomatic processes. The Vendée profile demonstrates that
this process occurs over several tens of metres and affects both N concentrations and N isotopic
compositions.
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Journal articles by Ralf Halama
CO2 may impact global carbon cycling.
systematics in high-pressure metamorphic rocks, we investigated three different profiles representing
three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan
(ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced
blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations
from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an
eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show
only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent
with a batch devolatilization process. A second profile from the Tianshan represents a retrograde
eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the
eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to
+5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern
Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed
eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses
have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations
become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for
the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N
isotopic compositions (δ15N = + 2.1 to +3.6‰).
Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N
in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances
and isotopic composition of N are controlled by the stability and presence of white mica. Phengite
breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid.
Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace
the extent of N transport during metasomatic processes. The Vendée profile demonstrates that
this process occurs over several tens of metres and affects both N concentrations and N isotopic
compositions.
CO2 may impact global carbon cycling.
systematics in high-pressure metamorphic rocks, we investigated three different profiles representing
three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan
(ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced
blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations
from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an
eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show
only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent
with a batch devolatilization process. A second profile from the Tianshan represents a retrograde
eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the
eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to
+5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern
Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed
eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses
have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations
become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for
the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N
isotopic compositions (δ15N = + 2.1 to +3.6‰).
Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N
in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances
and isotopic composition of N are controlled by the stability and presence of white mica. Phengite
breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid.
Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace
the extent of N transport during metasomatic processes. The Vendée profile demonstrates that
this process occurs over several tens of metres and affects both N concentrations and N isotopic
compositions.