Hydrogen in rutile and the onset of cold subduction
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Abstract
The evolution of plate tectonics and subduction conditions on Earth is widely debated in geosciences. Modern-style cold subduction is characterised by the formation of low-temperature – high-pressure (low-T–high-P) and ultra-high-pressure (UHP) metamorphic rocks from subducted oceanic crust. However, such low-T eclogites and UHP metamorphic rocks are largely absent in the Precambrian geological record. This requires the use of other tools to trace the evolution of subduction conditions throughout Earth’s history. One way to assess pressure-temperature conditions of Precambrian metamorphic rocks is to investigate their sedimentary equivalents. This however requires single grain pressure (P), temperature (T), age (t) and source lithology (X) estimates (P-T-t-X).
A potential candidate to trace metamorphic conditions in the sedimentary record is rutile. Rutile is one of the most common accessory minerals in high grade metamorphic rocks and is very stable during sedimentation processes. Additionally, rutile trace element composition can be used as petrogenetic indicator for temperature (Zr-in-rutile thermometry), age (U-Pb dating), and source lithology (discrimination diagrams based on e.g., Cr, Nb, W, Sn, Sb). However, a pressure proxy based on rutile geochemistry is missing to use detrital rutile as single grain P-T-t-X indicator.
In this thesis, I evaluate the use of hydrogen in rutile as such a potential pressure proxy. I develop a novel protocol for in-situ analysis of H+ in rutile through Fourier Transform Infrared (FTIR) spectroscopy. This protocol allows for the precise and accurate quantification of H2O contents in rutile. Moreover, deconvolution of measured FTIR spectra enables the identification of different H+ defects, as hydrogen incorporation into rutile is linked to Ti3+, Fe3+, Al3+, Fe2+, Mg2+ and Cr2+. Combined with trace element data, this can be used to evaluate charge balance in rutile.
I applied this measurement protocol to a sample set of metamorphic rutile from a variety of P-T conditions and bulk rock compositions across the rutile stability field. The obtained H2O contents are highly variable. Most granulite facies rutile are nominally dry; high-P granulite facies rutile has H2O contents of ∼300–400 μg/g. Amphibolite- and high-T eclogite facies rutile have ∼200–450 μg/g H2O. Rutile from UHP conditions show variable H2O contents up to ∼700 μg/g in mafic rutile. The highest H2O contents can be seen in low-T eclogite facies rutile at >450–1000 μg/g. Comparing rutile formed at similar temperatures but different pressures (e.g., amphibolite facies rutile and low-T eclogite facies rutile, granulite facies rutile and high-P granulite facies rutile), rutile formed at relatively higher pressure shows significantly higher H2O contents. Thus, H+ incorporation into rutile is at least partially pressure dependent, making H2O in rutile a viable pressure proxy. Combining H2O in rutile with Zr in rutile as well established temperature indicator, rutile from different thermal gradients can be differentiated. However, magmatic and hydrothermal rutile has similar H2O-Zr signatures as cold subduction related rutile. Thus, lithology needs to be distinguished to identify cold subduction in detrital rutile. Using H2O/Zr ratios together with total Nb+W+Sn contents, mafic cold subduction related rutile can be clearly identified at high H2O/Zr ratios and low Nb+W+Sn contents.
As hydrogen diffusion in rutile has been shown to be very fast, evaluating hydrogen retention is vital to validate the use of H2O in rutile to identify cold subduction conditions. FTIR mapping shows distinct zoning patterns of H2O in low-T rutile. This reveals H+-retention up to temperatures of ∼650 °C, due to linked substitution with di- and trivalent cations. As charge balance needs to be preserved, diffusion of hydrogen in natural rutile is limited by the diffusion of other cations involved in the charge balance. Thus, H2O contents are preserved at low-T eclogite facies conditions, while at higher temperatures diffusive re-equilibration occurs.
The intra-grain variability of trace elements in rutile has been evaluated through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) mapping, to evaluate possible implications for the use of rutile as petrogenetic indicator mineral. Most rutile shows distinct trace element zoning, with no consistent patterns. Zoning of Zr in rutile reveals an important influence of Zr-undersaturation on apparent Zr-in-rutile temperatures calculated for natural rutile. Niobium and Ta zonation leading to highly variable Nb/Ta ratios might shed light on fractionation processes.
Hydration experiments of synthetic pure and Cr-doped rutile confirm a pressure dependence of H+ incorporation into rutile. Additionally, a strong influence of oxygen fugacity and rutile composition can be seen. The presence of trivalent defects in rutile promotes the incorporation of H+ into rutile. Comparing experimentally determined H+ saturation with H2O contents of natural rutile shows that H+ related to the Ti3+ defect is retained in natural low-T eclogite facies rutile. For H+ coupled with di- and trivalent trace elements (Fe3+, Al3+, Mg2+, Fe2+, Cr2+) H+ retention is explained by the necessity to maintain charge balance, and consequently couled diffusion of hydrogen and other, slow diffusing cations. Contrary, Ti3+ could be oxidised to Ti4+, removing the necessity for coupled diffusion. This should allow H+ linked to Ti3+ to diffusive more easily. Thus Ti3+-related H+ retention in natural rutile cannot be fully explained.
The retention of the observed modern-style cold subduction signature during sedimentation processed is evaluated using modern fluvial detrital rutile from Alpine rivers with significant proportion of low-T eclogites within their respective catchment areas. Detrital rutile from these rivers shows dominantly a cold subduction signature, indicating the preservation of the modern-style cold subduction signature during sedimentary transport and deposition.
I investigated rutile in an arkose from the Hebridean Terrane, NW Scotland, to evaluate a potential cold subduction signature in Paleoproterozoic rocks. The detrial rutile ages range between 1.1–1.9 Ga. Generally, high Zr-in-rutile temperatures can be observed and many rutile grains have trace element signatures indicative of felsic, magmatic origin. Only few rutile grains have a cold subduction signature, indicating active local cold subduction in the Paleoproterozoic.
This thesis establishes H2O in rutile as a viable pressure proxy that is retained up to temperatures of ∼650 °C. This allows for rutile to be used as single grain P-T-t-X indicator. Hence, detrital rutile can be used to trace modern-style cold subduction conditions in the sedimentary record and help in understanding the evolution of subduction on Earth.
A potential candidate to trace metamorphic conditions in the sedimentary record is rutile. Rutile is one of the most common accessory minerals in high grade metamorphic rocks and is very stable during sedimentation processes. Additionally, rutile trace element composition can be used as petrogenetic indicator for temperature (Zr-in-rutile thermometry), age (U-Pb dating), and source lithology (discrimination diagrams based on e.g., Cr, Nb, W, Sn, Sb). However, a pressure proxy based on rutile geochemistry is missing to use detrital rutile as single grain P-T-t-X indicator.
In this thesis, I evaluate the use of hydrogen in rutile as such a potential pressure proxy. I develop a novel protocol for in-situ analysis of H+ in rutile through Fourier Transform Infrared (FTIR) spectroscopy. This protocol allows for the precise and accurate quantification of H2O contents in rutile. Moreover, deconvolution of measured FTIR spectra enables the identification of different H+ defects, as hydrogen incorporation into rutile is linked to Ti3+, Fe3+, Al3+, Fe2+, Mg2+ and Cr2+. Combined with trace element data, this can be used to evaluate charge balance in rutile.
I applied this measurement protocol to a sample set of metamorphic rutile from a variety of P-T conditions and bulk rock compositions across the rutile stability field. The obtained H2O contents are highly variable. Most granulite facies rutile are nominally dry; high-P granulite facies rutile has H2O contents of ∼300–400 μg/g. Amphibolite- and high-T eclogite facies rutile have ∼200–450 μg/g H2O. Rutile from UHP conditions show variable H2O contents up to ∼700 μg/g in mafic rutile. The highest H2O contents can be seen in low-T eclogite facies rutile at >450–1000 μg/g. Comparing rutile formed at similar temperatures but different pressures (e.g., amphibolite facies rutile and low-T eclogite facies rutile, granulite facies rutile and high-P granulite facies rutile), rutile formed at relatively higher pressure shows significantly higher H2O contents. Thus, H+ incorporation into rutile is at least partially pressure dependent, making H2O in rutile a viable pressure proxy. Combining H2O in rutile with Zr in rutile as well established temperature indicator, rutile from different thermal gradients can be differentiated. However, magmatic and hydrothermal rutile has similar H2O-Zr signatures as cold subduction related rutile. Thus, lithology needs to be distinguished to identify cold subduction in detrital rutile. Using H2O/Zr ratios together with total Nb+W+Sn contents, mafic cold subduction related rutile can be clearly identified at high H2O/Zr ratios and low Nb+W+Sn contents.
As hydrogen diffusion in rutile has been shown to be very fast, evaluating hydrogen retention is vital to validate the use of H2O in rutile to identify cold subduction conditions. FTIR mapping shows distinct zoning patterns of H2O in low-T rutile. This reveals H+-retention up to temperatures of ∼650 °C, due to linked substitution with di- and trivalent cations. As charge balance needs to be preserved, diffusion of hydrogen in natural rutile is limited by the diffusion of other cations involved in the charge balance. Thus, H2O contents are preserved at low-T eclogite facies conditions, while at higher temperatures diffusive re-equilibration occurs.
The intra-grain variability of trace elements in rutile has been evaluated through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) mapping, to evaluate possible implications for the use of rutile as petrogenetic indicator mineral. Most rutile shows distinct trace element zoning, with no consistent patterns. Zoning of Zr in rutile reveals an important influence of Zr-undersaturation on apparent Zr-in-rutile temperatures calculated for natural rutile. Niobium and Ta zonation leading to highly variable Nb/Ta ratios might shed light on fractionation processes.
Hydration experiments of synthetic pure and Cr-doped rutile confirm a pressure dependence of H+ incorporation into rutile. Additionally, a strong influence of oxygen fugacity and rutile composition can be seen. The presence of trivalent defects in rutile promotes the incorporation of H+ into rutile. Comparing experimentally determined H+ saturation with H2O contents of natural rutile shows that H+ related to the Ti3+ defect is retained in natural low-T eclogite facies rutile. For H+ coupled with di- and trivalent trace elements (Fe3+, Al3+, Mg2+, Fe2+, Cr2+) H+ retention is explained by the necessity to maintain charge balance, and consequently couled diffusion of hydrogen and other, slow diffusing cations. Contrary, Ti3+ could be oxidised to Ti4+, removing the necessity for coupled diffusion. This should allow H+ linked to Ti3+ to diffusive more easily. Thus Ti3+-related H+ retention in natural rutile cannot be fully explained.
The retention of the observed modern-style cold subduction signature during sedimentation processed is evaluated using modern fluvial detrital rutile from Alpine rivers with significant proportion of low-T eclogites within their respective catchment areas. Detrital rutile from these rivers shows dominantly a cold subduction signature, indicating the preservation of the modern-style cold subduction signature during sedimentary transport and deposition.
I investigated rutile in an arkose from the Hebridean Terrane, NW Scotland, to evaluate a potential cold subduction signature in Paleoproterozoic rocks. The detrial rutile ages range between 1.1–1.9 Ga. Generally, high Zr-in-rutile temperatures can be observed and many rutile grains have trace element signatures indicative of felsic, magmatic origin. Only few rutile grains have a cold subduction signature, indicating active local cold subduction in the Paleoproterozoic.
This thesis establishes H2O in rutile as a viable pressure proxy that is retained up to temperatures of ∼650 °C. This allows for rutile to be used as single grain P-T-t-X indicator. Hence, detrital rutile can be used to trace modern-style cold subduction conditions in the sedimentary record and help in understanding the evolution of subduction on Earth.
Year of graduation
2025
Theses Type
dissertation
Subject(s)
Language(s)
en
Author(s)
Faculty/Graduate School
Institute
Access(Rights)
open.access
Primary OA Publication
true