The hydrothermal alteration of mantle rocks (referred to as serpentinization) occurs in submarine environments extending from mid-ocean ridges to subduction zones. Serpentinization affects the ...physical and chemical properties of oceanic lithosphere, represents one of the major mechanisms driving mass exchange between the mantle and the Earth's surface, and is central to current origin of life hypotheses as well as the search for microbial life on the icy moons of Jupiter and Saturn. In spite of increasing interest in the serpentinization process by researchers in diverse fields, the rates of serpentinization and the controlling factors are poorly understood. Here we use a novel in situ experimental method involving olivine micro-reactors and show that the rate of serpentinization is strongly controlled by the salinity (water activity) of the reacting fluid and demonstrate that the rate of serpentinization of olivine slows down as salinity increases and H
O activity decreases.
Melt inclusions (MI) are considered the best tool available for determining the pre-eruptive volatile contents of magmas. H2O and CO2 concentrations of the glass phase in MI are commonly used both as ...a barometer and to track magma degassing behavior during ascent due to the strong pressure dependence of H2O and CO2 solubilities in silicate melts. The often unstated and sometimes overlooked requirement for this method to be valid is that the glass phase in the MI must represent the composition of the melt that was trapped at depth in the volcanic plumbing system. However, melt inclusions commonly contain a vapor bubble that formed after trapping owing to differential shrinkage of the melt compared to the host crystal, and/or crystallization at the inclusion-host interface. Such bubbles may contain a substantial portion of volatiles, such as CO2, that were originally dissolved in the melt. In this study, we determined the contribution of CO2 in the vapor bubble to the overall CO2 content of MI based on quantitative Raman analysis of the vapor bubbles in MI from the 1959 Kilauea Iki (Hawaii), 1960 Kapoho (Hawaii), 1974 Fuego volcano (Guatemala), and 1977 Seguam Island (Alaska) eruptions. We found that the bubbles typically contain 40 to 90% of the total CO2 in the MI. Reconstructing the original CO2 content by adding the CO2 in the bubble back into the melt results in an increase in CO2 concentration by as much as an order of magnitude (thousands of parts per million). Reconstructed CO2 concentrations correspond to trapping pressures that are significantly greater than one would predict based on analysis of the volatiles in the glass alone. Trapping depths can be as much as 10 km deeper than estimates that ignore the CO2 in the bubble. In addition to CO2 in the vapor bubbles, many MI showed the presence of a carbonate mineral phase. Failure to recognize the carbonate during petrographic examination or analysis of the glass and to include its contained CO2 when reconstructing the CO2 content of the originally trapped melt will introduce additional errors into the calculated volatile budget. Our results emphasize that accurate determination of the pre-eruptive volatile content of melts based on analysis of melt inclusions must consider the volatiles contained in the bubble (and carbonates, if present). This can be accomplished either by analysis of the bubble and the glass followed by mass-balance reconstruction of the original volatile content of the melt, or by re-homogenization of the MI prior to conducting microanalysis of the quenched, glassy MI.
Melt inclusions (MI) represent the best source of information concerning the pre-eruptive volatile contents of magmas. If the trapped melt is enriched in volatile species, following trapping the MI ...may generate a vapor bubble containing volatiles that have exsolved from the melt. Thermodynamic modeling of vapor-saturated albitic composition (NaAlSi3O8) MI shows that the CO2 content of the melt phase in the MI is sensitive to small amounts of post-entrapment crystallization (PEC), whereas the H2O content of the melt is less sensitive to PEC. During PEC, CO2 is transferred from the melt to the vapor phase and the vapor bubble may contain a significant amount, if not most, of the CO2 in the MI. The contrasting behaviors of H2O and CO2 during PEC lead to H2O-CO2 trends that are similar to those predicted for open-system degassing during magma ascent and decompression. Thus, similar H2O-CO2 trends may be produced if (1) vapor-saturated MI are trapped at various depths along a magmatic ascent path, or (2) MI having the same volatile content are all trapped at the same depth, but undergo different amounts of PEC following trapping. It is not possible to distinguish between these two contrasting interpretations based on MI volatile data alone. However, by examining the volatile trends within the context of other geochemical monitors of crystallization or magma evolution progress, it may be possible to determine whether the volatile trends were generated along a degassing path or if they reflect various amounts of PEC in an originally homogeneous melt inclusion assemblage. The volatile trends resulting from PEC of MI described in this study are directly applicable to silica-rich (granitic) MI trapped in non-ferromagnesian host phases, and are only qualitatively applicable to more mafic melt compositions and/or host phases owing to modifications resulting from Fe exchange with the host and to post-entrapment re-equilibration processes.
Fluid inclusions approximated by the system H₂O–CO₂–NaCl are common in many geologic environments. In order to apply microthermometric data from these inclusions to infer P–T (pressure–temperature) ...trapping conditions, the composition of the inclusions, including the salinity, must be known. Normally, salinities of aqueous inclusions are determined from ice-melting temperatures obtained during microthermometry. However, when CO₂-bearing aqueous fluid inclusions are cooled they often form a hydrate that incorporates H₂O into the structure, and salinities estimated from ice-melting temperatures are therefore higher than the actual salinity. A technique that combines data from Raman spectroscopic and microthermometric analyses of individual inclusions was developed to determine the salinity of CO₂-bearing aqueous inclusions based on measured clathrate melting temperatures and CO₂ pressures obtained from Raman analyses. In this study, the pressure within inclusions was determined using Raman spectroscopy based on the splitting of the Fermi diad of CO₂, measured at the clathrate melting temperature. The CO₂ densities (and pressures) predicted by the equation developed in this study are in relatively good agreement with previously published equations, except for very low densities and correspondingly low pressures. The combined Raman spectroscopy – microthermometry technique thus provides both the temperature and the pressure in the inclusion at clathrate melting. For inclusions in which the clathrate melts in the presence of CO₂ liquid, the salinity can be determined with a precision of a few tenths of a wt% NaCl, whereas for inclusions in which clathrate melts in the presence of CO₂ vapor the salinity error may be a few wt% NaCl. Applying the method to synthetic fluid inclusions with known salinity suggests that the technique is valid for determining salinity of H₂O–CO₂–NaCl fluid inclusions in which clathrate melts in the presence of liquid CO₂ only or vapor CO₂ only.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We have examined the suitability of a quartz-inclusions-in-epidote (qtz-in-ep) mineral barometer to better constrain
histories of epidote-bearing lithologies. Theoretical calculations applying an ...isotropic elastic model suggest that the qtz-in-ep barometer exhibits minimal temperature dependence, and thus, offers the potential to constrain growth conditions of epidote in various geologic environments, including skarn deposits, epidote-bearing granitoids, and metamorphic rocks.
To test if the applied equations of state and isotropic elastic model reasonably simulate the elastic evolution of two anisotropic minerals, we measured Raman shifts of the 464 cm
band of quartz inclusions relative to that of an unencapsulated quartz standard. We calculated a quartz inclusion pressure
at various temperatures and compared these values with temperature-dependent
predicted by elastic modeling
at elevated temperatures. Three epidote-bearing samples with reasonably well-constrained
histories were also examined: (1) sample HF14C from the Upper Schieferhuelle in the Western Tauern Window, Italy
(2) sample LdC-31C from Lago di Cignana, Italy
GPa); and (3) sample FT1E from the Frosnitz Tal in the Western Tauern region, Austria
Entrapment pressures
calculated from
determined at various temperatures show nominal differences from
calculated from
suggesting that for qtz-in-ep pairs, the calculated
does not significantly vary with the temperature of measurement. Furthermore, our calculated
for a sample from the Upper Schieferhuelle is in agreement with petrographic context and previously established
conditions, and the
determined for the Frosnitz Tal sample closely approximate previously reported pressures. The Lago di Cignana sample is derived from an epidote vein that is encased in a high-
foliation, and the calculated
is consistent with early, low-
epidote vein formation that pre-dates high-
metamorphism, or alternatively, late vein formation during exhumation, and confirms that the epidote did not form at or near peak conditions (~2.0 GPa). The results of this study indicate that the qtz-in-ep barometer potentially provides another tool that geoscientists can employ to better constrain
conditions in some epidote-bearing environments, where conventional thermobarometric techniques cannot be applied.
Inclusions of basaltic melt trapped inside of olivine phenocrysts during igneous crystallization provide a rich, crystal-scale record of magmatic processes ranging from mantle melting to ascent, ...eruption, and quenching of magma during volcanic eruptions. Melt inclusions are particularly valuable for retaining information on volatiles such as H
2
O and CO
2
that are normally lost by vesiculation and degassing as magma ascends and erupts. However, the record preserved in melt inclusions can be variably obscured by postentrapment processes, and thus melt inclusion research requires careful evaluation of the effects of such processes. Here we review processes by which melt inclusions are trapped and modified after trapping, describe new opportunities for studying the rates of magmatic and volcanic processes over a range of timescales using the kinetics of post-trapping processes, and describe recent developments in the use of volatile contents of melt inclusions to improve our understanding of how volcanoes work.
Inclusions of silicate melt (magma) trapped inside of crystals formed by magma crystallization provide a rich, detailed record of what happens beneath volcanoes.
These inclusions record information ranging from how magma forms deep inside Earth to its final hours as it ascends to the surface and erupts.
The melt inclusion record, however, is complex and hazy because of many processes that modify the inclusions after they become trapped in crystals.
Melt inclusions provide a primary archive of dissolved gases in magma, which are the key ingredients that make volcanoes erupt explosively.
Fluid inclusions (FI) that homogenize by halite disappearance are common in some geological environments, and interpretation of microthermometric data from these inclusions has been limited by the ...lack of a model describing the PVTX relationships over the complete range of PTX conditions found in nature. In this study, a system of equations has been developed that can be used to estimate salinity, pressure and density of FI that homogenize by halite disappearance. The salinity, pressure, density and dP/dT slope of the FI isochore are calculated as functions of liquid–vapor homogenization temperature (Th) and halite dissolution temperature (Tm). The equations are based on a numerical model describing the isochoric pressure–temperature trajectory followed by halite-saturated fluids during heating. The model is valid for Th and Tm from 100 to 600°C, and for pressures along the liquid–vapor–halite curve to 300MPa.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Constraining conditions and mechanisms of the early stages of exhumation from within subduction zones is challenging. Although pressure, temperature, and age can be inferred from the exhumed rock ...record, it is generally difficult to derive each of these parameters from any single rock, thus demanding assumptions that diverse data from multiple samples can be safely combined into a single pressure‐temperature‐time (P‐T‐t) path that might then be used to infer tectonic context and mechanisms of exhumation. Here, we present new thermobarometric and geochronologic information preserved in a single sample from Syros, Greece, to deduce the conditions and rates of the earliest phase of exhumation as a part of the well‐preserved high‐pressure metamorphic rocks of the Cycladic Blueschist Unit (CBU). The sample studied here is a garnet‐bearing, quartz‐mica schist that records two distinct metamorphic events. Results from thermodynamic models and quartz‐in‐garnet elastic geobarometry show that metamorphic garnet cores formed as P‐T conditions evolved from ∼485°C and 2.2 GPa to 530°C and 2.0 GPa, and that garnet rims formed as conditions evolved from ∼560°C and 2.1 GPa to ∼550°C and 1.6 GPa. Sm‐Nd geochronology on garnet cores and rims yields ages of 45.3 ± 1.0 and 40.5 ± 1.9 Ma, respectively, thus indicating a 4.8 ± 2.1 Myr growth span. Given the decompression path calculated based on garnet core and rim P‐T estimates, we conclude that the distinct phases of garnet growth preserve evidence of the initial exhumation of portions of the CBU.
Plain Language Summary
Constraining the evolution of rocks formed at great depths (>60 km) and their subsequent returned to Earth's surface is a challenging endeavor, because information regarding the pressure, temperature, and timing of formation is typically elucidated through the study of multiple samples and thus through the comparison of different datasets. Here, we present pressure, temperature, and age data that were derived by analyzing garnet crystals within a single sample from a well‐known paleo‐subduction zone locate in Syros, Greece. Our results show that the sample records two metamorphic events that occurred as a result of initial ascent to the surface from depths of ∼66 to 48 km.
Key Points
Metamorphic garnets record two stages of growth during HP/LT metamorphism and exhumation of the Cycladic Blueschist Unit (CBU) on Syros, Greece
Garnet growth spanned 4.8 ± 2.1 Myr including decompression from 2.1 to 1.6 GPa
Garnets in this sample record a part of the CBU that was exhumed as a series of tectonic slices on Syros
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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