Raman spectroscopy for fluid inclusion analysis Frezzotti, Maria Luce; Tecce, Francesca; Casagli, Alessio
Journal of geochemical exploration,
2012, 2012-1-00, 20120101, Letnik:
112
Journal Article
Recenzirano
Raman spectroscopy is a versatile non-destructive technique for fluid inclusion analysis, with a wide field of applications ranging from qualitative detection of solid, liquid and gaseous components ...to identification of polyatomic ions in solution. Raman technique is commonly used to calculate the density of CO
2 fluids, the chemistry of aqueous fluids, and the molar proportions of gaseous mixtures present as inclusions. Raman spectroscopy has been applied to measure the
pH range and oxidation state of fluids. The main advantages of this technique are the minimal sample preparation and the high versatility. Present review summarizes the recent developments of Raman spectroscopy in fluid inclusions research to provide support for laboratory analyses.
Primary multiphase solid (MS) inclusions without preserved fluid are found within peak minerals in kyanite quartzite
±
topaz and kyanite–phengite–epidote eclogite from Donghai area (Su-Lu terrane). ...Typical mineral association in inclusions is: paragonite
+
muscovite
+
anhydrite
±
corundum
±
“alunite-type” sulphate
±
zircon
±
calcite
±
chlorite
±
SiO
2
±
barite
±
pyrite
±
apatite in quartzites, and paragonite
+
rutile
+
apatite
±
amphibole
±
Zn-staurolite
±
magnetite
±
plagioclase
±
zircon
±
pyrite
±
“alunite-type” sulphate
±
Zn–Mg–Fe–Al–Ti spinel in eclogites.
On the basis of the fluid inclusion textures and of the daughter-phase assemblage, calculated fluid composition is as follows: in quartzites 24 wt.% SiO
2, 30 wt.% Al
2O
3, 9 wt.% CaO, 5 wt.% K
2O, 3 wt.% Na
2O, 11 wt.% SO
3, 18 wt.% H
2O, with traces of TiO
2, Fe
2O
3, FeO, MgO, BaO, P
2O
5, Cl
−, F, and (CO
3)
2−, and in eclogites 26 wt.% SiO
2, 21 wt.% TiO
2, 20 wt.% Al
2O
3, 2 wt.% MgO, 4 wt.% FeO, 6 wt.% Fe
2O
3, 7 wt.% CaO, 3 wt.% Na
2O, 3 wt.% P
2O
5, 7 wt.% H
2O, 1 wt.% Cl, and traces of ZnO, MnO, K
2O, SO
3, and F. Originally trapped chloride-poor aqueous fluids contained very high amounts – in the order of tens of wt.% – of Si
4+, Al
3+, and Ti
4+. Solute species reflect the chemical composition of the host rocks: Mg
2+, Fe
2+, Ti
4+, P
5+, and Na
+, are abundant in the fluids present in eclogites, while fluids are enriched in Al
3+, K
+, Na
+, (SO
4)
2−, (CO
3)
2− in quartzites.
We propose that multiphase solid inclusions represent remnants of high-density supercritical silicate-rich aqueous fluids that were in equilibrium with peak minerals at UHP conditions. These fluids show characters which are transitional between aqueous fluids and silicate melts, and were probably produced by dehydration reactions of the host rocks during the latest stages of subduction.
The ultrahigh‐pressure pyrope whiteschists from the Brossasco‐Isasca Unit of the Southern Dora‐Maira Massif represent metasomatic rocks originated at the expense of post‐Variscan granitoids by the ...influx of fluids along shear zones. In this study, geochemical, petrological and fluid‐inclusion data, correlated with different generations of pyrope‐rich garnet (from medium, to very‐coarse‐grained in size) allow constraints to be placed on the relative timing of metasomatism and sources of the metasomatic fluid. Geochemical investigations reveal that whiteschists are strongly enriched in Mg and depleted in Na, K, Ca and LILE (Cs, Pb, Rb, Sr, Ba) with respect to the metagranite. Three generations of pyrope, with different composition and mineral inclusions, have been distinguished: (i) the prograde Prp I, which constitutes the large core of megablasts and the small core of porphyroblasts; (ii) the peak Prp II, which constitutes the inner rim of megablasts and porphyroblasts and the core of small neoblasts; and (iii) the early retrograde Prp III, which locally constitutes an outer rim. Two generations of fluid inclusions have been recognized: (i) primary fluid inclusions in prograde kyanite that represent a NaCl‐MgCl2‐rich brine (6–28 wt% NaCleq with Si and Al as other dissolved cations) trapped during growth of Prp I (type‐I fluid); (ii) primary multiphase‐solid inclusions in Prp II that are remnants of an alumino‐silicate aqueous solution, containing Mg, Fe, alkalies, Ca and subordinate P, Cl, S, CO32‐, LILE (Pb, Cs, Sr, Rb, K, LREE, Ba), U and Th (type‐II fluid), at the peak pressure stage. We propose a model that illustrates the prograde metasomatic and metamorphic evolution of the whiteschists and that could also explain the genesis of other Mg‐rich, alkali‐poor schists of the Alps. During Alpine metamorphism, the post‐Variscan metagranite of the Brossasco‐Isasca Unit experienced a prograde metamorphism at HP conditions (stage A: ∼1.6 GPa and ≤ 600 °C), as indicated by the growth of an almandine‐rich garnet in some xenoliths. At stage B (1.7–2.1 GPa and 560–590 °C), the influx of external fluids, originated from antigorite breakdown in subducting oceanic serpentinites, promoted the increase in Mg and the decrease of alkalies and Ca in the orthogneiss toward a whiteschist composition. During stage C (2.1 < P < 2.8 GPa and 590 < T < 650 °C), the metasomatic fluid influx coupled with internal dehydration reactions involving Mg‐chlorite promoted the growth of Prp I in the presence of the type‐I MgCl2‐brine. At the metamorphic peak (stage D: 4.0–4.3 GPa and 730 °C), Prp II growth occurred in the presence of a type–II alumino‐silicate aqueous solution, mostly generated by internal dehydration reactions involving phlogopite and talc. The contribution of metasomatic external brines at the metamorphic climax appears negligible. This fluid, showing enrichment in LILE and depletion in HFSE, could represent a metasomatic agent for the supra‐subduction mantle wedge.
At active volcanoes, petrological studies have been proven to be a reliable approach in defining the depth conditions of magma transport and storage in both the mantle and the crust. Based on fluid ...inclusion and mineral geothermobarometry in mantle xenoliths, we propose a model for the magma plumbing system of the Island of El Hierro (Canary Islands). The peridotites studied here were entrained in a lava flow exposed in the El Yulan Valley. These lavas are part of the rift volcanism that occurred on El Hierro at approximately 40–30 ka. The peridotites are spinel lherzolites, harzburgites, and dunites which equilibrated in the shallow mantle at pressures between 1.5 and 2 GPa and at temperatures between 800 and 950 °C (low-temperature peridotites; LT), as well as at higher equilibration temperatures of 900 to 1100 °C (high-temperature peridotites; HT). Microthermometry and Raman analyses of fluid inclusions reveal trapping of two distinct fluid phases: early type I metasomatic CO
2
-N
2
fluids (
X
N2
= 0.01–0.18; fluid density (d) = 1.19 g/cm
3
), coexisting with silicate-carbonate melts in LT peridotites, and late type II pure CO
2
fluids in both LT
(d
= 1.11–1.00 and 0.75–0.65 g/cm
3
) and HT (
d
= 1.04–1.11 and 0.75–0.65 g/cm
3
) peridotites. While type I fluids represent metasomatic phases in the deep oceanic lithosphere (at depths of 60–65 km) before the onset of magmatic activity, type II CO
2
fluids testify to two fluid trapping episodes during the ascent of xenoliths in their host mafic magmas. Identification of magma accumulation zones through interpretation of type II CO
2
fluid inclusions and mineral geothermobarometry indicate the presence of a vertically stacked system of interconnected small magma reservoirs in the shallow lithospheric mantle between a depth of 22 and 36 km (or 0.67 to 1 GPa). This magma accumulation region fed a short-lived magma storage region located in the lower oceanic crust at a depth of 10–12 km (or 0.26–0.34 GPa). Following our model, the 40–30-ka-old volcanic activity of El Hierro is related to this mantle-based magma system, a system that we propose fed the recent 2011–2012 eruption.
•Thermodynamic analysis of UHP fluid inclusion chemistry and phase equilibria.•Marbles release solute-bearing COHS fluids during UHP Alpine subduction.•Post-entrapment chemical evolution of UHP ...inclusions is reconstructed.•Electrolytic fluid models are consistent with the inferred evolution.
Subduction fluids play a crucial role in regulating long-term chemical cycles. Their characterisation is essential to understand the processes responsible for metasomatism, oxidation and melting of the mantle wedge. Both direct (fluid inclusion studies) and indirect (thermodynamic modelling) approaches to study subduction fluids have reliability issues due to the complexity of the investigated processes. Post-entrapment processes (e.g., solvent loss by diffusion or decrepitation and/or chemical reactions between host mineral and trapped fluid) are likely to modify the chemical fingerprint of ultra-high pressure (UHP) fluid inclusions, while thermodynamic modelling of solute-bearing fluids at UHP conditions is still at the beginning of its application. In this work, we apply and compare data obtained by both approaches for fluid inclusions trapped within UHP clinopyroxene from a chemically simple Ol-Cpx-Dol-Cal marble (Brossasco-Isasca Unit, Dora-Maira Massif, Western Italian Alps). Classical molecular-fluid thermodynamics is adequate to qualitatively describe the post-entrapment reactions between fluid inclusions and host clinopyroxene. However, an electrolytic fluid model is necessary to describe the chemical composition of the solute-bearing aqueous fluids at the peak metamorphic condition (H2O: 96.30 mol%/88.49 wt%; solutes: 3.61 mol%/11.34 wt%/2.08 mol/kg; other volatiles: 0.09 mol%/0.17 wt%) generated by progressive rock dissolution. Comparison of the model fluid composition with that inferred from the analysis of fluid inclusions clarifies the types and the extent of post-trapping chemical modification of the UHP fluid inclusions. Our data reveal that the fluid-host reactions carry up to 42 mol% of host clinopyroxene component in the fluid inclusion bulk composition, whereas the fluid inclusion decrepitation and the water diffusion in the host clinopyroxene (through dislocations and/or micro-fractures) cause an H2O loss ranging from 18 mol% to 99 mol%. Applying these approaches, we demonstrate that the most relevant post-entrapment process is H2O loss. We also demonstrate that some fluid inclusions did not experience post-entrapment fluid-host modification and, thus, preserve the original fluid geochemistry.
The study of the oxidation state of lithospheric mantle-derived rocks allows modelling the deep cycle of volatiles (e.g., C, H, O, N and S) in the Earth's interior, which in turn plays a role in ...magma genesis, metasomatism and volcanic degassing. At the oxygen fugacity (i.e., fO2) recorded by residual abyssal peridotites, volatile elements like carbon are predicted to be in the immobile form of graphite. However, the compilation of the redox state of worldwide-distributed continental xenoliths shows evidence of their oxidation and refertilization through time by deeply formed subduction-related metasomatic fluids. The analyses of fluid inclusions in mantle-derived minerals like olivine (or pyroxenes) represent a snapshot of the volatile circulation in depth, whose noble gases signature (He, Ar, Ne) is used to identify their possible source. This study aims to reconstruct the origin of mantle metasomatism underneath the Hyblean Plateau (Sicily, Italy) and its redox history through the investigation of spinel-peridotite nodules, combining fO2 estimates with noble gases and fluid inclusions chemistry from hand-picked olivine grains. We analyzed eight mantle xenoliths classified as spinel lherzolites and spinel harzburgites from the Valle Guffari (Hyblean Plateau, Sicily). The calculated logfO2 is higher than that of most cratonic xenoliths worldwide ranging between 0.28 and 1.27 log units above to the fayalite-magnetite-quartz (FMQ) reference buffer. Micro-Raman measurements on olivine grains with dendritic trails of (metasomatic) fluid inclusions reveal an assemblage made of Mg-Ca carbonates ± sulfide ± elemental sulfur ± CO2 in the most reduced sample, and Mg-Ca carbonates ± sulfates ± CO2 in the most oxidized sample, the latter associated with a silicate glass and (secondary) hydrous phases. Both assemblages are taken as evidence of the product of crystallization of deeply originated volatile-bearing silicate melts. Analyses of He, Ar, and Ne in olivine grains confirm the evidence of a mantle source reworked by metasomatic processes. Our data suggest that an initially residual Hyblean lithospheric mantle was affected by extensive oxidizing events at several depths caused by the interaction with slab-derived CO2-rich silicate metasomatic liquids.
•Hyblean mantle peridotites record fO2s higher than modern abyssal peridotites.•Fluid inclusions analyses reveal an oxidized mineral assemblage.•Metasomatism of the lithospheric mantle driven by subduction-related liquids.
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