Journal of Petrology Advance Access originally published online on December 3, 2004
Journal of Petrology 2005 46(3):603-614; doi:10.1093/petrology/egh090
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FTIR Spectrum of Phenocryst Olivine as an Indicator of Silica Saturation in Magmas
1 DEPARTMENT OF EARTH AND ATMOSPHERIC SCIENCES, UNIVERSITY OF ALBERTA, 1–26 EARTH SCIENCES BUILDING, EDMONTON, ALTA., TG6 2E3, CANADA
2 INSTITUT FÜR MINERALOGIE, UNIVERSITÄT MÜNSTER, CORRENSSTR. 24, 48149 MÜNSTER, GERMANY
3 LEIBNIZ INSTITUTE FOR MARINE SCIENCES, DYNAMICS OF THE OCEAN FLOOR, WISCHHOFSTR. 1–3, 24148 KIEL, GERMANY
4 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF BRISTOL, QUEENS ROAD, BRISTOL BS8 1RJ, UK
5 INSTITUT FÜR GEOWISSENSCHAFTEN, UNIVERSITÄT KIEL, LUDEWIG-MEYN-STRAßE 10, 24118 KIEL, GERMANY
RECEIVED SEPTEMBER 15, 2003; ACCEPTED OCTOBER 7, 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSAMPLESPROTONATION EXPERIMENTSANALYTICAL PROCEDUREANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Fourier Transform infrared (FTIR) absorption spectra of hydroxyl were measured on olivine phenocrysts from hydrous basaltic melts that originated in island-arc tectonic settings. The basaltic melts encompass a wide range of silica activities from orthopyroxene-saturated hypersthene-normative to nepheline-normative compositions. The intensities and wavenumber placement of hydroxyl absorption bands correlate with the degree of silica saturation of the parent melt from which the olivine crystallized. Olivines from silica-undersaturated nepheline-normative melts absorb IR radiation in the wavenumber range 3430–3590 cm–1 (Group 1). In contrast, olivines from orthopyroxene-saturated boninitic melts exhibit hydroxyl absorption bands in the wavenumber range 3285–3380 cm–1 (Group 2). Olivines crystallized at intermediate silica activities exhibit a combination of the two groups of hydroxyl IR bands, where the proportion of Group 2 bands increases with increasing silica saturation of the parent melt. The positions of hydroxyl absorption peaks observed here for natural samples are consistent with previous measurements on experimentally annealed olivines. Thus protonation experiments can be employed to make spectroscopically dry olivine structures visible by IR, yielding information on the silica saturation of the parental magmas. Hydroxyl concentrations in the studied olivines were estimated to be 1–2 ppm, corresponding to an olivine–melt partition coefficient of
(1·0 ± 0·3) x 10–4. KEY WORDS: nominally anhydrous minerals; olivine; water; mantle; silica activity; melt inclusions
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSAMPLESPROTONATION EXPERIMENTSANALYTICAL PROCEDUREANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Following the recognition that nominally anhydrous rock-forming silicates can incorporate small amounts of hydrogen (Beran, 1970
; Wilkins & Sabine, 1973), much work has been undertaken to document the hydroxyl concentrations and possible hydroxyl substitution mechanisms in olivine using Fourier Transform infrared (FTIR) spectroscopy (e.g. Beran & Putnis, 1983; Aines & Rossman, 1984; Miller et al., 1987; Mackwell & Kohlstedt, 1990; Bai & Kohlstedt, 1993; Kohlstedt et al., 1996; Kohn, 1996; Libowitzky & Rossman, 1997; Ingrin & Skogby, 2000; Matveev et al., 2001; Lemaire et al., 2004), and also using computational methods (Wright & Catlow, 1994; Braithwaite et al., 2003). FTIR spectroscopic studies of natural olivine show that hydroxyl stretching bands generally occur in two wavenumber regions, one between 3430 and 3630 cm–1 and the other between 3285 and 3380 cm–1. These are, following the work of Bai & Kohlstedt (1993), referred to as Group 1 and Group 2 hydroxyl bands, respectively. In addition, synthetic pure forsterite and rare natural samples show lower-frequency hydroxyl bands ranging down to 3100 cm–1 (Miller et al., 1987; Demouchy & Mackwell, 2003; Lemaire et al., 2004).
Matveev et al. (2001) have shown experimentally that the IR absorption of dissolved hydroxyl relates to the silica activity (aSiO2) at which the natural olivine crystallized or finally equilibrated. When olivine is equilibrated with magnesiowüstite (i.e. at low aSiO2), the hydroxyl stretching bands occur between 3430 and 3630 cm–1 (Group 1). On the other hand, when olivine is equilibrated with orthopyroxene (i.e. at high aSiO2), the hydroxyl bands occur at lower energies between 3285 and 3380 cm–1 (Group 2). Hydroxyl in olivine could be associated with the presence of cation vacancies whose concentrations are largely controlled by aSiO2 (Stocker & Smyth, 1978; Nakamura & Schmalzried, 1983), whereby with decreasing aSiO2 the concentration of silicon vacancies increases and the concentration of metal vacancies decreases. Under this premise, Matveev et al. (2001) assigned Group 1 absorption bands to hydroxyl groups associated with silicon vacancies (i.e. a hydrogarnet-type substitution) and Group 2 absorption bands to hydroxyl associated with octahedral M-site (Fe, Mg) vacancies. Thus the possibility of locating hydrogen in silicon vacancies might stabilize these defects and promote their formation during olivine crystallization at low aSiO2, even though the concentration of silicon vacancies in the anhydrous olivine structure might be regarded as insignificant (e.g. Nakamura & Schmalzried, 1983; Mackwell et al., 1988).
These experimental results are supported by recent computer simulations undertaken for the pure forsterite (Mg2SiO4) crystal structure (Braithwaite et al., 2003). The simulations were made to determine the energetics associated with various possible hydroxyl substitutional mechanisms, including both cation vacancies and interstitials (nominally unoccupied crystallographic positions). Braithwaite et al. concluded that hydroxyl groups should occur in either vacant octahedral or tetrahedral sites so as to produce neutral defect complexes, which are energetically favorable relative to other substitution mechanisms. The simulations indicate that hydroxyl stretching bands occur in two wavenumber regions. Their calculated spectra show that in the case where Si is replaced by four H+ atoms, four hydroxyl stretching bands occur at relatively high wavenumbers, whereas in the case where two H+ atoms substitute for one Mg, two hydroxyl bands occur at lower energies. The calculations agree with the experimental observations of Lamaire et al. (2004), which showed a significant effect of aSiO2 on the FTIR spectrum of hydroxyl-bearing synthetic forsterite. These results are in line with the experimental observations of Matveev et al. (2001), although quantitative comparisons of OH stretching frequencies measured on pure forsterite with data obtained for natural samples are not possible because of the effects of Fe and trace elements, such as Ti (Berry et al., 2004), on the defect structure of olivine.
In this paper, we test the postulate of Matveev et al. (2001) that the hydroxyl substitution mechanism could serve as an indicator of the silica activity at which the crystal structure of natural olivine equilibrates. To do this, we measured the FTIR absorption spectra of hydroxyl in olivine phenocrysts from a range of primitive mafic volcanic rocks. From bulk-rock compositions as well as the composition of olivine melt inclusions, the parental melts were all water-bearing and cover a wide range in aSiO2.
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SAMPLES |
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TOPABSTRACTINTRODUCTIONSAMPLESPROTONATION EXPERIMENTSANALYTICAL PROCEDUREANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The basaltic lavas from which olivines were separated are listed in Table 1. The eight samples are volcanic rocks from subduction-related tectonic settings and include lavas from (1) Troodos, Cyprus, (2) Avacha Volcano, Kamchatka, Russia, and (3) Mount Mahimba, New Georgia Archipelago, Solomon Islands. All the samples are primitive in the sense that the melts from which olivine phenocrysts crystallized could be in Fe–Mg exchange equilibrium with typical mantle olivine (Fo88–92). In the most silica-rich samples from Cyprus, olivine is joined by orthopyroxene, and in the samples from Avacha volcano and the Solomon Islands clinopyroxene is the early liquidus mineral along with olivine.
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Compositions of basaltic lavas and homogenized olivine melt inclusions (e.g. Danyushevsky et al., 2002
) are plotted in Fig. 1 on projections from the olivine (a) and diopside (b) apices onto the base of the ‘basalt tetrahedron’. The molar concentrations of normative olivine (Ol), diopside (Di), quartz (Qtz), jadeite (Jd), Ca-Tschermak pyroxene (CaTs), and leucite (Lc) were calculated following procedures given by Falloon & Green (1987) (Tables 2 and 3). To quantify the silica saturation degree of the primary melts we use projections of plotted compositions on the (Jd + CaTs + Lc)–(Qtz) join parameterized as the Qtz/(Qtz + Jd + CaTs + Lc) molar ratio, subsequently referred to as the Silica Saturation Index (SSI) (Tables 2 and 3). The SSI is independent of changes in normative olivine and diopside concentrations, and thus remains largely unchanged during magma fractionation or during reaction of melt with host olivine. Thus the SSI of melt inclusions or basaltic lavas adequately reflects the silica-saturation degree of the primary melt. Another advantage of SSI is that unlike aSiO2 it can be easily calculated for magmas where olivine is not coexisting with orthopyroxene or magnesiowüstite (compare Carmichael, 2002). A negative correlation between SSI and TiO2 concentration in both basaltic lavas and melt inclusions is consistent with the origin of the Troodos primary magmas from variably depleted sources as a result of different degrees of melting (Cameron, 1985; Sobolev et al., 1993; Falloon & Danyushevsky, 2000) (Fig. 2).
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) and average compositions of melt inclusions (
) shown on projections onto the base of the normative ‘basalt tetrahedron’ (Falloon & Green, 1987
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Lavas from the Troodos ophiolite and Mamonia complex, Cyprus
The Troodos ophiolite and disaggregated ophiolitic blocks in the Mamonia complex in Cyprus are tectonically exhumed fragments of Upper Cretaceous oceanic-like crust interpreted to have been formed in a supra-subduction zone (Pearce, 1975
; Cameron, 1985; Rautenschlein et al., 1985). They represent the ‘birth’ and ‘youth’ stages of island-arc volcanism (Shervais, 2001). During the initial stages of subduction the asthenospheric mantle sources produced high-magnesian hypersthene- to quartz-normative basaltic melts including island-arc tholeiites and refractory high-CaO boninites (Cameron, 1985; Sobolev et al., 1993). The presence of water and hydroxyl in pillow-rim glasses and in melt inclusions in phenocrysts suggests involvement of water during mantle melting (e.g. Rautenschlein et al., 1985; Sobolev & Chaussidon, 1996). In the samples studied, bulk-rock MgO ranges from 22 to 32·7 wt %, and the lavas contain up to 50 vol. % of 0·5–3 cm diameter euhedral olivine phenocrysts. The unusually MgO-rich samples were shown by Sobolev et al. (1993) to be olivine cumulates in an evolved melt matrix.
Previously reported compositions of rocks and homogenized melt inclusions from forsteritic olivine phenocrysts (>Fo88) are summarized in Tables 2 and 3. Water contents are based on ion-probe analysis of homogenized melt inclusions and may be as high as 2·5 wt % in the most silica-rich, refractory boninitic melts. Further details concerning the petrography of the samples and the melt inclusion data have been given by Sobolev et al. (1993), Portnyagin et al. (1996, 1997), Sobolev & Chaussidon (1996) and Portnyagin (1998).
Lava from Avacha volcano
Basaltic lava AV-2, from Avacha volcano in the Kamchatka arc, with 15 wt % MgO (Tables 2 and 3) contains about 35 vol. % of large (up to 2 cm) phenocrysts of magnesian olivine (Fo91–80) and clinopyroxene (Mg-number 92–73). This represents one of the most primitive basalts of the Quaternary volcanic series of the Kamchatka arc. Based on the compositions of homogenized melt inclusions in olivine, the parental magmas were strongly silica undersaturated under crustal conditions, and were probably derived from a refractory asthenospheric mantle source that was refertilized prior to, or during, partial melting by a subduction-related component rich in H2O, CO2, and incompatible trace elements. In contrast to the parental melt composition inferred from melt inclusions, the host lava is hypersthene-normative and was interpreted to be a mush of olivine and clinopyroxene crystals in an evolved andesitic matrix (Portnyagin et al., 2004).
Lava from the Solomon Islands
Sample SS-1 is an olivine ± clinopyroxene-phyric lava with 24 wt % MgO, derived from Mt. Mahimba, Kolo Caldera, New Georgia Island. The sample is a cumulate derived from a highly oxidized olivine–hypersthene-normative parental melt with 13·2 wt % MgO and 48·9 wt % SiO2. Rohrbach (2003) identified two generations of primitive olivine phenocrysts (Fo90–92), one with 0·1 and the other with >0·3 wt % CaO, of which only the high-CaO generation is considered to have been in equilibrium with the CaO content of the parent melt. The low-CaO olivines are probably xenocrysts from the lithospheric upper mantle. Consequently, FTIR spectra were collected from only the high-CaO olivines.
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PROTONATION EXPERIMENTS |
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TOPABSTRACTINTRODUCTIONSAMPLESPROTONATION EXPERIMENTSANALYTICAL PROCEDUREANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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To make the defect structure of anhydrous olivine visible by IR spectroscopy, olivine can be protonated experimentally (Bai & Kohlstedt, 1993
). Unlike equilibration experiments in which the olivine defect structure is largely controlled by buffered aSiO2 (e.g. Bai & Kohlstedt, 1993; Kohlstedt et al., 1996; Matveev et al., 2001), the protonation experiments are designed to keep the original olivine defect structure intact. Therefore, pressure–temperature conditions and run durations must be such that the relaxation time of cation point defect equilibration significantly exceeds the run time of an experiment. To optimize experimental conditions, we have used the diffusion coefficients for hydrogen and metal vacancies reported by Kohlstedt & Mackwell (1998) and references therein. We also assumed that the diffusion rate of silicon vacancies does not exceed that of metal vacancies (Mackwell et al., 1988; Matveev et al., 2001). Experiments were performed in a piston-cylinder apparatus and held at 1000°C and 2 GPa for 4 h. At these run conditions, we calculate that hydrogen may penetrate and protonate a 0·5–1 mm diameter olivine grain (the typical grain size of our olivine separates), with the defect structure remaining intact.
For the protonation experiments we have chosen olivines from Solomon Islands basalt (SS-1) in which the primary hydroxyl content was found to be below the detection limit of the FTIR analysis. To ensure that the olivine defect structure did not change in the course of protonation we performed two aSiO2-buffered experiments: in one, olivine crystals were embedded in periclase powder; in the other, an orthopyroxene powder was used. Dry experimental charge (150 mg) along with 25 µl of water were welded in 5 mm outer diameter platinum capsules. Only fluid-saturated experiments that released water upon recovery are reported in this study.
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ANALYTICAL PROCEDURE |
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TOPABSTRACTINTRODUCTIONSAMPLESPROTONATION EXPERIMENTSANALYTICAL PROCEDUREANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Olivine grains were separated from the host rocks or removed from the protonation experiments and polished on both sides to give crystal platelets of 100–600 µm thickness. The studied crystals were anhedral, so their crystallographic orientation could not be visually determined prior to polishing. To have a range of crystal orientations available, at least 10 crystals per sample were separated from each rock sample and prepared for FTIR analysis.
For the FTIR measurements, the crystal platelets were placed in an IR microscope attached to a Nicolet 800 spectrometer. Spectra were collected in transmission mode in regions free of cracks or inclusions using an unpolarized beam. The size of the measuring spot was defined by choosing either 75 or 100 µm diameter apertures. The IR spectra were collected in the wavenumber range from 600 to 6000 cm–1 with a resolution of 4 cm–1. The IR microscope was kept inside a plastic box and purged with dried nitrogen gas [see Jamtveit et al. (2001) for technical details]. The IR spectra were measured after holding the crystals in a dry nitrogen gas atmosphere for at least 12 h, resulting in low spectral noise in the wavenumbers above 3600 cm–1, where IR stretching of water vapour occurs (e.g. Bernath, 2002).
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ANALYTICAL RESULTS |
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TOPABSTRACTINTRODUCTIONSAMPLESPROTONATION EXPERIMENTSANALYTICAL PROCEDUREANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The FTIR absorption spectra were examined in two wavenumber ranges: from 3100 to 3700 cm–1, where hydroxyl bands absorb; and from 1600 to 2100 cm–1, where second-order Si–O overtones occur. The latter are used to estimate the crystal orientation.
The FTIR spectra exhibit hydroxyl absorption bands consistent with the Group 1 and Group 2 classification of Bai & Kohlstedt (1993). Group 1 bands were observed between 3430 and 3590 cm–1. The rims of experimentally protonated crystals show a further absorption band at 3615 cm–1, which is probably due to high fH2O in the experiments. The Group 2 hydroxyl bands occur between 3285 and 3380 cm–1. At room temperature, these bands have FWHM (full width at half maximum) of 20–40 cm–1.
Second-order Si–O overtones were used to deduce the crystallographic orientation of the studied olivine grains following Jamtveit et al. (2001), but it should be noted that the directions [100] and [010] were swapped as they appear to have been wrongly assigned for the commonly accepted olivine unit-cell settings, where b > c > a. For several olivine grains, spectroscopically estimated orientations were confirmed by single-crystal X-ray diffractometry (XRD; Fig. 4; Table 4). Below we compare FTIR spectra that were measured with the IR beam parallel to [010]. Such spectra are characterized by the strongest absorption of unpolarized IR radiation in both the Group 1 and Group 2 wavenumber regions (e.g. Kohlstedt et al., 1996).
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Olivines from Cyprus and Kamchatka
IR absorption spectra of olivine from the Cyprus lavas are summarized in Fig. 3a. Olivines from orthopyroxene-bearing samples MAM-25 and TRD-64 absorb in the Group 2 wavenumber range (spectra 1–4), whereas absorption in the Group 1 range is at the limit of the FTIR detection. Olivines from the orthopyroxene-free Cyprus samples show both the Group 2 and the Group 1 bands (spectra 5–8). The proportion of Group 2 bands decreases progressively in the sample sequence TRD-39, TRD-41–TRD-75–TRD-150, so that olivine from the tholeiitic sample TRD-150 shows mainly Group 1 hydroxyl bands (spectrum 8). The FTIR spectra of the olivine phenocrysts from the Avacha lava AV-2 contain only Group 1 bands (Fig. 3b).
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Placement and intensity of hydroxyl bands in the FTIR spectra of olivines correlate well with SSI calculated for basaltic lavas and melt inclusions (Fig. 1). SSI is highest for the samples in which olivine exhibits Group 2 hydroxyl bands and lowest for the samples in which olivine exhibits mainly Group 1 bands. To better illustrate the influence of silica saturation on IR absorption, we have integrated the Group 1 absorption bands from 3466 to 3620 cm–1 and the Group 2 bands from 3260 to 3412 cm–1, and then determined the absorption intensity ratio Aint, ratio = Aint, Group 1/(Aint, Group 1 + Aint, Group 2), where Aint denotes integrated absorption coefficients. The ratio systematically increases with decreasing SSI of basaltic lavas (Fig. 5a) and melt inclusions (Fig. 5b and c). Lesser scatter observed for melt inclusions illustrates that melts trapped in olivine preserve the primary composition better than the host lavas. Based on the FTIR results obtained for the Cyprus samples, the transition from the Group 1 dominating spectrum to the Group 2 dominating spectrum occurs within the hypersthene-normative field of parental magmas and in the relatively narrow range of SSI between 0·6 and 0·75.
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Water solubility in olivines from the Cyprus lavas was estimated from the Group 1 and Group 2 integrated absorption coefficients using the calibration of Libowitzky & Rossman (1997)
. The weighted mean wavenumber for the Group 1 hydroxyl bands was located at 3500 cm–1 and for Group 2 at 3330 cm–1. Because FTIR spectra were measured with the unpolarized beam parallel to [010], an additional orientation factor 
= 0·5 was applied (Mackwell & Kohlstedt, 1990; Lemaire et al., 2004). Water concentrations are given in Table 5, and for olivines from the Cyprus lavas fall in the concentration range from 1 to 2 ppm. However, it should be noted that the poorly constrained value of orientation factor ( 0·3–0·5; Paterson, 1982; Mackwell & Kohlstedt, 1990), spectral noise and unknown olivine matrix correction of the calibration make these calculations rather qualitative. Using water concentrations measured on homogenized melt inclusions in olivine from the Cyprus lavas as a proxy for water content in the primary melt, an olivine–melt partition coefficient for H2O can be estimated as Kd (1·0 ± 0·3) x 10–4. Because the baselines were positioned at the flanks of each group of absorption bands, the broad plateau of underlying absorption commonly attributed to small amounts of molecular water in submicroscopic inclusions (Miller et al., 1987; Matveev et al., 2001) did not contribute to the total calculated water content. Thus water contents reported here are somewhat lower than those that would result from integration of the entire hydroxyl absorption area (e.g. Matveev et al., 2001). The obtained partition coefficient is in good agreement with the coefficient from Hirth & Kohlstedt (1996) calculated for shallow mantle and crustal conditions (<300 MPa), and consistent with a shallow depth of olivine crystallization.
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Hydroxyl in olivine from AV-2 absorbs IR only in the Group 1 wavenumber range, which is consistent with the highly silica-undersaturated compositions of the melt inclusions (Fig. 1b). Low water concentrations in AV-2 melt inclusions (<0·1 wt %) might be indicative of higher pressures of olivine crystallization compared with that of Cyprus olivines, and thus a somewhat higher olivine–melt H2O partition coefficient (Hirth & Kohlstedt, 1996
). Alternatively, water could have been lost from the melt inclusions during ascent, as a result of decrepitation (Portnyagin et al., 2004).
Experimentally protonated olivines from Solomon Islands picrites
FTIR spectra of experimentally protonated olivine from sample SS-1 (Mt. Mahimba, Solomon Islands) were measured from core to rim with 100 µm step increments using a beam diameter of 100 µm (Fig. 6). Both experiments produced olivine grains with increased water contents. The resulting spectra show a 
100 µm wide rim whose defect structure corresponds to the aSiO2 of the buffer used (the lowest spectra in Fig. 6a and b). In contrast, the cores have lower water contents, but all show the spectra features related to low silica activity regardless of the buffer used. Hydroxyl incorporation in the rim may involve dissolution–precipitation mechanisms (e.g. Matveev et al., 2001), whereas core protonation occurs probably according to reduction–oxidation reactions as described by Kohlstedt et al. (1997). Good correlation between FTIR spectra measured on olivine cores and the low aSiO2 implied by the SSI calculated for SS-1 basalt (Tables 2 and 3, Fig. 5a) implies that the protonated structure of initially anhydrous olivine can be used to estimate the degree of silica saturation of the parent melts.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONSAMPLESPROTONATION EXPERIMENTSANALYTICAL PROCEDUREANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The experiments of Matveev et al. (2001)
, and the resulting FTIR spectra of hydroxyl-bearing mantle olivines, allow clear discrimination between the effects of aSiO2 and other important parameters such as pressure, temperature, water and oxygen fugacities, and olivine major and trace element compositions. Olivine grains separated from the same sample at equal P, T, aH2O, and fO2 exhibited Group 1 OH bands when experimentally equilibrated with magnesiowüstite (low aSiO2) and Group 2 OH bands when equilibrated with orthopyroxene (high aSiO2). The IR data on olivines from hydrous basaltic melts obtained in this study support the experimental results of Matveev et al. (2001), confirming that natural hydroxyl speciation in olivine is also largely controlled by aSiO2.
In natural samples, the influence of fO2 and trace element composition on the FTIR spectrum of olivine may often be hard to isolate from the effect of aSiO2 and grain orientation inconsistencies. As fO2 controls concentration of Fe3+ in olivine, it may also affect the capacity of olivine to store hydrogen during natural or experimental protonation (Kohlstedt et al., 1996). In the FTIR spectra of natural olivines fO2 appears to affect the intensities of individual OH bands (e.g. Matsyuk & Langer, 2004, and references therein), but because ferric iron is likely to occur in complexes where it substitutes in neighbouring tetrahedral and octahedral positions (Nakamura & Schmalzried, 1983) it is unlikely that fO2 will significantly affect the Aint, ratio.
The effect of trace impurities and the associated extrinsic defects in olivine is yet more cryptic. Relative concentrations of trace elements were estimated from the melt inclusion compositions (Table 3) assuming similar olivine–melt partition coefficients for the variety of the studied rocks. The Aint, ratio systematically changes only with the Ti content of the melt inclusions and thus the likely Ti content of the olivine (Nikogosian & Sobolev, 1997; Canil & Fedortchouk, 2001). However, the experimental data of Matveev et al., (2001) suggest that the spectra features assigned to high and low aSiO2 can be reproduced in olivines with identical Ti contents and are, therefore, not primarily controlled by this parameter. Thus the apparent correlation between olivine FTIR spectrum and Ti content is caused by decreasing TiO2 concentration with increasing aSiO2 of the olivine parent melt (Fig. 2). Nevertheless, the effect of Ti on the olivine hydroxyl speciation is significant; the data of Berry et al. (2004) suggest that Ti in olivine strongly affects the exact position of Group 1 peaks.
Another important effect is fH2O (P, T, aH2O), which affects the solubility of hydroxyl in the olivine structure. At higher fH2O not only do the intensities of both Group 1 and Group 2 bands increase, but also the number of resolvable OH bands, particularly at relatively higher frequencies (compare the rim and core spectra in Fig. 4; Kohlstedt et al., 1996; Matveev, 1997; Matveev et al., 2001; Matsyuk & Langer, 2004). However, there is no indication that fH2O notably affects the Aint,ratio (Matveev, 1997; Matveev et al., 2001).
Considering aSiO2 as a key variable controlling the frequency of OH IR absorption in olivine, we suggest that the FTIR spectrum of olivine can be used to deduce the aSiO2 at crystallization or final equilibration. The good correlation between IR spectra and the composition of melt inclusions or host lavas implies that the defect structure of the studied olivines has survived changes in pressure–temperature and even aSiO2 conditions during ascent.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONSAMPLESPROTONATION EXPERIMENTSANALYTICAL PROCEDUREANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The FTIR spectra of olivine phenocrysts in basaltic lavas correlate well with the degree of silica saturation of the parent melt and therefore aSiO2. Liquidus olivines crystallized from nepheline-normative basaltic melts have OH bands between 3430 and 3590 cm–1 (Group 1). Olivines that coexist at magmatic temperatures with orthopyroxene have OH bands in the wavenumber range from 3285 to 3380 cm–1 (Group 2). Olivines from orthopyroxene-undersaturated hypersthene-normative basaltic melts exhibit both groups of hydroxyl absorption bands, with the proportion of Group 2 bands increasing with increasing SSI. The compositions of melt inclusions correlate better with the FTIR spectra of olivine than the composition of the host basaltic lava, and thus more accurately preserve information on the silica saturation of primary melts.
FTIR results from this study are consistent with the experimental results of Matveev et al. (2001), implying that natural hydroxyl occurrence in olivine is similar to that imposed on natural olivine in high-pressure experiments. Therefore spectroscopically ‘dry’ olivine defect structures that have lost hydrogen during magma ascent can be re-protonated experimentally to reveal the aSiO2 at their last vacancy equilibration. Hydrogen diffusion and reduction of ferric iron in the olivine structure to OH and ferrous iron are relatively rapid, such that the original defect structure may survive a short experimental run time, and may be made visible by subsequent IR spectroscopy to reveal the original aSiO2 conditions.
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ACKNOWLEDGEMENTS |
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We thank the lapidary workshop at Münster University for sample preparation. Financial support by the DFG (Deutsche Forschungsgemeinschaft) through grants Ba 964/16-1 and Ge 659/11-1 (to C.B. and C.G.), the European Commission IHP Programme grant, which allowed IR analyses at the University of Bristol (to S.M.), as well as BMBF (Bundesministerium für Bildung und Forschung) funded KOMEX-2 project and RFBR (Russian Foundation for Basic Research) through grant 03-0564629 (to M.P.) are gratefully acknowledged. Earlier discussions with A. Sobolev (MPI für Chemie, Mainz) were invaluable for structuring the study. We also thank R. W. Luth, T. Chacko (University of Alberta) and J. Harris (University of Glasgow) for their comments, which helped to improve the manuscript. We thank J. Loens (Münster University) for performing single-grain XRD analyses.
* Corresponding author. Present address: Department of Earth and Atmospheric Sciences, University of Alberta, 1–26 Earth Sciences Building, Edmonton, Alta., TG6 2E3, Canada. Telephone: ++1 780 492 3191. Fax: ++1 780 492 2030. E-mail: smatveev{at}ualberta.ca
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REFERENCES |
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TOPABSTRACTINTRODUCTIONSAMPLESPROTONATION EXPERIMENTSANALYTICAL PROCEDUREANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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