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Mineralogisches Institut, Universität Heidelberg, INF 236, D-69120 Heidelberg e-mail: dlattard{at}min.min.uni-heidelberg.de
This paper was presented at the EMPG VIII meeting in Bergamo, Italy (April 2000)
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Abstract |
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 Top Abstract IntroductionExperimental procedureTesting the method: does...Application: preliminary...Appendix: Discussion of the...AcknowledgementsReferences |
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The experiments are conducted in two steps. Step 1: in order to fix the initial oxygen fugacity (fO2), small charges of the glassy starting materials are either pressed onto a loop of thin Pt-wire or into a small AgPd crucible and equilibrated at super-liquidus temperatures (>1180 °C) with CO/CO2 gas mixtures. Step 2: to achieve equilibrium crystallization under closed-system conditions, the charges are subsequently placed together with their metal holder/container in evacuated silica-glass ampoules and re-equilibrated under sub-liquidus conditions (1050–1170 °C).
To test whether this experimental approach really ensures closed-system conditions, a series of experiments was conducted at near-liquidus temperatures with a synthetic ferro-basaltic starting composition. Within the analytical uncertainties, the bulk ferrous iron contents of the samples remain constant during the step-2 experiments, pointing to systems closed to oxygen. There are, however, indications for a slight oxidation related to a small loss of iron from the sample to the AgPd container.
Using the same synthetic ferro-basaltic composition, preliminary equilibrium crystallization experiments under closed-system conditions were performed at 1091–1146 °C with an initial superliquidus fO2) corresponding to FMQ. The crystallization sequence of the mineral phases is the same as under open system conditions but magnetitess appears at higher temperatures. The FeOtot content of the residual melt shows the same increase with decreasing temperature under closed and open system conditions down to about 1100 °C. At lower temperatures, however, the values drop drastically under closed-system conditions, in contradiction with previous modelling. This exemplifies the need for further experiments in closed systems.
Key-words: experimental technique, magmatic crystallization, systems closed to oxygen, Fenner differentiation trend, redox state.
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Introduction |
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TopAbstractIntroductionExperimental procedureTesting the method: does...Application: preliminary...Appendix: Discussion of the...AcknowledgementsReferences |
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In experimental studies on the crystallization of magmatic liquids, the oxygen fugacity is generally controlled, either by a solid oxygen buffer or by gas mixtures, to parallel the log fO2-T curves of solid buffers (e.g. Snyder et al., 1993; Toplis & Carroll, 1995). In both cases, the system is open to oxygen, i.e. the sample may exchange oxygen with the external buffering medium.
In natural magmatic systems, however, the oxygen fugacity is most probably not imposed externally, even if fluids or melts may influx the magma chamber or conduits, but must be controlled by the magma itself. Most authors "model the crystallization of magmatic liquids by forcing them to follow the log fO2 relations of known oxygen fugacity buffers, largely because we think that that is approximately the way nature behaves. How the magma manifests this capacity for self-buffering remains a mystery, except that it must depend on phases which contain substantial quantities of ferric and ferrous iron, acting in combination" (quoted from Carmichael & Ghiorso, 1990). Based on experimental work in complex systems (e.g. Juster et al., 1989; Snyder et al., 1993; Toplis & Carroll, 1995, 1996) and on observations on natural volcanic suites and layered intrusions (cf. discussions in Juster et al., 1989; Snyder et al., 1993), there is however, growing evidence that, during magmatic crystallization and differentiation with decreasing temperature, fO2 does not follow a buffer curve. Instead, relative to a buffer curve, it increases before Fe-Ti oxide saturation and subsequently decreases. This evolution of oxygen fugacity may take place if the differentiation occurs under conditions closed to oxygen (Juster et al., 1989; Snyder et al., 1993; Toplis & Carroll, 1996).
Conditions closed to oxygen have also been inferred for some layered intrusions (e.g. Morse, 1980a; Morse et al., 1980) to explain the characteristic "Fenner differentiation trend" which leads to a strong iron enrichment in the residual melts (e.g. Skaergaard or Kiglapait intrusions; Wager & Deer, 1939; Wager, 1960; Morse, 1981). In contrast, the normal tholeiitic differentiation trend ("Bowen trend") is often thought to occur under conditions open to oxygen (e.g. Morse, 1980b). Indeed, Osborn (1959) and Presnall (1966) demonstrated with experiments in simplified model systems (MgO-FeO-Fe2O3-SiO2 and CaMgSi2O6-Mg2SiO4-Fe2O3-FeO) that magmatic systems crystallizing closed to oxygen should show a continuing iron enrichment of the melt, even after the appearance of magnetite, whereas in systems open to oxygen such iron-rich liquids should not be produced. These trends were confirmed by thermodynamical modelling in complex systems (Ghiorso & Carmichael, 1985, 1987). These calculations also pointed to a very strong increase in oxygen fugacity with crystallization under closed-system conditions (Ghiorso & Carmichael, 1987). However, it must be stressed that the models were not well calibrated for Fe-rich and Ti-bearing compositions because of the lack of experimental data at this time. Therefore it is not surprising that recent model calculations based on equilibrium phase relations, mineral-melt partitioning of major elements and mass balance constraints in an 8-component Ti-bearing system come to quite different results. They do not reveal any drastic difference in the liquid lines of descent of closed vs. open systems for ferro-basaltic magmas and show only a moderate increase of 
FMQ during the first stages of crystallization in closed systems (Toplis & Carroll, 1995, 1996).
Discussions on the control of the oxidation state of magmas in response to crystallization under closed or open system conditions are hampered by the fact that there are no available experimental data gained under conditions closed to oxygen. Such experiments were reputed to be difficult or even impossible (e.g. Toplis & Carroll, 1996). In fact, it is possible to simulate conditions closed to oxygen at 1 bar in the laboratory, by performing high-temperature experiments in evacuated silica-glass tubes.
The silica-glass tube technique has been extensively used for synthesis in dry sulfide systems at temperatures up to 1000 °C (e.g. Kullerud, 1971). The technique was also applied to sub-solidus synthesis and re-equilibration experiments on oxide parageneses in the temperature range 700–1300 °C (Knecht et al., 1977; Lattard, 1987, 1995). To our knowledge, however, only a few runs have been performed in which partially molten silicate samples were equilibrated alone in silica-glass ampoules, i.e. without solid oxygen buffer (Fudali, 1972).
The purpose of the present paper is essentially to show how the silica-glass tube technique has been adapted to the special requirements of crystallization experiments from silicate melts under closedsystem conditions and to demonstrate that the sample systems remain closed during the experiments, in particular to oxygen. To illustrate the use of the method we present a few preliminary experiments pertaining to the crystallization-differentiation of a ferrobasalt under closed-system conditions. We wish to stress that no definitive conclusions will be drawn on the basis of this restricted number of experiments. A complete study on this topic is in progress and will be presented elsewhere.
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Experimental procedure |
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TopAbstractIntroductionExperimental procedureTesting the method: does...Application: preliminary...Appendix: Discussion of the...AcknowledgementsReferences |
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The experiments are conducted in two steps:
High-temperature techniques
All high-temperature experiments (step 1 and 2) were conducted in a vertical quench furnace, with a hot zone (± 0.5 °C) of approximately 3 cm in length. The temperature was measured before and after the runs with a type S Pt-Pt90Rh10 thermocouple calibrated against the melting point of silver (960.8 °C), gold (1064.4 °C) and copper (1083.5 °C). The temperature was controlled within ±0.2 °C by commercial controllers. All experiments were terminated by drop-quenching the samples into water.
In the step-1 experiments, the oxygen fugacities in the logfO2 range –7.2 to –10.6 were fixed by mixing high-purity CO and CO2 gas with electronic valves (Tylan) and are estimated to have remained within ±0.2 log unit of the target value. The fugacities were controlled at the end of the experiments with an yttria-stabilized zirconia sensor with air as the reference. The sensor was calibrated against the Ni-NiO (O'Neill, 1987) and magnetite-wüstite (O'Neill, 1988) equilibria. The run durations were between 8 h (Pt-loops as sample containers) and 24–48 h (AgPd crucibles as sample containers).
For the step-2 experiments, the samples, placed in silica-glass ampoules, were initially heated above their liquidus and subsequently cooled at constant rates of 2–3 °C/h to the final temperature of interest. Samples were held at the final temperature for a duration of up to 150 h to allow equilibration between crystalline phases and coexisting melts.
Sample container
In the step-1 experiments a sample container is necessary to hold the sample in the hot zone of the vertical furnace. For small samples (40 mg), loops of about 3 mm diameter, made of a platinum wire of 0.1 mm diameter, were used. For larger samples (approximately 120 mg) we prefered small crucibles (5 mm outer diameter; 0.3 mm wall thickness:) machined from an Ag60Pd40 alloy.
In the step-2 experiments the sample container aimed essentially at avoiding a direct contact of the sample with the silica-glass tube, which could lead to a contamination of the sample and to a premature destruction of the ampoule during the experiments. Samples equilibrated in crucibles were simply transferred with their container in the silica ampoules. Small samples equilibrated in wire loops were placed in AgPd crucibles before they were introduced into the ampoule (Fig. 1).
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Although AgPd alloys with high Ag contents were expected to take up only very small amounts of iron (Muan, 1963), it proved necessary to "presaturate" the crucibles in the same way as the Pt wire. We also used them for several experiments at the same starting oxygen fugacity.
Preparation of the silica-glass ampoules
The silica-glass tubes were evacuated with a rotary vane pump to a vacuum in the order of 10–2 mbar prior to sealing. A silica-glass filler rod was used to minimize the internal volume of the ampoule (Fig. 1). To prevent a premature crystallization of the ampoule during the experiments, it proved useful to heat the empty silica-glass tube and the silica-glass filler rod for several minutes with a welding torch, prior to evacuating and sealing.
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Testing the method: does the system remain closed? |
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TopAbstractIntroductionExperimental procedureTesting the method: does...Application: preliminary...Appendix: Discussion of the...AcknowledgementsReferences |
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The main requirement to the method is that the sample system remains closed to oxygen during the step-2 experiments. There are three potential weaknesses in the procedure which may lead to a change of the oxygen content during the experiments: (1) the vacuum established in the ampoule may be insufficient, (2) the ampoule may leak during the high-temperature run, due to crystallization of the silica glass to cristobalite, and (3) the loss of iron from the sample to its container may lead to a slight oxidation of the sample system.
The effect of oxidation caused by insufficient vacuum in the ampoule can be easily estimated, if we assume that the atmosphere in the evacuated ampoule is similar in composition to air. If the volume of the filled AgPd crucible (5 mm in length and in diameter) is substracted from the volume of the inner space of the silica-glass ampoule (6 mm in length and in diameter; Fig. 1) the remaining free space (Vvac) inside the silica-glass ampoule amounts to about 72 mm3. Considering that the prevailing vacuum pressure (p) inside the ampoule is in the order of 10–2 mbar and that oxygen represents 21 vol.% of the air, the remaining mass of oxygen (moxygen) can be calculated from:
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In contrast, a 100 mg sample, as used in the closed-system experiments, contains about 1.4 *10–4 mole FeO (assuming 10 wt.% FeO). The amount of ferrous iron in the sample is about 7 to 8 orders of magnitude higher than the amount of oxygen in the remaining gas phase in the ampoule, which means that no detectable oxidation of the sample can be expected, even if the vacuum reaches only 10–1 mbar.
Considering, as pointed out by a reviewer, that the residual gas in an evacuated ampoule consists essentially of species not easily pumped by the machinery, and of those that desorb from surfaces or diffuse through the walls, it can be expected to be dominated by hydrogen. At high temperature, the quantity of gas desorbed will increase considerably, spoiling the vacuum, but decreasing the oxygen fraction in the residual gas. Alltogether, it is impossible to quantify the composition of the residual gas, but its oxygen content must be still smaller than the one calculated before and its redox effect on the sample can only be negligible.
Leakage of the ampoules during the high temperature experiments is improbable because, after quenching, the ampoules are always very clear with no detectable cracks and no apparent recrystallization even after run durations of 44 hours at 1178 °C or of 138 hours at 1133 °C (Fig. 2). No visible water intrudes the ampoule during the quench process.
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The only relatively thin-walled (1 mm) part of our ampoules is that around the sample (Fig. 1), with an outer surface of about 1.5 cm2. With a gas pressure of 10–2 mbar inside the ampoule and an oxygen content of 21% of the surrounding air, the volume of oxygen penetrating an ampoule in 100 h at 1100 °C can be calculated as: 3*10–4 cm3, which corresponds to 1.4*10–8 mole. This value represents an upper limit because the lag time for steady state diffusion must be much longer for our 1 mm thick ampoules. In any case, this oxygen input is too low to oxidize samples with a typical FeO content of 1.4* 10–4 mole (see FeO estimate in previous section).
The loss of iron to the sample container induces a change in the oxygen content of the sample system. Because only metallic iron may alloy with the container material, part of the ferrous iron of the sample must disproportionate according to the model reaction:
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Analytical control of the composition of the samples
In order to assess the real importance of the factors enumerated before (especially the iron loss to the container) and to test whether the system can be considered as closed to oxygen or not, the most efficient way is to check both the total iron and the ferrous iron content of the samples before and after step-2 experiments.
To maximize a possible oxygen diffusion through the silica-glass ampoule and the loss of iron to the sample container, we have chosen to perform both step-1 and step-2 experiments at near-liquidus temperatures (1180–1190 °C), with relatively long run durations (24–48 h). Moreover, we have used "pre-saturated" AgPd crucibles which have a large contact surface between sample and container.
The starting material (SC1-P) was a synthetic glass powder of ferro-basaltic composition, with about 13 wt.% FeOtot, i.e. nearly identical to that used by Toplis & Carroll (1995) in their open-system experiments aimed at modelling the liquid line of descent in basic layered intrusions. Batches of about 120 mg of the SC1-P starting material were first equilibrated (step-1 experiments; see Table 1) in "pre-saturated" Ag60Pd40 crucibles at near-liquidus temperatures (1189 or 1178 °C) and different fo2 for up to 44 h and quenched in distilled water. Half of each charge was removed from the crucibles and analysed. The rest of the charges was returned into the same crucibles, sealed in evacuated silica-glass ampoules and held again for 23 h at practically the same temperatures (step-2 experiments; see Table 1). After quenching, these glassy samples were analysed as well. The series of runs at 1178°C was performed after that at 1189 °C, using at each fo2 the same crucibles, which means that the crucibles were expected to absorb less iron during the second series of experiments.
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The results of the wet chemical ferrous iron analyses are encouraging: within analytical uncertainties, the FeO contents are the same before and after the step-2 experiments. However, the general tendency is that of a slight decrease of these contents, i.e. a slight oxidation, during the step-2 runs (Table 2; Fig. 3a). It is very improbable that this oxidation might be due to oxygen diffusion from the surrounding atmosphere into the silica-glass ampoule. Otherwise, the clear inverse correlation between the FeO contents and the oxygen fugacities fixed during the step-1 experiments would not be kept (Fig. 3a). The most probable explanation for a slight oxidation is that some iron is lost from the sample to its AgPd container.
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Summarizing, during the step-1 (open system) experiments there is indeed a small loss of iron from the sample to its AgPd container, which is strongest at low oxygen fugacities but does not exceed about 10 % relative (at FMQ-1; Fig. 3; Table 2). This is on the same order of magnitude as the iron loss reported for experiments performed with platinum loops (e. g. Toplis & Carroll, 1995). In contrast, the loss of iron to the AgPd crucibles during the step-2 (closed system) runs is apparently within the analytical uncertainties (Fig. 3, Table 2). Considering that the present test experiments were all performed at high, near-liquidus, temperatures, still smaller iron loss can be expected for experiments at temperatures 50 to 150 °C lower. The correlated oxidation of the sample should also be negligible.
At lower, sub-liquidus temperatures, however, another effect has to be considered. If minerals containing no or little Fe3+ are crystallizing (e.g. olivine), the residual melt will be oxidized and the oxygen fugacity will increase (cf. Kress & Carmichael, 1991). A reviewer suggested that this could result in the oxidation of part of the metallic iron alloyed with AgPd which could move back as Fe2+ into the melt. In this case, the system would not be closed anymore. This effect cannot be ruled out but we do not think that it plays a significant role. First, the changes in fO2 induced by partial crystallization of basaltic liquids in closed systems during cooling over about 100 °C are predicted not to exceed 1.5 log units (e.g. Toplis & Carroll, 1996). As shown in Fig. 4, this should not translate in a drastic change of the Fe content of the crucible, inasmuch as the saturation level is not reached. Moreover, Johannes & Bode (1978) have shown that metallic iron electroplated as a thin film onto the inner wall of a Pt capsule preferentially migrates into the capsule wall and not into the sample melt during superliquidus experiments conducted at about FMQ.
Altogether, we are confident that under subliquidus closed-system conditions the transfer of iron between sample and container and the correlated redox changes in the sample are kept to a minimum. As sodium evaporation is not expected during runs performed in evacuated silica-glass ampoules, the sample system in these experiments may well be considered as closed, for all elements including oxygen.
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Application: preliminary experiments on the differentiation of a ferrobasalt under closed-system conditions |
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TopAbstractIntroductionExperimental procedureTesting the method: does...Application: preliminary...Appendix: Discussion of the...AcknowledgementsReferences |
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In all six experiments reported here, the oxygen fugacities fixed in the first step at superliquidus conditions (1171–1200 °C) corresponded to those of the FMQ buffer (Table 2). The main reason for this choice was that it enabled a direct comparison of our experimental results not only with those of Toplis & Carroll (1995) in open system conditions but also with their predicted values under closed-system conditions. We are aware that these starting oygen fugacities might be too high to model the initial redox conditions of a parental tholeiitic magma, especially in the case of layered basic intrusions.
The final temperatures during the step-2 experiments were in the range 1091–1146 °C, i.e. significantly above the solidus (T < 1040 °C, according to Toplis & Carroll, 1995). To ensure a good crystallization of the mineral phases, the samples held in silica-glass ampoules were brought above their liquidus temperatures for a short time (0.5 h) and slowly cooled to their final temperature (ramp: 2 or 3 °C/h; Table 1). To allow equilibration, the final temperatures were held for at least 3 days (Table 1). At the end of the runs the samples were dropquenched into water.
The run products consist of euhedral crystalline phases homogeneously distributed within the quenched melt. The grain size is not only positively correlated with the equilibrium temperature but also dependent on the type of sample container. If a Pt-loop was used, the crystals are coarse-grained (up to 2 mm in length), whereas in samples directly held in AgPd crucibles the size is significantly smaller (up to 300 µm in length). This is probably related to heterogeneous nucleation at the large contact surface of the sample with the crucible. Electron-microprobe analyses of olivine, magnetite, ilmenite and quenched melts show their compositions to be homogeneous, irrespective of their location within the sample. Clinopyroxene and plagioclase crystals are slightly zoned. The maximum compositional ranges, recorded at the lowest temperature (1091 °C; run CS20; Table 1) span ± 4 mole% An for plagioclase and ± 4 mole% Wo, En, Fs for clinopyroxene. The crystalline phases are the same as those observed by Toplis & Carroll (1995) in their open-system experiments and they appear in the same order with decreasing temperature, i.e. plagioclase, olivine, clinopyroxene, magnetite-ulvöspinelss, and ilmenite-hematitess. The main difference to the open-system results is that magnetite first crystallizes at significantly higher temperatures (T > 30 °C). In contrast, ilmenite appears at practically the same temperature as under open-system conditions (Fig. 6). The first observation is easily explained, considering that under closed-system conditions, in which the bulk Fe3+/Fetot remains constant, the crystallization of the Fe2+-bearing phases (olivine, clinopyroxene) leads to a relative oxidation of the residual melt, favouring the crystallization of magnetite. In contrast, in an open system with a constant oxygen fugacity, the Fe3+/Fetot value of the melt is kept constant, preventing the crystallization of magnetite at high temperatures.
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In the temperature interval 1146–1103 °C there is an excellent agreement between our experimental data and the results retrieved by Toplis & Carroll (1996) for closed-system conditions from mass balance calculations (Fig. 6). At lower temperatures, however, the model of Toplis & Carroll (1996) fails to reproduce our FeOtot values in the melt. This is not that surprising because mass balance calculations with 6 phases in an 8-component system are extremely uncertain. This exemplifies the need for results from closed-system experiments.
One of the main question to be addressed in such forthcoming experiments is that of the liquid evolution in basic layered intrusions. The present preliminary results have already shown, in agreement with the model of Toplis & Carroll (1996), that closed-system conditions do not lead to high iron concentrations in the melt during equilibrium crystallization if the initial redox conditions are moderatly high (FMQ). With lower initial fO2 conditions, as generally proposed for basic intrusions, the maximum iron content of the residual melt is expected to be significantly higher. Whether it may reach the exceptionally high values typical of the Fenner Trend critically depends on the evolution of the modal proportions of the Fe-Ti oxides with decreasing temperature. Equilibrium crystallization experiments in closed system complemented with modelling of fractional crystallization processes will help in solving these open questions.
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Appendix: Discussion of the analytical results on glassy run products of near-liquidus experiments (Table 2) |
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TopAbstractIntroductionExperimental procedureTesting the method: does...Application: preliminary...Appendix: Discussion of the...AcknowledgementsReferences |
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In the other run products (from step-1 or from step-2 experiments at 1189 °C), however, plagioclase could not be found. Interestingly, the same material re-equilibrated at 1380 °C either in air or in a CO/CO2 gas mixture corresponding to FMQ+1 does not show higher TiO2, CaO or MgO contents.
Therefore we think that the slightly higher TiO2, CaO and MgO contents in products of runs at 1178–1189 °C cannot be due to any contamination and probably reflect problems with the correction procedure of the EMPA.
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Acknowledgements |
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TopAbstractIntroductionExperimental procedureTesting the method: does...Application: preliminary...Appendix: Discussion of the...AcknowledgementsReferences |
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Received 13 September 2000
Modified version received 15 November 2000
Accepted 4 December 2000
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TopAbstractIntroductionExperimental procedureTesting the method: does...Application: preliminary...Appendix: Discussion of the...AcknowledgementsReferences |
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standard deviations over n single analyses). Further comments in the 
