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The solubility of TiO2 in olivine: implications for the mantle wedge environment
Chemical Geology 160 Ž1999. 357–370 www.elsevier.comrlocaterchemgeo
The solubility of TiO 2 in olivine: implications for the mantle wedge environment L. Dobrzhinetskaya ) , K.N. Bozhilov, H.W. Green
1
II
Institute of Geophysics and Planetary Physics and Department of Earth Sciences, UniÕersity of California, RiÕerside, CA 92521, USA Received 27 May 1998; accepted 10 September 1998
Abstract One characteristic of many subduction-zone garnet peridotites is that they contain titanium-bearing phases not otherwise found in mantle rocks. In particular, titanoclinohumite andror its breakdown assemblage consisting of symplectic intergrowths of olivine and ilmenite is common in many of these bodies. The Alpe Arami garnet lherzolite of the Swiss Alps, while lacking titanoclinohumite, displays instead large numbers of FeTiO 3 rod-shaped precipitates in the oldest generation of olivine, amounting to approximately 1% by volume, indicating that at some time in its past, the peridotite experienced conditions under which the solubility of TiO 2 in olivine was ) 0.6 wt.%. In order to test the hypothesis that the environment of very high solubility of TiO 2 in olivine is to be found at very high pressures, we have conducted experiments on lherzolite compositions with added ilmenite at pressures between 5 and 12 GPa and temperatures of 1350–1700 K. Our results on anhydrous compositions show that whereas solubility of TiO 2 was not detected in olivine at 5 GPa, 1400 K where it coexists with rutile, when rutile disappeared from the paragenesis, the solubility climbed to 0.4 wt.% at 8 GPa, 0.5 wt.% at 10 GPa and to ) 1.0 wt.% at 12 GPa, 1700 K. These results support our previous interpretations from titanate morphology and abundance that the Alpe Arami massif has surfaced from P s 10 GPa but remove the need to suggest a deeper origin and possible precursor phase such as wadsleyite. They also support the hypothesis that garnet peridotites with unusual Ti-bearing phases reflect a unique mantle environment occurring in the mantle wedge overlying subduction zones. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Olivine; Titanium solubility; Mantle wedge; Deep subduction
1. Introduction It is now generally accepted that emanations of hydrous fluids andror hydrous partial melts from subducted lithosphere flux melting of mantle peridotite at temperatures below the anhydrous solidus contribute to the formation of island-arc volcanism. ) Corresponding author. Tel.: q1-909-787-5535; fax: q1-909787-5924 1 E-mail: [email protected].
Although it is clear that such fluids carry slab signatures, the behavior of the high field-strength elements ŽHFSE. during this process is a matter of some discussion. Garnet peridotites that have surfaced in areas of previous deep subduction commonly Žubiquitously?. carry titaniferous phases that are different from or in greater abundance than those present in mantle peridotites from other environments. For example, titanoclinohumite ŽTiCH. andror its symplectitic dehydration products, olivine q ilmenite, occur in peridotites of the microdia-
0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 1 0 7 - 2
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L. Dobrzhinetskaya et al.r Chemical Geology 160 (1999) 357–370
mond-bearing Kokchetav massif, Kazakhstan ŽEfimov, 1964; Mockel, 1969; Udovkina, 1985., the ¨ Cima de Gagnoni body of the high pressure Adula nappe of the Swiss Alps ŽTrommsdorff and Evans, 1980; Evans and Trommsdorff, 1983. and in the ultra-high-pressure metamorphic terrane of the Dabie Mountains of China ŽYang et al., 1993; Zhang et al., 1995.. Xenoliths in diatremes from the Colorado Plateau of the Southwestern United States also have been reported to contain TiCH and have been interpreted as reflecting hydrous alteration of the mantle wedge overlying a subduction zone ŽMcGetchin et al., 1970; Aoki et al., 1976.. The Alpe Arami massif of the Adula nappe lacks TiCH but contains up to 1 vol.% FeTiO 3 precipitates in olivine that imply a previous solubility of ) 0.6 wt.% TiO 2 ŽDobrzhinetskaya et al., 1996; Green et al., 1997a,b.. Evidence for such extensive solubility of TiO 2 in olivine had not been reported previously; mantle olivine from a variety of environments had shown solubility increasing with depth but with a maximum of ; 0.05 wt.% TiO 2 in ‘‘hot garnet lherzolites’’ from South African kimberlites ŽHervig et al., 1986. which have a maximum depth of about 200 km Že.g., Boyd, 1973.. This contrast caused Dobrzhinetskaya et al. Ž1996. to conclude that Alpe Arami represents a mantle environment not previously sampled at the surface, perhaps representing very great depth. Indeed, these authors and Green et al. Ž1997b. suggested that there might be no conditions under which olivine could dissolve such large amounts of TiO 2 , and proposed that the FeTiO 3 precipitates in Alpe Arami olivine might be evidence for a precursor phase that preceded olivine in the paragenesis, perhaps wadsleyite, stable only at depths ) 400 km. These observations clearly distinguish subductionzone peridotites from all other mantle rocks and raise critical questions about the source of their unusual Ti-bearing phases. To pursue these questions, we are investigating the solubility of TiO 2 in olivine as a function of pressure, temperature, and H 2 O content. We present here the results of our dry experiments conducted on powders of natural minerals with bulk composition close to that of the Alpe Arami lherzolite with the addition of excess ilmenite. We find a profound pressure effect on the solubility of TiO 2 in olivine, which correlates strongly with the stable coexisting titanate phase; the amount formerly pre-
sent in Alpe Arami olivine can be dissolved at pressures in excess of 10 GPa, consistent with our previous estimates of minimum pressure based upon precipitate abundance, type, and morphology ŽDobrzhinetskaya et al., 1996; Green et al., 1997a,b.. 2. Materials and methods 2.1. Apparatus and conditions of the experiments Our experimental program was conducted with a multianvil apparatus ŽWalker-style ‘‘hatbox’’; Walker, 1991; Walker et al., 1990.. Room temperature calibration for pressure was established with Bi phase transitions at 2.55 ŽI–II. and 7.7 GPa ŽIII–IV.. The calibration was within experimental uncertainty of that of Walker’s similar calibration for his identical apparatus; we adopted Walker’s high-temperature calibration for pressure. Pressure-medium octahedra were made from semi-sintered MgO–5% Cr2 O 3 ceramic ŽCeramacast 584-OF, Aremco Products.. Within the internal space of the apparatus, eight 25-mm WC cubes, each with a 6-mm octahedral truncation on one corner, are arranged in the form of a larger cube with their octahedral truncations facing inwards to form an octahedral cavity into which the pressure medium and sample are placed. To minimize friction and to provide electrical insulation, sheets of fiberglass laminate ŽG-10. coated with a thin film of dry non-silicon mold release Ž811 DFVMR. are placed between the WC cubes and the six anvils. Table 1 summarizes the conditions of all experiments. Those conducted at pressures of 5–10 GPa were slowly cold-compressed at room temperature to the desired pressure followed by slow heating to the desired temperature. Experiments conducted at 12 GPa were pressurized initially to 9 GPa at room temperature, heated to approximately 750 K to allow more stable compression of the pressure medium, further pressurized to 12 GPa, and finally heated to the desired temperature. All but five experiments consisted of ‘‘one stage’’, meaning they were subjected to one set of experimental conditions and then quenched to room temperature in seconds by shutting off the power to the apparatus, followed by programmed depressurization to atmospheric pressure over 24–35 h.
L. Dobrzhinetskaya et al.r Chemical Geology 160 (1999) 357–370
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Table 1 Experimental conditions. All experiments were performed ‘‘dry’’. Asterisks are put after the specimen numbers for which starting materials were Ol Ž90%. and Ilm Ž10%. P ŽGPa.
T ŽK.
Time Žh.
Ol:TiO 2 Žwt.%., EDS ŽSEM.
Ol:TiO 2 Žwt.%., EDS ŽTEM.
Run number
5 8 8 8 8 10 10 12 12 12 12 12 12 12 12 12 12 12 12 12 10 12 12
1400 1500 1500 1600 1600 1500 1600 1450 1500 1600 1650 1700 1800 1600 1350 1700 1550 1700 1550 1650 1650 1900 1700
24 0.8 14 5 18 24 12 2 24 24 20 2.2 0.8 20 10 24 24 24 72 24 57 17 24
- 0.02 0.26 0.42 0.38 0.47 0.46 0.52 0.50 0.66 0.98 1.02 0.78 0.90
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 1.17 n.d. n.d. n.d.
ma71 ma58 ma61 ma64 ma67 ma57 ma62 ma56 ma65 ma63 ma79 ma43 ma40 ma68
0.99
0.50–1.33
0.98
0.87–1.1
0.91
n.d.
ma74U ma78 ma81U 0.57
0.57–1.00 ma82U
1.12
In addition to the single stage experiments, five ‘‘two-stage’’ experiments Žma68, ma74, ma78, ma81, and ma82. were conducted to approach the equilibrium concentration of dissolved TiO 2 from the oversaturated side ŽTable 1.. The specimens were first held at one set of conditions followed by a second set of conditions at lower temperature andror pressure. The first stage of these experiments occupied 17–24 h at a pressure of 12 GPa and at temperatures of 1600–1900 K to establish a high dissolved TiO 2 content in olivine. The second stage of all except ma81 consisted of further annealing at 12 GPa but under lower temperature conditions. The temperature was reset to the second set of conditions by adjustment of power to the Eurotherm temperature controller as quickly as possible Ž1–3 min. while in the manual mode. In the case of ma81, following the first stage of heating Ž24 h. the temperature was reduced to room temperature, pressure was slowly reduced to 10 GPa and the temperature was returned to 1650 K for 57 h. At the end of these experiments,
n.d.
they also were temperature-quenched by turning off the power to the apparatus, followed by programmed depressurization over 30–35 h. 2.1.1. Sample assemblies We explored various sample assemblies to develop one well-suited for this particular study. Graphite furnaces were unacceptable because our experiments are all in the diamond stability field and at temperatures high enough to expect reaction to proceed and destroy the resistance-furnace function. Early on, we performed garnet–olivine thermometry experiments to determine the temperature gradient in various assemblies with lanthanum chromate and rhenium furnaces. This technique is based on the temperature dependence of the compositions of coexisting garnet and olivine. The temperature distribution within the sample charge encapsulated in Pt and placed adjacent to the thermocouple was calculated using the empirical equation of O’Neill and Wood Ž1979.. We found that for both lanthanum chromate
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L. Dobrzhinetskaya et al.r Chemical Geology 160 (1999) 357–370
and rhenium furnaces, temperature gradients from sample center to end were unacceptably large Ž150– 250 K., non-reproducible, and not symmetrical Žeither from end-to-end or radially.. Thereafter, we evolved the following assembly that was used for all of the experiments reported here ŽFig. 1.: a cylindrical hole with diameter 4.76 mm was drilled normal to one pair of octahedral faces of the ceramic octahedron that serves as a pressure cell. The assembly consists of a resistance furnace made from Re-foil surrounding a tube of semi-sintered pure MgO ceramic manufactured by Ozark Tech. The powdered sample was placed at the center of the assembly, with semi-sintered MgO spacers on either side to complete sample encapsulation. W–3% Re–W–25% Re thermocouple junctions were placed within the sample to ensure that chemical analyses were made on materials subjected to the temperature recorded for the run. All reported chemical measurements were made within a few hundred microns of the thermocouple, greatly reducing the problem of uncertainty in temperature measurement that has been widely discussed in the literature Že.g., Zhang and Herzberg, 1994; Walter et al., 1995; Bertka and Fei, 1997.. However, as for all multianvil experiments, we have reported our temperatures assuming a zero
pressure effect on thermocouple emf. That has the effect of reporting temperatures that are too high by an unknown amount. This is a problem for all highpressure apparatus using thermocouples and is minimized by using W–Re. No presence of either W nor Re has been detected in specimen crystals. 2.1.2. Starting materials This study was initiated to test the hypothesis that the conditions experienced by the Alpe Arami peridotite that were responsible for the former very high solubility of TiO 2 involved extremely high pressures ŽDobrzhinetskaya et al., 1996; Green et al., 1997a,b.. Therefore, the starting material for most experiments consisted of lherzolite powder made of minerals taken from that massif, with the exception of olivine for which San Carlos, AZ, olivine has been substituted to avoid the FeTiO 3 precipitates abundant in Alpe Arami olivine. The powders consist of 66% olivine, 8% each of Cpx, Opx, and Grt from Alpe Arami, and 10% pure FeTiO 3 ilmenite of unknown source. Three experiments Žma74, ma79, ma81 and ma82. were conducted on the composition 90% ŽSan Carlos. olivine and 10% ilmenite. Experiment ma78 had sample composition of 90% of San Carlos olivine, 7% ilmenite and 3% chromite. The minerals
Fig. 1. Schematic cross section showing the sample assembly and materials used for this study.
L. Dobrzhinetskaya et al.r Chemical Geology 160 (1999) 357–370
for both starting mixes were powdered by grinding separately in an agate mortar under ethanol and dried for 24 h at 383 K in vacuum. Grain sizes of the final mix were evaluated under the optical microscope as 3–5 mm Ž75%. and 10–15 mm Ž25%.. 2.1.3. Sample preparation for analysis The dimensions of the samples after the experiments is usually about 2 = 2 mm2 . After recovery from the apparatus, the samples were cut into two pieces and two doubly-polished thin sections with thickness ; 30 mm were prepared for each experiment. One thin section was usually used for optical microscopy and scanning electron microscopy ŽSEM. studies, the another one for transmission electron microscopy ŽTEM. thin foil preparation. TEM foils were prepared by standard Ar-ion milling ŽPIPS, Gatan. at an angle of incidence of 58–68 at an applied voltage of 5 kV and beam current of 10 mA for 30–45 min. Before milling, the sample was transferred onto a diamond grid to avoid contamination produced by copper holders in cases for which the sample size was not large enough to cover up the entire surface of the support grid. Specimens were carbon coated for electron microscopy. 2.2. SEM SEM and microanalysis were performed at our Analytical Electron Microscopy Facility with a Philips XL30 instrument, equipped with FEG operated at 15 and 20 kV. The EDAX microanalytical system of this microscope consists of an energy dispersive spectrometer ŽEDS. equipped with Si detector with super-ultra-thin window, resolution of 137 eV at MnK a. The spectral data were acquired at 1500 to 2000 countsrs with dead time below 25%, beam current of about 1 nA, effective spot size about 1.5 mm, 200–300 s counting time, and were reduced using the eDXi software from EDAX and the ZAF correction scheme ŽArmstrong, 1988.; results are presented in Table 2. Natural and synthetic mineral standards of olivine and rutile were used for the quantification procedure. The oxide concentrations were calculated by normalization. The analytical error was estimated by analyzing a control grain of olivine from San Carlos, AZ with known composi-
361
tion. The estimated error from the control measurements together with the counting statistics at 2 s level is - 2% for Si and Mg, - 4.5% for Fe, and in the range of 2%–10% for Ti and Ni. This leads to an absolute error in determination of TiO 2 concentration to between "0.05 and "0.1 wt.% for grains where the concentration is above 0.4 wt.% and error of "0.15 wt.% for concentrations - 0.4 wt.%. Minor amounts Ž- 0.2%. of Al and Cr were detected in a few spectra; such spectra were not included in averaging of the analyses. 2.3. Analytical TEM Analytical TEM studies also were conducted at our Analytical Electron Microscopy Facility using a Philips CM300 electron microscope with twin lens and LaB 6 cathode, operated at 200 and 300 kV. Images in bright and dark field mode as well as diffraction patterns were obtained to study the specimens of interest. Lightly carbon-coated specimens Ž; 5 nm C film thickness. were mounted on a Philips low-background double-tilt holder for examination in the TEM. The analytical equipment consists of EDAX system for acquisition and processing of energy dispersive X-ray spectra ŽEDS., equipped with Si detector with resolution of 135 eV at MnK a , ultra-thin window and the MX-TEM software. The EDS spectra were collected with specimen tilt of about 258 towards the detector resulting in effective take-off angle of 388. EDS analyses were quantified using the CliffLorimer k factor method ŽCliff and Lorimer, 1975.. Experimentally calculated k factors for Si, Mg, Al, Ti, Fe, Ni were obtained by using natural mineral standards of olivine, diopside, titanite, kyanite, and ferrosilite and following the procedure described by Livi and Veblen Ž1987. and Klein et al. Ž1997.. Thirty spectra were collected from each standard specimen at the same conditions as the experimental material. All analyses were taken at edges and thin areas of interest where absorption was negligible. The standard deviation for the averaged analyses of the standard silicates is 2% or less which corresponds to error of 2% or less in determination of the k factors.
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Table 2 EDS microanalysis of the olivine synthesized in the experiments. The standard deviation is given in parentheses. Analysis labeled by U s EDS, TEM; analysis without labelings EDS, SEM; n s number of analysis Oxides
SiO 2
TiO 2
MgO
FeO
NiO
MnO
n
ma71 ma58 ma61 ma64 ma67 ma57 ma62 ma56 ma65 ma63 ma63U r1 ma63U r2 ma63U r3 ma63U r4 ma63Ur5 ma63U r6 ma63U r7 ma63U r8 ma63U r9 ma63U r10 ma63U avr ma79 ma43 ma40 ma68 ma68U ra ma68U rb ma68U rc ma68U avr ma74 ma74U ra ma74Urc ma74U avr ma78 ma78U ra ma78U rb ma78U rc ma78U avr ma81 ma81U ra ma81U rb ma81U rc ma81U rd ma81U avr ma82avr
40.55Ž0.45. 41.10Ž0.11. 41.30Ž0.37. 41.66Ž0.28. 41.13Ž0.16. 41.21Ž0.09. 41.33Ž0.44. 41.34Ž0.07. 41.64Ž0.51. 41.45Ž0.64. 40.40 41.455 41.477 40.814 41.129 41.371 41.290 40.881 40.770 40.998 41.059Ž0.33. 41.59Ž0.25. 41.50 40.96 41.06Ž0.19. 40.457 40.676 41.890 41.014Ž0.77. 41.12Ž0.36. 41.137 40.676 40.907Ž0.23. 41.54Ž0.23. 42.819 41.101 41.991 41.970Ž0.23. 41.90Ž0.58. 42.400 42.594 42.326 41.961 42.320Ž0.27. 41.47Ž0.28.
0.01Ž0.007. 0.26Ž0.01. 0.42Ž0.10. 0.38Ž0.02. 0.47Ž0.07. 0.46Ž0.05. 0.52Ž0.04. 0.50Ž0.11. 0.66Ž0.10. 0.98Ž0.16. 1.164 1.087 1.456 1.172 1.173 1.315 1.138 1.106 1.065 1.062 1.174Ž0.12. 1.02Ž0.04. 0.78 0.90 0.99Ž0.18. 1.332 0.872 0.503 0.902Ž0.41. 0.98Ž0.05. 1.098 0.872 0.985Ž0.16. 0.91Ž0.19. 0.731 0.622 0.597 0.650Ž0.19. 0.57Ž0.06. 0.573 0.626 0.615 0.997 0.703Ž0.20. 1.12Ž0.16.
49.99Ž0.14. 49.55Ž0.29. 48.83Ž0.39. 48.78Ž0.30. 49.34Ž0.24. 49.94Ž0.17. 49.20Ž0.47. 50.58Ž0.24. 50.00Ž0.30. 50.09Ž0.25. 50.711 50.58 49.941 50.919 50.683 50.313 50.428 50.669 51.172 50.917 50.633Ž0.33. 49.91Ž0.61. 50.42 50.47 50.56Ž0.27. 50.333 51.018 50.505 50.619Ž0.36. 50.42Ž0.44. 50.849 51.018 50.934Ž0.12. 50.06Ž0.24. 49.879 50.893 50.232 50.335Ž0.24. 49.00Ž0.57. 49.315 48.902 49.194 48.705 49.029Ž0.28. 50.16Ž0.27.
8.95Ž0.27. 8.82Ž0.21. 9.18Ž0.32. 8.78Ž0.09. 8.85Ž0.21. 8.14Ž0.07. 8.61Ž0.59. 7.37Ž0.17. 7.40Ž0.33. 7.14Ž0.41. 7.262 6.605 6.702 6.751 6.726 6.746 6.751 6.889 6.616 6.618 6.817Ž0.18. 7.15Ž0.56. 7.02 7.25 7.04Ž0.37. 7.484 7.130 6.707 7.107Ž0.39. 7.21Ž0.37. 6.485 7.130 6.807Ž0.46. 7.16Ž0.28. 6.055 6.955 6.797 6.602Ž0.28. 8.33Ž0.46. 7.343 7.498 7.498 7.894 7.558Ž0.24. 7.04Ž0.29.
0.50Ž0.03. 0.27Ž0.18. 0.27Ž0.11. 0.40Ž0.05. 0.21Ž0.20. 0.27Ž0.09. 0.31Ž0.12. 0.21Ž0.16. 0.30Ž0.11. 0.20Ž0.25. 0.462 0.274 0.425 0.345 0.290 0.256 0.393 0.455 0.376 0.406 0.368Ž0.07. 0.15Ž0.04. 0.28 0.42 0.35Ž0.08. 0.394 0.303 0.395 0.363Ž0.05. 0.27Ž0.14. 0.431 0.303 0.367Ž0.09. 0.33Ž0.14. 0.517 0.428 0.383 0.443Ž0.14. 0.20Ž0.11. 0.364 0.380 0.367 0.443 0.390Ž0.04. 0.21Ž0.09.
n.d. n.d. n.d. n.d. n.d. n.d. 0.22Ž0.11. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.19Ž0.04. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
2 2 3 3 3 3 5 5 6 7 1 1 1 1 1 1 1 1 1 1 10 5 1 1 3 1 1 1 3 3 1 1 2 5 1 1 1 3 8 1 1 1 1 4 5
The experimental spectra were acquired at rates between 1000 and 2000 countsrs with dead time in the range of 25%–10% and were collected for 150–
200 s. The error due to the counting statistics at 2 s level for Mg and Si is in the range of 0.6%; for Ti, Ni, Fe, the relative error depends on concentration
L. Dobrzhinetskaya et al.r Chemical Geology 160 (1999) 357–370
363
and varies between 1% and 15%. This leads to a maximum relative error in determination of Ti concentration of about 12% or in absolute units between "0.07 and "0.15 wt.% depending on concentration; results are presented in Table 2.
3. Results All of our experiments were performed in the olivine stability field ŽFig. 2.. However, the range of conditions encompasses regions of stability of three titanium oxides — rutile, ilmenite, and the high pressure polymorph of ilmenite which has a perovskite structure ŽLeinenweber et al., 1991; Mehta et
Fig. 3. TiO 2 solubility in olivine as a function of time under experimental conditions. Symbols are the same as in Fig. 2.
Fig. 2. Phase diagram showing stability fields of olivine and titanates and conditions of the experiments of this study. Boxes labeled 1–3 are P – T determinations of Alpe Arami peridotite by Ernst Ž1981., Medaris and Carswell Ž1990., and Brenker and Brey Ž1997., respectively. Olivine phase equilibria from Akaogi et al. Ž1989.; Il-pv boundary for FeTiO 3 from Mehta et al. Ž1994.. Solid circles indicate conditions of experiments in which the Si-bearing titanate is absent; open circles represent experiments in which Si-titanate is present. A lower case ‘r’ is printed adjacent to the rutile-bearing experiment at 5 GPa. Ellipses and squares represent two-stage experiments. Asterisks are experiments from Green et al. Ž1997b..
al., 1994.. Although we have not directly determined the existence of the perovskite phase Žor its quench product which has a lithium niobate crystal structure., we distinguish it from ilmenite and magnesian ilmenite by its different optical reflectivity and its tendency to dissolve Si. Open symbols in Fig. 2 denote conditions in which the titanate was Sibearing; closed symbols denote the presence of ŽMg,Fe.TiO 3 ilmenite; the lower-case ‘r’ adjacent to
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L. Dobrzhinetskaya et al.r Chemical Geology 160 (1999) 357–370
the symbol for the 1400 K experiment indicates the formation of rutile. 3.1. Saturation time for Ti-solubility in oliÕine To determine the minimum experimental time that would be necessary to reach a plateau of TiO 2 solubility in olivine under the PT conditions of our experiments, we performed a series of experiments with different durations. Fig. 3 shows the TiO 2 content of olivine in specimens run at the same condi-
tions but for different periods of time. At 8 GPa, 1500 K, increasing run duration from 0.8 to 14 h resulted in an increase in the TiO 2 concentration by 0.16 wt.%; at 1600 K, the increase was somewhat less between 5 and 18 h ŽFig. 3a.. At 10 GPa and T s 1600–1650 K, the TiO 2 content differed only nominally when duration of the experiment was increased from 12 to 57 h ŽFig. 3b.. With increase of the run duration from 2.2 to 24 h at T s 1700 K and P s 12 GPa, the content of TiO 2 in olivine increased by 0.34 wt.%, but at the same pressure and 1500 K,
Fig. 4. ŽA. TEM image of olivine crystal from experiment ma63 with numbered spot locations of EDS analyses. High magnification insert of the area of spot 6 shows the absence of even very small inclusions of other phases that might conceivably contribute to the TiO 2 signal of the chemical analyses. ŽB. EDS spectrum from spot 3 of this crystal ŽTiO 2 s 1.47 wt.% — see Table 1.. ŽC. EDS spectrum collected with the SEM from a much larger area of the same sample ŽTiO 2 s 1.05 wt.% — see Table 1..
L. Dobrzhinetskaya et al.r Chemical Geology 160 (1999) 357–370
365
Fig. 4 Žcontinued..
increase from 24 to 72 h showed a much smaller decrease ŽFig. 3c.. On the basis of these results, we concluded that run times of 12 or more hours pro-
vide a reliable measure of the solubility of TiO 2 in olivine at the temperatures and pressures of these experiments. For the remainder of this paper, we will
366
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restrict our discussions to specimens held at temperature for 12 or more hours. 3.2. 1400 K experiment Experiment ma71 was conducted at T s 1400 K and P s 5 GPa on the lherzoliteq ilmenite powder starting material. These conditions correspond to the conditions of last equilibration calculated recently for the Alpe Arami massif by Brenker and Brey Ž1997.. These conditions are also close to the thermobarometric determinations for the Dabie Mountains peridotites of China ŽYang et al., 1993.. The experiment was targeted to determine the solubility of TiO 2 in olivine under the maximum PrT conditions that have been calculated using the chemistry of coexisting mineral phases in subduction zone garnet peridotites. These conditions represent the maximum PrT conditions for which the rocks retain a ‘‘chemical memory’’, meaning the most extreme conditions frozen in when interdiffusion ceased in the most refractory phases. This was the only experiment in this series of nominally anhydrous experiments in which rutile was produced by reaction of the starting ilmenite with the silicate phases; ŽMg,Fe.TiO 3 ilmenite was also produced. Under these conditions, we were unable to detect any TiO 2 in olivine Ž- 0.1–0.2 wt.% TiO 2 .. 3.3. 1500 K experiments Three single-stage experiments were performed at T s 1500 K, one each at 8, 10 and 12 GPa Žma61, ma57, and ma65, respectively.. All of them had the lherzoliteq ilmenite powder starting material. Titanium concentration in olivine increased with increasing pressure: 0.42 wt.% at 8 GPa; 0.46 wt.% at 10 GPa; 0.66 wt.% at 12 GPa. In experiments at 8 and 10 GPa, ilmenite developed a significant Mg-component; at 12 GPa the titanate also exhibited significant amounts of Si ŽDobrzhinetskaya and Green, 1997.. The Si-component developed only under conditions where the stable phase is the perovskite structure ŽFig. 2.. Small ŽMg,Fe.SiO 3 pyroxene Žhigh pressure clinoenstatite, ŽPacalo and Gasparik, 1989; Angel et al., 1992. neoblasts of 1–3 mm diameter formed adjacent to the titanate crystal aggregates that replaced the original ilmenite.
3.4. 1600 K experiments Three single-stage lherzoliteq ilmenite powder experiments were also performed at T s 1600 K, one each at 8, 10 and 12 GPa Žma67, ma62, and ma63, respectively.. At all pressures, olivine showed higher amounts of TiO 2 than at 1500 K; at 8 and 10 GPa, concentrations were only slightly elevated, but at 12 GPa, the increase was considerable. Investigation of this specimen Žma63. by TEM ŽFig. 4. showed that the crystals were free of inclusions that might have contributed to the high TiO 2 concentration in olivine measured by SEM, and confirmed that the TiO 2 concentration was in excess of 1 wt.% and homogeneous ŽTable 2.. Small grains of clinoenstatite adjacent to the titanate were more abundant than at 1500 K. 3.5. ReÕersal experiments Experiments ma68, ma74, ma78, ma81, and ma82, were all initially held at 12 GPa and temperature 1600 K to establish a TiO 2 composition in olivine 1 wt.%, followed by re-annealing at lower temperature or, in the case of ma81, at lower pressure. All experiments yielded olivine compositions consistent with the systematics of the single-stage results ŽTables 1 and 2.. These specimens also exhibited titanate precipitates in olivine Žnot shown., reflecting higher TiO 2 content established during the first stage of the experiment. We interpret the ranges of TiO 2 concentration in olivine of these specimens as due to failure of the specimens to fully re-equilibrate during the down-temperature reaction. The lowest values of TiO 2 measured in olivine in the TEM are the most reliable analyses in this case because we know the reaction is running from higher to lower concentration and because the SEM analyses may be averaging in precipitates produced during the second stage of the experiment. For example, specimen ma68 that had the second stage at only 1350 K shows a broad spread of TEM analyses and the SEM analysis is approximately the average of the TEM analyses ŽTable 1.. 3.6. Summary of results Our results show a range of TiO 2 solubility in olivine from less than the detection limit of our
L. Dobrzhinetskaya et al.r Chemical Geology 160 (1999) 357–370
Fig. 5. TiO 2 content of olivine vs. pressure for olivine held at various temperatures for 12 h. Symbols are the same as in Fig. 2.
method Ž- 0.1 wt.%. at 5 GPa, 1400 K, to ) 1.0 wt.% at 12 GPa, 1700 K ŽFig. 5.. There is general agreement between experiments starting with olivine having extremely low TiO 2 concentrations Žsinglestage experiments. and those approaching equilibrium from higher concentrations induced by an original treatment at higher pressure andror temperature Žtwo-stage experiments.. The presence of titanate precipitates in the latter class of experiments confirms that the first stage did, indeed, induce higher concentration of TiO 2 than measured in ‘‘clean’’ olivine at the end of the experiment. We also find a temperature dependence of solubility which is moderate at 8 and 10 GPa and greater at 12 GPa.
4. Discussion Our results fall into three general categories — very low solubility of TiO 2 in olivine at 5 GPa, moderate solubility at 8–10 GPa, and greater solubility at 12 GPa ŽFig. 5.. These three regimes were found to correlate with the presence of different titanate phases ŽFig. 2.. At 5 GPa, the starting FeTiO 3 ilmenite reacted with the silicate assemblage to yield rutile and ŽMg,Fe.TiO 3 ilmenite. Under these conditions, the solubility of TiO 2 in olivine was below our detection limit. In all dry experiments at 8 GPa and in 1500 K experiments at 10 GPa, only ŽMg,Fe.TiO 3 ilmenite was produced by Mg–Fe exchange with the silicate phases. At still higher pressure and temperature, the stable titanate was ŽMg,Fe.ŽTi,Si.O 3 per-
367
ovskite ŽFig. 2. and Ti-dissolution in olivine increased further. The increase in solubility was particularly enhanced at 12 GPa ŽFig. 5.. Thus, when olivine coexisted with rutile, solubility of TiO 2 was very low, but when it coexisted only with FeTiO 3 phases Žwith or without Si incorporation into the titanate. a solubility in excess of 0.4 wt.% TiO 2 was observed. Although only one experiment in this series produced rutile, our preliminary experiments in a hydrous environment have found rutile persisting to higher pressures and the hydrous experiments that are rutile-bearing also show no measurable TiO 2 in olivine ŽDobrzhinetskaya and Green, 1997; Bozhilov et al., 1999.. TiO 2 solubility in a hydrous fluid follows a similarly buffered path. At low pressures, in rutile-bearing assemblages, the solubility is significant, but decreases markedly with pressure ŽFoley and Wheller, 1990; Ayers and Watson, 1993., resulting in predictions that metasomatizing fluids in a mantle wedge environment should be impoverished in TiO 2 . It is interesting to note, however, that Fig. 3 Žp. 325. of Ayers and Watson Ž1993. shows the pressure-dependence of the solubility to be decreased at 3 GPa. Moreover, a subsequent study by Iizuka and Nakamura Ž1995. has shown that between 6 and 8 GPa, the stable titanate in ecoglite changes from rutile to ilmenite and correlates with a jump in solubility of TiO 2 in hydrous fluid from a vanishingly small value to a much larger one. These latter authors conducted experiments at 8508C Ž1123 K. and P s 4, 6, 8 GPa on samples consisting of 1r2 blueschist Žto simulate hydrous oceanic crust. and 1r2 olivine Žto simulate the overlying mantle wedge.. They showed that at 4 and 6 GPa, the blueschist was converted to rutile-bearing eclogite and that the fluid released by dehydration of blue amphibole reacted with the olivine to produce enstatite, indicating that the fluid carried significant amounts of SiO 2 ; no other phases grew, suggesting that the concentration of other components in the fluid was very low. At 8 GPa, however, the residual eclogite of their experiments was ilmenite-bearing and the olivine portion of the charge reacted to form TiCH as well as enstatite Žclinoenstatite?., indicating that destabilization of rutile led to a large increase of TiO 2 solubility in the fluid which carried it out of the ‘‘slab’’ and into the adjacent ‘‘mantle wedge’’ where it reacted with
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olivine to form TiCH. Our preliminary results on hydrous peridotite with excess TiO 2 are consistent with these results. Thus, when the activity of TiO 2 is fixed at 1 by rutile stability, both hydrous fluids and olivine exhibit very low TiO 2 solubility, but when rutile disappears from the paragenesis and is replaced by ilmenite, TiO 2 solubility increases significantly. In the case of olivine, the solubility of TiO 2 increases further when the ilmenite–perovskite phase boundary is crossed, but our results are not sufficient to distinguish between this being simply a PrT effect or whether the further enhancement of TiO 2 solubility in olivine is controlled by the coexisting titanate.
5. Geological significance of the results Our experimental results show that under nominally anhydrous conditions, the solubility of TiO 2 in olivine reaches 1.1 " 0.1 wt.% at P s 12 GPa, T s 1600–1700 K and that it is approximately half that at 10 GPa and comparable temperatures. These data are consistent with the solubility inferred by Dobrzhinetskaya et al. Ž1996. from the abundance of FeTiO 3 precipitates in Alpe Arami and further support origin of that body at depths ) 300 km Ž P ) 10 GPa.. Further, these results simplify the minimum depth of origin required for the Alpe Arami massif because they demonstrate the possibility of origin within the olivine stability field and remove the need to propose that such large amounts of dissolved TiO 2 may require the previous presence of a phase precursor to olivine, such as wadsleyite ŽGreen et al., 1997b.. These results and those of Iizuka and Nakamura Ž1995. suggest that there are two possible origins of the differences between subduction-zone garnet peridotites and their xenolithic and ophiolitic counterparts from the subcontinental and suboceanic lithosphere. Either additional TiO 2 has been added to subduction-zone peridotites, probably from fluid emanations from subducted lithosphere at 8 GPa, or existing TiO 2 in these rocks has been redistributed, very likely also in response to infiltration of fluids from subducted lithosphere. We propose that at least some peridotites bearing significant amounts of TiCH
have obtained this excess TiO 2 by dehydration of subducted oceanic crust at P ) 6 GPa. On the other hand, if the TiO 2 signature of mantle wedge peridotites is obtained from hydrous fluids, then how does one explain the case of the Alpe Arami lherzolite in which the assemblage is anhydrous and there is no evidence of the former presence of TiCH ŽErnst, 1977.. The simplest explanation is that the conditions under which the TiO 2 signature of Alpe Arami was impressed on the rocks was under conditions of higher P andror T than the maximum stability of TiCH. Our continuing experiments will investigate this upper stability limit. In summary, our experimental results resolve the geochemical problem of how such large quantities of TiO 2 could have been dissolved in Alpe Arami olivine and provide further evidence of the extraordinary depth from which this peridotite has been exhumed. They also support the hypothesis that garnet peridotites with unusual Ti-bearing phases such as TiCH or relict olivine with ilmenite precipitates reflect a unique mantle environment. The geological context in which these rocks outcrop strongly suggests that this environment is that of the mantle wedge overlying a subduction zone. Expanded geochemical studies of these rocks and the country rocks in which they are now found potentially can provide many new insights into this complicated environment.
Acknowledgements We thank Connie Bertka, Robert Liebermann, Robert Luth, Keith Putirka, and Eric Riggs for sharing their knowledge and experience with multianvil apparatus and for helpful discussions about experimental technique. We especially thank David Walker who provided extensive instruction and advice, and shared his calibrations with us during set up of our multianvil. We also thank Frank Forgit for assistance with apparatus alignment and maintenance. Bill Carey provided WDS analysis of our San Carlos olivine standard. Gary Ernst and David Walker provided critical reviews which significantly improved the manuscript. This work was supported by NSF Grant aEAR96-283432.
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References Akaogi, M., Ito, E., Navrotsky, A., 1989. Olivine-modified spinel–spinel transitions in the system Mg 2 SiO4 –Fe 2 SiO4 : calorimetric measurements, thermochemical calculation, and geophysical application. J. Geophys. Res. 94, 15671–15685. Angel, R.J., Chopelas, A., Ross, N., 1992. Stability of high-density clinoenstatite at upper mantle pressures. Nature 358, 322–324. Aoki, K.-I., Fujino, K., Akaogi, M., 1976. Titanochondrodite and titanoclinohumite derived from the upper mantle in Buell Park kimberlite, Arizona, USA. Contrib. Mineral. Petrol. 56, 243– 253. Armstrong, J.T., 1988. Quantitative analysis of silicate and oxide minerals: comparison of Monte Carlo, ZAF and phi–rho–z procedures. In: Newbury, D.E. ŽEd.., Microbeam Analysis — 1988. San Francisco Press, San Francisco, pp. 239–246. Ayers, J., Watson, B., 1993. Rutile solubility and mobility in supercritical aqueous fluids. Contrib. Mineral. Petrol. 114, 321–330. Bertka, C.M., Fei, Y., 1997. Mineralogy of the Martian interior up to core–mantle boundary pressures. JGR 102, 5251–5264. Boyd, F.R., 1973. A pyroxene geotherm. Geochim. Cosmochim. Acta 37, 2533–2546. Bozhilov, K.N., Green, H.W. II, Dobrzhinetskaya, L.F., 1999. Clinoenstatite in Alpe Arami: additional evidence of very high pressure. Science 284, 128–132. Brenker, F.E., Brey, G.P., 1997. Reconstruction of the exhumation path of the Alpe Arami garnet–peridotite body from depths exceeding 160 km. J. Metamorph. Geol. 15 Ž3.. Dobrzhinetskaya, L.F., Green, H.W., II, 1997. TiO 2 — solubility in olivine and composition of coexisting titanate at very high pressure and temperature: preliminary results. EOS, AGU, 1997 Fall Meeting, p. F786. Dobrzhinetskaya, L.F., Green, H.W. II, Wang, S., 1996. Alpe Arami: a peridotite massif from depths of more than 300 kilometers. Science 271, 1841–1845. Cliff, G., Lorimer, G.W., 1975. The quantitative analysis of thin specimens. J. Microsc. 103, 203–207. Efimov, I.A., 1964. Regularities in the genesis and occurrence of the eclogitic rock in northern and southern Kazakhstan. In: Physico-Chemical Conditions of Metamorphism and Metasomatism. Nauka Press, Novosibirsk, pp. 70–79 Žin Russian.. Ernst, W.G., 1977. Mineralogic study of eclogitic rocks from Alpe Arami, Lepontine Alps, Southern Switzerland. J. Petrol. 18, 371–398, Part 3. Ernst, W.G., 1981. Petrogenesis of eclogites and peridotites from the Western and Ligurian Alps. Am. Mineral. 66, 443–472. Evans, B.W., Trommsdorff, V., 1983. Fluorine hydroxyl titanian clinohumite in alpine recrystallized garnet peridotite: compositional controls and petrologic significance. Am. J. Sci. 283A, 355–369. Green, H.W. II, Dobrzhinetskaya, L., Bozhilov, K., 1997a. Determining the origin of ultra-high pressure lherzolites Žresponse.. Science 278, 704–707. Green, H.W. II, Dobrzhinetskaya, L., Riggs, E., Jin, Z.-M., 1997b.
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Alpe Arami: a peridotite massif from the mantle transition zone?. Tectonophysics 279, 1–21. Foley, S.F., Wheller, G.E., 1990. Parallels in the origin of the geochemical signatures of island arc volcanics and continental potassic igneous rocks: the role of residual titanates. Chem. Geol. 85, 1–18. Hervig, R.L., Smith, J.V., Dawson, J.B., 1986. Lherzolite xenoliths in kimberlites and basalts: petrogenetic and crystallochemical significance of some minor and trace elements in olivine, pyroxenes, garnets and spinel. R. Soc. Edinburgh, Trans.: Earth Sci. 77, 181–201. Iizuka, Y., Nakamura, E., 1995. Experimental study of the slab– mantle interaction and implications for the formation of titanoclinohumite at deep subduction zone. Proc. Jpn. Acad., Ser. B 71, 159–165. Klein, U., Sharp, T.G., Schumacher, J.C., 1997. Analytical electron microscopy of nanometer-scale hornblende lamellae: low-temperature exsolution in cummingtonite. Am. Mineral. 82, 1079–1090. Leinenweber, K., Utsumi, W., Tsuchida, Y., Yagi, T., Kurita, K., 1991. Unquenchable high-pressure perovskite polymorphs of MnSnO 3 and FeTiO 3 . Phys. Chem. Miner. 18, 244–250. Livi, K.J.T., Veblen, D.R., 1987. ‘‘Eastonite’’ from Easton, Pennsylvania: a mixture of phlogopite and a new form of serpentine. Am. Mineral. 72, 113–125. McGetchin, T.R., Silver, L.T., Chodos, A.A., 1970. Titanoclinohumite: a possible mineralogical site for water in the upper mantle. J. Geophys. Res. 75, 255–259. Medaris, L.G., Carswell, D.A., 1990. The petrogenesis of Mg–Cr garnet peridotites in European metamorphic belts. In: Carswell, D.A. ŽEd.., Eclogite Facies Rocks. Blackie-GlasgowLnd, pp. 260–290. Mehta, A., Leinenweber, K., Navrotsky, A., Akaogi, M., 1994. Calorimetric study of high pressure polymorphism in FeTiO 3 : stability of perovskite phase. 21, 207–212. Mockel, J.R., 1969. Structural petrology of the garnet peridotite of ¨ Alpe Arami ŽTicino, Switzerland.. Leidse Geol. Meded. 42, 61–130. O’Neill, H.St.C., Wood, B.J., 1979. An experimental study of Fe–Mg partitioning between garnet and olivine and its calibration as a geothermometer. Contrib. Mineral. Petrol. 70, 59–70. Pacalo, R.E.G., Gasparik, T., 1989. Reversals of the orthoenstatite–clinoenstatite transition at high pressures and temperatures. EOS 70, 508. Trommsdorff, V., Evans, B.W., 1980. Titanian hydroxylclinohumite: formation and breakdown in antigorite rocks ŽMalenco, Italy.. Contrib. Mineral. Petrol. 72, 229–242. Udovkina, N.G., 1985. Eclogites of the USSR. Nauka Press, Moscow, p. 285 Žin Russian.. Walker, D., 1991. Lubrication, gasketing, and precision in multianvil experiments. Am. Mineral. 76, 1092–1100. Walker, D., Carpenter, M.A., Hitch, C.M., 1990. Some simplifications to multianvil devices for high pressure experiments. Am. Mineral. 75, 1020–1028. Walter, M.J., Thibault, Y., Wei, K., Luth, R.W., 1995. Characterizing experimental pressure and temperature conditions in multianvil apparatus. Can. J. Phys. 73, 273–286.
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The solubility of TiO 2 in olivine: implications for the mantle wedge environment L. Dobrzhinetskaya ) , K.N. Bozhilov, H.W. Green
1
II
Institute of Geophysics and Planetary Physics and Department of Earth Sciences, UniÕersity of California, RiÕerside, CA 92521, USA Received 27 May 1998; accepted 10 September 1998
Abstract One characteristic of many subduction-zone garnet peridotites is that they contain titanium-bearing phases not otherwise found in mantle rocks. In particular, titanoclinohumite andror its breakdown assemblage consisting of symplectic intergrowths of olivine and ilmenite is common in many of these bodies. The Alpe Arami garnet lherzolite of the Swiss Alps, while lacking titanoclinohumite, displays instead large numbers of FeTiO 3 rod-shaped precipitates in the oldest generation of olivine, amounting to approximately 1% by volume, indicating that at some time in its past, the peridotite experienced conditions under which the solubility of TiO 2 in olivine was ) 0.6 wt.%. In order to test the hypothesis that the environment of very high solubility of TiO 2 in olivine is to be found at very high pressures, we have conducted experiments on lherzolite compositions with added ilmenite at pressures between 5 and 12 GPa and temperatures of 1350–1700 K. Our results on anhydrous compositions show that whereas solubility of TiO 2 was not detected in olivine at 5 GPa, 1400 K where it coexists with rutile, when rutile disappeared from the paragenesis, the solubility climbed to 0.4 wt.% at 8 GPa, 0.5 wt.% at 10 GPa and to ) 1.0 wt.% at 12 GPa, 1700 K. These results support our previous interpretations from titanate morphology and abundance that the Alpe Arami massif has surfaced from P s 10 GPa but remove the need to suggest a deeper origin and possible precursor phase such as wadsleyite. They also support the hypothesis that garnet peridotites with unusual Ti-bearing phases reflect a unique mantle environment occurring in the mantle wedge overlying subduction zones. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Olivine; Titanium solubility; Mantle wedge; Deep subduction
1. Introduction It is now generally accepted that emanations of hydrous fluids andror hydrous partial melts from subducted lithosphere flux melting of mantle peridotite at temperatures below the anhydrous solidus contribute to the formation of island-arc volcanism. ) Corresponding author. Tel.: q1-909-787-5535; fax: q1-909787-5924 1 E-mail: [email protected].
Although it is clear that such fluids carry slab signatures, the behavior of the high field-strength elements ŽHFSE. during this process is a matter of some discussion. Garnet peridotites that have surfaced in areas of previous deep subduction commonly Žubiquitously?. carry titaniferous phases that are different from or in greater abundance than those present in mantle peridotites from other environments. For example, titanoclinohumite ŽTiCH. andror its symplectitic dehydration products, olivine q ilmenite, occur in peridotites of the microdia-
0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 1 0 7 - 2
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mond-bearing Kokchetav massif, Kazakhstan ŽEfimov, 1964; Mockel, 1969; Udovkina, 1985., the ¨ Cima de Gagnoni body of the high pressure Adula nappe of the Swiss Alps ŽTrommsdorff and Evans, 1980; Evans and Trommsdorff, 1983. and in the ultra-high-pressure metamorphic terrane of the Dabie Mountains of China ŽYang et al., 1993; Zhang et al., 1995.. Xenoliths in diatremes from the Colorado Plateau of the Southwestern United States also have been reported to contain TiCH and have been interpreted as reflecting hydrous alteration of the mantle wedge overlying a subduction zone ŽMcGetchin et al., 1970; Aoki et al., 1976.. The Alpe Arami massif of the Adula nappe lacks TiCH but contains up to 1 vol.% FeTiO 3 precipitates in olivine that imply a previous solubility of ) 0.6 wt.% TiO 2 ŽDobrzhinetskaya et al., 1996; Green et al., 1997a,b.. Evidence for such extensive solubility of TiO 2 in olivine had not been reported previously; mantle olivine from a variety of environments had shown solubility increasing with depth but with a maximum of ; 0.05 wt.% TiO 2 in ‘‘hot garnet lherzolites’’ from South African kimberlites ŽHervig et al., 1986. which have a maximum depth of about 200 km Že.g., Boyd, 1973.. This contrast caused Dobrzhinetskaya et al. Ž1996. to conclude that Alpe Arami represents a mantle environment not previously sampled at the surface, perhaps representing very great depth. Indeed, these authors and Green et al. Ž1997b. suggested that there might be no conditions under which olivine could dissolve such large amounts of TiO 2 , and proposed that the FeTiO 3 precipitates in Alpe Arami olivine might be evidence for a precursor phase that preceded olivine in the paragenesis, perhaps wadsleyite, stable only at depths ) 400 km. These observations clearly distinguish subductionzone peridotites from all other mantle rocks and raise critical questions about the source of their unusual Ti-bearing phases. To pursue these questions, we are investigating the solubility of TiO 2 in olivine as a function of pressure, temperature, and H 2 O content. We present here the results of our dry experiments conducted on powders of natural minerals with bulk composition close to that of the Alpe Arami lherzolite with the addition of excess ilmenite. We find a profound pressure effect on the solubility of TiO 2 in olivine, which correlates strongly with the stable coexisting titanate phase; the amount formerly pre-
sent in Alpe Arami olivine can be dissolved at pressures in excess of 10 GPa, consistent with our previous estimates of minimum pressure based upon precipitate abundance, type, and morphology ŽDobrzhinetskaya et al., 1996; Green et al., 1997a,b.. 2. Materials and methods 2.1. Apparatus and conditions of the experiments Our experimental program was conducted with a multianvil apparatus ŽWalker-style ‘‘hatbox’’; Walker, 1991; Walker et al., 1990.. Room temperature calibration for pressure was established with Bi phase transitions at 2.55 ŽI–II. and 7.7 GPa ŽIII–IV.. The calibration was within experimental uncertainty of that of Walker’s similar calibration for his identical apparatus; we adopted Walker’s high-temperature calibration for pressure. Pressure-medium octahedra were made from semi-sintered MgO–5% Cr2 O 3 ceramic ŽCeramacast 584-OF, Aremco Products.. Within the internal space of the apparatus, eight 25-mm WC cubes, each with a 6-mm octahedral truncation on one corner, are arranged in the form of a larger cube with their octahedral truncations facing inwards to form an octahedral cavity into which the pressure medium and sample are placed. To minimize friction and to provide electrical insulation, sheets of fiberglass laminate ŽG-10. coated with a thin film of dry non-silicon mold release Ž811 DFVMR. are placed between the WC cubes and the six anvils. Table 1 summarizes the conditions of all experiments. Those conducted at pressures of 5–10 GPa were slowly cold-compressed at room temperature to the desired pressure followed by slow heating to the desired temperature. Experiments conducted at 12 GPa were pressurized initially to 9 GPa at room temperature, heated to approximately 750 K to allow more stable compression of the pressure medium, further pressurized to 12 GPa, and finally heated to the desired temperature. All but five experiments consisted of ‘‘one stage’’, meaning they were subjected to one set of experimental conditions and then quenched to room temperature in seconds by shutting off the power to the apparatus, followed by programmed depressurization to atmospheric pressure over 24–35 h.
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359
Table 1 Experimental conditions. All experiments were performed ‘‘dry’’. Asterisks are put after the specimen numbers for which starting materials were Ol Ž90%. and Ilm Ž10%. P ŽGPa.
T ŽK.
Time Žh.
Ol:TiO 2 Žwt.%., EDS ŽSEM.
Ol:TiO 2 Žwt.%., EDS ŽTEM.
Run number
5 8 8 8 8 10 10 12 12 12 12 12 12 12 12 12 12 12 12 12 10 12 12
1400 1500 1500 1600 1600 1500 1600 1450 1500 1600 1650 1700 1800 1600 1350 1700 1550 1700 1550 1650 1650 1900 1700
24 0.8 14 5 18 24 12 2 24 24 20 2.2 0.8 20 10 24 24 24 72 24 57 17 24
- 0.02 0.26 0.42 0.38 0.47 0.46 0.52 0.50 0.66 0.98 1.02 0.78 0.90
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 1.17 n.d. n.d. n.d.
ma71 ma58 ma61 ma64 ma67 ma57 ma62 ma56 ma65 ma63 ma79 ma43 ma40 ma68
0.99
0.50–1.33
0.98
0.87–1.1
0.91
n.d.
ma74U ma78 ma81U 0.57
0.57–1.00 ma82U
1.12
In addition to the single stage experiments, five ‘‘two-stage’’ experiments Žma68, ma74, ma78, ma81, and ma82. were conducted to approach the equilibrium concentration of dissolved TiO 2 from the oversaturated side ŽTable 1.. The specimens were first held at one set of conditions followed by a second set of conditions at lower temperature andror pressure. The first stage of these experiments occupied 17–24 h at a pressure of 12 GPa and at temperatures of 1600–1900 K to establish a high dissolved TiO 2 content in olivine. The second stage of all except ma81 consisted of further annealing at 12 GPa but under lower temperature conditions. The temperature was reset to the second set of conditions by adjustment of power to the Eurotherm temperature controller as quickly as possible Ž1–3 min. while in the manual mode. In the case of ma81, following the first stage of heating Ž24 h. the temperature was reduced to room temperature, pressure was slowly reduced to 10 GPa and the temperature was returned to 1650 K for 57 h. At the end of these experiments,
n.d.
they also were temperature-quenched by turning off the power to the apparatus, followed by programmed depressurization over 30–35 h. 2.1.1. Sample assemblies We explored various sample assemblies to develop one well-suited for this particular study. Graphite furnaces were unacceptable because our experiments are all in the diamond stability field and at temperatures high enough to expect reaction to proceed and destroy the resistance-furnace function. Early on, we performed garnet–olivine thermometry experiments to determine the temperature gradient in various assemblies with lanthanum chromate and rhenium furnaces. This technique is based on the temperature dependence of the compositions of coexisting garnet and olivine. The temperature distribution within the sample charge encapsulated in Pt and placed adjacent to the thermocouple was calculated using the empirical equation of O’Neill and Wood Ž1979.. We found that for both lanthanum chromate
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and rhenium furnaces, temperature gradients from sample center to end were unacceptably large Ž150– 250 K., non-reproducible, and not symmetrical Žeither from end-to-end or radially.. Thereafter, we evolved the following assembly that was used for all of the experiments reported here ŽFig. 1.: a cylindrical hole with diameter 4.76 mm was drilled normal to one pair of octahedral faces of the ceramic octahedron that serves as a pressure cell. The assembly consists of a resistance furnace made from Re-foil surrounding a tube of semi-sintered pure MgO ceramic manufactured by Ozark Tech. The powdered sample was placed at the center of the assembly, with semi-sintered MgO spacers on either side to complete sample encapsulation. W–3% Re–W–25% Re thermocouple junctions were placed within the sample to ensure that chemical analyses were made on materials subjected to the temperature recorded for the run. All reported chemical measurements were made within a few hundred microns of the thermocouple, greatly reducing the problem of uncertainty in temperature measurement that has been widely discussed in the literature Že.g., Zhang and Herzberg, 1994; Walter et al., 1995; Bertka and Fei, 1997.. However, as for all multianvil experiments, we have reported our temperatures assuming a zero
pressure effect on thermocouple emf. That has the effect of reporting temperatures that are too high by an unknown amount. This is a problem for all highpressure apparatus using thermocouples and is minimized by using W–Re. No presence of either W nor Re has been detected in specimen crystals. 2.1.2. Starting materials This study was initiated to test the hypothesis that the conditions experienced by the Alpe Arami peridotite that were responsible for the former very high solubility of TiO 2 involved extremely high pressures ŽDobrzhinetskaya et al., 1996; Green et al., 1997a,b.. Therefore, the starting material for most experiments consisted of lherzolite powder made of minerals taken from that massif, with the exception of olivine for which San Carlos, AZ, olivine has been substituted to avoid the FeTiO 3 precipitates abundant in Alpe Arami olivine. The powders consist of 66% olivine, 8% each of Cpx, Opx, and Grt from Alpe Arami, and 10% pure FeTiO 3 ilmenite of unknown source. Three experiments Žma74, ma79, ma81 and ma82. were conducted on the composition 90% ŽSan Carlos. olivine and 10% ilmenite. Experiment ma78 had sample composition of 90% of San Carlos olivine, 7% ilmenite and 3% chromite. The minerals
Fig. 1. Schematic cross section showing the sample assembly and materials used for this study.
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for both starting mixes were powdered by grinding separately in an agate mortar under ethanol and dried for 24 h at 383 K in vacuum. Grain sizes of the final mix were evaluated under the optical microscope as 3–5 mm Ž75%. and 10–15 mm Ž25%.. 2.1.3. Sample preparation for analysis The dimensions of the samples after the experiments is usually about 2 = 2 mm2 . After recovery from the apparatus, the samples were cut into two pieces and two doubly-polished thin sections with thickness ; 30 mm were prepared for each experiment. One thin section was usually used for optical microscopy and scanning electron microscopy ŽSEM. studies, the another one for transmission electron microscopy ŽTEM. thin foil preparation. TEM foils were prepared by standard Ar-ion milling ŽPIPS, Gatan. at an angle of incidence of 58–68 at an applied voltage of 5 kV and beam current of 10 mA for 30–45 min. Before milling, the sample was transferred onto a diamond grid to avoid contamination produced by copper holders in cases for which the sample size was not large enough to cover up the entire surface of the support grid. Specimens were carbon coated for electron microscopy. 2.2. SEM SEM and microanalysis were performed at our Analytical Electron Microscopy Facility with a Philips XL30 instrument, equipped with FEG operated at 15 and 20 kV. The EDAX microanalytical system of this microscope consists of an energy dispersive spectrometer ŽEDS. equipped with Si detector with super-ultra-thin window, resolution of 137 eV at MnK a. The spectral data were acquired at 1500 to 2000 countsrs with dead time below 25%, beam current of about 1 nA, effective spot size about 1.5 mm, 200–300 s counting time, and were reduced using the eDXi software from EDAX and the ZAF correction scheme ŽArmstrong, 1988.; results are presented in Table 2. Natural and synthetic mineral standards of olivine and rutile were used for the quantification procedure. The oxide concentrations were calculated by normalization. The analytical error was estimated by analyzing a control grain of olivine from San Carlos, AZ with known composi-
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tion. The estimated error from the control measurements together with the counting statistics at 2 s level is - 2% for Si and Mg, - 4.5% for Fe, and in the range of 2%–10% for Ti and Ni. This leads to an absolute error in determination of TiO 2 concentration to between "0.05 and "0.1 wt.% for grains where the concentration is above 0.4 wt.% and error of "0.15 wt.% for concentrations - 0.4 wt.%. Minor amounts Ž- 0.2%. of Al and Cr were detected in a few spectra; such spectra were not included in averaging of the analyses. 2.3. Analytical TEM Analytical TEM studies also were conducted at our Analytical Electron Microscopy Facility using a Philips CM300 electron microscope with twin lens and LaB 6 cathode, operated at 200 and 300 kV. Images in bright and dark field mode as well as diffraction patterns were obtained to study the specimens of interest. Lightly carbon-coated specimens Ž; 5 nm C film thickness. were mounted on a Philips low-background double-tilt holder for examination in the TEM. The analytical equipment consists of EDAX system for acquisition and processing of energy dispersive X-ray spectra ŽEDS., equipped with Si detector with resolution of 135 eV at MnK a , ultra-thin window and the MX-TEM software. The EDS spectra were collected with specimen tilt of about 258 towards the detector resulting in effective take-off angle of 388. EDS analyses were quantified using the CliffLorimer k factor method ŽCliff and Lorimer, 1975.. Experimentally calculated k factors for Si, Mg, Al, Ti, Fe, Ni were obtained by using natural mineral standards of olivine, diopside, titanite, kyanite, and ferrosilite and following the procedure described by Livi and Veblen Ž1987. and Klein et al. Ž1997.. Thirty spectra were collected from each standard specimen at the same conditions as the experimental material. All analyses were taken at edges and thin areas of interest where absorption was negligible. The standard deviation for the averaged analyses of the standard silicates is 2% or less which corresponds to error of 2% or less in determination of the k factors.
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Table 2 EDS microanalysis of the olivine synthesized in the experiments. The standard deviation is given in parentheses. Analysis labeled by U s EDS, TEM; analysis without labelings EDS, SEM; n s number of analysis Oxides
SiO 2
TiO 2
MgO
FeO
NiO
MnO
n
ma71 ma58 ma61 ma64 ma67 ma57 ma62 ma56 ma65 ma63 ma63U r1 ma63U r2 ma63U r3 ma63U r4 ma63Ur5 ma63U r6 ma63U r7 ma63U r8 ma63U r9 ma63U r10 ma63U avr ma79 ma43 ma40 ma68 ma68U ra ma68U rb ma68U rc ma68U avr ma74 ma74U ra ma74Urc ma74U avr ma78 ma78U ra ma78U rb ma78U rc ma78U avr ma81 ma81U ra ma81U rb ma81U rc ma81U rd ma81U avr ma82avr
40.55Ž0.45. 41.10Ž0.11. 41.30Ž0.37. 41.66Ž0.28. 41.13Ž0.16. 41.21Ž0.09. 41.33Ž0.44. 41.34Ž0.07. 41.64Ž0.51. 41.45Ž0.64. 40.40 41.455 41.477 40.814 41.129 41.371 41.290 40.881 40.770 40.998 41.059Ž0.33. 41.59Ž0.25. 41.50 40.96 41.06Ž0.19. 40.457 40.676 41.890 41.014Ž0.77. 41.12Ž0.36. 41.137 40.676 40.907Ž0.23. 41.54Ž0.23. 42.819 41.101 41.991 41.970Ž0.23. 41.90Ž0.58. 42.400 42.594 42.326 41.961 42.320Ž0.27. 41.47Ž0.28.
0.01Ž0.007. 0.26Ž0.01. 0.42Ž0.10. 0.38Ž0.02. 0.47Ž0.07. 0.46Ž0.05. 0.52Ž0.04. 0.50Ž0.11. 0.66Ž0.10. 0.98Ž0.16. 1.164 1.087 1.456 1.172 1.173 1.315 1.138 1.106 1.065 1.062 1.174Ž0.12. 1.02Ž0.04. 0.78 0.90 0.99Ž0.18. 1.332 0.872 0.503 0.902Ž0.41. 0.98Ž0.05. 1.098 0.872 0.985Ž0.16. 0.91Ž0.19. 0.731 0.622 0.597 0.650Ž0.19. 0.57Ž0.06. 0.573 0.626 0.615 0.997 0.703Ž0.20. 1.12Ž0.16.
49.99Ž0.14. 49.55Ž0.29. 48.83Ž0.39. 48.78Ž0.30. 49.34Ž0.24. 49.94Ž0.17. 49.20Ž0.47. 50.58Ž0.24. 50.00Ž0.30. 50.09Ž0.25. 50.711 50.58 49.941 50.919 50.683 50.313 50.428 50.669 51.172 50.917 50.633Ž0.33. 49.91Ž0.61. 50.42 50.47 50.56Ž0.27. 50.333 51.018 50.505 50.619Ž0.36. 50.42Ž0.44. 50.849 51.018 50.934Ž0.12. 50.06Ž0.24. 49.879 50.893 50.232 50.335Ž0.24. 49.00Ž0.57. 49.315 48.902 49.194 48.705 49.029Ž0.28. 50.16Ž0.27.
8.95Ž0.27. 8.82Ž0.21. 9.18Ž0.32. 8.78Ž0.09. 8.85Ž0.21. 8.14Ž0.07. 8.61Ž0.59. 7.37Ž0.17. 7.40Ž0.33. 7.14Ž0.41. 7.262 6.605 6.702 6.751 6.726 6.746 6.751 6.889 6.616 6.618 6.817Ž0.18. 7.15Ž0.56. 7.02 7.25 7.04Ž0.37. 7.484 7.130 6.707 7.107Ž0.39. 7.21Ž0.37. 6.485 7.130 6.807Ž0.46. 7.16Ž0.28. 6.055 6.955 6.797 6.602Ž0.28. 8.33Ž0.46. 7.343 7.498 7.498 7.894 7.558Ž0.24. 7.04Ž0.29.
0.50Ž0.03. 0.27Ž0.18. 0.27Ž0.11. 0.40Ž0.05. 0.21Ž0.20. 0.27Ž0.09. 0.31Ž0.12. 0.21Ž0.16. 0.30Ž0.11. 0.20Ž0.25. 0.462 0.274 0.425 0.345 0.290 0.256 0.393 0.455 0.376 0.406 0.368Ž0.07. 0.15Ž0.04. 0.28 0.42 0.35Ž0.08. 0.394 0.303 0.395 0.363Ž0.05. 0.27Ž0.14. 0.431 0.303 0.367Ž0.09. 0.33Ž0.14. 0.517 0.428 0.383 0.443Ž0.14. 0.20Ž0.11. 0.364 0.380 0.367 0.443 0.390Ž0.04. 0.21Ž0.09.
n.d. n.d. n.d. n.d. n.d. n.d. 0.22Ž0.11. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.19Ž0.04. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
2 2 3 3 3 3 5 5 6 7 1 1 1 1 1 1 1 1 1 1 10 5 1 1 3 1 1 1 3 3 1 1 2 5 1 1 1 3 8 1 1 1 1 4 5
The experimental spectra were acquired at rates between 1000 and 2000 countsrs with dead time in the range of 25%–10% and were collected for 150–
200 s. The error due to the counting statistics at 2 s level for Mg and Si is in the range of 0.6%; for Ti, Ni, Fe, the relative error depends on concentration
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and varies between 1% and 15%. This leads to a maximum relative error in determination of Ti concentration of about 12% or in absolute units between "0.07 and "0.15 wt.% depending on concentration; results are presented in Table 2.
3. Results All of our experiments were performed in the olivine stability field ŽFig. 2.. However, the range of conditions encompasses regions of stability of three titanium oxides — rutile, ilmenite, and the high pressure polymorph of ilmenite which has a perovskite structure ŽLeinenweber et al., 1991; Mehta et
Fig. 3. TiO 2 solubility in olivine as a function of time under experimental conditions. Symbols are the same as in Fig. 2.
Fig. 2. Phase diagram showing stability fields of olivine and titanates and conditions of the experiments of this study. Boxes labeled 1–3 are P – T determinations of Alpe Arami peridotite by Ernst Ž1981., Medaris and Carswell Ž1990., and Brenker and Brey Ž1997., respectively. Olivine phase equilibria from Akaogi et al. Ž1989.; Il-pv boundary for FeTiO 3 from Mehta et al. Ž1994.. Solid circles indicate conditions of experiments in which the Si-bearing titanate is absent; open circles represent experiments in which Si-titanate is present. A lower case ‘r’ is printed adjacent to the rutile-bearing experiment at 5 GPa. Ellipses and squares represent two-stage experiments. Asterisks are experiments from Green et al. Ž1997b..
al., 1994.. Although we have not directly determined the existence of the perovskite phase Žor its quench product which has a lithium niobate crystal structure., we distinguish it from ilmenite and magnesian ilmenite by its different optical reflectivity and its tendency to dissolve Si. Open symbols in Fig. 2 denote conditions in which the titanate was Sibearing; closed symbols denote the presence of ŽMg,Fe.TiO 3 ilmenite; the lower-case ‘r’ adjacent to
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the symbol for the 1400 K experiment indicates the formation of rutile. 3.1. Saturation time for Ti-solubility in oliÕine To determine the minimum experimental time that would be necessary to reach a plateau of TiO 2 solubility in olivine under the PT conditions of our experiments, we performed a series of experiments with different durations. Fig. 3 shows the TiO 2 content of olivine in specimens run at the same condi-
tions but for different periods of time. At 8 GPa, 1500 K, increasing run duration from 0.8 to 14 h resulted in an increase in the TiO 2 concentration by 0.16 wt.%; at 1600 K, the increase was somewhat less between 5 and 18 h ŽFig. 3a.. At 10 GPa and T s 1600–1650 K, the TiO 2 content differed only nominally when duration of the experiment was increased from 12 to 57 h ŽFig. 3b.. With increase of the run duration from 2.2 to 24 h at T s 1700 K and P s 12 GPa, the content of TiO 2 in olivine increased by 0.34 wt.%, but at the same pressure and 1500 K,
Fig. 4. ŽA. TEM image of olivine crystal from experiment ma63 with numbered spot locations of EDS analyses. High magnification insert of the area of spot 6 shows the absence of even very small inclusions of other phases that might conceivably contribute to the TiO 2 signal of the chemical analyses. ŽB. EDS spectrum from spot 3 of this crystal ŽTiO 2 s 1.47 wt.% — see Table 1.. ŽC. EDS spectrum collected with the SEM from a much larger area of the same sample ŽTiO 2 s 1.05 wt.% — see Table 1..
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Fig. 4 Žcontinued..
increase from 24 to 72 h showed a much smaller decrease ŽFig. 3c.. On the basis of these results, we concluded that run times of 12 or more hours pro-
vide a reliable measure of the solubility of TiO 2 in olivine at the temperatures and pressures of these experiments. For the remainder of this paper, we will
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restrict our discussions to specimens held at temperature for 12 or more hours. 3.2. 1400 K experiment Experiment ma71 was conducted at T s 1400 K and P s 5 GPa on the lherzoliteq ilmenite powder starting material. These conditions correspond to the conditions of last equilibration calculated recently for the Alpe Arami massif by Brenker and Brey Ž1997.. These conditions are also close to the thermobarometric determinations for the Dabie Mountains peridotites of China ŽYang et al., 1993.. The experiment was targeted to determine the solubility of TiO 2 in olivine under the maximum PrT conditions that have been calculated using the chemistry of coexisting mineral phases in subduction zone garnet peridotites. These conditions represent the maximum PrT conditions for which the rocks retain a ‘‘chemical memory’’, meaning the most extreme conditions frozen in when interdiffusion ceased in the most refractory phases. This was the only experiment in this series of nominally anhydrous experiments in which rutile was produced by reaction of the starting ilmenite with the silicate phases; ŽMg,Fe.TiO 3 ilmenite was also produced. Under these conditions, we were unable to detect any TiO 2 in olivine Ž- 0.1–0.2 wt.% TiO 2 .. 3.3. 1500 K experiments Three single-stage experiments were performed at T s 1500 K, one each at 8, 10 and 12 GPa Žma61, ma57, and ma65, respectively.. All of them had the lherzoliteq ilmenite powder starting material. Titanium concentration in olivine increased with increasing pressure: 0.42 wt.% at 8 GPa; 0.46 wt.% at 10 GPa; 0.66 wt.% at 12 GPa. In experiments at 8 and 10 GPa, ilmenite developed a significant Mg-component; at 12 GPa the titanate also exhibited significant amounts of Si ŽDobrzhinetskaya and Green, 1997.. The Si-component developed only under conditions where the stable phase is the perovskite structure ŽFig. 2.. Small ŽMg,Fe.SiO 3 pyroxene Žhigh pressure clinoenstatite, ŽPacalo and Gasparik, 1989; Angel et al., 1992. neoblasts of 1–3 mm diameter formed adjacent to the titanate crystal aggregates that replaced the original ilmenite.
3.4. 1600 K experiments Three single-stage lherzoliteq ilmenite powder experiments were also performed at T s 1600 K, one each at 8, 10 and 12 GPa Žma67, ma62, and ma63, respectively.. At all pressures, olivine showed higher amounts of TiO 2 than at 1500 K; at 8 and 10 GPa, concentrations were only slightly elevated, but at 12 GPa, the increase was considerable. Investigation of this specimen Žma63. by TEM ŽFig. 4. showed that the crystals were free of inclusions that might have contributed to the high TiO 2 concentration in olivine measured by SEM, and confirmed that the TiO 2 concentration was in excess of 1 wt.% and homogeneous ŽTable 2.. Small grains of clinoenstatite adjacent to the titanate were more abundant than at 1500 K. 3.5. ReÕersal experiments Experiments ma68, ma74, ma78, ma81, and ma82, were all initially held at 12 GPa and temperature 1600 K to establish a TiO 2 composition in olivine 1 wt.%, followed by re-annealing at lower temperature or, in the case of ma81, at lower pressure. All experiments yielded olivine compositions consistent with the systematics of the single-stage results ŽTables 1 and 2.. These specimens also exhibited titanate precipitates in olivine Žnot shown., reflecting higher TiO 2 content established during the first stage of the experiment. We interpret the ranges of TiO 2 concentration in olivine of these specimens as due to failure of the specimens to fully re-equilibrate during the down-temperature reaction. The lowest values of TiO 2 measured in olivine in the TEM are the most reliable analyses in this case because we know the reaction is running from higher to lower concentration and because the SEM analyses may be averaging in precipitates produced during the second stage of the experiment. For example, specimen ma68 that had the second stage at only 1350 K shows a broad spread of TEM analyses and the SEM analysis is approximately the average of the TEM analyses ŽTable 1.. 3.6. Summary of results Our results show a range of TiO 2 solubility in olivine from less than the detection limit of our
L. Dobrzhinetskaya et al.r Chemical Geology 160 (1999) 357–370
Fig. 5. TiO 2 content of olivine vs. pressure for olivine held at various temperatures for 12 h. Symbols are the same as in Fig. 2.
method Ž- 0.1 wt.%. at 5 GPa, 1400 K, to ) 1.0 wt.% at 12 GPa, 1700 K ŽFig. 5.. There is general agreement between experiments starting with olivine having extremely low TiO 2 concentrations Žsinglestage experiments. and those approaching equilibrium from higher concentrations induced by an original treatment at higher pressure andror temperature Žtwo-stage experiments.. The presence of titanate precipitates in the latter class of experiments confirms that the first stage did, indeed, induce higher concentration of TiO 2 than measured in ‘‘clean’’ olivine at the end of the experiment. We also find a temperature dependence of solubility which is moderate at 8 and 10 GPa and greater at 12 GPa.
4. Discussion Our results fall into three general categories — very low solubility of TiO 2 in olivine at 5 GPa, moderate solubility at 8–10 GPa, and greater solubility at 12 GPa ŽFig. 5.. These three regimes were found to correlate with the presence of different titanate phases ŽFig. 2.. At 5 GPa, the starting FeTiO 3 ilmenite reacted with the silicate assemblage to yield rutile and ŽMg,Fe.TiO 3 ilmenite. Under these conditions, the solubility of TiO 2 in olivine was below our detection limit. In all dry experiments at 8 GPa and in 1500 K experiments at 10 GPa, only ŽMg,Fe.TiO 3 ilmenite was produced by Mg–Fe exchange with the silicate phases. At still higher pressure and temperature, the stable titanate was ŽMg,Fe.ŽTi,Si.O 3 per-
367
ovskite ŽFig. 2. and Ti-dissolution in olivine increased further. The increase in solubility was particularly enhanced at 12 GPa ŽFig. 5.. Thus, when olivine coexisted with rutile, solubility of TiO 2 was very low, but when it coexisted only with FeTiO 3 phases Žwith or without Si incorporation into the titanate. a solubility in excess of 0.4 wt.% TiO 2 was observed. Although only one experiment in this series produced rutile, our preliminary experiments in a hydrous environment have found rutile persisting to higher pressures and the hydrous experiments that are rutile-bearing also show no measurable TiO 2 in olivine ŽDobrzhinetskaya and Green, 1997; Bozhilov et al., 1999.. TiO 2 solubility in a hydrous fluid follows a similarly buffered path. At low pressures, in rutile-bearing assemblages, the solubility is significant, but decreases markedly with pressure ŽFoley and Wheller, 1990; Ayers and Watson, 1993., resulting in predictions that metasomatizing fluids in a mantle wedge environment should be impoverished in TiO 2 . It is interesting to note, however, that Fig. 3 Žp. 325. of Ayers and Watson Ž1993. shows the pressure-dependence of the solubility to be decreased at 3 GPa. Moreover, a subsequent study by Iizuka and Nakamura Ž1995. has shown that between 6 and 8 GPa, the stable titanate in ecoglite changes from rutile to ilmenite and correlates with a jump in solubility of TiO 2 in hydrous fluid from a vanishingly small value to a much larger one. These latter authors conducted experiments at 8508C Ž1123 K. and P s 4, 6, 8 GPa on samples consisting of 1r2 blueschist Žto simulate hydrous oceanic crust. and 1r2 olivine Žto simulate the overlying mantle wedge.. They showed that at 4 and 6 GPa, the blueschist was converted to rutile-bearing eclogite and that the fluid released by dehydration of blue amphibole reacted with the olivine to produce enstatite, indicating that the fluid carried significant amounts of SiO 2 ; no other phases grew, suggesting that the concentration of other components in the fluid was very low. At 8 GPa, however, the residual eclogite of their experiments was ilmenite-bearing and the olivine portion of the charge reacted to form TiCH as well as enstatite Žclinoenstatite?., indicating that destabilization of rutile led to a large increase of TiO 2 solubility in the fluid which carried it out of the ‘‘slab’’ and into the adjacent ‘‘mantle wedge’’ where it reacted with
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olivine to form TiCH. Our preliminary results on hydrous peridotite with excess TiO 2 are consistent with these results. Thus, when the activity of TiO 2 is fixed at 1 by rutile stability, both hydrous fluids and olivine exhibit very low TiO 2 solubility, but when rutile disappears from the paragenesis and is replaced by ilmenite, TiO 2 solubility increases significantly. In the case of olivine, the solubility of TiO 2 increases further when the ilmenite–perovskite phase boundary is crossed, but our results are not sufficient to distinguish between this being simply a PrT effect or whether the further enhancement of TiO 2 solubility in olivine is controlled by the coexisting titanate.
5. Geological significance of the results Our experimental results show that under nominally anhydrous conditions, the solubility of TiO 2 in olivine reaches 1.1 " 0.1 wt.% at P s 12 GPa, T s 1600–1700 K and that it is approximately half that at 10 GPa and comparable temperatures. These data are consistent with the solubility inferred by Dobrzhinetskaya et al. Ž1996. from the abundance of FeTiO 3 precipitates in Alpe Arami and further support origin of that body at depths ) 300 km Ž P ) 10 GPa.. Further, these results simplify the minimum depth of origin required for the Alpe Arami massif because they demonstrate the possibility of origin within the olivine stability field and remove the need to propose that such large amounts of dissolved TiO 2 may require the previous presence of a phase precursor to olivine, such as wadsleyite ŽGreen et al., 1997b.. These results and those of Iizuka and Nakamura Ž1995. suggest that there are two possible origins of the differences between subduction-zone garnet peridotites and their xenolithic and ophiolitic counterparts from the subcontinental and suboceanic lithosphere. Either additional TiO 2 has been added to subduction-zone peridotites, probably from fluid emanations from subducted lithosphere at 8 GPa, or existing TiO 2 in these rocks has been redistributed, very likely also in response to infiltration of fluids from subducted lithosphere. We propose that at least some peridotites bearing significant amounts of TiCH
have obtained this excess TiO 2 by dehydration of subducted oceanic crust at P ) 6 GPa. On the other hand, if the TiO 2 signature of mantle wedge peridotites is obtained from hydrous fluids, then how does one explain the case of the Alpe Arami lherzolite in which the assemblage is anhydrous and there is no evidence of the former presence of TiCH ŽErnst, 1977.. The simplest explanation is that the conditions under which the TiO 2 signature of Alpe Arami was impressed on the rocks was under conditions of higher P andror T than the maximum stability of TiCH. Our continuing experiments will investigate this upper stability limit. In summary, our experimental results resolve the geochemical problem of how such large quantities of TiO 2 could have been dissolved in Alpe Arami olivine and provide further evidence of the extraordinary depth from which this peridotite has been exhumed. They also support the hypothesis that garnet peridotites with unusual Ti-bearing phases such as TiCH or relict olivine with ilmenite precipitates reflect a unique mantle environment. The geological context in which these rocks outcrop strongly suggests that this environment is that of the mantle wedge overlying a subduction zone. Expanded geochemical studies of these rocks and the country rocks in which they are now found potentially can provide many new insights into this complicated environment.
Acknowledgements We thank Connie Bertka, Robert Liebermann, Robert Luth, Keith Putirka, and Eric Riggs for sharing their knowledge and experience with multianvil apparatus and for helpful discussions about experimental technique. We especially thank David Walker who provided extensive instruction and advice, and shared his calibrations with us during set up of our multianvil. We also thank Frank Forgit for assistance with apparatus alignment and maintenance. Bill Carey provided WDS analysis of our San Carlos olivine standard. Gary Ernst and David Walker provided critical reviews which significantly improved the manuscript. This work was supported by NSF Grant aEAR96-283432.
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