Werner Schreyer

Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz

Following and extending the early work of Velde (1965) the pressure-temperature dependence of the compositions of potassic white micas coexisting with K-feldspar, quartz, and phlogopite in the model system K2O-MgO-Al2O3-SiO2-H2O was investigated up to fluid pressures of 24 kbar by synthesis experiments. There is a strong, almost linear increase of the Si content per formula unit (p.f.u.) of phengite, ideally KAl2−xMgx[Al1−xSi3+xO10] (OH)2 with pressure, as well as a moderate decrease of Si (or x) with temperature. The most siliceous phengite with Si near 3.8 p.f.u. becomes stable near 20 kbar depending on temperature. However, contrary to Veldes assumption, these phengites coexisting with the limiting assemblage are invariably not of an ideal dioctahedral composition (as given by the above formula) but have total octahedral occupancies as high as about 2.1 p.f.u.The stability field of the critical assemblage phengite — K-feldspar — phlogopite — quartz ranges, in the presence of excess H2O, from at least 350° C to about 700° C but has an upper pressure limit in the range 16–22 kbar, when K-feldspar and phlogopite react to form phengite and a K, Mg-rich siliceous fluid.For the purpose of using these phase relationships as a new geobarometer for natural rocks, the influence of other components in the phengite (F, Fe, Na) is evaluated on the basis of literature data. Water activities below unity shift the Si isopleths of phengite towards higher pressures and lower temperatures, but the effects are relatively small. Tests of the new geobarometer with published analytical and PT data on natural phengite-bearing rocks are handicapped by the paucity of reliable values, but also by the obvious lack of equilibration of phengite compositions in many rocks that show zonation of their phengites or even more than one generation of potassic white micas with different compositions. From natural phengites that do not coexist with the limiting assemblage studied here but still with a Mg, Fe-silicate, at least minimum pressures can be derived with the use of the data presented.

35 Ma old ultrahigh-pressure metamorphism and evidence for very rapid exhumation in the Dora Maira Massif, Western Alps

Abstract Ion microprobe (SHRIMP) data on zircons from various rock types of an ultrahigh-pressure (UHP) metamorphic whiteschist-type pyrope quartzite lens of the Dora Maira Massif (DMM) consistently show domains giving a Late Eocene age of 35.4 ± 1.0 Ma which is taken as the age of UHP metamorphism. These domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 Ma. Zircons of a metagranitic country rock next to the UHP metamorphic lens have these primary features best preserved. All zircons measured also yield intermediate ‘ages’ between 275 and 35 Ma with a statistical concentration between 260 and 210 Ma. Thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith, that is a granite intruded during the Late Herynian. While the intermediate ages are at least partly due to incomplete resetting of the zircons during UHP metamorphism, those in the 260–210 Ma range may be related to rifting episodes in the Permotriassic. The Mg-rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite, perhaps by fluids derived from evaporitic sediments dating as early as the Permotriassic as well. The more pervasive resetting of zircon ages during UHP metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and/or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous Mg-rich protoliths and not in their drier granitic neighborhood. Fission track ages determined partly on the same zircon samples yielding 29.9 ± 1.4 Ma mark the time when the UHPM unit had reached about 290°C at a shallow location within the crust. Thus exhumation over a vertical distance of about 120 km must have occurred within about 5–6 Ma indicating an average uplift rate of about 20–24 km/Ma and an average cooling rate of about 85–100°C/Ma. The radiometric data obtained do not lend any support to an Eo-Alpine Cretaceous subduction event so that deep subduction and immediately following exhumation all took place during Early to Mid Tertiary time. This scenario seems to apply to large portions of the Western and Central Alps as well calling for drastic geodynamic reinterpretations of these parts of the Alps.

The pyrope-coesite rocks and their country rocks at Parigi, Dora Maira Massif, Western Alps ; Detailed petrography, mineral chemistry and PT-path

Both the coarse- and fine-grained varieties of the partly coesite-bearing pyrope-quartzites, their interlayered jadeite-kyanite rocks, and the biotite-phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents, which were also studied analytically by microprobe. Prior to pyrope and coesite growth, the Mg-rich metapelites were talc-kyanite-chlorite-rutile-ellenbergerite schists which — upon continued prograde metamorphism — developed first pyrope megacrysts in silica-deficient local environments at the expense of chlorite + talc + kyanite, and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc + kyanite. Based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations, a prograde PT-path — simplified for water activity = 1 — is constructed which passes through the approximate PT-conditions 16 kbar and 560° C, 29 kbar and 720° C, and finally up to 37 kbar at about 800° C, where the Mg-rich metapelite was a pyrope-coesite rock with phengite, kyanite, and talc still present. During the retrograde path, pyrope was altered metasomatically either into phlogopite + kyanite + quartz or, at a later stage, to chlorite + muscovite + quartz. Both assemblages yield PT-constraints, the latter about 7–9 kbar, 500–600° C. The country rock gneisses have also endured high-pressures of at least 15 kbar, but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies (about 5 kbar, 450° C). Microprobe data are presented for the following minerals: pyrope, ellenbergerite, dumortierite (unusually MgTi-rich), jadeite, vermiculite (formed after Na-phlogopite?), paragonite, and for several generations of phengite, chlorite, talc, phlogopite, dravite, and glaucophane in the high-pressure rocks, as well as for biotite, chlorite, phengites, epidote, garnet, albite, and K-feldspar in the country rock gneisses. An outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their PT-stability limits.

Antigorite: High-pressure stability in the system MgOSiO2H2O (MSH)

Abstract The breakdown reactions of antigorite: (1) forming talc + forsterite + water at low pressures and (2) forming forsterite + clinoenstatite + water at high pressures were determined in reversed equilibrium experiments. Results on reaction (1) were found to be in good agreement with former experimental determinations by both Johannes [Johannes, W., 1975. Zur Synthese und thermischen Stabilitat von Antigorit. Fortschr. Mineral. Beih. 53, 36.] and Evans et al. [Evans, B.W., Johannes, W., Oterdoom, H., Trommsdorff, V., 1976. Stability of crysotile and antigorite in the serpentinite multisystem. Schweiz. Mineral. Petrogr. Mitt. 56, 79–93.]. From our experiments the invariant point (I1), interconnecting the two reactions, can be located at about 15 kbar/650°C. This is consistent with the thermodynamic calculations using the dataset of Berman [Berman, R.G., 1988. Internally consistent thermodynamic data for minerals in the system J. Petrol. 29, 445–522.]; however, it is in contrast to recent experimental studies of Ulmer and Trommsdorff [Ulmer, P., Trommsdorff, V., 1995a. Serpentine stability to mantle depths and subduction-related magmatism. Science 268, 858-861.] who determined I1 at 21 kbar/730°C. Our PT-conditions for I1 could be confirmed by equilibrium experiments on reaction (10) talc + forsterite ↔ clinoenstatite + water, which is generated at I1 as well. Up to about 25 kbar the breakdown reaction (2) is nearly pressure-independent. Towards still higher pressures the dP/dT-slope of reaction (2) bends and becomes negative. Schreinemakers analysis as well as thermodynamic calculations of the upper pressure-stability of antigorite show that the possible antigorite breakdown reaction (3) antigorite ↔ clinoenstatite + brucite + water and reaction (4) brucite + clinoenstatite ↔ forsterite + water could originate at a new invariant point I2, provided that the reactions (2) and (11) brucite + antigorite ↔ forsterite + water intersect. Bracketing equilibrium (4) and combining these results with those on reaction (2), I2 was located at only about 51 kbar/490°C, compared to 77 kbar/680°C according to Bermans data. However, when taking into account the dense hydrous magnesium silicate (= DHMS)-phase A, Mg7Si2O8(OH)6, the phase relations of antigorite are changed resulting (i) in the metastability of I2 and reaction (3) and (ii) in a new invariant point I7 at about 44 kbar and 580°C generating the new antigorite breakdown-reaction (16) antigorite ↔ phase A + clinoenstatite + water. On the basis of these new data on the stability of antigorite, earlier conclusions about dehydration depths in subducted serpentine-bearing oceanic lithosphere have to be reconsidered. The maximum pressure stability of antigorite according to reaction (16) extends between 44 and 55 kbar, that is between about 130 and 160 km depths, as opposed to about 75 kbar (220 km) following Ulmer and Trommsdorff (see above). Because many different thermal regimes are possible in subduction zones, no specific dehydration depth can be expected but rather more continuous dehydration fronts in space and time.

Pb−Sr−Nd isotopic behavior of deeply subducted crustal rocks from the Dora Maira Massif, Western Alps, Italy-II: what is the age of the ultrahigh-pressure metamorphism?

Pb, Nd and Sr isotope data are reported from two localities on mineral separates from Mg-rich metapelites and associated rocks that have been subducted to depths of at least 100 km, for which metamorphic conditions are estimated at 28–33 kilobars pressure and 700°–800° C, and then returned to the surface. Initial isotope ratio data from the granitoid country rock are similar to those found in the metapelites. The initial ratios indicate predominantly recycled, aged granitic crustal materials for the sources of all of the samples. Five zircon samples, 4 from pyrope megacrysts and 1 from fine-grained pyrope quartzite lenses in the metapelites accurately define a chord yielding intercept ages of 304±10 and 38.0±1.4 Ma in a concordia diagram. Zircon from the country rock also plots along the chord. The zircon data, together with initial Nd and Sr data, indicate that the sedimentary sources of the rocks were derived mainly or entirely from sialic Hercynian rocks. Ellenbergerite from pyrope megacrysts and monazite from the fine-grained ground mass yield slightly younger ages of 30–34 Ma, apparently reflecting lower blocking temperatures than that of zircon. Sm−Nd data from a pyrope megacryst give an errorchron corresponding to an age of 38 Ma, in agreement with the zircon date. A major question concerns the timing of the ultrahigh-pressure metamorphism. Experimental data suggest that pyrope and quartz/coesite as well as ellenbergerite formed by various metamorphic reactions. If, as generally assumed, the ultrahigh-pressure metamorphism occurred ca. 100 Ma ago, our data require that the zircon did not experience measurable lead loss at that time, but lost major amounts of lead 38 Ma ago during late Alpine metamorphism. Estimates of diffusion rates for Nd in pyrope further suggest that the apparent Sm/Nd age of 38 Ma for the megacryst is not consistent with that model. Those problems are resolved if the ultrahigh-pressure metamorphism occurred 38–40 Ma ago, but problems remain from Ar/Ar dates of 100 Ma on phengite, an inferred 120 Ma age for zircon lead loss from another study, and possibly by the very rapid uplift required if the metamorphism is that young.

Ultradeep metamorphic rocks: The retrospective viewpoint

Ultradeep, or ultra-high-pressure (UHP), metamorphic rocks, formed from crustal protoliths within the stability field of coesite at pressures >2.5–3.0 GPa corresponding to depths >80–120 km, occur locally though regionally distributed in at least five continental areas. Their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high-pressure studies. Detailed petrographic and microprobe work, especially on mineral inclusions, in favorable cases allows the derivation of prograde PT paths during subduction and of retrograde ones during exhumation. Commonly, the gneisses adjacent to the UHP rocks do not exhibit signs of ultradeep metamorphism, apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration to shallower PT conditions during the retrograde path. It is also possible, however, that UHP equilibria were not attained throughout the rock volumes subducted, but only along zones of shearing and fluid introduction. If it is true that not all UHP metamorphic rocks return to the crustal orogenic belts, but some continue to be subducted to greater mantle depths, the classical geochemical pattern of a one-way mass transfer from mantle to crust throughout the Earths history is at stake. The assumed gradual growth of continents may have had a counterpart of continent destruction during collision events. Most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous (Mg)A1-silicates exist that were not found in nature thus far, but may be characteristic minerals in the cold portions of old subduction zones, thus extending the water retentivity of subducting slabs to greater, and hitherto unexpected, depths.

Whiteschists: Their compositions and pressure-temperature regimes based on experimental, field, and petrographic evidence

Abstract Whiteschists exhibiting the characteristic assemblage kyanite—talc are chemically almost completely contained in the model system MgO—Al 2 O 3 —SiO 2 —H 2 O. Their unusual bulk chemistry may at least partly be due to a derivation from mudstones associated with evaporites. The paragenesis talc—kyanite is stable at water pressures above 10 kbar in the temperature range of about 600–850°C. Geologically this would imply mean depths of burial near the continental Mohorovicic discontinuity. The talc—kyanite pair exhibits reaction relationships with lower-pressure assemblages such as chlorite—quartz, yoderite-quartz, and cordierite—corundum, which have also been found to occur as late metamorphic products in Whiteschists. Whiteschists and related rock types occur most widely in Central Africa, but are also known from Europe, Afghanistan, Tasmania, and Brazil. Estimated total pressures of metamorphism depend on the experimentally as yet unknown influence of water deficiency on the talc—kyanite stability field. There are indications from the variable stability of Mg-cordierite under wet and dry conditions that the talc—kyanite field may be shifted to total pressures as low as 7–8 kbar, but the influence of other gas species such as CO 2 that can also be incorporated in the cordierite structure, may possibly counteract this shift. The three-stage metamorphic history of the Afghanistan whiteschist, with the mineral succession chlorite + quartz → talc + kyanite → cordierite + corundum all preserved in one thin section, strongly suggests, however, that compositional changes of the coexisting metamorphic fluids with time may be far more decisive for the mineralogy developed than total pressure and temperature changes. The variability of fluid compositions during whiteschist metamorphism is, at least for this locality, due to the mobilization of a former evaporite deposit. Total pressures during whiteschist formation may not always have been as extreme as initially expected.

Garnet–omphacite–phengite thermobarometry of eclogites from the coesite-bearing unit of the southern Dora-Maira Massif, Western Alps

Abstract Using relevant geothermobarometric methods, P – T -data were collected for the reconstruction of the metamorphic evolution of 34 eclogite samples taken from small lenses and boudins within the ultrahigh-pressure (UHP) metamorphic coesite-bearing Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif. The mineral phases used (clinopyroxene, garnet, phengite), or growth zones thereof, were identified as being coexistent for different stages of metamorphism on the basis of careful petrographic studies. Of several published geothermobarometers, the garnet–clinopyroxene thermometer of Powell [Powell, R., 1985. Regression diagnostics and robust regression in geothermometer/geobarometer calibration: the garnet–clinopyroxene geothermometer revisited. J. Metamorph. Geol., 3, pp. 231–243.] combined with the garnet–clinopyroxene–phengite barometer after Waters and Martin [Waters, D., Martin, H.N., 1993. The garnet–clinopyroxene–phengite barometer. Terra Abstr., 5, pp. 410–411.] was chosen here, because it provided the most reliable results. Nevertheless, the scatter of P – T -data points for the prograde (stage I), peak metamorphic (stage II), and retrograde (stage III) development of the eclogites is still considerable. Among the many possible reasons for this inconsistency discussed, a partial lack of equilibration of some of the eclogites during their metamorphic history should be taken into account. Despite the data scatter, an average P – T -path could be estimated, which includes the following coordinates: for stage I: 15 kbar/500°C; 25 kbar/570°C; 32 kbar/650°C; for stage II: 36 kbar/720°C; and for stage III: 24 kbar/680°C and 14 kbar/620°C. This is in fair agreement with P – T -paths derived earlier for other rock types of the BIU on the basis of other geothermobarometers.

Instability of sapphirine at high pressures

AbstractThe stability field of Mg-sapphirines is limited at high pressures through the solid-solid breakdown reaction sapphirine⇌pyrope = corundum+spinel, the univariant curve originating from an invariant point located at 22 kb, 880°C to 30 kb, 1350°C. Under water pressures less than 22 kb sapphirines exhibit the same low-temperature breakdown into the assemblage chlorite+corundum+spinel as determined by Seifert (1974) between 1 kb and 7 kb thus resulting in one continuous univariant lower stability limit extending from 1 kb, about 650°C through 10 kb, 770°C to the invariant point at 22 kb, 880°C. If