Journal of Petrology Volume 42 Number 9 Pages 1781-1787 2001
© Oxford University Press 2001
‘New Constraints on the P–T Evolution of the Alpe Arami Garnet Peridotite Body (Central Alps, Switzerland)’: Reply to Comment by Nimis & Trommsdorff (2001)
MINERALOGISCHES INSTITUT, UNIVERSITÄT HEIDELBERG, IM NEUENHEIMER FELD 236, 69120 HEIDELBERG, GERMANY
Received May 14, 2001; Revised typescript accepted May 18, 2001
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INTRODUCTION |
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 TOP INTRODUCTION REPLYCONCLUSIONSREFERENCES |
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Nimis & Trommsdorff (2001a)
(hereafter N&T) offer an alternative and somewhat challenging interpretation of the textural and mineral compositional features displayed by the Alpe Arami (AA) peridotite body. Although N&T dispute some of the arguments used by Paquin & Altherr (2001) (hereafter P&A), they now agree that the AA peridotite has indeed experienced high temperatures of 1100–1200°C as indicated by the partitioning of elements with high diffusivities (Fe, Mg, Ni, Co) between the cores of grt, ol, opx and cpx grains (Brenker & Brey, 1997; P&A). We are therefore left with the key problem of why the application of two-pyroxene thermometers based on elements with low diffusivities (Ca, Sc, Cr, V, Ti) results in low and strongly scattering temperature values (i.e. 690–925°C; P&A; Nimis & Trommsdorff, 2001b). P&A conclude that the discrepancy between the high and low temperature values is due to a lack of equilibration during rapid subduction and exhumation and, therefore, assume peak metamorphic conditions of 
1180°C and 5·9 GPa. N&T postulate a two-stage evolution whereby the AA peridotite was first subducted and equilibrated with respect to all elements at low T and P (800°C and 3 GPa) and subsequently subjected to a short-lived thermal event at T 
1100 °C and P 
3 GPa. The arguments that led N&T to promote their hypothesis will be considered under various aspects: (1) effective bulk compositions and system equilibration; (2) selection of thermobarometers; (3) geological and geochronological constraints. We will demonstrate that the textural and mineral compositional features of the AA garnet peridotite are not in favour of the hypothesis put forward by N&T. |
REPLY |
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TOPINTRODUCTIONREPLYCONCLUSIONSREFERENCES |
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Ca partitioning between Opx and Cpx
N&T correctly state that Ca/(1 - Na) = Ca* is ‘a much more robust indicator of temperature than is Ca alone’ but even in their fig. 1 showing experimentally determined Ca*opx–Ca*cpx relationships as a function of T and P (data of Brey & Köhler, 1990
), it is obvious that the value of Ca*opx Ib is too low to be in equilibrium with the value of Ca*cpx Ib, unless the apparent T–P values of 920°C and 5–6 GPa are accepted. Disequilibrium between opx Ib and cpx Ib is also indicated by the fact that the temperature values given by the pyroxene solvus thermometer and the Ca-in-opx thermometer of Brey & Köhler (1990) is exceptionally large (794 ± 23°C vs 926 ± 23°C at 6 GPa; see table 3 of P&A) and not <40°C as claimed by N&T. However, because pyroxene solvus relationships at low temperatures are not very well constrained, as a result of the relative insensitivity of Ca* to temperature changes and lack of coverage by reversed experiments, we agree with N&T that average Ca* values of pyroxene Ib cores alone are not necessarily a proof for both pyroxenes being in chemical disequilibrium.
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A much better argument for disequilibrium among the first-generation pyroxenes is provided by the overall variability in pyroxene core compositions in terms of elements with low diffusivity. Although, for example, the element zonation patterns of all cpx Ib grains are fairly similar, their cores show significantly different absolute abundances of Ca, Na, Al, Ti and Sc (P&A, table 2). These compositional heterogeneities, both within the cores of individual pyroxene Ib grains and among different grains, should not be overlooked when postulating an equilibration between cpx Ib and opx Ib grains that have never been found in contact with each other. Therefore, we still think that the compositions of pyroxene Ib grains, in particular their budget in Ca and Na, were to a large degree controlled by variable effective bulk compositions depending on next-neighbour mineral phases’ element diffusivity and grain size.
Al in opx and Cr in cpx
N&T point out that the Al-in-opx barometer of Taylor (1998) (hereafter PT98) and the Cr-in-cpx barometer of Nimis & Taylor (2000) (hereafter PNT00) show a very good agreement when applied to AA pyroxenes assuming nominal temperatures equal to those derived from the pyroxene solvus (800–850°C) but diverge significantly at higher temperatures (see N&T, fig. 2). Furthermore, N&T claim that the poor accord between the Al-in-opx barometer of Brey & Köhler (1990) (hereafter PBK90) and PNT00 at high T ‘renders P&A’s barometric data for peak metamorphic conditions unreliable and suggests that they can have been significantly overestimated’. Our reply to these comments is as follows:
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The discrepancy between the results obtained by the different barometer formulations at high nominal temperatures cannot be used as an argument for the correctness of the postulated low equilibration temperatures of AA pyroxene Ia and Ib grains. A careful evaluation of the database for both barometers reveals a severe problem. Whereas the pressures of the reversed experiments carried out by Brey & Köhler (1990) are excellently reproduced by PBK90 without any systematic dependence on either T or P (see Brey & Köhler, 1990, fig. 10), there is a systematic divergence between experimental and calculated pressures for PNT00 (Fig. 1). PNT00 tends to overestimate at low pressures (4·2 GPa) and increasingly underestimates at higher pressures. To date, the experimental dataset of Brey & Köhler (1990) is the only one that covers pressures in excess of 3·5 GPa; these data were also used by Nimis & Taylor (2000) for the formulation of their PNT00 (see Nimis & Taylor, 2000, fig. 2a). As the experiments by Nickel (1989) and Taylor (1998) (all at P 3·5 GPa) that were also used to formulate PNT00 were not reversed, the observed systematic divergence of PNT00 relative to the more reliable PBK90 may, at least in part, be an artefact of incomplete equilibration in the experiments. As this reply is not meant to become a comment on Nimis & Taylor (2000), we will not address further problems that are related to the solubility of Cr and Al in clinopyroxene.
Al zoning in pyroxenes
We agree with N&T that the rimward decrease in Al and Na observed in cpx Ib porphyroclasts can, in principle, be caused by jadeite-consuming reactions producing edenite. Indeed, a careful re-inspection of our data revealed a more or less systematic variation of the Al zoning patterns shown by cpx Ia inclusions in grt, dependent on the presence or absence of secondary clinoamphibole at the cpx–grt interface. The occurrence of M-shaped Al zoning profiles is restricted to those cpx Ia inclusions that are virtually not accompanied by secondary amphibole. Whenever edenitic hornblende is present at the grt–cpx Ia interface, cpx Ia grains display a more or less regular decrease of Al from core to rim (Fig. 2; see also P&A, fig. 5).
A much more important issue is the explanation of M-shaped Al zoning profiles in first-generation opx grains. We agree with N&T that the initial rimward increase of Al in opx Ib porphyroclasts can be, in principle, equally well explained by near-isothermal decompression (P&A) or near-isobaric heating (N&T). It is, however, important to note that only in some of the opx Ib grains is the initial rimward increase in Al accompanied by a minor increase in Ca (see P&A, fig. 3). Furthermore, opx Ia inclusions in grt never show an increase in Ca from core to rim. Instead, the abundances of Ca in these grains are either constant or they decrease slightly from core to rim (Fig. 2; see also P&A, fig. 4). If the rimward increase of Al in all these opx grains is really caused by a short-lived thermal pulse at 3 GPa as suggested by N&T, then the variable behaviour of Ca is difficult to explain. Moreover, there are some opx Ia inclusions in grt that show a decrease in Al from core to rim, accompanied by a slight decrease in Ca (AA-3P1/1 in Fig. 2). On account of all these observations, we still favour our original explanation that the M-shaped Al zoning profiles of opx Ib grains are the result of a near-isothermal decompression at high T, followed by enhanced cooling at low P, whereby the abundances of Ca were not controlled by changing P–T conditions alone, but also by effective bulk compositions with more or less Ca.
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The high-T stage
We will first discuss the constraints on the duration of the postulated high-T stage. As suggested earlier (Paquin & Altherr, 2000), we agree with N&T that the homogenization of grt I grains in terms of Fe, Mg and Ni at T = 1150°C and 3 GPa would require only a relatively short time span. However, for the Mn-poor AA garnets with XMg = 0·82 and XFe = 0·18, an interdiffusion coefficient D(Mg–Fe) of 8·8 x 10-16 cm2/s (Ganguly et al., 1998) seems more appropriate than the value of 10-14 cm2/s used by N&T. This would result in a time span of 1 m.y. as opposed to the value of 0·1 m.y. suggested by N&T. This is, however, not a critical point, as absolute values of D have an uncertainty of about two orders of magnitude (e.g. Dimanov & Sautter, 2000).
N&T propose that the thermal event was short enough to leave ‘the composition of pyroxene cores virtually unaffected’. In fact, the partioning of Ni and Co between opx Ib and cpx Ib cores indicates high temperatures (see ‘The partitioning of transition elements between pyroxene porphyroclasts’ in P&A). This is, however, attributed by N&T to ‘uncertainties in the determinations of trace element abundances, especially in opx’ without giving further concrete arguments.
The rather speculative assumption of N&T that ‘accurate barometry of this high-T phase is not anyway practicable on the basis of available data, because of lack of high-T pyroxene–garnet equilibrium pairs’ is incorrect. Our detailed investigations have shown that primary contacts between garnet and opx Ia/Ib grains (without secondary kelyphite) are still preserved in the AA peridotite. At their contacts with grt, all opx grains consistently show Al contents of 0·037 cations per formula unit (c.p.f.u.) as shown in Fig. 2 (see also P&A, fig. 4), irrespective of opx core compositions (0·032–0·045 Al c.p.f.u.). Assuming that the initial rimward increase of Al in opx Ib porphyroclasts and in most opx Ia grains was indeed caused by a short thermal pulse with peak temperatures of 1180°C as suggested by N&T, the constant Al content of opx at the interface with grt should reflect the P–T conditions reached during the thermal pulse. Assuming peak temperatures of 1180 and 1100°C, the application of the Al-in-opx barometer of Brey & Köhler (1990) yields pressure values of 5·4 and 4·9 GPa, respectively. Even if the maximum observed Al content in opx (0·045 c.p.f.u.) is used for the calculation, one still obtains a nominal pressure of 4·8 GPa (at 1180°C). These values are much higher than the 3 GPa postulated by N&T and we therefore conclude that Al contents in opx at the interface with grt are not compatible with the hypothesis of a short-lived thermal event with peak temperatures above 1100°C at P 3 GPa as suggested by N&T.
N&T dispute the usability of Ca–Cr relations in peridotitic garnet (Brenker & Brey, 1997) as an independent check for P–T estimates and propose that Nimis & Trommsdorff (2001b) ‘demonstrated the inconsistency of the thermobarometric formulations of Brenker & Brey’. However, the evaluation of Brenker & Brey’s formulation by Nimis & Trommsdorff (2001b) is somewhat incorrect. The thermometer and barometer expressions of Brenker & Brey’s formulation given by Nimis & Trommsdorff (2001b, fig. 2) show that pressure-corrected T and temperature-corrected P depend on both the Ca and the Cr contents of grt. Therefore the ‘scatter’ of the experimental data of Brey et al. (1990) is partly due to variable Cr contents of their garnets. Because, for any given P and T, the Ca content in grt increases linearly with Cr (with a slope of 0·045 Ca c.p.f.u. per 0·100 Cr c.p.f.u.), a linear regression for garnets with Cr < 0·2 c.p.f.u. is not allowed from a mathematical standpoint. For any given Cr content in grt, a difference of 0·045 c.p.f.u. Ca corresponds to 
T of 421°C (at constant P) or P of 3·08 GPa (at constant T). The statement ‘For Ca contents equal to those in Alpe Arami garnets (dashed line) T and P can be greatly overestimated’ (see Nimis & Trommsdorff, 2001b, caption to fig. 2) is therefore incorrect and misleading. In fact, the Ca–Cr relationships of AA garnets are compatible with an equilibration at high T and P and they are clearly not compatible with 840°C and 3·2 GPa (see P&A, fig. 8). Moreover, the Ca–Cr relationships of garnets from the Cima di Gagnone peridotite (Nimis & Trommsdorff, 2001b; Paquin, 2001) are also consistent with estimated peak metamorphic conditions (740°C at 3·0 GPa; Nimis & Trommsdorff, 2001b). Unfortunately, the analysis of grt in the Mt. Duria peridotite that was presented by Nimis & Trommsdorff (2001b) and used by these authors as another argument to discriminate the Brenker & Brey’s Ca–Cr formulation is of very poor quality as it shows only 2·957 Si c.p.f.u. and a total of 8·041 cations.
Igneous event being responsible for high temperatures
The two-stage evolutionary model proposed by N&T for the AA peridotite implies advective transport of heat to raise temperatures from 850 to 1150°C within a short time span (stage 2 of N&T’s model). Assuming that heat was provided by an intruding mafic magma with T = 1300°C requires a minimum volume of melt that is 2·5 times larger than the volume of the heated AA peridotite. By losing heat, this melt should have crystallized as eclogite (at high P) or mafic granulite (at lower P). The AA peridotite body comprises boudins and layers of (garnet-bearing) spinel pyroxenite of centimetre to decimetre thickness. The total volume of these pyroxenites is estimated to be <3% of the whole peridotite body. The pyroxenites contain 35–36 Ma zircon domains characterized by oscillatory growth banding and thought to have crystallized from a melt or a fluid phase (Gebauer, 1996). As similar 35 Ma zircon domains were also found in the peridotite itself (Gebauer, 1996), one might speculate that the peridotite comprises a certain amount of cryptic infiltrated melt. This amount, however, must be very limited, as indicated by the fact that peridotitic ol, opx and cpx are generally characterized by high mg-number (90–93), even in the vicinity of pyroxenitic layers.
The eclogite slivers surrounding the AA peridotite body could, in principle, also represent remnants of a former mafic intrusion that was responsible for the thermal pulse postulated by N&T. Interestingly enough, these eclogites also contain 35 Ma zircon domains showing oscillatory zoning (Gebauer, 1996, 1999). It is, therefore, necessary to compare the thermal evolutions of the AA eclogite and peridotite. Apart from retrograde effects at their rims (e.g. grt grains show a rimward increase in Fe and Mn, and decrease in Mg), garnet and clinopyroxene grains in the AA eclogites show homogeneous core compositions (Ernst, 1977; Tappert et al., 1999). The application of thermometers based on the partitioning of Fe–Mg between grt and cpx (Krogh, 1988; Krogh Ravna, 2000) to the mineral compositions given by Ernst (1977) results in temperatures of 750–800°C for a nominal pressure of 3 GPa. For all nominal pressures between 3 and 6 GPa, calculated equilibration temperatures for the eclogites are at least 200°C lower than those obtained for the AA peridotite, irrespective of the chosen thermometer version (Fig. 3). Equilibration of the eclogites at low temperatures of 800°C implies a relatively long time span of 10–30 m.y. (Tappert et al., 1999). On the other hand, the compositions and zoning patterns of mineral grains in the AA peridotite indicate rapid cooling from 1150°C to temperatures below 700°C (P&A) and are clearly not compatible with a 10 m.y. period of equilibration at 800°C. We conclude that the eclogites and peridotites were tectonically amalgamated after equilibration of the eclogites at 750–800°C. It is, therefore, highly unlikely that the eclogite precursor magma was responsible for the thermal pulse postulated by N&T.
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CONCLUSIONS |
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TOPINTRODUCTIONREPLYCONCLUSIONSREFERENCES |
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We are grateful to N&T for their comments, which helped us to clarify certain aspects of our paper (P&A) that were formulated in a somewhat brief manner, such as the interpretation of retrograde Al and Na zonation profiles in cpx Ib porphyroclasts and of Ca*opx–Ca*cpx relationships. None the less, with respect to the key problem, we have demonstrated that the compositions and zoning patterns of first-generation mineral grains in the AA peridotite are not compatible with the two-stage evolutionary model postulated by N&T. In particular, the Al contents of opx grains at the contact with grt cannot be explained by assuming temperatures of 1100–1180°C at 3 GPa, but instead, require high temperatures in combination with high pressures (
4·8 GPa). Furthermore, simple heat balance calculations demonstrate that the short-lived thermal event postulated by N&T implies advective heat input by a magma volume that is significantly larger than that of the AA garnet peridotite body as a whole. Even if the pyroxenitic layers within the peridotite were generated by melt infiltration at or after the peak of subduction-related metamorphism, the magma volume was by far too small to cause the postulated heating of the peridotite to temperatures in excess of 1100°C. Furthermore, because the thermal evolution of the eclogite associated with the AA peridotite is different from that of the peridotite, the eclogite precursor magma is also excluded as a possible heat source.
N&T state that ‘unique early features are shared by the Alpe Arami and Cima di Gagnone garnet peridotites’, such as ilmenite rods and palisades (Risold et al., 2001) and crystal preferred orientations of olivine Ib with [100] perpendicular to foliation (Frese et al., 2001). These common features are, however, accompanied by a number of significant differences between the two peridotite bodies. In marked contrast to the AA peridotite, the mineral grains in the Cima di Gagnone (CdG) peridotite show a higher degree of chemical heterogeneity and significantly different trace element concentrations (Paquin, 2001; Paquin et al., 2001). Moreover, in contrast to the CdG peridotite, unequivocal primary amphibole was never found at Alpe Arami (Nimis & Trommsdorff, 2001b; Paquin, 2001; P&A) and the two peridotites show different oxygen isotope compositions (Paquin et al., 2000).
As already pointed out by P&A, it is, from a geodynamic point of view, by no means necessary to assume that there exists ‘a prograde, high-pressure Alpine metamorphic sequence of the Adula–Cima Lunga nappe’ (N&T). Although the Adula and the Cima Lunga units interpreted as lithospheric mélanges by Trommsdorff (1990) occupy comparable positions within the Penninic nappe system, this does not imply their tectonostratigraphic equivalence. During subduction, collision and exhumation, HP and UHP mantle rocks such as the garnet peridotites from the Cima Lunga unit may have become tectonically mingled with continental materials at various stages, as indicated by the different thermal evolutions of the AA eclogite and peridotite bodies as discussed above.
In conclusion, we think that the textural and mineral compositional features of the AA peridotite and its relations with the country rocks can best be explained by our original hypothesis whereby rapid subduction to depths equivalent to a pressure of 5·9 GPa was followed by a near-isothermal decompression with subsequent enhanced cooling. Within the frame of this model the discrepancy between high and low temperature values obtained from thermometers based on the partitioning of elements with high and low diffusivities, respectively, can easily be explained by a lack of equilibration during prograde evolution. Nevertheless, there is still a lot of information that waits to be extracted from one of the most fascinating rocks on Earth.
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ACKNOWLEDGEMENTS |
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We are grateful to Paolo Nimis and Volkmar Trommsdorff for stimulating discussions, and to Kurt Bucher for his editorial handling. Emily Lowe improved the English style. J.P. was financially supported by the Studienstiftung des Deutschen Volkes e. V.
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FOOTNOTES |
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*Corresponding author. Telephone: +49-6221-544651. Fax: +49-6221-544805. E-mail: jpaquin{at}min.uni-heidelberg.de
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REFERENCES |
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