Ti diffusion in quartz inclusions: implications for metamorphic time scales

Transcription

1 Contrib Mineral Petrol (212) 164: DOI 1.17/s z ORIGINAL PAPER Ti diffusion in quartz inclusions: implications for metamorphic time scales Frank S. Spear Kyle T. Ashley Laura E. Webb Jay B. Thomas Received: 3 April 212 / Accepted: 28 July 212 / Published online: 24 August 212 Ó Springer-Verlag 212 Abstract Quartz inclusions in garnet from samples collected from the staurolite zone in central New England are zoned in cathodoluminescence (CL). The CL intensity is interpreted to be a proxy for Ti concentration and the zoning attributed to Ti diffusion into the quartz grains driven by Ti exchange between quartz and enclosing garnet as a function of changing temperature. The CL zoning has been interpreted using a numerical diffusion model to constrain the time scales over which the diffusion has occurred. Temperature time histories are sensitive to the presumed peak temperature but not to other model parameters. The total time of the metamorphic heating and cooling cycle from around 45 C to the peak temperature (55 6 C) back to 45 C is surprisingly short and encompasses only.2 2 million years for peak temperatures of 6 55 C. The metamorphism was accompanied by large-scale nappe and dome formation, and it is suggested that this occurred as a consequence of in-sequence thrusting resulting in a mid-crustal ductile duplex structure. Communicated by T. L. Grove. F. S. Spear (&) J. B. Thomas Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, 11 8th Street, Troy, NY 1218, USA spearf@rpi.edu K. T. Ashley Department of Geosciences, 444 Derring Hall, Virginia Tech, Blacksburg, VA 2461, USA L. E. Webb Department of Geology, University of Vermont, 18 Colchester Avenue, Burlington, VT 545, USA Keywords Cathodoluminescence Quartz Ti diffusion Vermont metamorphism TitaniQ Introduction Spear and Wark (29) described the appearance of quartz grains in cathodoluminescence (CL) images in rocks crystallized at metamorphic grades ranging from the biotite to the migmatite zone. They observed that in rocks of the staurolite and staurolite kyanite zones, quartz inclusions within garnet (and staurolite) were typically zoned with increasing CL intensity from the quartz core to the quartz garnet interface. They attributed this zoning to have been caused by diffusion of Ti in quartz, driven by Ti Si exchange between quartz and enclosing garnet. Quartz inclusions from all 15 of the staurolite-zone samples examined revealed this type of zoning. In contrast, quartz inclusions from lower-grade samples were unzoned, and quartz inclusions from higher-grade samples displayed either complex internal CL patterns, or were zoned with decreasing CL intensity (and Ti contents) toward the quartz garnet interface. The purpose of this paper is to present observations on additional staurolite-zone samples and describe the results of diffusion modeling of the zoning profiles. The objective of this study is to constrain the time scales of the metamorphism of these samples and to evaluate possible tectonic processes that may have been responsible for these time scales. Analytical methods CL imaging and Ti analyses were performed on the Cameca SX-1 electron microprobe at Rensselaer

2 978 Contrib Mineral Petrol (212) 164: Polytechnic Institute following the methods described in Spear and Wark (29). CL images were collected with a Gatan Mono-CL detector equipped with red, green, and blue filters. The blue filter was used exclusively for the images used in this study because of the linear relationship between Ti concentration and CL emission at blue wavelengths (e.g. Wark and Spear 25; Spear and Wark 29; Leeman et al. 212). Ti analyses were collected on four PET crystals simultaneously, yielding a precision of around 6 7 ppm for each spot analysis (Spear and Wark 29). Sample selection and modeling approach Samples were selected for analysis from the authors collections from east-central Vermont and west-central New Hampshire after routine CL imaging of dozens of samples from different metamorphic grades (e.g. Spear and Wark 29). With the exception of sample TM-549, all are from the staurolite or staurolite kyanite zones (Table 1) and all samples contain garnet with abundant quartz inclusions. The approach to be followed in this paper is to present in detail the zoning character and diffusion modeling results from a single sample from eastern Vermont (79-149D) and then to apply this method to diffusion profiles from the entire suite of samples. Table 1 contains the complete list of samples to be discussed and their locations. Sample D is a metapelite from the Orfordville belt of eastern Vermont and western New Hampshire (Fig. 1. Also see Fig. 3 of Spear and Rumble 1986, for a map showing the location of this sample). The assemblage is garnet? biotite? staurolite? kyanite? muscovite? quartz? plagioclase? ilmenite? monazite? zircon. Garnet preserves growth zoning of, from core to rim, decreasing Mn and Ca and increasing Fe/Mg. The P T path calculated for this sample from garnet zoning shows an episode of nearly isothermal loading followed by heating with decompression (Spear and Rumble 1986, Fig. 7) with a peak P T conditions of 58 C, 5.1 kbar. Figure 2 shows a CL image of the garnet from this sample at different scales (see also Spear and Wark 29, Figs. 1c, d, e; 6a, b, c). Quartz in the matrix is relatively unzoned with Ti concentrations of 5 14 ppm. In contrast, all of the quartz inclusions in the garnet are zoned with increasing Ti content from core to rim. In addition, quartz inclusions inside of staurolite are zoned similarly to those in garnet, although not as cleanly and quartz inclusions in kyanite from this sample are unzoned. The Ti contents in one inclusion measured on the electron probe range from 3 ppm to 14 ppm (Figs. 3, 6c of Spear and Wark 29) and correlate linearly with CL intensity in the blue region, although the uncertainty in these measurements is on the order of ±7 ppm (1 sigma). Additional data supporting the correlation of CL intensity in quartz in the blue region (ca. 415 nm wavelength) with Ti concentration have been presented by Spear and Wark (29), Wark and Spear (25), and Leeman et al. (212). Based on these studies, Ti will be assumed to be a linear function of CL intensity in the blue region, thus enabling rapid semi-quantitative assessment of the Ti zoning profiles in quartz. In contrast, CL images of quartz inclusions in garnet in samples from other metamorphic grades display distinctly different zoning. In garnet zone samples, quartz inclusions are relatively unzoned (e.g. Fig. 2 of Spear and Wark 29), and in sillimanite-zone and higher-grade samples, quartz inclusions display complex internal textures and are typically zoned with decreasing Ti contents toward the garnet interface (e.g. Fig 8. of Spear and Wark 29). Two causes of the observed CL (i.e. Ti) zoning in quartz are considered. In the first, prior to entrapment by garnet, the quartz in the matrix is assumed to be zoned with lower Ti cores and higher Ti rims. In the extreme, the rims might have been step functions, although no such zoned quartz Table 1 Sample locations and metamorphic grade Sample Latitude Longitude Grade Peak a Peak P(Kb) a Assemblage St Ky 58 ± Qtz? Pl? Grt? Bt? Ms? St? Ky? Ilm? Mnz TM Grt 48 ± Qtz? Grt? Bt? Ms? Chl TM St 555 ± Qtz? Pl? Grt? Bt? Chl(r)? Rt TM St 575 ± Qtz? Pl? Grt? Bt? Ms? Rut? Gra? Tur TM St 595 ± Qtz? Pl? Grt? Bt? Ms? Rut? Ap? Tur TM St 55 ± Qtz? Pl? Grt? Bt? Ms? Chl? Rt? Ap 9SD8A St 55 ± Qtz? Pl? Grt? Bt? Ms? Rt TM St 55 ± Qtz? Pl? Grt? Bt? Ms? Chl(r)? Rt? Gra? Tur BF-38B St 55 ± Qtz? Pl? Grt? Bt? Ms? St? Ilm a Peak P and T sources: Samples TM-xxx are from Menard and Spear (1994); sample D from Spear and Rumble (1986); sample BF-38B from Spear and Wark (29). Mineral abbreviations after Kretz (1983)

3 Contrib Mineral Petrol (212) 164: Fig. 1 Geologic sketch map of a part of eastern Vermont and western New Hampshire showing location of samples studied with the exception of BF-38B, which is described by Spear and Wark (29) and located in Spear et al. (1995). Diamonds with numbers are locations of samples from Wing et al. (23) discussed in the text with monazite ages shown. Inset shows general location of study area Fig. 2 CL images of sample D. a Low-magnification image showing garnet (black) and distribution of quartz inclusions. Box shows location of (b). b CL image showing zoning in quartz inclusions. Box shows location of Fig. 3a grains are now observed in the matrix. Mechanisms for producing quartz zoned in this fashion were discussed by Spear and Wark (29) and include (a) prograde reactions and (b) recrystallization due to fabric reorientation. It is not expected that pre-garnet prograde reactions could have produced quartz zoned with increasing Ti concentrations toward the rim because matrix quartz in garnet- and subgarnet-grade samples shows no such zoning and the first major quartz-producing reaction experienced by these rocks is the staurolite-in reaction (garnet? chlorite? muscovite = staurolite? biotite? quartz? H 2 O). It is still possible that quartz recrystallization as a result of strain could produce pre-garnet rims of higher Ti concentrations, but again evidence for this in lower-grade samples is lacking. The second cause considered is that the pre-garnet quartz grains were relatively homogeneous with low Ti concentrations on the order of 3 5 ppm, similar to the

4 98 Contrib Mineral Petrol (212) 164: (a) (b) Ti (ppm) (c) Ti (ppm) , y =.2x R 2 =.8 25 µm Distance (µm) Fig. 3 a CL image of quartz inclusion in garnet (sample D). Bright spots are electron microprobe analytical spots with Ti (ppm) indicated for each. Garnet host is anomalously bright (it should be dark) because of secondary luminescence of oil on the CL mirror. Rectangle shows location of zoning traverse in (c). b Plot showing CL intensity (gray scale value) against measured Ti concentration. c Plot of Ti zoning in the quartz inclusion in a based on calibration shown in (b) cores of the quartz inclusions in garnet, with the Ti concentration governed by the local TiO 2 activity. Sample D contains ilmenite in the matrix and as inclusions in garnet, so it is expected that the activity of TiO 2 was close to 1. (Ghent and Stout, 1984). The interior of the quartz at the time of garnet overgrowth was not necessarily in equilibrium with the rim as garnet nucleation is believed to have occurred at around 5 C (Spear and Rumble 1986), whereas the Ti concentration of the quartz interiors, which ranges from 3 to 5 ppm in both matrix and inclusion grains, reflects a temperature of approximately 35 4 C at around 5 kbar (calibration of Thomas et al. 21). Once garnet completely overgrew a quartz grain, equilibrium with the matrix TiO 2 activity is no longer relevant and is replaced by exchange equilibria between garnet and quartz. The substitution mechanism for the incorporation of Ti into garnet is not known, so it is not possible to write a specific exchange reaction and associated equilibrium constant. However, garnet contains more Ti than coexisting quartz. Analyses for Ti contents in garnet from sample D using LA-ICPMS reveal concentrations ranging from ca. 6 ppm near the rim to 3 ppm near the core. Microprobe analyses of garnet crystals examined in this study range up to a thousand ppm Ti. Regardless of the nature of the exchange reaction between garnet and quartz, the equilibrium constant for the reaction should approach 1. (equal partitioning) with increasing temperature with the result that the quartz rim should become more Ti-rich and the adjacent garnet more Ti-poor with increasing metamorphic grade. This exchange would set up a gradient in the quartz, which would drive diffusion. The quartz rim would continue to become increasingly Ti-rich up to the metamorphic peak, and the exchange would reverse on cooling. The final zoning profile in the quartz would thus be a function of the time-dependent composition of the quartz rim, the diffusivity of Ti in quartz, and the temperature time history. Of these two mechanisms, the second is preferred in part because of the reasons given above that argue for homogeneous quartz at the time of garnet growth, and in part because of the symmetry of the Ti zoning in the inclusions. In any case, the results of this study are not strongly dependent on whether the pre-garnet quartz grains had step functions in Ti concentration or were modified by the exchange of Ti with the host garnet. In either case, the present-day zoning profile is modified by diffusion over length scales of less than 1 lm, which, as will be discussed below, requires quite rapid heating and cooling. Ideally, the zoning profiles in quartz should be modeled as Ti concentration versus distance. However, the linear relationship between CL intensity in the blue spectrum and Ti concentration provides a more sensitive, both spatially and compositionally, approach. As can be seen in Fig. 3, the linear correlation between CL intensity and Ti concentration permits the use of CL intensity as a proxy for Ti concentration and permits the modeling of CL intensity profiles directly. Although the scatter in Fig. 3b is

5 Contrib Mineral Petrol (212) 164: relatively large, more detailed justification for this correlation has been given by Wark and Spear (25), Spear and Wark (29), and Leeman et al. (212). The observed CL intensity profile in many quartz inclusions is somewhat asymmetric (Fig. 3c), as is the measured Ti profile, for unknown reasons. Both sides of the profile are systematic with respect to the garnet quartz interface, and both sides show an increase from core to rim to a maximum value, then a decrease right at the rim. It should be noted in Fig. 3a that garnet is the brightest phase in the image, even though the luminescence is almost nil. This has occurred because, at the time this CL image was collected, a thin film of oil had contaminated the parabolic light focusing mirror on the CL light finger. This oil luminesces in the blue region and garnet is bright in this image because backscattered electrons are activating the oil causing it to luminesce (ilmenite luminesces even brighter in similar images). This artifact was corrected by cleaning the mirror but has the fortuitous advantage of showing exactly the position of the garnet quartz interface. It can be clearly seen in Fig. 3a that a dark border, indicative of a decrease in Ti concentration, mantles the edge of the quartz at the garnet interface. This decrease is important in the modeling as it is the change in concentration due to diffusion during cooling. Results Figure 4 shows an example of the results of a diffusion model of a CL zoning profile from a quartz grain from sample D based on the assumptions inherent in the second mechanism discussed above (initial conditions of homogenous quartz of low Ti concentration included in garnet). CL intensities were calculated as average pixel intensities using ImageJ (Rasband ) software and a traverse width of typically 1 pixels. The diffusivities measured by Cherniak et al. (27) were incorporated into a finite difference code using a linear geometry, fixed boundary position, and boundary composition that changed with temperature. The initial condition was that of an unzoned quartz grain with the composition of the core of the grain. The CL intensity at the quartz garnet interface was modeled using an expression to mimic a partitioning expression, namely: Log(CL intensity) ¼ a þ b=t ð1þ The parameters a and b were fit by a two-point fit using the core CL intensity with an assumed temperature of quartz core formation (typically 45 C) and the estimated peak metamorphic temperature (Table 1) and an assumed CL intensity at the quartz garnet interface at the peak temperature. This peak CL intensity is not known because of diffusional modification on cooling, but was adjusted as a model fit parameter. For example, for sample D with an assumed peak temperature of 6 C (Fig. 4a), the initial and peak CL intensities were taken to be 28 CL units, 45 C and 25 CL units, 6 C, respectively. In practice, it was discovered that the fit of the model profile to the observed profile was quite sensitive to the choice of the peak CL intensity, but the overall T t history was not. On the other hand, the overall T t history was found to be quite sensitive to the choice of the peak metamorphic temperature. The fit of CL intensity with temperature (Eq. 1) was adjusted depending on the assumed peak metamorphic temperature. For example, for the model in which the assumed peak temperature was 575 C (Fig. 4b), the peak CL intensity was taken to be 25 CL units, 575 C. The three diffusion models in which the peak metamorphic temperature was assumed to be 6, 575, and 55 C display equally good fits (Fig. 4). The only major difference is the times scales for the metamorphic episode (heating and cooling) of approximately.25 m.y.,.5 m.y., and 1. m.y., respectively, for the three peak temperatures. That is, the time scale approximately halves for every 25 C increases in peak metamorphic temperature. Spear and Rumble (1986) report the peak metamorphic temperature for the southwest Orfordville quadrangle (NH and VT) where sample D was collected as 58 ± 25 C, 5.1 kbar, so the metamorphic time scale consistent with this peak condition is on the order of.5 m.y. Considerations regarding the diffusion model Sources of possible error in the diffusion modeling are not simple to evaluate. It is not believed that the diffusivities measured by Cherniak et al. (27) are significantly in error unless a different mechanism operates in the natural samples compared with the experimental study. The Cherniak et al. (27) experiments were conducted dry, and it is possible that diffusion under hydrous conditions such as those found in an evolving schist might be faster. This possibility is difficult to evaluate, but if diffusivities were faster it would only serve to shorten the time scales of metamorphism. It is expected that Ti diffusion in garnet is slower than that in quartz, especially if a coupled substitution in garnet is required. If Ti diffusion in garnet is infinitely slow, this raises the question whether equilibrium between garnet and quartz inclusion could be maintained. Clearly, it is based on the observed pervasiveness of zoned quartz inclusions. What is required is that the flux out of garnet equals the flux into quartz. Inasmuch as garnet may contain several hundred ppm Ti, compared with 5 2 in quartz, the flux balance would require only a very small shell of garnet to exchange with the quartz, so it is not

6 982 Contrib Mineral Petrol (212) 164: (a) d Peak T = 6 C (b) 6 Peak T = 6 C 55 CL intensity T C (c) CL intensity Radius microns d 2. Peak T = 575 C (d) T C Peak T = 575 C (e) CL intensity Radius microns d 2. Peak T = 55 C (f) T C Peak T = 55 C Radius microns Fig. 4 Results of diffusion models for Ti diffusion in quartz from sample D assuming peak temperatures of 6 C (a, b), 575 C (c, d) and 55 C (e, f). (a, c, e) Plots of CL intensity versus distance. Dashed line shows assumed profile at the peak temperature Solid line shows the final calculated profile. b, d, f Temperature time plots for each of the diffusion models. Note the scale difference along the time axis likely that diffusion in garnet has limited the diffusion observed in quartz. Even if it had to some extent, the diffusion models are not dependent on a priori assumptions about the partitioning of Ti between quartz and garnet, but rather on the geometric shape of the zoning profile in quartz (see peak lines in Fig. 4). The major determining factor in generating a good fit to the diffusion profile is the penetration distance of the diffusion profile (e.g. around 5 6 lm in Fig. 4), which, coupled with the peak metamorphic temperature, determines the time scale for profile development. The alternative mechanism for generating the zoning profiles (mechanism 1 discussed above) has as the initial condition a step function in the quartz inclusion. The initial

7 Contrib Mineral Petrol (212) 164: step would need to be symmetrical around a quartz grain (not a likely possibility), and the length of the step would need to have been on the order of the midpoint of the diffusion profile (e.g. around 3 5 lm). Although diffusion models involving this geometry were not attempted, the time scale for the development of the final profile would have to be shorter than with the preferred model because the length scale for diffusion is only around half of that with the preferred model. It should also be noted that the linear temperature time histories used in the model experiments are unrealistic and considerations of heat flow would suggest that any realistic T t history should be curved concave downward somewhat like a parabola. Models with this type of T t history were not attempted, but it is clear that such models would result in even shorter time scales for the same peak temperature because the time the rock would spend near the peak temperature would be larger than with the linear models. Thus, the results presented here utilize the more conservative temperature time paths. Other samples CL zoning profiles from quartz inclusions in garnet from samples collected along a traverse across the Strafford Dome in eastern Vermont (see Fig. 1 for locations) and from a single staurolite-grade sample from New Hampshire are shown in Fig. 5, and temperature time plots from the diffusion modeling are shown in Fig. 6. With the exception of the single garnet zone sample (TM-549), which shows little or no zoning, all samples have similar length scales for diffusion of around 5 1 lm. The temperature time plots indicate metamorphic time scales of.1 2 m.y., and the major reason for the difference in time scales is the differences in peak metamorphic temperatures, which range from around 55 6 C (Table 1). The single sample from New Hampshire (BF-38B) has a similar time scale of 1.5 m.y. If the results of the diffusion modeling are taken at face value, then the time scales for metamorphism across this segment of Vermont must have varied by an order of magnitude for all diffusion profiles to have similar length scales. The shortest duration, as well as the highest peak metamorphic temperature, is recorded by the sample from the core of the Strafford Dome (TM-732), and it is possible that this sample did, in fact, experience a shorter metamorphic heating cooling cycle by virtue of its structural setting. For example, it may have experienced rapid burial and subsequent exhumation during dome formation, whereas samples on the flanks of the dome were heated and cooled more slowly as limited by heat conduction. Alternatively, it is certainly possible that the estimates of the metamorphic temperature, which were made using garnet biotite thermometry and are only accurate to ca. ±25 3 C, actually are much more similar than reported. Further work refining the peak temperatures should help resolve this issue. Discussion Metamorphic heating and cooling cycles of 2 m.y or less are rapid and somewhat unanticipated for this Barrovian terrane where the models of England and Thompson (1984) for continental collision would suggest a time scale of tens of millions of years. However, other recent studies have reached similar conclusions based on independent methods. Dachs and Proyer (22) concluded from a study of diffusional relaxation of Mn zoning profiles in garnet that the total time elapsed following formation of the garnet overgrowth (below the metamorphic peak of 57 C) to around 45 C was 1 m.y. or less, based on their finite difference diffusion modeling. They conclude that this required very rapid exhumation rates of cm/year, but it also required rapid heating to the metamorphic peak as well. A similar rapid heating cooling cycle was reported by Ague and Baxter (27) for samples from the classic Barrovian terrane in the Dalradian of Scotland based on diffusion modeling of Sr zoning in apatite, and Viete et al. (211) argue for short (a few million years or less) thermal pulses to explain Mn zoning profiles in garnet. Ague and Baxter (27) attribute the short thermal pulse to advective heat transfer by magmas and associated fluid flow, and Viete et al. (211) also suggest that shear zones may play a critical role in providing significant advective heat transport. Geochronologic studies have also suggested rapid time scales for metamorphism. Exhumation at a rate of several cm/year is required by data reported by Harrison et al. (1997) and Catlos et al. (21) for the Main Central Thrust of the Central Himalaya, and similar rapid rates are required for exhumation of eclogites in the Alps (Rubatto and Hermann 21; Smye et al. 211) and the Papua New Guinea eclogites (Baldwin et al. 24). Although the time scales inferred from our diffusion studies are similar to those obtained for Barrovian metamorphism in the Dalradian (e.g. Ague and Baxter 27; Viete et al. 211), thermal pulses due to magmas or fluids only are not likely in Vermont. No evidence of such plutons is evident in the immediate geology of the study area, and those that do exist along strike are 1 3 m.y. older than the presumed time of metamorphism based on monazite ages (e.g. Wing et al. 23). Furthermore, the P T paths of the Vermont samples display an episode of nearly isothermal loading (e.g. Spear and Rumble 1986; Menard and Spear 1994), which requires tectonic thickening for a significant part of the P T history. This thickening must

8 984 Contrib Mineral Petrol (212) 164: D TM TM TM-828A TM SD8A TM TM BF-38B Distance (µm) Distance (µm) Distance (µm) Fig. 5 Plots of CL intensity versus distance for the samples studied. Horizontal scales are equal to facilitate comparison, whereas vertical (intensity) scales are not comparable. Note the similarities in length have been rapid and involved placement of hot rocks above cooler rocks to cause the rapid heating. Exhumation and cooling must have followed immediately and may have been caused by the rocks under study having been placed onto cooler rocks. A possible kinematic scenario is a series of in-sequence ductile thrust faults or shear zones in the general form of a mid-crustal duplex structure. Significantly, geochronologic studies in Vermont do not provide any inklings about the rates of tectonic processes because the duration of the heating and cooling revealed by the diffusion study is shorter than the typical uncertainty associated with age determinations. For example, monazite ages from this area of Vermont from SIMS analyses have been presented by Wing et al. (23) and are shown in Fig. 1. Well-crystallized monazites are found only above the staurolite isograd and are interpreted to have formed from the breakdown of allanite. Ages of three samples from the staurolite kyanite zone are , , and Ma (Fig. 1). In contrast, the ages reported for scales for the CL zoning in all samples from the staurolite zone (e.g. around 5 lm) monazite from a chlorite zone sample are Ma and that monazite was poorly crystallized. The ages of the staurolite kyanite zone samples are all within error, and even the best of the samples, which gave an average age of ± 8.9 (2 sigma), has an error that is outside of the entire metamorphic history recorded by the diffusion modeling in this study. The younger ages recorded by the chlorite zone sample are difficult to interpret without additional study, but possibly indicate that a distinct metamorphic heating cooling cycle has affected these rocks and may suggest that a significant post-metamorphic fault separates them from the higher-grade rocks. In another study, Janots et al. (29) report ages of allanite and monazite from the central Alps and interpret the age difference of around 13 m.y. (31.5 and 18.5 Ma, respectively) in terms of the duration of the metamorphic event. However, their results are also consistent with a T t history that involves a short thermal spike at around 19 Ma (to produce the monazite) imposed on a background

9 Contrib Mineral Petrol (212) 164: TM-543 9SD8A TM-828A TM TM BF-38B Fig. 6 Plots showing results of diffusion modeling of CL zoning for all samples from this study. a Observed and modeled CL versus distance plots. Horizontal scales are equivalent. b Temperature time plots. All results have been scaled the same to facilitate comparison metamorphic grade in the allanite stability field. The implication is that, whereas accessory mineral geochronology can serve to place the metamorphic event in an absolute time scale and may also serve to differentiate distinct metamorphic heating/cooling events, it cannot be expected to resolve the relationships between metamorphism and tectonics at the same resolution possible from diffusion studies. Obviously, there is considerable room for substantial additional work using this potentially powerful new method to estimate T t histories in metamorphic rocks. Acknowledgments The authors wish to thank helpful discussions with Daniele Cherniak, Bruce Watson, Michael Ackerson, Ben Hallett, Kenny Horkley, and the able assistance of Dan Ruscitto with the microprobe and LA-ICPMS. Thoughtful reviews by J. M. Ferry and J. J. Ague are also gratefully acknowledged. This work was funded in part by grants from the National Science Foundation (EAR to Spear and Thomas and EAR to Webb) and the Edward P. Hamilton Distinguished Professor Chair at Rensselaer Polytechnic Institute. References Ague JJ, Baxter EF (27) Brief thermal pulses during mountain building recorded by Sr diffusion in apatite and multicomponent diffusion in garnet. Earth Planet Sci Lett 261:5 516 Baldwin SL, Monteleone BD, Webb LE, Fitzgerald PG, Grove M, Hill EJ (24) Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea. Nature 431: Catlos EJ, Harrison TM, Kohn MJ, Grove M, Ryerson FJ, Manning CE, Upreti BN (21) Geochronologic and thermobarometric constraints on the evolution of the Main Central Thrust, central Nepal Himalaya. J Geophys Res 16(B8): Cherniak DJ, Watson EB, Wark DA (27) Ti diffusion in quartz. Chem Geol 236:65 74 Dachs E, Proyer A (22) Constraints on the duration of highpressure metamorphism in the Tauern Window from diffusion modelling of discontinuous growth zones in eclogite garnet. J Metamorp Geol 2: England PC, Thompson AB (1984) Pressure temperature time paths of regional metamorphism, part I: heat transfer during the evolution of regions of thickened continental crust. J Petrol 25: Ghent ED, Stout MZ (1984) TiO 2 activity in metamorphosed pelitic and basic rocks: principles and applications to metamorphism in southeastern Canadian Cordillera. Contrib Mineral Petrol 86: Harrison TM, Ryerson FJ, Le Fort P, Yin A, Lovera OM, Catlos EJ (1997) A late Miocene-Pliocene origin for the central Himalayan inverted metamorphism. Earth Planet Sci Lett 146:1 7 Janots E, Engi M, Rubatto D, Berger A, Gregory C, Rahn M (29) Metamorphic rates in collisional orogeny from in situ allanite and monazite dating. Geology 37:11 14 Kretz R (1983) Symbols for rock-forming minerals. Am Miner 68: Leeman WP, MacRae CM, Wilson NC, Torpy A, Lee C-TA, Student JJ, Thomas JB, Vicenzi EP (212) Quantitative application of

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