T. Mark Harrison

Zircon saturation revisited : temperature and composition effects in a variety of crustal magma types

Abstract Hydrothermal experiments in the temperature range 750–1020°C have defined the saturation behavior of zircon in crustal anatectic melts as a function of both temperature and composition. The results provide a model of zircon solubility given by: In D Zr zircon/melt = −3.80−[0.85(M−1)]+12900/T where D Zr zircon/melt is the concentration ratio of Zr in the stoichiometric zircon to that in the melt, T is the absolute temperature, and M is the cation ratio (Na + K + 2Ca)/(Al · Si). This solubility model is based principally upon experiments at 860°, 930°, and 1020°C, but has also been confirmed at temperatures up to 1500°C for M = 1.3. The lowest temperature experiments (750° and 800°C) yielded relatively imprecise, low solubilities, but the measured values (with assigned errors) are nevertheless in agreement with the predictions of the model. For M = 1.3 (a normal peraluminous granite), these results predict zircon solubilities ranging from ∼ 100 ppm dissolved Zr at 750°C to 1330 ppm at 1020°C. Thus, in view of the substantial range of bulk Zr concentrations observed in crustal granitoids (∼ 50–350 ppm), it is clear that anatectic magmas can show contrasting behavior toward zircon in the source rock. Those melts containing insufficient Zr for saturation in zircon during melting can have achieved that condition only by consuming all zircon in the source. On the other hand, melts with higher Zr contents (appropriate to saturation in zircon) must be regarded as incapable of dissolving additional zircon, whether it be located in the residual rocks or as crystals entrained in the departing melt fraction. This latter possibility is particularly interesting, inasmuch as the inability of a melt to consume zircon means that critical geochemical “indicators” contained in the undissolved zircon (e.g. heavy rare earths, Hf, U, Th, and radiogenic Pb) can equilibrate with the contacting melt only by solid-state diffusion, which may be slow relative to the time scale of the melting event.

The behavior of apatite during crustal anatexis: Equilibrium and kinetic considerations

The solubility and dissolution kinetics of apatite in felsic melts at 850°–1500°C have been examined experimentally by allowing apatite crystals to partially dissolve into apatite-undersaturated melts containing 0–10 wt% water. Analysis of P and Ca gradients in the crystal/melt interfacial region enables determination of both the diffusivities and the saturation levels of these components in the melt. Phosphorus diffusion was identified as the rate-limiting factor in apatite dissolution. Results of four experiments at 8 kbar run in the virtual absence of water yield an activation energy (E) for P diffusion of 143.6 ± 2.8 kcal-mol−1 and frequency factor (D0) of 2.23+2.88−1.26 × 109cm2-sec−1. The addition of water causes dramatic and systematic reduction of both E and D0 such that at 6 wt% H2O the values are ~25 kcal-mol−1 and 10−5 cm2-sec−1, respectively. At 1300°C, the diffusivity of P increases by a factor of 50 over the first 2% of water added to the melt, but rises by a factor of only two between 2 and 6%, perhaps reflecting the effect of a concentration-dependent mechanism of H2O solution. Calcium diffusion gradients do not conform well to simple diffusion theory because the release of calcium at the dissolving crystal surface is linked to the transport rate of phosphorus in the melt, which is typically two orders of magnitude slower than Ca. Calcium chemical diffusion rates calculated from the observed gradients are about 50 times slower than calcium tracer diffusion. Apatite solubilities obtained from these experiments, together with previous results, can be described as a function of absolute temperature (T) and melt composition by the expression: In Dapatite/meltP = [(8400 + ((SiO2 − 0.5)2.64 × 104))/T] − [3.1 + (12.4(SiO2 − 0.5))] where SiO2 is the weight fraction of silica in the melt. This model appears to be valid between 45% and 75% SiO2, 0 and 10% water, and for the range of pressures expected in the crust. The diffusivity information extracted from the experiments can be directly applied to several problems of geochemical interest, including I) dissolution times for apatite during crustal anatexis, and 2) pileup of P, and consequent local saturation in apatite, at the surfaces of growing major-mineral phases.

Kinetics of zircon dissolution and zirconium diffusion in granitic melts of variable water content

The experimental dissolution of zircon into a zircon-undersaturated felsic melt of variable water content at high pressure in the temperature range 1,020° to 1,500° C provides information related to 1) the solubility of zircon, 2) the diffusion kinetics of Zr in an obsidian melt, and 3) the rate of zircon dissolution. Zirconium concentration profiles observed by electron microprobe in the obsidian glass adjacent to a large, polished zircon face provide sufficient information to calculate model diffusion coefficients. Results of dissolution experiments conducted in the virtual absence of water (<0.2% H2O) yield an activation energy (E) for Zr transport in a melt ofM=1.3 [whereM is the cation ratio (Na+K+2Ca)/(Al·Si)] of 97.7±2.8 kcal-mol−1, and a frequency factor (D0) of 980−580+1,390 cm2-sec−1. Hydrothermal experiments provide an E=47.3±1.9 kcal-mol−1 andD0=0.030−0.015+0.030 cm2-sec−1. Both of these results plot close to a previously defined diffusion compensation line for cations in obsidian. The diffusivity of Zr at 1,200° C increases by a factor of 100 over the first 2% of water introduced into the melt, but subsequently rises by only a factor of five to an apparent plateau value of ∼2×10−9 cm2-sec−1 by ∼6% total water content. The remarkable contrast between the wet and dry diffusivities, which limits the rate of zircon dissolution into granitic melt, indicates that a 50 μm diameter zircon crystal would dissolve in a 3 to 6% water-bearing melt at 750° C in about 100 years, but would require in excess of 200 Ma to dissolve in an equivalent dry system. From this calculation we conclude that zircon dissolution proceeds geologically instantaneously in an undersaturated, water-bearing granite. Estimates of zircon solubility in the obsidian melt in the temperature range of 1,020° C to 1,500° C confirm and extend an existing model of zircon solubility to these higher temperatures in hydrous melts. However, this model does not well describe zircon saturation behavior in systems with less than about 2% water.

Rapid early Miocene acceleration of uplift in the Gangdese Belt, Xizang (southern Tibet), and its bearing on accommodation mechanisms of the India-Asia collision

Abstract Five samples from a biotite-hornblende granodiorite phase of the 42.5 Ma Quxu pluton, Gangdese batholith, southern Tibet, have been collected at 250 m vertical intervals. Biotite from these rocks yields monotonically decreasing40Ar/39Ar isochron ages with decreasing elevation of 26.8 ± 0.2, 23.3 ± 0.5, 19.7 ± 0.3, 18.4 ± 0.4,and17.8 ± 0.1Ma (Tc = 335°C). Coexisting K-feldspars have virtually identical minimum apparent40Ar/39Ar ages of 17.0 ± 0.4Ma (Tc = 285°C). These data indicate parts of southern Tibet experienced a pulse of uplift in the early Miocene with the rate of uplift rising from 0.07 to ∼ 4.4 mm/year in the interval 20 to 17 Ma. An apatite fission track age of 9.9 ± 0.9Ma from this locality constrains the average uplift rate at this site to about 0.81 mm/year between 17 and 9.9 Ma and 0.30 mm/year from 9.9 Ma to present. K-feldspar from the Dagze granite, 30 km to the east, near Lhasa, yields a minimum apparent40Ar/39Ar age of 35.9 ± 0.9Ma (Tc = 227°C) which indicates an average uplift rate there of 0.21 mm/year since then. The marked pulse of uplift of the Quxu granodiorite and the difference in uplift history between the Dagze and Quxu plutons suggests southern Tibet has experienced discrete pulses of uplift variable in both space and time. These data are not consistent with models which require a large proportion of uplift of the Tibetan plateau to have occurred in the last 2 Ma. The data support the suggestion that convergence between India and Asia was largely accommodated by tectonic escape during the opening of the South China Sea 32 to 17 Ma ago and permit distributed shortening as a mechanism for crustal thickening and uplift of this part of the Tibetan plateau subsequent to 20 Ma.

Exsolution in hornblende and its consequences for 40Ar39Ar age spectra and closure temperature

Abstract Step-heating studies of hornblende and biotite which coexist in an amphibolite near North Walpole, Vermont, reveal dramatically different 40 Ar 39 Ar age spectra. The biotite (Mg#59) is characterized by a plateau over 98 percent 39Ar release at 343 ± 5 Ma, in strong contrast to the amphibole which exhibits both a complex age spectrum and a K-Ar age significantly younger than the biotite. The amphibole release pattern contains three complete age cycles which range between extremes of 199 Ma and 408 Ma. Interestingly, the K Ca ratio (calculated from reactor-produced isotopes) also varies with Ar-release, but follows a trend which is antipathetic to the 40 Ar 39 Ar age pattern. Detailed microstructural examination, including transmission electron microscopy (TEM), reveals a complex exsolution structure that is consistent with both the complexity of the release pattern as well as the low degree of radiogenic 40Ar (40Ar∗) retentivity in the amphibole. This result underscores the variability of closure temperature (Tc) for 40Ar∗ in metamorphic amphiboles and should warn against accepting a single, universal value of Tc.

Diffusion of Sm, Sr, and Pb in fluorapatite

Abstract Diffusion coefficients for Sm, Sr, and Pb in fluorapatite at 900°–1250°C were obtained by measuring experimentally-induced diffusional uptake profiles of these elements in the margins of gem-quality apatite crystals. The crystals were immersed in synthetic melts enriched in the trace elements of interest and presaturated in apatite, and the resulting diffusion gradients were characterized by electron microprobe analysis. Except in the case of Pb, the diffusivities define good Arrhenius lines for the respective elements: DSm = 2.3 × 10−6exp(−52,200/RT) DSr = 412 exp(−100,000/RT). (Diffusion perpendicular to and parallel to c is measurably different in the case of Sr; the Arrhenius equation given above is an average for the two directions). Results on Pb were erratic, probably because extremely Pb-rich melts were used for some of the experiments. Data believed to be reliable define the following Arrhenius line: DPb = 0.035 exp(−70,000/RT). Constraints based on closure of natural apatites with respect to Pb suggest that the experimental data can be extrapolated, with sizeable uncertainty, to temperatures as low as 550°C. When applied to the question of isotopic and trace-element equilibration of residual or entrained apatites with crustal melts, the measured diffusivities indicate that 0.05-cm crystals will rarely preserve the original Pb-isotope characteristics of the source; the same is not true, however, of Sr (and, under some conditions, the REE), which may be unaffected at crystal cores during typical melting events.

Diffusion of 40Ar in metamorphic hornblende

Isothermal, hydrothermal experiments were performed on two compositionally contrasting hornblendes from amphibolites in order to examine Ar diffusion behavior in metamorphic hornblendes. Ten experiments on sample RF were performed at temperatures of 750°C, 800°C, and 850°C and pressures of 1 kbar using measured grain radii of 158, 101, and 34 μm. Eight experiments on sample 118576 were performed under the same conditions using measured grain radii of 145, 77, and 25 μm. Minor (<5%) alteration was observed in high temperature runs. Diffusion coefficients were calculated from measured radiogenic 40Ar loss following treatment assuming a spherical geometry for the mineral aggregate. Diffusivities calculated for different grain sizes vary by up to an order of magnitude for a given temperature indicating that the effective diffusion radius was less than the measured grain radius. Diffusivities for RF and 118576 calculated for grain radii of 101 and 145 μm, respectively, form a linear array on an Arrhenius diagram with slopes indicating activation energies of ∼ 60 kcal/mol. No correlation between Mg number (100 Mg/(Mg+Fe)) and activation energy was observed. Diffusivities calculated for these experiments are higher than previously reported results from similar experiments performed on hornblendes. A comparison of results for 34 μm splits from these two studies indicates higher apparent diffusivities (by a factor of 5), which probably result from observed phyllosilicate inter-growths (chlorite) and/or exsolution lamellae that partition the metamorphic hornblendes into smaller subdomains. Diffusivities calculated for experiments performed on 65 μm and 34 μm splits of 40Ar/39Ar standard MMhb-1 at 800°C and 1 kbar are consistent with a previously reported activation energy of 65 kcal/mol. Arrhenius parameters which emerge from the empirical model of Fortier and Giletti (1989) agree with experimental results to within analytical uncertainty. Although results of these experiments support previously reported estimates of the activation energy of 40Ar in hornblende (∼60 kcal/mol), phyllosilicate intergrowths and/or microstructures such as exsolution lamellae within the two metamorphic hornblendes result in extremely small diffusion domains, which may lead to lower Ar retentivities and lower closure temperatures. The effective diffusion dimension for 40Ar in hornblende is not likely to be defined by dislocations but rather by some larger structure within the crystal. TEM and SEM studies may provide some insight into the effective diffusion dimension for 40Ar in amphiboles, thereby enabling better estimates of closure temperatures and more precise temperature-time reconstructions.

Multiple trapped argon isotope components revealed by 40AR39AR isochron analysis

Abstract Isotope correlation diagrams can be used to obtain geologically meaningful age information from complex, seemingly meaningless, 40 Ar 39 Ar step heating results. A violation of the common assumption that the trapped argon component has a 40 Ar 36 Ar equal to 295.5, the atmospheric value, has been observed for K-feldspar, biotite, muscovite and hornblende, the four terrestrial minerals most commonly dated by the 40 Ar 39 Ar method. This approach has revealed up to three separate trapped argon components in a single sample which are thermally distinct during laboratory heating; that is, as one 40 Ar 36 Ar component is exhausted, another dominates as the trapped phase. The recognition of these trapped reservoirs of excess 40 Ar explain why many release spectra have complex patterns. When multiple trapped argon components do not remain thermally distinct during laboratory heating, only limits on age and trapped argon composition can be obtained.

Age and cooling history of the Manaslu granite: implications for Himalayan tectonics

Abstract The Manaslu granite is one of the High Himalayan leucogranites which are believed to be derived by partial melting of quartzofeldspathic gneissess of the High Himalayan Crystallines (HHC) and emplaced at the contact between the HHC and the Tethyan Sedimentary Series. We have analyzed a suite of muscovite (15), biotite (3), and alkali feldspar (10) by the 40 Ar/ 39 Ar method to help constrain the age and cooling history of the granite. 40 Ar/ 39 Ar ages ranges are 18.4 to 13.3 Ma, 17.0 to 14.7, and 16.4 to 3.4 for muscovite, biotite, and K-feldspar, respectively. Based on the muscovite analyses and a re-interpretation of a previously published U-Pb monazite age we conclude that the crystallization age of the Manaslu granite is everywhere — 20 Ma. The postcrystallization cooling history given by the micas and the feldspars suggests the presently exposed parts of the pluton were emplaced at depths of 8 to 15 km. These data place a minimum age of movement on the MCT in this area of 20 Ma. We estimate the time of magma segregation and transport to be no more than - 5 Ma.

Effects of excess argon within large diffusion domains on K-feldspar age spectra

Abstract Unusual shapes of 40Ar/39Ar age spectra have long been attributed to the presence of excess argon. We recognize a distinctive age spectrum type brought about by extraneous argon trapped in large diffusion domains. Spectra of this type are characterized by little or no excess argon in low temperature steps, sensible apparent ages over the first ~40 to 50% 39Ar released, an abrupt increase to ages exceeding the permissible age of the sample, followed by either a gradual decrease in age until fusion or an anomalously old plateau segment. Detailed study of one such orthoclase reveals that the excess argon within these samples is likely located in radiogenic argon sites within large diffusion domains. Argon contained within these features is trapped when the domains close to argon loss at ~400°C. Thus, there exists a genetic difference between large-domain-trapped-argon-effect spectra and those saddle-shaped age spectra which result from excess argon trapped in anion vacancies at temperatures below ~350°C.