D.J. Cherniak

Diffusion in Zircon

Despite its low abundance in most rocks, zircon is extraordinarily useful in interpreting crustal histories. The importance of U-Th-Pb isotopic dating of zircon is well and long established (Davis et al., this volume; Parrish et al., this volume). Zircon also tends to incorporate trace elements useful as geochemical tracers, such as the REE, Y, and Hf. A number of characteristics of zircon encourage the preservation of internal isotopic and chemical variations, often on extremely fine scale, which provide valuable insight into thermal histories and past geochemical environments. The relative insolubility of zircon in crustal melts and fluids, as well as its general resistance to chemical and physical breakdown, often result in the existence of several generations of geochemical information in a single zircon grain. The fact that this information is so frequently retained (as evidenced through backscattered electron or cathodoluminescence imaging that often reveal fine-scale zoning down to the sub-micron scale) has long suggested that diffusion of most elements is quite sluggish in zircon. In this chapter, we present an overview of the findings to date from laboratory measurements of diffusion of cations and oxygen in zircon. Because of its importance as a geochronometer, attempts have been made to measure diffusion (especially of Pb) for over 30 years. But only in the last decade or so have profiling techniques with adequate depth resolution been employed in these studies, resulting in a plethora of new diffusion data. These findings have important implications for isotopic dating, interpretation of stable-isotope ratios, closure temperatures, and formation and preservation of primary chemical composition and zoning in zircon. Efforts have been made for some time to quantify and characterize diffusion in zircon, most notably of Pb, in deference to its significance in interpreting Pb isotopic signatures and refining understanding of thermal histories. As is evident from …

Lead diffusion in titanite and preliminary results on the effects of radiation damage on Pb transport

Lead diffusion has been measured in natural titanite using two different methods to introduce the diffusant: (1) implantation of Pb ions; and (2) immersion of the crystals in a PbS powder reservoir. In both sets of experiments, Rutherford backscattering spectrometry (RBS) was used to obtain concentration profiles, which were then fit with appropriate solutions to the diffusion equation. Experiments using the PbS powder source, run over the temperature range 650–1027°C, define the Arrhenius relationship: D=1.11+1.18−0.57exp−78.5±2.7 (kcal mol−1)RT(cm2s−1) for diffusion parallel to the (100) plane. Diffusivities are not significantly affected by orientation or differences in preannealing treatment. The ion implantation experiments, in contrast, deviate from simple linear Arrhenius behavior, with the high-temperature data (850–950°C) displaying good agreement with the above relation but the lower-temperature data (600–800°C) exhibiting enhanced diffusivities and a lowered activation energy (19 kcal mol−1). These results suggest that radiation damage induced by the ion implantation is repaired at high temperatures, thus leading to agreement of the two data sets. At lower temperatures, lattice repair proceeds much more slowly in relation to the duration of the diffusion anneal, resulting in elevated diffusion coefficients typical of materials that have become amorphous. A comparison with earlier work on apatite and zircon places the rate of repair of implantation-induced damage in titanite between these minerals, with apatite exhibiting a more rapid rate of repair and zircon a much slower rate for the same implant dose and similar annealing conditions. This difference is largely a function of structural and compositional characteristics that affect the rate of accumulation and repair of damage in each mineral. The results presented here may have important implications for Pb transport and interpretation of isotope ratios in naturally radiation-damaged minerals, as ion implantation produces damage comparable to that resulting from α-recoil.

Rare-earth diffusion in zircon

Diffusion rates for three rare-earth elements (REEs: Sm, Dy, Yb) have been measured in synthetic and natural zircon. REE-phosphate powders were used as the source of diffusant, with Rutherford backscattering spectrometry (RBS) used to measure REE depth profiles. Over the temperature range 1150–1400°C, the following Arrhenius relations were obtained (diffusion coefficients in m2 s−1 ): log DYb = (7.40±1.15)+(−769±34 kJ mol−12.303 RT) log DDy = (5.36±0.21)+(−734±35 kJ mol−12.303 RT) log DSm = (8.46±1.61)+(−841±57 kJ mol−12.303 RT) Results for synthetic: and natural zircons were quite similar, and no evidence of significant anisotropy was observed when comparing transport normal and parallel to the c-axis. The data show a systematic increase in diffusivity with decreasing ionic radius (i.e. faster diffusion rates for the heavier REEs). Given these trends the diffusion rates of Lu and La should differ by over two orders of magnitude. Diffusive fractionation is unlikely in the Sm-Nd system because differences in diffusivities are relatively small, but may be a factor in the Lu-Hf system given the much slower diffusion rates of tetravalent cations. The very slow diffusion rates measured for the REEs suggest that they are essentially immobile under most geologic conditions, thus permitting the preservation of fine-scale chemical zoning and isotopic signatures of inherited cores.

OXYGEN DIFFUSION IN ZIRCON

Oxygen diffusion in natural, non-metamict zircon was characterized under both dry and water-present conditions at temperatures ranging from 765°C to 1500°C. Dry experiments were performed at atmospheric pressure by encapsulating polished zircon samples with a fine powder of18O-enriched quartz and annealing the sealed capsules in air. Hydrothermal runs were conducted in cold-seal pressure vessels (7–70 MPa) or a piston cylinder apparatus (400–1000 MPa) on zircon samples encapsulated with both18O-enriched quartz and18O water. Diffusive-uptake profiles of18O were measured in all samples with a particle accelerator, using the18O(p, α)15N reaction. For dry experimental conditions at 1100–1500°C, the resulting oxygen diffusivities (24 in all) are well described by:Ddry (m2/s) = 1.33 × 10 −4exp(−53920/T) There is no suggestion of diffusive anisotropy. Under wet conditions at 925°C, oxygen diffusion shows little or no dependence uponPH2O in the range 7–1000 MPa, and is insensitive to total pressure as well. The results of 27 wet experiments at 767–1160°C and 7–1000 MPa can be described a single Arrhenius relationship:Dwet (m2/s) = 5.5 × 10−12exp(−25280/T) The insensitivity of oxygen diffusion toPH2O means that applications to geologic problems can be pursued knowing only whether the system of interest was ‘wet’ (i.e.,PH2O > 7MPa) or ‘dry’. Under dry conditions (presumably rare in the crust), zircons are extremely retentive of their oxygen isotopic signatures, to the extent that δ18O would be perturbed at the center of a 200 μm zircon only during an extraordinarily hot and protracted event (e.g., 65 Ma at 900°C). Under wet conditions, δ18O may or may not be retained in the central regions of individual crystals, cores or overgrowth rims, depending upon the specific thermal history of the system.

Pre-eruption recharge of the Bishop magma system

The 650 km 3 rhyolitic Bishop Tuff (eastern California, USA), which is stratigraphically zoned with respect to temperatures of mineral equilibration, refl ects a corresponding thermal gradient in the source magma chamber. Consistent with previous work, application of the new TitaniQ (Ti-in-quartz) thermometer to quartz phenocryst rims documents an ~100 °C temperature increase with chamber depth at the time of eruption. Application of TitaniQ to quartz phenocryst cores, however, reveals lower temperatures and an earlier gradient that was less steep, with temperature increasing with depth by only ~30 °C. In many late-erupted crystals, sharp boundaries that separate low-temperature cores from high-temperature rims cut internal cathodoluminescent growth zoning, indicating partial phenocryst dissolution prior to crystallization of the high-temperature rims. Rimward jumps in Ti concentration across these boundaries are too abrupt (e.g., 40 ppm across a distance of <10 µm) to have survived magmatic temperatures for more than ~100 yr. We interpret these observations to indicate heating-induced partial dissolution of quartz, followed by growth of high-temperature rims (made possible by lowering of water activity due to addition of CO 2 ) within 100 yr of the climactic 760 ka eruption. Hot mafi c melts injected into deeper parts of the magma system were the likely source of heat and CO 2 , raising the possibility that eruption and caldera collapse owe their origin to a recharge event.

Rare earth elements in synthetic zircon: Part 1. Synthesis, and rare earth element and phosphorus doping

Abstract Zircon crystals were grown from a Li-Mo flux (7.5 mol% Li2MoO4; 86.5 mol% MoO3) to which equal molar proportions ZrO2 and Li2SiO3 were added (3 mol% each). The crystals were initially grown undoped, but later Dy was added to the flux without any other charge-compensating species. With Dy and P added, in equal molar proportions, the zircon crystals incorporated 1.37 mol% (6.99 wt%) Dy and 1.36 mol% (1.33 wt%) P, a factor of 5.3 increase in Dy over the crystals grown without P intentionally added to the flux. The other P+REE-doped zircon crystals displayed an approximately 1000-fold increase in REE and P from La through Lu, a result of decreasing ionic radii. The incorporation of P5+ allowed, in a general sense, the xenotime-type coupled substitution of (REE3+, Y3+) + P5+ = Zr4+ + Si4+. From La to Nd, however, P exceeds REE, from Sm to Gd, the REE are generally equal to P, and from Tb to Lu the REE exceed P. The Y- and P-doped zircon behaved more like middle-REE (MREE)-doped zircon than heavy-REE (HREE)-doped zircon crystals in their ability to incorporate Y (and P) and to maintain charge neutrality. To investigate the incorporation of Dy with no P added to the flux, the P to LREE excess, and the HREE to P excess in the doped zircon, secondary ion mass spectrometry (SIMS) analyses were completed on selected crystals. In the Dydoped crystals, the SIMS analyses revealed minor amounts of P, Li, and Mo in the crystals. These elements contributed to charge balance required by the excess Dy. In REE- and P-doped zircon, the SIMS analyses detected Li and Mo, and the Li and Mo may also provide charge balance for excess REE in the HREE+P-doped crystals.

Pb diffusion in rutile

Abstract Diffusion of Pb was measured in natural and synthetic rutile under dry, 1 atmosphere conditions, using mixtures of Pb titanate or Pb sulfide and TiO2 as the sources of diffusant. Pb depth profiles were then measured with Rutherford Backscattering Spectrometry (RBS). Over the temperature range 700–1100 °C, the following Arrhenius relation was obtained for the synthetic rutile: D=3.9 × 10−10exp(−250 ± 12 kJ mol−1/RT) m2s−1. Results for diffusion in natural and synthetic rutile were quite similar, despite significant differences in trace element compositions. Mean closure temperatures calculated from the diffusion parameters are around 600 °C for rutile grains of ∼100 μm size. This is about 100 °C higher than rutile closure temperature determinations from past field-based studies, suggesting that rutile is more resistant to Pb loss through volume diffusion than previously thought.

A study of strontium diffusion in plagioclase using Rutherford backscattering spectroscopy

Strontium chemical diffusion has been measured in plagioclase of three intermediate compositions (between An23 and An67) under anhydrous, 1-atmosphere conditions. A strontium oxide-aluminosilicate powder mixture was used as the diffusant source material, with Rutherford backscattering spectroscopy (RBS) used to measure diffusion profiles. Over the temperature range 725–1075°C, the following Arrhenius relations were obtained for diffusion normal to (001) (D in m2 s−1): Oligoclase (An23): log D = (−6.07 ± 0.58) + (−273 ± 13 kJ mol−1)/RT. Andesine (An43): log D = (−6.75 ± 0.33) + (−265 ± 8 kJ mol−1)/RT. Labradorite (An67): log D = (−7.03 ± 0.37) + (−268 ± 8 kJ mol−1)/RT. In the labradorite, diffusion of Sr normal to (010) was somewhat slower (by ~0.7 log units) than diffusion perpendicular to (001). The activation energy for diffusion normal to (010) appears to follow a trend similar to that observed for diffusion perpendicular to (001). In oligoclase, diffusion normal to (001) also appears to be somewhat faster than diffusion normal to (010), but the differences are less pronounced than in labradorite. No significant crystallographic anisotropy was noted in the andesine. The results show a clear trend of increasing Sr diffusivity with decreasing An content of the plagioclase. Profiles of Ca and Na, measured with RBS and Nuclear Reaction Analysis (NRA) respectively, indicate that Sr is exchanging with both of these species in the plagioclase. In each feldspar composition, decreases in Ca and Na in the near-surface region are nearly proportional to their bulk concentrations, indicating that there is no significant substitutional preference of Sr for either cation in those feldspars investigated. NRA measurements of Al, showing a corresponding uptake of Al and Sr, suggest that a coupled exchange, possibly Sr+2 + Al+3 → Na+1 + Si+4, is taking place when Sr exchanges with Na. Chemical diffusion of Sr in calcic plagioclase is dominated by Sr+2 → Ca+2 exchange.

A study of strontium diffusion in K-feldspar, Na-K feldspar and anorthite using Rutherford Backscattering Spectroscopy

Abstract Sr chemical diffusion has been measured in orthoclase, anorthoclase and anorthite under dry, 1-atm conditions. A strontium oxide-aluminosilicate powder mixture was used as the source material, with Rutherford Backscattering Spectroscopy (RBS) used to measure diffusion profiles. Over the temperature range 725–1075°C the following Arrhenius relations were obtained. Orthoclase (normal to (001)):D=5.97 × 10+0.0060−0.0030exp−67900 ± 1600RTcm 2 s −1 Measurements of diffusion perpendicular to (010) and (100) indicate little diffusional anisotropy in orthoclase. Anorthoclase: normal to (010): D=4.51 × 10 +1 +358−5.6exp−89100 ± 4800RT cm 2 s −1 normal to (001): D=2.25 × 10 +2 +1615−31exp − 89300 ± 4600RT cm 2 s −1 Anorthite (normal to (010)): D=3.85 × 10 −2 +0.380−0.0039 exp − 78800 ± 5400RT cm 2 s −1 The correlation of Sr uptake with a reduction of K and Ca in orthoclase and anorthite, respectively, indicates that Sr is exchanging with these species. Nuclear reaction analysis (NRA) measurements of Al in orthoclase suggests that the coupled exchange Sr 2+ + Al 3+ → K 1+ + Si 4+ is taking place, as expected. Sr closure temperatures for feldspars, calculated with the above parameters, are relatively high compared to those of some other minerals (e.g., biotite, muscovite, apatite). However, closure temperatures of feldspars may be significantly depressed if they are exsolved or twinned, leading to small effective diffusion lengths. Feldspar crystals of millimeter size should retain 87 /Sr 86 Sr ratios except when subjected to thermal events of extreme temperature or duration.

Rare earth element diffusion in apatite

Abstract Diffusion of rare earth elements (REEs) in natural and synthetic fluorapatite has been characterized under anhydrous conditions. Three types of experiments were run. In the first set of experiments, Sm was introduced into the apatite by means of ion implantation, with diffusivities extracted through measurement of the “relaxation” of the implanted profile after diffusion anneals. The second group consisted of “in diffusion” experiments, in which apatite was immersed in reservoirs of synthetic REE apatite analogs of various compositions. The final set of experiments was “out-diffusion” experiments run on synthetic Nd-doped apatite immersed in a reservoir of synthetic (undoped) fluorapatite. REE depth profiles in all cases were measured with Rutherford Backscattering Spectrometry. Diffusion rates for the REE vary significantly among these sets of experiments. For the ion-implantation experiments, the following Arrhenius relation was obtained for Sm, over the temperature range 750°C to 1100°C: D imp =6.3×10 −7 exp(−298±17 kJ/mol/RT) m 2 /s Diffusion of a series of REE, from light to heavy, was investigated in the “in-diffusion” experiments. Over the temperature range 800°C to 1250°C, the following Arrhenius relations are obtained for La, Nd, Dy, and Yb, for in-diffusion experiments using REE silicate oxyapatite sources: D La =2.6×10 −7 exp(−324±9 kJ/mol/RT) m 2 /s D Nd =2.4×10 −6 exp(−348±13 kJ/mol/RT) m 2 /s D Dy =9.7×10 −7 exp(−340±11 kJ/mol/RT) m 2 /s D Yb =1.3×10 −8 exp(−292±23 kJ/mol/RT) m 2 /s Diffusivities of the REE in these “in-diffusion” experiments are all quite similar, suggesting little difference in diffusion rates in apatite with increasing ionic radii of the REEs. The “out-diffusion” experiments on the Nd-doped synthetic apatite, over the temperature range 950°C to 1400°C, yield the Arrhenius law: D out =9.3×10 −6 exp(−392±31 kJ/mol/RT) m 2 /s The differences in REE diffusion among these three sets of experiments (i.e., ion implantation, in-diffusion, and out-diffusion) may be attributable to the differences in substitutional processes facilitating REE exchange. The fastest diffusion, found in the ion-implantation experiments, is likely largely governed by simple light REE+3 ↔ REE+3 exchange, with no charge compensating species necessary. REE transport in the in-diffusion experiments requires movement of an additional charge-compensating species, either through the substitutions REE+3 + Si+4 ↔ Ca+2 + P+5 or REE+3 + Na+1 ↔ 2 Ca+2, and thus proceeds more slowly than simple REE exchange. Slowest of all is Nd out-diffusion from the synthetic Nd-doped apatite, for which neither charge compensating species are present nor REEs available in the surrounding reservoir to facilitate Nd exchange. This observed dependence of REE diffusion rates on the exchange process involved has important geochemical implications. These findings indicate that REE isotope and chemical signatures can become decoupled in apatite, with light REE isotope exchange proceeding much more rapidly than REE chemical diffusion altering total REE concentrations. Under temperatures typical of thermal events, REE zoning (involving differences in REE concentration across zones) of a given dimension might persist over time periods two orders of magnitude greater than those under which zoning of REE isotopes (without significant changes in total REE) on similar scale is preserved.