Peter S. Dahl

A crystal-chemical basis for Pb retention and fission-track annealing systematics in U-bearing minerals, with implications for geochronology

This study develops an empirical crystal-chemical framework for systematizing the kinetics of Pb loss and fission-track annealing in U-bearing minerals. Ionic porosity, Z (the fraction of a minerals unit-cell volume not occupied by ions) potentially accounts for kinetic behavior by monitoring mean metal-oxygen bond length/strength. Various tests of a general kinetics-porosity relationship are presented, based upon diverse mineral data including: (1) Pb diffusion parameters; (2) measured closure temperatures (TC) for fission-track annealing and (3) retentivities of both Pb and fission tracks, from apparent-age data. Every kinetic parameter (including TC and mineral age for both the U/Pb and fission-track systems) is inversely correlated with Z within the sub-assemblage: zircon (Z ≈ 29%), titanite (∼ 34%) and apatite (∼ 38%). Assuming a diffusional closure model, Pb isotopic transport phenomena are described by a TC-Z scale “calibrated” with field-based TC data for titanite (≥ 680 ± 20°C) and apatite (∼ 500°C). Extrapolation of this scale yields TC estimates for the following minerals: staurolite (TC ≥ 1060°C, Z ≈ 25%); garnet (≥ 1010°C, ∼ 26.5%); zircon (≥900°C); monazite, xenotime, and epidote (≥ 750°C, ∼ 32%); and Ca-clinopyroxene (≥ 670 ± 30°C, ∼ 34 ± 1%, depending on composition). These empirical results imply that a (U/)Pb/Pb date for staurolite or garnet records the time of mineral growth, not post-growth isotopic closure, as also concluded in recent field studies. Because Z systematizes fission-track annealing, this recrystallization process, like volume-diffusion, must also be rate-limited by the strength of chemical bonds. The extent to which other recrystallization processes are likewise rate-limited is important to U/Pb geochronology because they potentially compete with diffusion as mechanisms for Pb-isotopic resetting in nature.

The effects of composition on retentivity of argon and oxygen in hornblende and related amphiboles: A field-tested empirical model

The ionic porosity model of Fortier and Giletti (1989), which parameterizes variations in diffusion coefficients among different mineral structures, has been extended to estimate the variation in diffusivities in a single mineral structure as a function of composition (X). Applied to Ar and O diffusion in hornblende and related K-rich amphiboles, the extended model predicts that diffusivity, D, increases one-hundredfold as ionic porosity (Z, a monitor of atomic packing density) increases from 36.5% (edenite) to 39.7% (ferro-actinolite). Partitioning this trend into its separate activation energy (E) and frequency factor (D0) components leads to new E-Z and D0-Z expressions that predict antipathetic trends of closure temperature (Tc) vs. Z in the clinoamphiboles. The Ar model yields a ΔTC/ΔZ of −38 ±3°C/Z %, which translates into: (1) a ∼120°C increase in TC from synthetic ferro-actinolite to edenite and (2) a ∼70°C range in Tc for natural hornblendes amenable to 40Ar/39Ar dating. Model TC-X effects isolated from available amphibole data include increases in TC of up to: (1) 60 ± 10°C as Mg# ranges from 0–100; (2) 40 ± 15°C as A-sites progress from empty to full; (3) ∼38°C as Al-Tschermaks substitution progresses from zero to full; and (4) ∼17°C as Fe3+/(Fe2+ + Fe3+) varies from 0.2–0.5. Comparable results are obtained for O. The Ar diffusion model also predicts a 38 ± 3 Ma difference in 40Ar39Ar cooling age between adjacent, slowly-cooled (1°C/Ma) hornblendes differing in Z by 1% (absolute)—all other age-determining factors equal. At a cooling rate of 10°C/Ma, however, the model age discordance reduces to 3.8 ± 0.3 Ma, such that any compositional effects become lost within analytical uncertainty. As calibrated, the TC-Z-X relationship is more applicable for determining relative (rather than absolute) TC values among hornblendes. Preliminary evidence supportive of the model (in its relative form) is provided by antipathetic age-Z trends preserved in two lower crustal terranes. Finally, the model D-TC-Z-X results promote physical understanding of the diffusional closure process, as shown at both unit-cell and grain scales.

New constraints on the timing of Early Proterozoic tectonism in the Black Hills (South Dakota), with implications for docking of the Wyoming province with Laurentia

A question regarding the 1900–1600 Ma assembly of Laurentia is whether the Wyoming province (west-central United States) was part of the Hearne province (west-central Canada) prior to the Early Proterozoic Trans-Hudson orogeny, or a separate entity welded later to a Hearne-Superior continent (central Canada). New 40 Ar/ 39 Ar mica dates help to address this question by extending a Middle Proterozoic geochronologic front, long established along the southern Wyoming province, into the Black Hills of South Dakota. This suggests that previously unexplained, north-directed fold nappes (F 1 ) in the Black Hills resulted from island-arc accretion to the south ca. 1780 Ma. North-northwest–trending upright F 2 folds, which formed during east-west collision of the Wyoming and Superior provinces, must therefore be younger. New 40 Ar/ 39 Ar hornblende dates, recently published age data, and crustal heat-flow considerations further suggest that this collision began at or before ca. 1770 Ma and culminated with posttectonic magmatism beginning ca. 1715 Ma (the Harney Peak granite). This tectonic-magmatic interval is ∼50–60 m.y. younger than that reported for the Hearne-Superior collision (Trans-Hudson orogeny in Canada). Comparably young metamorphic dates (1810–1710 Ma) also typify the eastern and northern Wyoming province periphery (western Dakotas and southwestern Montana). Collectively, these data suggest that the Hearne and Wyoming provinces were once separate continents that were ultimately welded to the Superior province (and to each other) during distinct Early Proterozoic orogenies. Regional relationships further suggest that final docking of the eastern Wyoming province with Laurentia began during the ca. 1780–1740 Ma interval of island-arc accretion along the southern margin of the growing craton.

Comparative isotopic and chemical geochronometry of monazite, with implications for U-Th-Pb dating by electron microprobe: An example from metamorphic rocks of the eastern Wyoming Craton (U.S.A.)

Abstract Polygenetic monazite grains in diverse Precambrian crystalline rocks from the Black Hills, South Dakota, have been analyzed in situ by ion and electron microprobe methods (SHRIMP and EMP), to evaluate the accuracy and precision of EMP ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections. Parallel evaluations were conducted by comparing EMP chemical and SHRIMP isotopic ages at regional-, rock-, and grainscales. The monazite data set includes 354 EMP chemical analyses from 26 grains in six metamorphic rocks, which resolve into 54 age-composition domains, and 31 SHRIMP isotopic ages from 13 grains in one of the rocks, with six grains microanalyzed in common by the two methods. The data set also includes monazite-bearing garnets in two of the rocks, whose isotopic compositions were analyzed using Pb stepwise-leaching (PbSL) methods. Both the EMP and SHRIMP data sets reveal a continuum of apparent monazite ages spanning a ~1790.1680 Ma timeframe, with a relatively high probability of ages at ~1755 and ~1715 Ma that correspond spatially to core and rim domains. PbSL ages of ~1742 and ~1734 Ma obtained from monazite-bearing garnet in two rocks are intermediate compared to the corresponding EMP ages, and are thereby interpreted as mixed ages. EMP data for two grains in the structurally deepest of the six rocks record ~1785 and ~1755 Ma ages in the cores and (higher-Y and lower-Th) rims, respectively, and these results are duplicated by SHRIMP ages in these and/or other grains from the same rock. Overall, the EMP, SHRIMP, and PbSL ages are internally consistent at the various scales of observation, which serves to validate EMP chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains. The EMP data set is interpreted geologically as reflecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the Black Hills. Taking the most precise EMP data at face value, it is possible to resolve the timing of the two older events at ≤1784 ± 4 Ma (or ≤1786 ± 6 Ma) and 1756 ± 3 Ma (or 1753 ± 4 Ma), with 95% confidence. These events are considered to be related to sequential episodes of N-directed thrusting and ~E-W compression associated with Paleoproterozoic crustal assembly in the mid-continent. A younger metamorphism, related to granite intrusion known to have occurred at 1715 ± 3 Ma, is dated independently at 1717 ± 2 Ma from the EMP monazite ages.

Electron probe (Ultrachron) microchronometry of metamorphic monazite: Unraveling the timing of polyphase thermotectonism in the easternmost Wyoming Craton (Black Hills, South Dakota)

Abstract A metapelite from the easternmost Wyoming craton (Black Hills, South Dakota) has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800.1700 Ma assembly of southern Laurentia. Three deformational fabrics are recognized in oriented thin sections: an ENE-trending S1 fabric, preserved as oblique inclusion trails in garnet porphyroblasts; a NNWtrending S2 fabric, preserved as microlithons in the rock matrix; and a flattening fabric, S3, which transposed S1-S2 and dominates the matrix. A complex monazite porphyroblast has been analyzed in situ with the electron microprobe (Ultrachron) to constrain the timing of S1-S3 fabric formation associated with monazite growth. The core of this grain uniquely preserves the S1-S2 fabrics as sigmoidal inclusion trails. The mean total-Pb age of this domain is 1750 ± 10 Ma (all dates reported at 95% confidence; n = 39 spots), which is equivalent to the published 207Pb/206Pb age for the same domain. These results validate the total-Pb dating method in general and the Ultrachron in particular, for reliable age determination in low-Th monazite, and are interpreted as 1750 Ma minimum ages for the S1-S2 fabrics and sequential, D1-D2 collisional events that imposed them (~N-directed arc accretion and ~E-W continental collision, respectively). A higher-Th,Y rim of this same .Rosetta. grain truncates the S1-S2 sigmoid, and is associated with resorption textures in garnet porphyroblasts, coupled release of Y, and an S3 fabric that pervasively overprinted S1-S2 in the rock matrix. The mean Ultrachron date of this domain is 1692 ± 5 Ma (n = 17 spots), which is slightly younger that the published isotopic age for all monazite rims combined. These results support a ~1715.1690 Ma timeframe for localized doming (D3) related to granite magmatism, the onset of which has been dated independently at 1715 ± 3 Ma. The timing of post-D3 cooling through 350 and 300 °C is constrained by 40Ar/39Ar dates of ~1610 and ~1480 Ma obtained for separates of D3 matrix muscovite and biotite, respectively, which are interpreted as closure ages. This study shows that fabrics in poly-deformed rocks can be dated by linking monazite spot ages to key microtextures. Further, the results of this micrometer-scale study enhance previous knowledge of local thermotectonism (Black Hills) and regional terrane assembly (Laurentia).

Step-leach Pb-Pb dating of inclusion-bearing garnet and staurolite, with implications for Early Proterozoic tectonism in the Black Hills collisional orogen, South Dakota, United States

We report 207 Pb/ 206 Pb stepwise-leaching (PbSL) ages of 1762 ± 15 Ma, 1759 ± 8 Ma, and 1760 ± 7 Ma for almandine garnet, staurolite, and garnet-staurolite (combined) in an Early Proterozoic metapelite from the Black Hills collisional orogen, South Dakota. PbSL-derived 208 Pb/ 206 Pb (Th/U) trends reveal that staurolite contains inclusions of both cogenetic monazite (ca. 1760 ± 7 Ma) and older detrital zircon (≥2040 Ma), whereas coexisting garnet contains only the zircon. These results and petrologic data indicate prograde sequential growth of garnet, monazite, and staurolite during tectonic burial at temperatures between ∼400 °C and ∼550 °C (i.e., well below Pb closure temperatures). We interpret the 1760 ± 7 Ma date as a maximum age for Wyoming-Superior continental collision in the Black Hills, which apparently postdated the 1800–1900 Ma Trans-Hudson orogeny in Canada, but was nearly synchronous with north-directed accretion of the Central Plains orogen in southeastern Wyoming.

Obtaining geologically meaningful 40Ar–39Ar ages from altered biotite

Biotite is the most used 40Ar–39Ar geochronometer yet two significant problems arise from Ar–Ar step-heating. Dating altered biotite can be problematic, producing disturbed age spectra that reflect 39Ar recoil. However, unaltered biotite can yield disturbed ages with apparently meaningful plateau ages as a result of mineral breakdown during stepped heating. Obtaining meaningful ages from such spectra is very difficult. Because alteration of biotite is common and widespread in nature, and because many biotites affected by alteration are nonetheless unique geologically and/or representative of key localities, the capability to obtain reliable ages from altered material is extremely important. In this study, the effects of alteration progress on biotite age spectra were tested using both IR laser step-heating and UV laser microprobe 40Ar–39Ar dating techniques. Our aims were to extract geologically meaningful ages from altered biotite and to identify cases where the ages had been influenced by alteration. Three variably altered biotites from the Precambrian metamorphic terrain of southwestern Montana were selected for argon isotopic analysis. Sample A is an unaltered rock containing pristine biotite, sample B is a highly altered rock with chlorite and prehnite interlayers within biotite, and sample C contains biotite with only incipient alteration. For each sample, the biotite ages obtained with IR and UV laser techniques were compared and the validity of the apparent ages was assessed. IR step-heating analysis of biotite from sample A yielded a well-defined plateau of 1776±6 Ma. UV laser microprobe analysis of the same biotite yielded a concordant weighted mean age of 1771±8 Ma, based upon 65 spot analyses of eight grains that ranged in age from 1819±54 to 1722±62 Ma. In contrast, IR step-heating of biotite from sample B resulted in disturbed spectra, with two separate fragments yielding total gas ages of 1505±12 and 1540±16 Ma, whereas UV laser microprobe analysis yielded 14 spot ages ranging from 1806±72 to 1565±78 Ma. UV laser analysis of sample C produced apparent ages ranging from 1772±52 to 1359±200 Ma. Detailed analysis of age variations occurring perpendicular to (001) cleavage planes of biotite in sample B was accomplished by depth profiling using the UV laser microprobe. Depth profiling of single grains revealed considerable age variation among the biotite layers, with profiles 1 and 2 yielding apparent ages ranging from 1730±226 to 1511±186 Ma, and from 1741±290 to 956±230 Ma, respectively. In both samples B and C, younger apparent ages correspond to higher 36Ar/39Ar ratios. The youngest ages, therefore, are believed to have derived from altered layers, whereas the oldest ages are related to layers with relatively little or no alteration. In summary, incremental step-heating of altered biotite single grains yields 40Ar–39Ar age spectra that are compromised by recoil and variable release patterns. However, extraction of small samples via the UV laser microprobe provides a simpler pattern of apparent ages, all younger than the “true” cooling age yet inversely correlated with atmospheric argon. This result suggests that true biotite ages can be recovered from areas of pristine material if they are sufficiently large to be ablated by the UV laser.

The Systematics of Trace-Element Partitioning Between Coexisting Muscovite and Biotite in Metamorphic Rocks from the Black-Hills, South-Dakota, USA

Abstract Coexisting muscovite and biotite in forty-nine staurolite- and sillimanite-zone schists from the southern Black Hills, South Dakota, USA, have been analyzed by ICP spectrometry for major and selected trace elements. This study represents the first comprehensive effort to document and explain trace-element partitioning behavior between coexisting micas in metamorphic rocks. Overall, the data reveal systematic element distributions across a wide thermal-compositional range. Mean Henrys Law partition coefficients [ K ∗ D (bio/mus)] are as follows: Mn (14 ± 6 (1σ)), Ni (14 ± 6), Zn (13 ± 8), Li (4.0 ± 1.1), Ti(2.9 ± 0.7), Co (2.6 ± 1.6), Yb (2.0 ± 1.7), Cu (1.9 ± 1.2), Y (1.5 ± 1.5), Be (1.1 ± 1.5), Cr (1.0 ± 0.2), La (0.8 ± 0.7), V (0.7 ± 0.1), Zr (0.6 ± 0.2), Ba (0.5 ± 0.3), Sc (0.4 ± 0.1), Sr (0.4 ± 0.4), and Na (0.2 ± 0.1). This sequence is governed largely by the crystal structure of the micas and their major-element compositions. Several structural effects on K ∗ D have been identified. First, the observed Kast;D sequences Cr3+ > V3+ > Sc3+ and Ni2+ > Co2+ > Cu2+ are just as predicted from relative octahedral site preference energies, indicating that crystal-field effects influence the partitioning behavior of transition-metal cations. Second, the presence of smaller (i.e., more collapsed) interlayer sites in muscovite (relative to biotite) favors substitution in muscovite of cations smaller than K+, namely, Na+, Sr2+, Ba2+, and La3+. Likewise, interlayer cations larger than K+ (e.g., H3O+, Rb+, and Cs+) are predicted to substitute preferentially into biotite. Third, tetrahedral Fe3+ is predicted to favor biotite over muscovite because of larger tetrahedral sheets in biotite (due, in turn, to more Al and less Si). Fourth, the occurrence of heterovalent interlayer cations in micas suggests that their partitioning behavior is partly governed by charge-balance reactions. As a general compositional effect on K ∗ D , the preferences exhibited by biotite for Li+ and divalent cations of first-row transition elements reflect its high abundance of comparably sized vi(Mg, Fe2+) relative to coexisting muscovite. Likewise, relatively strong affinities of muscovite for Cr3+, V3+, and Sc3+ reflect its high stoichiometric vi(Al, Fe3+) abundance. Element-specific compositional effects on K ∗ D are less evident; but there are indications that Mg2+ partitioning affects that of Li+, Ni2+, and Mn2+. Temperatures inferred from Mg-Tschermak (MgSiAL2) exchange between coexisting muscovite and biotite ( Hoisch , 1989) provide a convenient datum by which to evaluate the thermal behavior of analogous vector components involving trace elements. Several of these components appear to possess exchange potentials (ΔGrxn) sufficiently large so that respective equilibrium constants approach unity with temperature increase. These components include the following: NiSiAl−2, MgSiCr−1Al−1, MgSiV−1Al−1, MgSiSc−1Al−1, CrAl−1, VAl−1, and Li2 Si viAl−1, vi□−1, ivAl−1. In contrast, any thermal sensitivity of other such components is masked by analytical scatter.

40Ar/39Ar evidence for Middle Proterozoic (1300–1500 Ma) slow cooling of the southern Black Hills, South Dakota, midcontinent, North America: Implications for Early Proterozoic P‐T evolution and posttectonic magmatism

40Ar/39Ar total gas and plateau dates from moscovite and biotite in the southern Black Hills, South Dakota, provide evidence for a period of Middle Proterozoic slow cooling. Early Proterozoic (1600–1650 Ma) mica dates were obtained from metasedimentary rocks located in a synformal structure between the Harney Peak and Bear Mountain domes and also south of Bear Mountain. Metamorphic rocks from the dome areas and undeformed samples of the ∼1710 Ma Harney Peak Granite (HPG) yield Middle Proterozoic mica dates (∼1270–1500 Ma). Two samples collected between the synform and Bear Mountain dome yield intermediate total gas mica dates of ∼1550 Ma. We suggest two end-member interpretations to explain the map pattern of cooling ages: (1) subhorizontal slow cooling of an area which exhibits variation in mica Ar retention intervals or (2) mild folding of a Middle Proterozoic (∼1500 Ma) ∼300°C isotherm. According to the second interpretation, the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure (and downwarping of relatively cold rocks) during a period of approximately east-west contraction and slow uplift during the Middle Proterozoic. The mica data, together with hornblende data from the Black Hills published elsewhere, indicate that the ambient country-rock temperature at the 3–4 kbar depth of emplacement of the HPG was between 350°C and 500°C, suggesting that the average upper crustal geothermal gradient was 25°–40°C/km prior to intrusion. The thermochronologic data suggest HPG emplacement was followed by a ∼200 m.y. period of stability and tectonic quiescence with little uplift. We propose that crust thickened during the Early Proterozoic was uplifted and erosionally(?) thinned prior to ∼1710 Ma and that the HPG magma was emplaced into isostatically stable crust of relatively normal thickness. We speculate that uplift and crustal thinning prior to HPG intrusion was the result of differential thinning of the subcrustal lithosphere beneath the Black Hills. If so, this process would have also caused an increase in mantle heat flux across the Moho and triggered vapor-absent melting of biotite to produce the HPG magma. This scenario for posttectonic granite generation is supported, in part, by the fact that in the whole of the Black Hills, the HPG is spatially associated with the deepest exposed Early Proterozoic country rock.

Influence of F(OH)−1 substitution on the relative mechanical strength of rock‐forming micas

Microtextural and experimental studies have yielded conflicting data on the relative mechanical strengths of muscovite and biotite [Wilson and Bell, 1979; Kronenberg et al., 1990; Mares and Kronenberg, 1993]. We propose a crystal-chemical resolution to this conflict, namely, that (001) dislocation glide in biotite is rate-limited by its fluorine content. Significant F(OH)−1 substitution, and concomitant removal of hydroxyl H+ directed into the interlayer cavity, potentially increases mechanical strength of biotite in two ways: (1) it eliminates K+-H+ repulsion, thereby strengthening the interlayer bonds, and (2) it allows K+ to “sink” deeper into the interlayer cavity, the resultant geometry being less favorable to basal slip. In testing this hypothesis we analyzed the naturally deformed biotite studied by Wilson and Bell [1979] and documented its very low F content (XF ≤ 0.02) compared to that of the biotite experimentally deformed by Kronenberg et al. [1990]. Our model and the comparative XF data explain why the biotite of Wilson and Bell [1979] deformed more easily in nature than its coexisting muscovite, whereas the biotite of Kronenberg et al. [1990] was mechanically stronger than muscovite similarly deformed by Mares and Kronenberg [1993]. Our reconciliation of these otherwise conflicting results provides a framework for predicting mechanical strength of natural micas based upon the extent of their F(OH)−1 substitution. Our synthesis highlights the potential role of crystal chemistry in determining mechanical behavior in multicomponent mineral families. Further testing of crystal-chemical effects on rheology will require mineral specimens of both appropriate composition and sufficient size.