Stable hydrogen isotopes in iron oxides—II. DH variations among natural goethites

Goethite samples analyzed for this study have a δD range from ?202 to ?98 per mil with a corresponding δD range of associated waters from about ?110 to +7 per mil. Goethites with the most positive δD values are from marine environments. Goethite-water equilibrium $\text{D}\text{H}$ fractionation factors measured in this laboratory at 100°C and 145°C and estimated for sedimentary temperatures from data on natural samples have values of about 0.900 at all three temperatures. These data suggest that goethite δD values may be a direct, temperature-independent measure of the δD values of the waters with which the goethite last equilibrated. Most of the goethite δD values in this study reflect the δD values of the modern waters in the locales of origin.Substitution of Mn and/or Al for Fe in the goethite structure may affect the mineral-water fractionation factor. Manganese appears to decrease the value of α. The effect of Al substitution on α has not yet been measured, but preliminary arguments suggest that increasing Al content could increase α values.     

SrCa and MgCa ratios in polygenetic carbonate allochems from a Michigan marl lake

Rapid accumulation of CaCO₃ is occurring in Littlefield Lake, a marl lake located in central Michigan. The sediment, which is 95% CaCO₃, primarily consists of eight different genetic groups of carbonate allochems. These include calcite muds, sands, algal oncoids and Chara encrustations, as well as the dominant aragonitic gastropods Valvota tricarinota. Gyraulus deflectus and Amnicola integra. and the dominant aragonitic pelecypod Sphaerium partumeium. Samples of each of these groups were analyzed for Ca, Sr and Mg. Molar $\text{Mg}\text{Ca}$ ratios are primarily controlled by allochem mineralogy, with calcitic forms having $\text{Mg}\text{Ca}$ ratios 5–10 times larger than aragonitic (shelled) forms. The $\text{Sr}\text{Ca}$ ratios are primarily controlled by biochemical fractionation, and are significantly lower than $\text{Sr}\text{Ca}$ ratios of inorganically precipitated aragonite from other settings. Partition coefficients were determined for both Sr and Mg for each carbonate allochem group and, based on comparisons with results reported by other workers, the partition coefficients determined here are generally considered ‘typical’ or representative values for biogeneous freshwater carbonates. An analysis of variance of the data indicates that most genera and species of carbonate-secreting organisms in marl lakes have highly characteristic $\text{Sr}\text{Ca}$ and $\text{Mg}\text{Ca}$ ratios. These ratios can potentially serve as geochemical tracers in future investigations of lacustrine carbonate diagenesis. Both Sr and Mg are influenced by grain size and/or surface area, probably due to the presence of these elements in non-lattice-held (exchangeable) positions.     

KRb ratios in Precambrian granulite terranes

Concentrations of potassium and rubidium are reported for 62 Precambrian shield whole-rock samples belonging to the pyroxene granulite facies of world-wide distribution. Compared with the Main Trend established by Shaw (1968) for igneous and quasi-igneous rocks of the upper crust, the high-grade metamorphic granulites exhibit a similar correlation but with a measurable depletion in Rb for a given K content. The Metamorphic Trend may be described by the equations: log₁₀ (ppm Rb) = 1.136 log₁₀ (per cent K) + 1.495 or ppm Rb = 31.28 (per cent K)^1.136. The square of the product moment correlation coefficient shows that 80 per cent of the variation in Rb is associated with variation in K.All suites studied have initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios less than 0.707. This low value argues against repeated extraction of Rb and K through time on a regional scale. Rather, the major depletion in these lithophile elements occurred in the early stages of crustal evolution.     

CaAl ratio and composition of the Earth's upper mantle

Undifferentiated meteorites (chondrites) have the same relative abundances of refractory lithophile elements (Ca, Al, Ti, Sc, REE, etc.), despite variable absolute concentrations. The reasonable assumption of chondritic ratios among refractory elements in the bulk Earth is used to constrain the chemical composition of the upper mantle in the following way: Correlations of the compatible refractory elements Ca, Al, Ti, Sc and Yb with MgO are worldwide very similar in suites of spinel-lherzolite xenoliths from basaltic rocks. Such suites represent upper mantle material depleted to differing degrees by extraction of partial melts. From these refractory elements vs. MgO correlations, ratios of pairs of refractory elements were calculated at various MgO contents. Chondritic $\text{Al}\text{Ti}$ and $\text{Sc}\text{Ti}$ ratios were only obtained for MgO contents below 36%. A chrondritic $\text{Sc}\text{Yb}$ ratio requires an MgO content above 35%. We therefore accept 35.5% as the most reasonable MgO content of undepleted upper mantle. This MgO content is slightly below the spinel-lherzolite with the lowest measured MgO content (36.22%). The corresponding Al₂O₃ content of 4.75% is higher than in previous estimates of upper mantle composition. The concentrations of other elements were obtained from similar correlations at a MgO content of 35.5%. The resulting present upper mantle composition is enriched in refractory elements by a factor of 1.49 relative to Si and Cl and by a factor of 1.12 for Mg relative to Si and Cl. These enrichments are in the same range as those for the Vigarano type carbonaceous chondrites. The Mg/Mg + Fe ratio of 89 is slightly lower than previous estimates.The $\text{Ca}\text{Al}$ ratio in spinel lherzolite suites is, however, uniformly higher worldwide than the chondritic ratio by about 15%. Orogenic peridotites as well as komatiites appear to have similar non-chondritic $\text{Ca}\text{Al}$ ratios. It is therefore suggested that this non-chondritic $\text{Ca}\text{Al}$ ratio is a characteristic of the upper mantle, possibly since the Archean. A minor fractionation of about 4% of garnet in an early, global melting event (deep magma ocean?) is presented as the most likely cause for the high $\text{Ca}\text{Al}\text{-}\text{ratio}$. In this case the addition of 4% of such a garnet component to the undepleted present upper mantle would be required to obtain the composition of the primordial upper mantle. The $\text{Ca}\text{Al}\text{-}\text{ratio}$ of this primordial mantle would be 15% higher than that of the undepleted present upper mantle, resulting in an enrichment of refractory elements of 1.70 ($\text{Al}\text{Si}$ relative to Cl) for the primordial upper mantle.     

Surface study of HF- and HFH2SO4-treated feldspar using Auger electron spectroscopy

Auger electron spectroscopy has been used to study K-feldspar that has been reacted with both aqueous 10% HF and a 50% mixture of a 10% HF/0.1 N H₂SO₄ solution. In the feldspar/HF system, the resulting feldspar surface was shown to have been fluorinated; depth profiling, using argon ion sputtering, showed the fluorination to have occurred substantially into the mineral bulk. In the feldspar/ $\text{HF}\text{H}{}_2\text{SO}{}_4$ system, the resulting surface contained both fluorine and sulfur. The fluorination had again penetrated into the bulk, but the sulfur could be removed with mild argon ion sputtering. The $\text{Al}\text{F}$ signal ratio was much lower on the feldspar surface treated with the 10% HF/0.1 N H₂SO₄ solution than the feldspar surface treated with the weaker 10% HF acid solution.     

HFSiF4 ratios in volcanic and magmatic gases

The possible importance of SiF₄ in volcanic and magmatic gases has been neglected due to the convention of reporting analyses and basing calculations on the presence of HF. Calculated $\text{HF}\text{SiF}{}_4$ ratios for natural gas compositions serve to justify this convention by showing that SiF₄ is not a significant F-bearing molecular species at high temperatures.     

40Ar39Ar dating,Ar diffusion properties,and cooling rate determinations of severely shocked chondrites

Determinations of ${}^{40}\text{Ar}{}^{39}\text{Ar}$ ages are reported for seven severely shock-heated chondrites. Shaw gives a plateau age of 4.29 Gyr. Louisville, Farmington, and Wickenburg give well-defined intercept ages of 0.5–0.6 Gyr. Orvinio, Arapahoe, and Lubbock show complex ${}^{40}\text{Ar}{}^{39}\text{Ar}$ release curves, with age minima of 0.7–1.0 Gyr. Degassing times of 0.5–1.0 Gyr are suggested for these meteorites. Most severely shocked chondrites were apparently not totally degassed of ⁴⁰Ar by the event, but retained from ~ 2 to ~45% of their ⁴⁰Ar. When calculated values of the diffusion parameter, $\text{D}\text{a}{}^2$, for Ar are examined in Arrhenius plots, they show two distinct linear relationships, which apparently correspond to the degassing of different mineral phases with distinct $\text{K}\text{Ca}$ ratios and different average temperatures for Ar release. The experimentally determined values of $\text{D}\text{a}{}^2$ for the high temperature phase of several severely shocked chondrites are ~10^?7 to 10^?5sec^?1 for their determined shock-heating temperatures of ~950°C to ~ 1200°C. The inferred reheating temperatures, $\text{D}\text{a}{}^2$ values, and fraction of ⁴⁰Ar loss during the reheating event for these seven chondrites suggest post-shock cooling rates and burial depth of ~ 10^?2 10^?4°C/sec and ~0.5–2m, respectively. For three chondrites these cooling rates agree with those determined from Ni diffusion in metal grains: for five chondrites the cooling rates derived from ⁴⁰Ar and Ni disagree by a factor of ~10⁵. It is suggested that five of these severely shocked chondrites were part of large ejecta blankets containing hot material and cold clasts with a distribution of sizes and that the cooling rate of this ejecta appreciably decreased as a function of time.     

The variation of the marine 87Sr86Sr ratio during Phanerozonic time: interpretation using a flux model

The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in sea water has varied over geologic time due to the addition of strontium to the sea from rocks with a variety of ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios. The measurements by Petermanet al. (Geochim. Cosmochim. Acta34, 105–120, 1970) of the value of the marine ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio have been confirmed by several other workers and by some new measurements on JOIDES samples. They form the basis of a model calculation of the relative proportions of ‘basaltic’ (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.704}$) and ‘granitic’ (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.718}$) strontium being supplied to the sea. For the last 200 million years, the proportions of these two sources appear to reflect the history of global tectonics; ‘basaltic’ during rifting and increasingly ‘granitic’ during the present episodes of uplift and continental collision     

The geochemical coherence of Pu and Nd and the 244Pu238U ratio of the early solar system

Two very different sets of ${}^{244}\text{Pu}{}^{238}\text{U}$ ratios have been reported for early solar system materials. One group of samples yields high (0.015–0.016) ratios (Podosek, 1970a, 1972; Drozd et al., 1977) and calculations based on another group of analyses yield low ratios (~0.004) (Marti et al., 1977). Recently measured partition coefficients for Pu and Sm are used to evaluate the data of Marti et al. and $\text{sol}{}^{244}\text{Pu}{}^{238}\text{U}$ ratios from other sources are also considered. A low $\text{sol}{}^{244}\text{Pu}{}^{238}\text{U}$ ratio (~0.005) is favored, and some implications of this low ratio to galactic nucleosynthesis and meteorite age dating are briefly discussed.     

Radium content and the 226Ra228Ra activity ratio in groundwater from bedrock

The relative abundance of ²²⁶Ra and ²²⁸Ra were determined in the groundwater from 125 drilled wells containing from < 0.1 to 51.3 pCi/l of ²²⁶Ra. The determination of ²²⁸Ra was carried out with a liquid scintillation counter by measuring only the weakly energetic β particles emitted from ²²⁸Ra. Thus the interference from the daughter nuclides of ²²⁶Ra was avoided, without specific separation of ²²⁸Ac. The direct measurement of ²²⁸Ra made the method decisively simpler and faster in terms of the chemistry involved.The concentration of ²²⁸Ra was found to be independent of the amount of ²²⁶Ra present in the samples. The concentrations of ²²⁸Ra were nearly the same over the whole range of ²²⁶Ra concentrations and the average $\text{sol}{}^{226}\text{Ra}{}^{228}\text{Ra}$ ratio sharply increased as the ²²⁶Ra content of water increased. The ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratio in the drilled wells varied from 0.3 to 26. Abnormally high ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratios were found in areas with known uranium deposits as well as in several drilled wells at other locations. The abnormally high ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratios present in groundwater suggest that the radioactivity anomaly is caused by uranium deposits and not by common rocks. In samples with a low radioactivity level the average ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratio was slightly below unity, corresponding to the typical U/Th ratio of granite, the most common kind of rock in the study area. The samples from the rapakivi area proved to be exceptional in that they had a low ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratio independent of the concentration of ²²⁶Ra.     

The thermal significance of potassium feldspar K-Ar ages inferred from 40Ar39Ar age spectrum results

${}^{40}\text{Ar}{}^{39}\text{Ar}$ age spectrum analyses of three microcline separates from the Separation Point Batholith, northwest Nelson, New Zealand, which cooled slowly (~5°C-Ma^?1) through the temperature zone of partial radiogenic ⁴⁰Ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages (~80 Ma) corresponding to the time that the samples cooled below about 100°C. The last 35 percent of ³⁹Ar released from the microclines yields plateau ages (103,99 and 93 Ma) which reflect the different bulk mineral ages, and correspond to cooling temperatures between about 130 to 160°C. Theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates ≤5°C-Ma^?1. Diffusion parameters calculated from the ³⁹Ar release yield an activation energy, E = 28.8 ± 1.9 kcal-mol^?1, and a frequency factor/grain size parameter, $\text{D}{}_0\text{l}{}^2\text{= 5.6}{}_{\mathrm{?3.9}}{}^{\mathrm{+14}}\text{sec}{}^{\mathrm{?1}}$. This Arrhenius relationship corresponds to a closure temperature of 132 ± 13°C which is very similar to the independently estimated temperature. From the observed diffusion compensation correlation, this $\text{D}{}_0\text{l}{}^2$ implies an average diffusion half-width of about 3 μm, similar to the half-width of the perthite lamellae in the feldspars. The range in microcline K-Ar ages from the Separation Point Batholith is the result of relatively small temperature differences within the pluton during cooling. Comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous K-feldspars (E = 40 to 52 kcal-mol^?1) reveals that Ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state. Previously published U-shaped age spectra are probably the result of the superimposition of excess ⁴⁰Ar upon diffusion profiles of the kind described here.     

Mg,Ca and O18O16 exchange in the sediment-pore water system,hole 149, DSDP

The diagenesis and geochemical evolution of deep-sea sediments are controlled by the interaction between sediments and their associated pore waters. With increasing depth, the pore water of Hole 149 (DSDP) exhibits a strong depletion in Mg and a corresponding enrichment in Ca, while the alkalinity remains relatively constant. Dissolved SiO₂ is nearly constant in the upper 100 m of sediment, but is highly enriched in the deepest pore waters. The pore waters exhibit a depletion in K with increasing depth, and $\text{O}{}^{18}\text{O}{}^{16}$ pore water ratios also decrease.The sediment section has three zones of sedimentary regimes with increasing depth in the drill hole: an upper 100 m section of detrital clays, a middle section enriched in calc-akalic volcanics which have undergone submarine weathering to a smectite phase, and a lower section of siliceous ooze which still has a diagenetic smectite phase. The quartz-feldspar ratios and $\text{O}{}^{18}\text{O}{}^{16}$ composition of the silicate phases are in agreement with these interpretations.The submarine weathering of volcanics to a smectite can account for the observed pore water gradients. Volcanics release Ca and Mg to the pore waters causing the alkalinity values to increase. Smectite is formed, depletes the pore waters in Mg and O¹⁸ and causes the alkalinity to decrease. The net reaction allows for the observed relationship between pore water Ca and Mg gradients with little net change in alkalinity. Given the abundance of volcanics in many deep-sea sediments, especially in lower sections which often form near ridge crests, the submarine formation of smectite may be an additional oceanic Mg sink which has not yet been fully considered.     

Determination of Rb/Sr and 87Sr86Sr ratios of some standard rocks and evaluation of X-ray fluorescence spectrometry in Rb1bSr geochemistry

New isotope dilution results are presented for Rb and Sr concentrations in U.S.G.S. standard rocks and NBS-70a K-feldspar. The results (based on at least five analyses of each rock), are generally accurate to ± 0.5% and resolve discrepancies in previously published data. X-ray fluorescence analyses of the same samples yield Rb and Sr determinations which are only accurate to ± 5%, but Rb/Sr ratios which are as precise and in excellent agreement with the isotope dilution values. It is concluded that X.R.F. determination of Rb/Sr ratios is perfectly suitable for whole-rock Rb1bSr geochronology.${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios have been determined on G-2, GSP-1, BCR-1 and AGV-1 as well as the Eimer and Amend SrCO₃ standard.     

U234U238 Ratios in limestone cave seepage waters and speleothem from West Virginia

Speleothem from West Virginia, ranging in age from 2000 to 200,000 yr B.P. contains uranium with $\text{U}{}^{234}\text{U}{}^{238}$ ratios significantly greater than unity. This ratio varies from one speleothem to another, as does average U content. Initial ratios, corrected for age, are remarkably constant for a given speleothem. By contrast, $\text{U}{}^{234}\text{U}{}^{238}$ ratios in seepage waters vary significantly from month to month at a given drip site, and their average values differ from that of the speleothem which they are depositing. This discrepancy is attributed either to long-term averaging-out of fluctuations, or fractional precipitation on the speleothem of a chemical species of uranium with a more constant ratio. Constancy of initial $\text{U}{}^{234}\text{U}{}^{238}$ ratios from $\text{Th}{}^{230}\text{U}{}^{234}$. datable portions of speleothems should permit $\text{U}{}^{234}\text{U}{}^{238}$-dating of older portions of the same speleothem, back to about 10⁶ yr B.P., with estimated precision of ±5 per cent.     

Application of the 227Th230Th method to dating Pleistocene carbonates and comparison with other dating methods

The ${}^{227}\text{Th}{}^{230}\text{Th}$ dating method is described in detail and its usefulness investigated by comparing ages of sixteen Pleistocene carbonates (mainly cave deposits) with those determined by the ${}^{231}\text{Pa}{}^{235}\text{U}$ and ${}^{230}\text{Th}{}^{234}\text{U}$ methods. The ${}^{227}\text{Th}{}^{230}\text{Th}$ ages are found to be critically dependent on corrections for decay of ²²⁷Th prior to alpha counting and ingrowth of daughter isotopes of ²³²Th derived from clastic detritus. Of nineteen sets of ages determined for the sixteen samples, good agreement is found for only seven sets. Differences are attributed to low U content of some samples and the possibility of excess ²²⁷Th in the calcite of samples younger than ~50 ky, possibly due to the coprecipitation of ²³¹Pa during formation. Calculated “negative” ${}^{227}\text{Th}{}^{230}\text{Th}$ ages may be a direct result of this process and the fact that, unlike the other methods, the activity ratio is non-zero at zero age. Nevertheless, the ${}^{227}\text{Th}{}^{230}\text{Th}$ is found to be a useful alternative dating technique for carbonates which are between ~50 and 300 ky, because no spiking is required. It also serves as a check for partial concordancy with ages dated by the other methods.     

Recognition of extraneous argon components through incremental-release 40Ar39Ar analysis of biotite and hornblende across the Grenvillian metamorphic gradient in southwestern Labrador

${}^{40}\text{Ar}{}^{39}\text{Ar}$ incremental-release ages have been determined for 15 hornblende and 20 biotite concentrates separated from rocks collected across the garnet and kyanite zones of Grenvillian metamorphism in southwestern Labrador. Most hornblende spectra from the kyanite zone are slightly discordant, with low-temperature increments yielding ages older than the ca 1000 Ma date suggested for culmination of Grenvillian metamorphism in the area. However, all the hornblende concentrates record well-defined plateau ages. These range from 968 to 905 Ma across the kyanite zone and date times of diachronous post-metamorphic cooling. The discordant spectra are interpreted to result from low-temperature liberation of excess ⁴⁰Ar components from grain margins. Two hornblende concentrates from the garnet zone display very discordant spectra (total-gas ages of 2100 and 3017 Ma) in which incremental dates systematically decrease during analysis. This pattern of discordance suggests that excess argon components are inhomogeneously distributed throughout these hornblende grains.Most biotites from the garnet and kyanite zones record total-gas or plateau ages in excess of 1000 Ma (2066-857 Ma), reflecting the widespread presence of excess argon components. Because most of the ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age spectra are internally concordant, the ratios of excess ⁴⁰Ar relative to radiogenic ⁴⁰Ar must have been uniform in the various gas fractions liberated from each sample. This is also reflected in the inability of isotope correlation diagrams to differentiate between excess, radiogenic, and atmospheric argon components. The biotite total-gas or plateau dates show marked local variation. This is interpreted to indicate that the biotite grains were in contact with a post-metamorphic intergranular vapor phase that was characterized by large and variable ${}^{40}\text{Ar}{}^{36}\text{Ar}$ ratios. Such ratios most likely resulted from widespread diffusion of the argon liberated from adjacent Archean basement gneisses during the Grenvillian metamorphic overprint.     

A high 87Sr86Sr mantle source for low alkali tholeiite,northern Great Basin

Olivine tholeiites, the youngest Tertiary units (about 8–11 m.y. old) at five widely spaced localities in northeastern Nevada, are geologically related to the basalts of the Snake River Plain, Idaho, to the north and are similar in major element and alkali chemistry to mid-ocean ridge basalts (MORB) and island arc tholeiites. The measured K (1250–3350 ppm), Rb (1·9–6·2 ppm) and Sr (140–240 ppm) concentrations overlap the range reported for MORB. Three of the five samples have low, unfractionated rare earth element (REE) patterns, the other two show moderate light-REE enrichment. Barium concentration is high and variable (100–780 ppm) and does not correlate with the other LIL elements. The rocks have ⁸⁷Sr/⁸⁶Sr = 0·7052–0·7076, considerably higher than MORB (~0·702–0·703). These samples are chemically distinct (i.e. less alkalic) from the olivine tholeiites from the adjacent Snake River Plain, but their Sr isotopic compositions are similar. They contain Sr that is distinctly more radiogenic than the basalts from the adjacent Great Basin. About 10 b.y. would be required for the mean measured Rb/Sr (~ 0·02) of these samples to generate, in a closed system, the radiogenic Sr they contain. The low alkali content of these basalts makes crustal contamination an unlikely mechanism. If the magma is uncontaminated, the time-averaged Rb/Sr of the source material must have been ~0·04. A significant decrease in Rb/Sr of the source material (a factor 2?) thus most probably occurred in the relatively recent (1?0⁹ yr) past. Such a decrease of Rb/Sr in the mantle could accompany alkali depletion produced by an episode of partial melting and magma extraction. In contrast, low ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios indicate that the source material of the mid-ocean ridge basalts may have been depleted early in the Earth's history.     

Investigations of an intrusive contact,northwest Nelson,New Zealand—II. Diffusion of radiogenic and excess 40Ar in hornblende revealed by 40Ar39Ar age spectrum analysis

The Rameka Gabbro, emplaced 367 Ma ago, experienced a well documented reheating on intrusion of the Separation Point Batholith 114 Ma ago. ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age spectrum analyses of hornblende from the Rameka Gabbro show diffusion gradients which provide information on the ⁴⁰Ar boundary concentration during reheating.Three samples of hornblende exhibit age spectra that conform to a model of ⁴⁰Ar loss by diffusion, implying a zero ⁴⁰Ar boundary concentration during heating. The calculated ⁴⁰Ar loss from these samples, together with a model of heat flow in the aureole, provide estimates of diffusion coefficients of ⁴⁰Ar in Mg-rich hornblende which correspond to an activation energy, E, of ~60 kcal-mol^?1 and a frequency factor. D₀, of ~ 10^?3 cm²-sec^?1. When combined with laboratory diffusion results, these data yield a well defined diffusion law (E = 63.3 ± 1.7 kcal-mol^?1, $\text{D}{}_0\text{= 0.022}{}^{\mathrm{+0.048}}{}_{\mathrm{?0.010}}\text{cm}{}^2\text{-}\text{sec}{}^{\mathrm{?1}}$).The age spectra of the eight other samples record steep gradients of excess ⁴⁰Ar over the first few percent of gas release. Although this effect causes high apparent conventional K-Ar ages, the plateau segments of many sampes still record the crystallization age of 367 ± 5 Ma. These measurements show that the excess ⁴⁰Ar phase developed locally in the intergranular regions of the gabbro, following intrusion of the batholith. on time scales that varied from 10⁴ to 10⁶years. The minimum average ${}^{40}\text{Ar}{}^{36}\text{Ar}$ ratio of this component was found to be 1300 ± 400. The partial pressure of Ar was at least 10^?2 bars in some places.A single ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age spectrum analysis of plagioclase reveals a ‘saddle-shaped” release pattern with a minimum at 140 Ma.In conjunction with theoretical diffusion models and a diffusion law, ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age spectrum analysis of hornblende that has experienced a post-crystallization heating can provide close estimates of the maximum temperature of the thermal event as well as both age of crystallization and reheating.