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.     

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     

RbSr studies of CI and CM chondrites

RbSr whole rock analyses have been performed on 2 CI and 3 CM chondrites. Four of these stones (Ivuna, Orgueil, Cold Bokkeveld and Erakot) were previously studied in this laboratory and were shown to be discordant from a 4.6 Gyr isochron. The fifth, Murchison, was not previously studied. The new data support the discordance of the first four stones, and indicate that Murchison is also discordant. Studies of Sr isotope ratios in unspiked Orgueil show that the discordance is not due to inhomogeneities in the $\text{Sr}{}^{84}\text{Sr}{}^{86}$ ratio caused by incomplete mixing of nucleosynthesis products.In order to gauge the effects of weathering, two leaching experiments were performed on fresh, interior samples of Murchison; one for a period of 1.5 hr and the other for 117 hr. The results indicate that the relative solubility of nonradiogenic Sr is approximately twice that of Rb and radiogenic Sr is more soluble than the nonradiogenic Sr. This gives the residue a lower model age than the whole rock both by increasing the $\text{Rb}\text{Sr}$ ratio and by decreasing the $\text{Sr}{}^{87}\text{Sr}{}^{86}$ ratio. This result is in contrast to that expected from studies of ordinary chondrite finds, which generally show higher model ages than falls. The constancy of $\text{K}\text{Rb}$ and $\text{K}\text{Sr}$ ratios between the two leaching experiments, and their difference from the unaltered whole rock ratios suggest that the bulk ratios are produced by dissolution of a single phase, and the higher radiogenic Sr content by selective leaching of other phases.     

Determination of the 87Rb decay constant

The decay constant ⁸⁷Rb has been redetermined by measuring the amount of radiogenic ⁸⁷Sr produced over a period of 19 years, in 20 g samples of purified RbClO₄, using isotope dilution techniques. The rubidium sample was spiked with ⁸⁴Sr and the nanogram quantities of strontium separated by coprecipitation with Ba(NO₃)₂. Analyses were carried out on a 25cm, 90° sector mass spectrometer equipped with a Spiraltron electron multiplier. Measurement of three independent ratios permitted continuous monitoring of the ion beam fractionation. The average of nine determinations gives a value for the decay constant of $\text{1.419(±0.012) × 10}{}^{\mathrm{?11}}\text{yr}{}^{\mathrm{?1}}\text{(2σ)}$. [$\text{τ}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 4.89(±0.04) × 10}{}^{10}\text{yr}$.]     

The RbSr isotope system as an index of origin and diagenetic evolution of southern Pacific red clays

Morphological, mineralogical, chemical and RbSr isotopic studies have been made on Fesmectites (nontronites) from southern Pacific red clays cored near the Marquisas Islands. These minerals have at the top of the core, an ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.70917 ± 0.00007, which indicates an authigenic origin in isotopic equilibrium with seawater. Weak leaching experiments with 1N HCl show that the nontronites also contain a volcanic component with a lower ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio which, combined with the morphology of the particles, suggests a transportation by bottom currents of clay formed elsewhere.During burial, the nontronites experience diagenetic modifications resulting in an increase in Fe, K and Rb contents, a concomitant decrease of Mg, Ca, Ti, Na and Sr, and a preferential migration of radiogenic ⁸⁷Sr from the clays into the surrounding pore waters.The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of the Sr adsorbed on the outermost surfaces of the nontronites does not change with depth in the core, and is, therefore, independent of diagenetic influence, which is rather characterized by the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the interstitial waters. The isotopic composition of both the adsorbed Sr and that of the pore fluids may yield useful information on the crystallization environment and subsequent history of deep sea red clays.     

Détermination de 87Sr86Sr dans les andésites cénozoïques et les laves associées de Sardaigne Nord occidentale (Italie)

Initial isotopic ratios of strontium have been determined in some calc-alkali rocks of Sardinia. The values of these ratios are from 0.7044 and 0.7047 in two basalts, and from 0.7063 to 0.7081 in five andesites. A dacite and a rhyolite have intermediate values of 0.7056 and 0.7058.It is suggested that increase of radiogenic strontium in andesite is most likely related to a contamination process involving water expelled from an underthrusting plate. This water also contributes to the increase in the potassium and related trace element contents in andesites.In spite of the${}^{87}\text{Sr}{}^{86}$Sr difference between basalt and andesite, a cogenetic relationship between these two kinds of rocks cannot be precluded     

Strontium isotope ratios in volcanic rocks from the northwestern part of the Hellenic arc

${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios have been determined in fifteen volcanic rocks from the northwestern part of the Hellenic arc. They range from 0.7041 to 0.7134. There is no apparent correlation of strontium isotope values with any major chemical component or with Rb/Sr ratios. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios appear to increase in a general way with increasing depth to the Benioff zone. The strontium isotope ratios are higher than from most island arcs; this is believed to be due to contamination.     

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.     

Rb—Sr and Sm—Nd geochronology of lower Proterozoic granite—greenstone terrains in French Guiana,South America

Nine samples of metavolcanic rock from the lower parts of greenstone belts in central French Guiana (the Paramaca series) and 14 granitic samples from the intrusive gneisses (the Degrad Roche and Arawa gneisses) were selected for Sm—Nd and Rb—Sr analysis.The Sm—Nd results from the metavolcanic series (including two tholeiites, five peridotitic komatiites and two andesites) yield an isochron age of 2.11±0.09 (2 σ) Ga with an initial ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratio (I_Nd) of 0.51002±9 (2 σ), corresponding to ?_Nd(T) = + 2.1 ± 1.8. This isochron is interpreted as representing the age of initial volcanism of the Paramaca series. Acid intrusives were dated by the Rb—Sr method. A whole rock Rb—Sr isochron, including data points from both the Degrad Roche and Arawa gneisses, yields an age of 2.00±0.07 (2 σ) Ga with initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio (I_Sr value) of 0.7019±4 (2 σ). This result is considered to be the time of emplacement of the orthogneiss protoliths.The positive ε_Nd value (+ 2.1 ± 1.8) obtained from the metavolcanic rocks of French Guiana suggests that their mantle sources have evolved in reservoirs slightly depleted in Light Rare Earth Elements (LREE). This result confirms the possible existence of ancient LREE-depleted reservoirs within the lower Proterozoic mantle. Moreover, the high ε_Nd(T) value for these rocks excludes any significant crustal contamination during magma genesis.The French Guianese orthogneisses yield a low I_Sr value (0.7019±4 (2 σ)) which, together with geochemical considerations, suggests that their granitic protoliths could have originated by partial melting of short-lived crustal precursors of basaltic to granodioritic composition.The present geochronological and isotopic study suggests that the Guiana Shield may represent a major continental accretion event during the lower Proterozoic.     

RbSr study of dolerite dikes and psammite from the Western Gneiss region of Norway

The Saetra Nappe, consisting of feldspathic psammite cut by pre-tectonic dolerite dikes, is a distinctive tectonostratigraphic unit of the Western Gneiss Region of Norway. Although the rocks were metamosphosed at amphibolite facies, RbSr whole-rock sf three well preserved dikes a date of $\text{745 ± 37}\text{Ma}$ with initial ${}^{87}\text{Sr}\text{/}{}^{86}\text{Sr}\text{= .7046}$, interpreted to record the dike intrusion. Analysis of small psammite samples collected in rows parallel to the foliation and in columns perpendicular to it, indicate that the RbSr isotopic systems were not equilibrated on the whole-rock scale. The Strata Nappe provides evidence of a strong Caledonian deformation/metamorphic event in the Western Gneiss Region of Norway.     

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.     

RbSr and KAr ages of paleozoic glauconites from Ohio—Indiana and Missouri,U.S.A.

Glauconite pellets from the Lower Silurian Brassfield Formation on both limbs of the Cincinnati Arch in Ohio and Indiana give a RbSr age of 370 ± 11 Ma, which is substantially younger than the stratigraphic age of the formation. The age is concordantcwith conventional KAr ages of 355 ± 6 and 368 ± 5 Ma for two of the same glauconites. Concordant ages were also obtained from the Viburnum Trend in Missouri, where glauconite pellets from the Davis Formation in an ore-zone collapse structure into the underlying Bonneterre Formation give a RbSr age of 387 ± 21 Ma and conventional KAr ages of 368 ± 5 and 369 ± 5 and 369 ± 5 Ma. A third suite of glauconite from the Bonneterre Formation in the Old Lead Belt ～ 10 km from the nearest ore body has given a RbSr age of 423 ± 7 Ma and slightly older conventional KAr ages of 434 ± 6, 445 ± 6 and 441 ± 11 Ma.Because these glauconite-bearing rocks have been buried to depths of less than 1 km, thermal resetting of the RbSr and KAr systematics appears unlikely. The initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the RbSr isochrons are similar to the ratios for vein- and vug-filling dolomite and calcite. This is consistent with resetting of the RbSr and KAr systems during diagenetic changes which included the isotopic equilibration (perhaps by cation exchange) of the Sr in the glauconite with that in the diagenetic and Mississippi Valley-type ore fluids. This interpretation implies that the age of the Mississippi Valley-type mineralization in the Viburnum Trend is Devonian rather than Carboniferous—Permian as has been inferred from paleomagnetic measurements.Cation-exchange experiments with a dilute Sr-bearing solution and an artificial oilfield brine indicate that glauconite adsorbs large amounts of Sr, some of which is sufficiently strongly attached to the glauconite lattice as to resist leaching with ammonium acetate. The introduction of this strongly attached Sr may be the first step in the resetting of the RbSr systematics of glauconite by cation exchange.     

The nature and origin of geochemical variation in Mid-Atlantic Ridge basalts from the Central North Atlantic

Seventy-two basalts from 58 dredge stations located along the Mid-Atlantic Ridge from 29°N to 59°N have been analyzed for ${}^{87}\text{Sr}{}^{86}\text{Sr}$ and for K, Rb, Cc, Sr and Ba. The Sr-isotope profile along the ridge has three distinct maxima, one coinciding with the Azores platform (0.70345), one at 45°N (0.70340) and the third at 35°N, in the vicinity of the Oceanographer Fracture Zone. Basalts from ridge segments between 29°N and 33°N, and 49°N and 59°N have ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios typical of ‘normal’ mid-ocean ridge basalts (0.70230–0.70280). Profiles of K, Rb, Cs, Sr, Bz, Rb/Sr and Ba/Sr are similar to the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ profile, but Rb/K, Cs/K and Ba/K show broad maxima between 35°N and 45°N.These variations result from chemical and isotopic heterogeneity in the mantle, and are interpreted as caused by a mantle plume beneath the Azores which mixes with the LIL-element-depleted asthenosphere. Additional plumes may exist beneath 45°N and 35°N.Compared to the LIL-element-depleted asthenosphere, the Azores mantle plume is 10 to 30 times enriched in LIL elements with very small (? 0.1) bulk crystal/melt partition coefficients (Rb, Cs, Ba, La). Mildly incompatible elements (0.1 < D < 1) (Sr, Sm, Yb) are only 0.8–3 times enriched. These, observations suggest that LIL element differences between these two mantle reservoirs resulted from processes involving solid-liquid equilibria and not vapor-solid or vapor-liquid equilibria. Isotope systematics indicate that neither mantle reservoir remained a closed system since the formation of the Earth, but it is not possible to determine the time at which heterogeneity first developed.     

A study of strontium in core 119K,Discovery deep,Red Sea

The isotopic composition of strontium of pore water and of authigenic minerals leached from the sediment of core 119K with hot aqua regia is similar to that of the brine in the Discovery deep and differs from that of normal seawater. The average ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of strontium removed by acid leaching is 0.7077 ± 0.0007 (1σ) compared to a value of 0.70904 for the Red Sea. The detrital silicate fraction exhibits an approximate inverse correlation between ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios and strontium concentrations which provides tentative support for a model in which the detrital silicate fraction of deep-sea sediment is considered to be a mixture of terrigenous dust of sialic composition enriched in radiogenic ⁸⁷Sr and of volcanogenic material of basaltic composition and low ⁸⁷Sr abundance. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the shells of foraminifers and pteropods, expressed as δ ⁸⁷Sr‰ relative to 0.70904 for seawater, decrease from $\text{?0.23 ± 0.17‰}$ at 90 cm to $\text{?0.82 ± 0.17‰}$ at 273 cm and remain constant at this value to a depth of 450 cm. The lowering of the δ ⁸⁷Sr values is attributed both to the presence of aragonite overgrowths on pteropod shells and to possible isotope exchange with strontium in the connate fluid.     

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.     

207Pb206Pb ages of individual mineral phases in Luna 20 material by ion microprobe mass analysis

Ion microprobe analyses of returned lunar material have helped to demonstrate that U, Th and radiogenic Pb are concentrated in small accessory mineral phases. It is possible to measure the isotopic composition of this Pb and obtain a radiometric ${}^{207}\text{Pb}{}^{206}\text{Pb}$ age for the mineral. The ages so derived compare favorably with crystallization ages determined by conventional methods. A grain mount ($\text{22003,2}\text{6}$) of Luna 20 material was searched for such accessory mineral phases and two were found. One of these phases gives an age of 4.12 ± 0.04 b.y. and the other an age of 4.42 ± 0.11 b.y. Ages of minerals dated by the ion probe in Apollo samples 14310 and 15555 are given for comparison. Data on the upper limit for Pb concentration in the outermost surface layers of free lunar soil particles are also given.     

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.     

3He4He ratio,noble gas abundance and K-Ar dating of diamonds—An attempt to search for the records of early terrestrial history

The ${}^3\text{He}{}^4\text{He}$ ratios measured in 27 Southern Africa diamond stones, four from Premier Mine and the rest of unidentified origin, range from 4.2 × 10^?8 to 3.2 × 10^?4, with three stones above 1 × 10^?4. We conclude that the initial helium isotopic ratio (${}^3\text{He}{}^4\text{He)}{}_0$ in the earth was significantly higher than that of the planetary helium-A (${}^3\text{He}{}^4\text{He = 1.42 × 10}{}^{\mathrm{?4}}$), but close to the solar helium (${}^3\text{He}{}^4\text{He ? 4 × 10}{}^{\mathrm{?4}}$).The apparent K-Ar ages for the twelve diamonds of unidentified origin show enormously old age, indicating excess argon-40. ${}^3\text{He}{}^4\text{He}$ evolution in diamonds suggests that the diamonds with the high ${}^3\text{He}{}^4\text{He}$ ratio (>2 × 10^?4) may be as old as the earth.Noble gas elemental abundance in the diamonds relative to the air noble gas abundance shows monotonie decrease with a decreasing mass number.This paper discusses the implications of these observations on the early solar system and the origin of diamonds.     

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.