大别山榴辉岩的氦、氩同位素组成及其岩石成因

论述了石榴石和绿辉石的氦、氩同位素地球化学特征,并讨论了大别山榴辉岩的成因。测得3 He/4He 值为(1-19 ～4-63) ×10 - 7 ,40Ar/36Ar 值为1209 ～4416 ,4 He/40Ar 值分布在0-07 ～1-39 范围内,3 He/36 Ar 值的变化范围为(0-59 ～2-85) ×10 - 4 。大别山榴辉岩中保留了原始的稀有气体同位素, 但是退变质作用使氦同位素优先丢失。氦、氩同位素地球化学资料表明,榴辉岩可能形成于亏损的地幔或者是一种亏损型“地幔岩”中     

Factors controlling the noble gas abundance patterns of deep-sea basalts

A number of processes may modify the noble gas composition of silicate liquids so that the composition of noble gases observed in glassy margins of deep-sea basalts is not that of the upper mantle. Differential solubility enhances the light noble gases relative to the heavier gases; however, we demonstrate that the observed abundance pattern cannot be attributed to solubility of noble gases with atmospheric proportions. Partial melting and fractional crystallization increase the noble gas content of all species relative to mantle concentrations, but do not fractionate their relative abundances. Noble gases may be lost from an ascending magma in various ways, the most important, however, may be exclusion of gas from crystals forming at the time of solidification, which is shown to result in marked loss of gas from the basalt. Small amounts of low-temperature alteration of solidified basalt can produce dramatic changes in the noble gas abundance pattern, since the adsorption coefficients for the different noble gas favor uptake of heavy species relative to the light species. Atmospheric contamination can account for observed variations in the ⁴⁰Ar/³⁶Ar ratio of oceanic basalts. The degree of crystallinity of glassy margins of deep-sea basalts may control the helium abundance of these samples; however, the uniform ³He/⁴He values reported apparently reflect a relatively constant proportion of radiogenic and primordial helium in the mantle.     

The application of isochron diagrams in40Ar-39Ar dating: A discussion

The presence of a non-radiogenic argon component within rocks and minerals at the time of their crystallization (termed initial argon) is now widely recognized. Present data indicate that the³⁶Ar concentration of this initial argon is in the order of 10^?9 cm³/g STP in a wide range of geological materials. In⁴⁰Ar-³⁹Ar analyses the preference for small samples and division of the gas evolved into a number of temperature steps means that the amount of initial³⁶Ar analyzed is often less than the argon extraction line blank. Thus if blank corrections are not made an isochron plot will represent mixing lines between atmospheric argon and the argon derived from the sample. In this case the isochron parameters will probably be in error. Secondary alteration of samples adds atmospheric argon not related to the initial argon present in a rock at the time of its formation and must be eliminated or avoided if the isochron technique is to be used. The inability to obtain isochrons for samples with excess radiogenic argon may be related to significant extraction system contamination. At present the application of the two-error regression technique is being misapplied in determining the quality of fit and in some cases underestimating the error limits assigned to the isochron parameters.     

Helium accumulation in groundwater, III. Limits on helium transfer across the mantle-crust boundary beneath Australia and the magnitude of mantle degassing

The groundwaters of the Great Artesian Basin (Australia) have been previously shown to be accumulating in-situ production helium for groundwaters ages < 50 kyr and an external helium flux equivalent to whole crustal production for groundwater ages > 100 kyr [1,2]. New helium isotope measurements show that the observed in-situ production helium (³He/⁴He 1.6 × 10⁻⁸) is isotopically distinct from the crustal degassing helium flux (³He/⁴He 6.6 × 10⁻⁸). Furthermore, the crustal degassing helium isotope ratio is marginally in excess of the whole crustal production ratio (³He/⁴He= 3.5 × 10⁻⁸) and the production ratio in a variety of continental rock types. This suggests that the upper limit on volatile transport across the mantle-crust boundary beneath the (relatively) stable and “complacent” Australian continent can be characterized by a “conductive-diffusive” helium/heat flux ratio of 2.6 × 10⁶⁴He atoms mW⁻¹ s⁻¹ which is two orders of magnitude less than the “intrusive-volcanic” ratio of 2.9 × 10⁸⁴He atoms mW⁻¹ s⁻¹ measured at the Galapagos [16]. These results constrain the transcrustal mantle degassing fluxes of⁴He and⁴⁰Ar to be much less than the mid-ocean ridge degassing fluxes; which are much less than the degassing of⁴He and⁴⁰Ar from continental crust. Thus, the degassing of the Earth's interior is dominated by magmatic processes but the dominant fluxes of⁴He and⁴⁰Ar to the atmosphere must come from the continental crust.     

He, Ne and Ar isotope compositions of fluid inclusions in hydrothermal sulfides from the TAG hydrothermal field Mid-Atlantic Ridge

Helium, neon and argon isotope compositions of fluid inclusions have been measured in hydrothermal sulfide samples from the TAG hydrothermal field at the Mid-Atlantic Ridge. Fluid-inclusion³He/⁴He ratios are 2.2—13.3 times the air value (Ra), and with a mean of 7.2 Ra. Comparison with the local vent fluids (³He/⁴He=7.5—8.2 Ra) and mid-ocean ridge basalt values (³He/⁴He=6—11 Ra) shows that the variation range of³He/⁴He ratios from sulfide-hosted fluid inclusions is significantly large. Values for²⁰Ne/²²Ne are from 10.2 to 11.4, which are significantly higher than the atmospheric ratio (9.8). And fluid-inclusion⁴⁰Ar/³⁶Ar ratios range from 287 to 359, which are close to the atmospheric values (295.5). These results indicate that the noble gases of fluid inclusions in hydrothermal sulfides are a mixture of mantle- and seawater-derived noble gases; the partial mantle-derived components of trapped hydrothermal fluids may be from the lower mantle; the helium of fluid inclusions is mainly from upper mantle; and the Ne and Ar components are mainly from seawater.     

Rare gas isotopic compositions in natural gases of Japan

Isotopic and elemental compositions of rare gases in various types of gas samples collected in the Japanese Islands were investigated. Excess³He was found in most samples. Many samples showed a regionally uniform high³He/⁴He ratio of about 7 times the atmospheric ratio. The He concentrations varied from 0.6 to 1800 ppm, and they were low in CO₂-rich gases and high in N₂-rich gases. Ne isotopic deviations from the atmospheric Ne were detected in most volcanic gases. The deviations and the elemental abundance patterns in volcanic gases can be explained by a mixing between two components, one is mass fractionated rare gases and the other is isotopically atmospheric and is enriched in heavy rare gas elements. Ar was a mixture of mass fractionated Ar, atmospheric Ar and radiogenic Ar, and the contribution of radiogenic⁴⁰Ar was small in all samples. Except for He, elemental abundance patterns were progressively enriched in the heavier rare gases relative to the atmosphere. Several samples were highly enriched in Kr and Xe relative to the abundance pattern of dissolution equilibrium of atmospheric rare gases in water. The component which is highly enriched in heavy rare gases may be released from sedimentary materials in the crust.     

The evolution of excess argon in alpine biotites — A40Ar-39Ar analysis

Alpine biotites containing excess⁴⁰Ar have been analysed by step-heating argon analysis of both neutron irradiated and unirradiated samples. In addition to age spectra the data are discussed in terms of the thermal release of⁴⁰Ar,³⁹Ar,³⁷Ar and³⁶Ar and also displayed on a correlation plot of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar which is used to interpret the data and present a model of isotopic evolution during metamorphic cooling. This diagram overcomes misleading complications of isochron plots. The samples exhibit the following argon systematics: (1) flat age spectra for 80–90%³⁹Ar release with anomalously old ages but early gas fractions that approximate the accepted cooling ages; (2) each sample shows decreasing³⁶Ar/⁴⁰Ar with increasing temperature of heating step with three samples having a negative correlation of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar and one a positive correlation; (3) there appear to be two³⁶Ar components, one released at high temperatures and correlated with radiogenic⁴⁰Ar and one released at low temperatures which is not correlated with radiogenic⁴⁰Ar; and (4) there is no significant effect of neutron irradiation on the release of⁴⁰Ar and³⁶Ar.Interpretation suggests that these biotites contain a record of the evolution and isotopic composition of ambient argon retained within the metamorphic host rocks during cooling. After incorporation of argon of high⁴⁰Ar/³⁶Ar another argon component, of atmospheric composition, was retained at lower temperature and argon partial pressures.     

Noble gases in submarine pillow volcanic glasses

Fifteen submarine glasses from the East Pacific Rise (CYAMEX), the Kyushu-Palau Ridge (DSDP Leg 59) and the Nauru Basin (DSDP Leg 61) were analysed for noble gas contents and isotopic ratios. Both the East Pacific Rise and Kyushu-Palau Ridge samples showed Ne excess relative to Ar and a monotonic decrease from Xe to Ar when compared with air noble gas abundance. This characteristic noble gas abundance pattern (type 2, classified by Ozima and Alexander) is interpreted to be due to a two-stage degassing from a noble gas reservoir with originally atmospheric abundance. In the Kyushu-Palau Ridge sample, noble gases are nearly ten times more abundant than in the East Pacific Rise samples. This may be attributed to an oceanic crust contamination in the former mantle source.There is no correlation between the He content and that of the other noble gas in the CYAMEX samples. This suggests that He was derived from a larger region, independent from the other noble gases.Except where radiogenic isotopes are involved, all other noble gas isotopic ratios were indistinguishable from air noble gas isotopic ratios. The³He/⁴He in the East Pacific Rise shows a remarkably uniform ratio of (1.21±0.07)×10^?5, while the⁴⁰Ar/³⁶Ar ranges from 700 to 5600.     

Noble gas elemental and isotopic abundances in deep-sea trenches in the western Pacific

Noble gas elemental and isotopic abundances were measured in seven deep-sea water samples from five different sampling sites in the Nankai Trough, the Japan Trench and the Kuril Trench. The samples were obtained by the manned submersible “Nautile”. Most of the sampling sites are associated with clam colonies and/or fluid venting. Excesses both in³He/⁴He ratio and He concentration are observed in a seawater sample collected a few kilometers off the clam colonies which were found at a depth of 3830 m at the mouth of the Tenryu Canyon. Concentrations of noble gases (Ne, Ar, Kr and Xe) in this sample show progressive depletion from Ne to Xe relative to those in 1°C air-saturated seawater, which can be attributed to mixing of hot water ( 15°C) with cold ambient water ( 1°C). Isotopic compositions of Ne, Ar, Kr and Xe in this sample are atmospheric. These observations may reflect venting of hot pore water around the Tenryu Canyon. All the other samples show a significant excess in concentration of all noble gases relative to 1°C air-saturated seawater and the isotopic compositions are atmospheric. This excess of noble gas concentrations may appear to be air contamination in the samples. However, results of hydrocarbon analyses of the Kaiko samples imply that such large amount of air contamination is improbable. Decomposition of gas hydrate in deep-sea sediments is a more likely explanation for the observed excess of noble gas concentration.     

The noble gas isotope geochemical composition of chert at the bottom of Cambrian in Tarim Basin,China

In the Tarim Basin, black shale series at the bottom of Cambrian is one of the important marine facies hydrocarbon source rocks. This research focuses on the analysis of the isotope of noble gas of 11 cherts. The R/R _a ratio of chert in the Keping area is 0.032–0.319, and ⁴⁰Ar/³⁶Ar is 338–430. In Quruqtagh the R/R _a ratio is 0.44–10.21, and ⁴⁰Ar/³⁶Ar is 360–765. The R/R _a ratio of chert increases with ⁴⁰Ar/³⁶Ar from the west to the east accordingly. They have evolved from the crust source area to the mantle source area in a direct proportion. Surplus argon ⁴⁰Ar_E in chert is in direct proportion to the R/R _a ratio, indicating that it has the same origin of excess argon as in fluid and mantle source helium. Comparison of the R/R _a ratios between the west and the east shows that the chert in the eastern part formed from the activity system of the bottom hydrothermal venting driven by the mantle source, where the material and energy of crust and mantle had a strong interaction in exchange; whereas in the western part, chert deposited from the floating of hydrothermal plume undersea bottom, which is far away from the centre of activities of the hydrothermal fluid of ocean bottom. In addition, from noble gas isotope composition of chert, it is suggested that the ocean anoxia incident happened at the black shale of the Cambrian bottom probably because of the large-scaled ocean volcanoes and the following hydrothermal activities.

     

Noble gas anomalies related to high-intensity methane gas seeps in the Black Sea

Dissolved noble gases and tritium were analyzed at a series of high-intensity methane gas seeps in the Black Sea to study the transport and gas exchange induced by bubble-streams in the water column. These processes affect marine methane emissions to the atmosphere and are therefore relevant to climate warming. The seep areas investigated are located in the Dnepr paleo-delta, west of Crimea, and in the Sorokin Trough mud volcano area, south-east of Crimea. Noble gas concentration profiles at active seep sites revealed prominent anomalies compared to reference profiles that are unaffected by outgassing. Supersaturations of the light noble gases helium and neon observed relatively close to the sea floor are interpreted as effects of gas exchange between the water and the rising bubbles. Depletions of the heavy noble gases argon, krypton and xenon that were detected above an active, bubble-releasing mud volcano appear to be related to the injection of fluids depleted in noble gases that undergo vertical transport in the water column due to small density differences. In both cases, the noble gas anomalies clearly document seep-specific processes which are difficult to detect by other methods. Helium is generally enriched in the deep water of the Black Sea due to terrigenic input. Although exceptionally high helium concentrations observed in one seep area indicate a locally elevated helium flux, most of the seeps studied seem to be negligible sources of terrigenic helium. Noble gas analyses of sediment pore waters from the vicinity of a mud volcano showed large vertical gradients in helium concentrations. The helium isotope signature of the pore waters points to a crustal origin for helium, whereas the deep water of the Black Sea also contains a small mantle-type component.     

Noble gas systematics for coexisting glass and olivine crystals in basalts and dunite xenoliths from Loihi Seamount

Noble gas isotopes including ³He/⁴He, ⁴⁰Ar/³⁶Ar and Xe isotope ratios were determined for coexisting glass and olivine crystals in tholeiitic and alkalic basalts and dunite xenoliths from Loihi Seamount.Glass and coexisting olivine crystals have similar ³He/⁴He ratios (2.8–3.4) × 10^?5, 20 to 24 times the atmospheric ratio (R_A), but different ⁴⁰Ar/³⁶Ar ratios (400–1000). Based on the results of noble gas isotope ratios and microscopic observation, some olivine crystals are xenocrysts. We conclude that He is equilibrated between glass and olivine xenocrysts, but Ar is not.The apparent high ³He/⁴He ratio (3 × 10^?5; = 21 R_A) coupled with a relatively high ⁴⁰Ar/³⁶Ar ratio (4200) for dunite xenoliths (KK 17-5) may be explained by equilibration of He between MORB-type cumulates and the host magma.Except for the dunite xenoliths, noble gas data for these Loihi samples are compatible with a model in which samples from hot spot areas may be explained by mixing between P (plume)-type and M (MORB)-type components with the addition of A (atmosphere)-type component.Excess ¹²⁹Xe has not been observed due to apparent large mass fractionation among Xe isotopes.     

Anomalous changes of CH4/Ar ratio in subsurface gas bubbles as seismogeochemical precursors at Matsuyama,Japan

The results of seismogeochemical monitoring of subsurface gases associated with ground-waters from deep artesian wells in Matsuyama, Japan, are presented. The weekly collections of bubble-gas samples from the two borehole wells and gas chromatographic determinations of their He/Ar, H₂/Ar, N₂/Ar, and CH₄/Ar ratios for the recent nineteen months have revealed that the bubble-gas CH₄/Ar ratios at the two stations are correlated with the seismic activity in this area. Positive anomalies of the CH₄/Ar ratio were accompanied by five earthquakes with relatively large magnitudes and/or relatively small epicentral distances among thirty earthquakes that occurred during the monitoring period. The anomalous increases in the CH₄/Ar ratio had evidently begun prior to the respective earthquakes.The occurrence of the geochemical anomaly appears to depend strongly upon the earthquake magnitude and distance, because the earthquakes that accompanied the anomalies and those that did not can be reasonably distinguished in the plot of magnitude vs. distance. Similar anomalies can be also seen in the variations of He/Ar and N₂/Ar ratios, but their magnitudes are much smaller than those of the CH₄/Ar ratio. These features have been discussed in relation to the possible mechanism of the seismogeochemical phenomena.     

Thermal release characteristics of spallogenic He,Ne, and Ar from the Carbo iron meteorite

The thermal release patterns of He, Ne and Ar from samples of the Carbo iron meteorite show that virtually no fractionation of³He,⁴He,²¹Ne and³⁸Ar occurs. Thus, conclusions about iron meteorites based on measured noble gas ratios will be unaffected by gas loss, and measurement of these ratios cannot yield information about possible loss. Further, noble gas loss cannot explain the abnormal elemental and isotopic patterns observed in some iron meteorites, notably hexahedrites.     

Dating young MORB of the Central Indian Ridge (19°S): Unspiked K-Ar technique limitations versus 40Ar/39Ar incremental heating method

We have applied the unspiked K-Ar and the ⁴⁰Ar/³⁹Ar methods to samples precisely collected and localised, on both Central Indian Ridge flanks, to test their effectiveness and reliability when applied to the dating of recent (i.e. less than 1 Ma) MORBs. Twenty six samples) from the sixty five samples collected every ∼500 m up to the Brunhes-Matuyama boundary on both ridge flanks, were selected based on their distance from the ridge axis. Therefore, we can evaluate whether the isotopic ages are a good indicator of the crystallisation age by considering their geographic position with respect to the ridge axis (zero age) and the B/M magnetic boundary. Direct comparison of the isotopic and model ages shows that only 9 out of 26 samples were successfully dated. The GIMNAUT – MORB's test case amply demonstrates that the unspiked K-Ar technique, when applied to submerged volcanic samples, is subject to potentially defective assumptions of trapped atmospheric argon, excess/fractionated argon and extremely sensitive to alteration. Although the unspiked K-Ar technique is theoretically capable to produce high precision ages, the comparison with the ⁴⁰Ar/³⁹Ar techniques reveals that only 15% (i.e. 4 samples out of 26) of the ages obtained here are geologically meaningful. Five of the seven ⁴⁰Ar/³⁹Ar incremental heating experiments provide meaningful ages. Because potential sources of systematic errors such as excess ⁴⁰Ar*, recoil of ³⁹Ar_K and ³⁷Ar_Ca can be identified and because effects of alteration are significantly reduced by the pre-heating of the samples up to 500–600°c, the ⁴⁰Ar/³⁹Ar incremental heating method appears to be the method of choice to date MORBs.     

Rare gas constraints on hydrocarbon accumulation, crustal degassing and groundwater flow in the Pannonian Basin

The isotopic composition and abundances of He, Ne and Ar have been measured in a sequence of vertically stacked gas reservoirs at Hajduszoboszlo and Ebes, in the Pannonian Basin of Hungary. The gas reservoirs occur at depths ranging from 727 to 1331 m, are CH₄ dominated and occupy a total rock volume of approximately 1.5 km³. There are systematic variations in both major species abundances and rare gas isotopic composition with depth: CO₂ and N₂ both increase from 0.47 and 1.76% to 14.1 and 30.5%, respectively, and ⁴⁰Ar/³⁶Ar and ²¹Ne/²²Ne increase systematically from 340 and 0.02990 at 727 m to 1680 and 0.04290 at 1331 m. A mantle-derived He component between 2 and 5% is present in all samples, the remainder is crustal-radiogenic He. The Ar and Ne isotope variations arise from mixing between atmosphere-derived components in groundwater, and crustally produced radiogenic Ar and Ne. The atmosphere-derived ⁴⁰Ar and ²¹Ne decreases from 85 and 97% of the total ⁴⁰Ar and ²¹Ne at 727 m to 18 and 68% at 1331 m. The deepest samples are shown to have both atmosphere-derived and radiogenic components close to the air-saturated water and radiogenic production ratios. The shallowest samples show significant fractionation of He/Ar and Ne/Ar ratios in atmosphere-derived and radiogenic rare gas components, but little or no fractionation of He/Ne ratios. This suggests that diffusive fractionation of rare gases is relatively unimportant and that rare gas solubility partitioning between CH₄ and H₂O phases controls the observed rare gas elemental abundances.The total abundance of atmosphere-derived and radiogenic rare gas components in the Hajduszoboszlo gas field place limits on the minimum volume of groundwater that has interacted with the natural gas, and the amount of crust that has degassed and supplied radiogenic rare gases. The radiogenic mass balance cannot be accounted for by steady state production either within the basin sediments or the basement complex since basin formation. The results require that radiogenic rare gases are stored at their production ratios on a regional scale and transported to the near surface with minimal fractionation. The minimum volume of groundwater required to supply the atmosphere-derived rare gases would occupy a rock volume of some 1000 km³ (assuming an average basin porosity of 5%), a factor of 670 greater than the reservoir volume. Interactions between groundwater and the Hajduszoboszlo hydrocarbons has been on a greater scale than often envisaged in models of hydrocarbon formation and migration.     

The noble gas isotope geochemical composition of chert at the bottom of Cambrian in Tarim Basin,China

In the Tarim Basin, black shale series at the bottom of Cambrian is one of the important marine facies hydrocarbon source rocks. This research focuses on the analysis of the isotope of noble gas of 11 cherts. The R/R _a ratio of chert in the Keping area is 0.032–0.319, and ⁴⁰Ar/³⁶Ar is 338–430. In Quruqtagh the R/R _a ratio is 0.44–10.21, and ⁴⁰Ar/³⁶Ar is 360–765. The R/R _a ratio of chert increases with ⁴⁰Ar/³⁶Ar from the west to the east accordingly. They have evolved from the crust source area to the mantle source area in a direct proportion. Surplus argon ⁴⁰Ar_E in chert is in direct proportion to the R/R _a ratio, indicating that it has the same origin of excess argon as in fluid and mantle source helium. Comparison of the R/R _a ratios between the west and the east shows that the chert in the eastern part formed from the activity system of the bottom hydrothermal venting driven by the mantle source, where the material and energy of crust and mantle had a strong interaction in exchange; whereas in the western part, chert deposited from the floating of hydrothermal plume undersea bottom, which is far away from the centre of activities of the hydrothermal fluid of ocean bottom. In addition, from noble gas isotope composition of chert, it is suggested that the ocean anoxia incident happened at the black shale of the Cambrian bottom probably because of the large-scaled ocean volcanoes and the following hydrothermal activities.     

Rare gases,water, and carbon in kaersutites

Kaersutites from Kakanui, New Zealand and from three localities in the southwestern United States have been analyzed for rare gases, water and carbon to investigate the volatile signature of the sub-continental mantle. This study does not confirm the high ³He/⁴He and ²¹Ne/²²Ne ratios reported by Saito et al. [1] for the Kakanui kaersutite. Instead, a ³He/⁴He ratio of 6 R_A and atmospheric ²¹Ne/²²Ne ratios were measured which are consistent with our current knowledge of the earth's mantle. A low ⁴⁰Ar/³⁶Ar of 320 and more than 10^?8 cm³/g of ³⁶Ar confirms the argon results of Saito et al. and indicates that significant quantities of ³⁶Ar reside in this portion of the mantle. Kaersutites from the southwestern United States (Arizona) have a heterogeneous helium isotope signature, ranging from 8.8 R_A at San Carlos to 0.46 at Hoover Dam. All D/H ratios for the water in kaersutites (?56‰ to ?78‰) represent typical mantle values with no apparent correlation with ³He/⁴He. The correlation of increasing carbon content (140–400 ppm) with increasing δ¹³C (?24.5‰ to ?16.7‰) may reflect differences in the proportions of oxidized and reduced carbon in these samples.     

Regional-Scale Excess Ar wave in a Barrovian type metamorphic belt, eastern Tibetan Plateau

Laser step heating ⁴⁰Ar/³⁹Ar analysis of biotite and muscovite single crystals from a Barrovian type metamorphic belt in the eastern Tibetan plateau yielded consistent cooling ages of ca. 40 Ma in the sillimanite zone with peak metamorphic temperatures higher than 600°C and discordant ages from 46 to 197 Ma in the zones with lower peak temperatures. Chemical Th‐U‐Total Pb Isochron Method (CHIME) monazite (65 Ma) and sensitive high mass‐resolution ion microprobe (SHRIMP) apatite (67 Ma) dating give the age of peak metamorphism in the sillimanite zone. Moderate amounts of excess Ar shown by biotite grains with ages of 46 to 94 Ma at metamorphic grades up to the high‐grade part of the kyanite zone probably represent incomplete degassing during metamorphism. In contrast, the high‐grade part of the kyanite zone yields biotite ages of 130 to 197 Ma. The spatial distribution of these older ages in the kyanite zone along the sillimanite zone boundary suggests they reflect trapped excess argon that migrated from higher‐grade regions. The most likely source is muscovite that decomposed to form sillimanite. The zone with extreme amounts of excess argon preserves trapped remnants of an ‘excess argon wave’. We suggest this corresponds to the area where biotite cooled below its closure temperature in the presence of an elevated Ar wave. Extreme excess Ar is not recognized in muscovite suggesting that the entrapment of the argon wave by biotite took place when the rocks had cooled down to temperatures lower than the closure temperature of muscovite. The breakdown of phengite during ultrahigh‐pressure (UHP) metamorphism may be a key factor in accounting for the very old apparent ages seen in many UHP metamorphic regions. This is the first documentation of a regional Ar‐wave spatially associated with regional metamorphism. This study also implies that resetting of the Ar isotopic systems in micas can require temperatures up to 600°C; much higher than generally thought.