首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Seven samples of the unique St. Mesmin meteorite have been analyzed by instrumental and radiochemical neutron activation analysis for Na, Ca, Sc, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, Se, In, Sm, Yb, Ir and Au. St. Mesmin is unique in being the only ordinary chondrite known to contain an unmelted xenolith of another ordinary chondrite. Data for two host matrix samples and three light clasts are consistent with their classification as LL chondrite material. The composition of the large dark xenolith confirms earlier evidence that it is an H chondrite; volatile abundances are consistent with it being highly shocked, petrologic type-4 material. In an olivine microporphyry, siderophile abundances are mostly about 0.13 times LL abundances, an apparent indication of metal loss during the shock melting which produced the clast. As in other regolithic chondrites, the dark host has higher contents of highly volatile elements than do the light clasts. We suggest that this results from a combination of differences in intensity of preexisting metamorphism as well as a redistribution of volatiles during regolith gardening.The H-group xenolith in St. Mesmin is a relatively recent addition to the parent body (< 1.4 Ga ago), but it is argued that this does not require regolith activity at that time. Rather the view is supported that the regolith period occurred very early in the meteorite's history (&gsim;4.0 Ga ago) and may have been related to the growth of the parent body. The H-group fragment may be part of the projectile whose impact excavated the St. Mesmin meteoroid from the LL parent body.  相似文献   

2.
Blithfield (EL6) is one of five known enstatite chondrite breccias. It consists of troilite-rich clasts (35 ± 5vol.%) embedded in an extremely metallic Fe,Ni-rich matrix (65 ± 5 vol.%) that contains metal nodules up to 17 mm in size. Clasts and matrix agglomerated independently in the solar nebula under conditions of high and lowpS2/pO2 ratios, respectively. The matrix accreted to an EL chondrite planetesimal and was metamorphosed to～ 1000°C, above the FeNiS eutectic; chondrule textures were obliterated. The S-rich eutectic melt was lost from the matrix. The matrix material was buried to a depth of at least 3 km; accreting troilite-rich material was incorporated into the matrix as clasts. The breccia cooled through～ 500°C at 1000–10,000°C/Myr. After cooling below～ 500°C, Blithfield was quenched, possibly by impact excavation from depth and deposition onto the surface.Clasts or inclusions that are enriched in sulfide and depleted in metallic Fe,Ni are common in brecciated enstatite chondrites. Variations in thepS2/pO2 ratio in the nebular regions where these materials formed may explain many of their petrologic properties. The silica-rich clasts in Adhi Kot (EH4) formed at very highpS2/pO2ratios(> 1027); niningerite, free silica and troilite were produced from the sulfurization of enstatite and metallic Fe. The troilite-rich clasts in Blithfield and Atlanta (EL6) as well as the troilite-rich regions of the Hvittis (EL6) matrix formed at somewhat lowerpS2/pO2 ratios where sulfurization of metalic Fe produced troilite. The Ni content of the residual metal increased, forming some metal of martensitic composition. The dark inclusions in Abee (EH 4), which contain up to 9 wt.% oldhamite, formed at highpS2/pO2 ratios in the presence of an additional Ca-rich component.  相似文献   

3.
The Marangudzi ring complex, Rhodesia, consists essentially of a gabbro mass intruded by ring dykes of quartz syenite and cone sheets of nepheline syenite. Five intrusive units (gabbro, two quartz syenite and two nepheline syenite units) have been studied using Rb-Sr and K-Ar methods. A total of 24 whole rock samples define a Rb-Sr isochron which gives an age of 186 ± 3m.y. and an initial (87Sr/86Sr)0 ratio of 0.70769 ± 0.00006 (±2sigma; based on λ = 1.42 × 10?11yr?1). K-Ar and Rb-Sr analyses on biotite and hastingsite separates are consistent with this age assignment. Whole rock Rb-Sr isochrons for the different units treated individually agree with the above age and initial Sr ratio within analytical uncertainties. This is believed to indicate that the different rock types are comagmatic forming by fractional crystallization of a parental, mantle-derived, K2O-rich basaltic magma, having an initial Sr ratio of 0.7077, without appreciable assimilation of the Precambrian country rock. The entire differentiation, emplacement and crystallization processes took place over a rather short time span.  相似文献   

4.
High resolution40Ar-39Ar age spectra have been measured on plagioclase and glass from two howardites. Both the plagioclase and glass from the gas-rich Bununu howardite show well-defined age plateaux, yielding distinct ages of 4.42 ± 0.04 and 4.24 ± 0.05 AE, respectively. These age patterns are rather well behaved and are interpreted as representing the distinct times of formation of plagioclase from igneous processes and of glass fragments produced by impact on the meteorite body. The release pattern for the glass from the heavily shocked Malvern howardite is undulating at low and intermediate temperatures but does have a high-temperature plateau. Its age spectrum indicates little apparent diffusion loss, but rather an extensive redistribution of either40Ar during the shock event or of39Ar during the neutron irradiation or both. The total K-Ar age of Malvern glass is 3.64 ± 0.04 AE and the high-temperature plateau is 3.73 ± 0.05 AE. The age spectrum of the Malvern plagioclase has an intermediate temperature “plateau” at 3.80 AE that represents 20% of the total40Ar content and increases towards a high-temperature plateau at 4.29 ± 0.04 AE containing 26% of the total gas release. It seems likely that the event which formed the Malvern glass also reset part of the plagioclase. The distinct histories observed for the different phases of these howardites are consistent with their formation from a regolith. The present results along with similar young ages for igneous clasts from Kapoeta clearly show that the regoliths were extant on the parent bodies of howardites and that they were subjected to violent impact events at least as recently as 3.7 AE ago.  相似文献   

5.
Whole rock and chondrules of the Dhajala chondrite were analyzed for Ne, Ar, Kr and Xe by total melting as well as by stepwise heating techniques. The cosmic ray exposure ages for the whole rock and the chondrules are6.2 ± 0.8 and6.3 ± 1.0m.y. as determined by the21Ne method and4.8 ± 1.5 and4.2 ± 2.0m.y. by the38Ar method, respectively. The K-Ar age of the whole rock is4.2 ± 0.4b.y. The elemental composition of the trapped gas in this chondrite is of “planetary” type. The radiogenic129Xe contents in the whole rock and chondrules are similar and this component is very retentively sited in the chondrules.  相似文献   

6.
K-Ar ages have been determined for sulfide minerals for the first time. The occurrence of adequate amounts of potassium-bearing sulfides with ideal compositions K3Fe10S14 (～10 wt.% K) and KFe2S3 (～16 wt.% K) in samples from a mafic alkalic diatreme at Coyote Peak, California, prompted an attempt to date these materials. K3Fe10S14, a massive mineral with conchoidal fracture, gives an age of 29.4 ± 0.5m.y.(40Ar/39Ar), indistinguishable from the 28.3 ± 0.4m.y.(40Ar/39Ar) and 30.2 ± 1.0m.y.8 (conventional K-Ar) ages obtained for associated phlogopite (8.7 wt.% K). KFe2S3, a bladed, fibrous sulfide, gives a younger age, 26.5 ± 0.5m.y.(40Ar/39Ar), presumably owing to Ar loss.  相似文献   

7.
The Abee E4 enstatite chondrite breccia consists of clasts (many rimmed by metallic Fe, Ni), dark inclusions and matrix. The clasts and matrix were well equilibrated by thermal metamorphism, as evidenced by uniform mineral compositions, recrystallized chondrules, low MnO content of enstatite and high abundance of orthoenstatite. The clasts acquired their metal-rich rims prior to this metamorphic episode. The occurrence in Abee of relatively unmetamorphosed dark inclusions, clasts with nearly random magnetic orientations and a matrix with a uniform magnetic orientation [18,19] indicates that clast and matrix metamorphism occurred prior to the agglomeration of the breccia.The dark inclusions are an unusual kind of enstatite chondritic material, distinguished from the clasts and matrix by their relative enrichments in REE [21–23], low relative abundances of kamacite, total metallic Fe, Ni and silica, lower niningerite/(total sulfide) ratios, high relative abundances of oldhamite and martensite, smaller euhedral enstatite, more heterogeneous enstatite and metallic Fe, Ni, more calcic enstatite and more nickeliferous schreibersite.We propose the following model for the petrogenesis of the Abee breccia: The maximum metamorphic temperature of breccia parent material was?- 840°C (the minimum temperature of formation of Abee niningerite) and perhaps near 950–1000°C (the Fe-Ni-S eutectic temperature). Euhedral enstatite crystals in metallic Fe, Ni- and sulfide-rich areas grew at these metamorphic temperatures into pliable metal and sulfide. Breccia parent material was impact-excavated from depth, admixed with dark inclusions and rapidly cooled (700 to 200°C in about 2 hours) [15]. During this cooling, clast and matrix material acquired thermal remanent magnetization. Random conglomeration of clasts and unconsolidated matrix materials caused the clasts to have random magnetic orientations and the matrix areas to have net magnetic intensities of zero (due to the cancellation of numerous randomly oriented magnetic vectors of equal intensity in the matrix). A subsequent ambient magnetic field imparted a uniform net magnetic orientation to the matrix and caused the magnetic orientations of the clasts to be somewhat less random. The Abee breccia was later consolidated, possibly by shock or by shallow burial and very long-period/low-temperature (< 215°C) metamorphism.  相似文献   

8.
Nitrogen contents and isotopic ratios have been determined for three clasts from the enstatite chondrite Abee by stepwise heating. The clasts possess a wide range in nitrogen content, ranging from 254 to 850 ppm, whereas the nitrogen isotopic ratios are nearly identical atδ15N= ?29.2±0.6‰. A refractory inorganic nitrogen-bearing phase contains about 90% of the nitrogen which is released at temperatures of 1000°C and above. The stepwise heating experiments suggest the possible existence of two other distinct nitrogen components, released at low (770°C) and high (1500°C) temperatures.  相似文献   

9.
Baogutu copper deposit in Western Junggar area is a mesoscopic porphyry deposit found in recent years. Study on its geochronology will help further understand ore genesis and regional ore-forming pattern. A series of small quartz-diorite and granodiorite stocks outcrop at Baogutu area, numbered I―X according to their size. A detailed exploration on Number V stock confirmed it as a mesoscopic scale copper deposit, and various exploration work has been carried out on other stocks with ore-forming evidence. Th...  相似文献   

10.
A combined geochronologic (K-Ar) and palaeomagnetic study has been conducted on a basalt lava sequence at Yixian Formation in Liaoning Province, northeastern China. The new K-Ar age obtained from thirteen lava flows is 120.93±0.88 Ma. Detailed rock-magnetic investigations were conducted on each lava flow to determine their remanence carriers. The modified version of the Thellier-Thellier palaeointensity method with systematic partial thermoremanent magnetization (pTRM) checks was used for the palaeointensity determination. Virtual dipole moment (VDM) value is (3.66±0.10)x1022 Am2. This low dipole-field intensity value is approximately forty-five percent of the today field VDM. Combined with all of the other published palaeointensity data, possible links between the earth’s interior process and its control on the variation of the earth’s magnetic field during the geological time were tentatively discussed.  相似文献   

11.
The reverse paleomagnetism of the lava flows of Laschamp and Olby, already discovered by Bonhommet and Babkine, is confirmed. The ages of these flows, measured by14C, thermoluminescence and K-Ar dating are respectively36,000 ± 4000 and42,000 ± 5000years. VGP's comparison of the “Laschamp event” with those of the 30,000-year Lake Mungo excursion does not show evidence of coincidence between these two events.  相似文献   

12.
We report on a40Ar-39Ar study of the Apollo 16 breccia 67435 and present ages of five samples representing matrix, lithic clasts and plagioclase clasts. While the matrix age spectrum does not have a well-defined plateau, the two lithic clasts gave plateau ages of 3.96 and 4.04 AE. Since all samples had apparent ages of ～1 AE in the fractions ≤600°C extraction temperature, the breccia might have been assembled in a rather mild process at about that time or even more recently out of material with different metamorphic ages. The two plagioclase samples, of which one was a single 9-mg mineral clast and the other a 15-mg composite of several clasts, also have ages of ～1 AE in the low-temperature release fractions, but are apparently undisturbed by any ～4-AE events since they both have well-defined plateaux at 4.42 AE. The age of these strongly calcic plagioclase clasts, believed to be remnants of the anorthositic lunar crust, establishes a lower age limit to the end of the early lunar differentiation and thus places a strong constraint to the lunar evolution.  相似文献   

13.
The Avanavero Dolerite intruding the Guiana Shield in western Suriname is a quartz gabbroic mass yielding a Rb-Sr isochron age of 1603 ± 27 Ma, while the mass has invaded basement with a Rb-Sr isochron age of 1810 ± 40 Ma (λ = 1.47 × 10?11/a; errors with 95% confidence level). From 61 K-Ar measurements on whole-rocks, sieve fractions of whole-rocks, density fractions and separated minerals it is evident that nearly all investigated samples, fractions and minerals (except microcline which suffered some argon loss) contain excess40Ar. This excess40Ar is very inhomogeneously distributed through the gabbroic mass. The bulk is stored in the plagioclase, in which mineral the excess40Ar attains unusually high values ranging from about 14 × 10?6 to around 150 × 10?6cm3 NTP/g. The excess40Ar contents of biotite, hornblende and the pyroxenes are comparable to published data.Two samples from a core drilling were studied in some detail. In graphs of radiogenic40Ar versus K, four mineral separates from one sample display a fairly good, but geochronologically meaningless linear arrangement roughly passing through the origin. Most density fractions from the other sample show a tendency towards an array parallel to the 1603 Ma reference isochron.It is assumed that the excess40Ar was acquired by the Avanavero Dolerite during a moderate tectonothermal event 1200 ± 100 Ma ago, when the40Ar partial pressure was much higher in the environmental basement complexes than in the basic mass.  相似文献   

14.
The palaeomagnetism of Middle Triassic (224 ± 5 m.y.) igneous rocks from the Ischigualasto-Ischichuca Basin (67°40′W, 30°20′S) was investigated through 86 oriented hand samples from 11 sites. At least one reversal of the geomagnetic field has been found in these rocks. Nine sites yield a palaeomagnetic pole at 239°E, 79°S (α95 = 15°, k = 13).The K-Ar age determinations of five igneous units of the Puesto Viejo Formation give a mean age of 232 ± 4 m.y. (Early Triassic). The palaeomagnetism of six igneous units of the Puesto Viejo Formation (68°W, 35°S) was investigated through 60 oriented samples. These units, two reversed relative to the present magnetic field of the Earth and four normal, yield a pole at 236°E, 76°S (α95 = 18°, k = 14).Data from the Puesto Viejo Formation indicate, for the first time on the basis of palaeomagnetic and radiometric data, that the Illawarra Zone, which defines the end of the Kiaman Magnetic Interval, extends at least down to 232 ± 4 m.y. within the Early Triassic. The palaeomagnetic poles for the igneous rocks of the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation form an “age group” with the South American Triassic palaeomagnetic poles (mean pole position: 239°E, 77°S; α95 = 6.6°, k = 190). The Middle and Upper Permian, Triassic and Middle Jurassic palaeomagnetic poles for South America would define a “time group” reflecting a quasi-static interval (mean pole position: 232°E, 81°S; α95 = 4°, k = 131).  相似文献   

15.
The Zerga meteorite, an LL6 ordinary chondrite found at Aouelloul crater in 1973, is a small fragment of a larger mass whose pre-atmospheric radius was most likely between 20 and 125 cm. A typical amphoterite, it is a monomict breccia that has undergone at least one recrystallization episode.3He and21Ne contents define a shielding-corrected, cosmic ray exposure age of 21–24 × 106 years and the26Al content is consistent with a terrestrial age ?500,000 years (2σ limit). The K—Ar gas-retention age is 3.1 × 109 years. The meteorite's areal association with the impact crater is merely coincidental. A new K-Ar age of the glassy impactite found at Aouelloul dates the crater at 3.1 ± 0.3 × 106 years, sensibly indistinguishable from the recently determined age of nearby Tenoumer crater (2.5 ± 0.5 × 106years). The similar ages of these two impact craters, and their almost perfect linear alignment with a third, morphologically similar crater (Temimichat Ghallaman) over a distance of 600 km, suggests a simultaneous triple impact occasioned by the disruption of a large meteorite moving on a very shallow atmospheric trajectory. If so, the concomitant low impact angles may be responsible for the unusually shallow original depths inferred for two of the craters from gravity data.  相似文献   

16.
The Blake excursion was among the first recognized with directional and intensity behavior known mainly from marine sediment and Chinese loess. Age estimates for the directional shifts in sediments are poorly constrained to about 118−100 ka, i.e., at the marine isotope stage (MIS) 5e/5d boundary. Moreover, sediments at Lac du Bouchet maar, France and along the Portuguese margin reveal what may be a "post-Blake" excursion at about 105−95 ka. The excursional directions are associated with a prominent paleointensity minimum between about 125 and 95 ka in global stacked records. Lava flow recordings of the Blake excursion(s) have, however, been questionable because precise ages required for correlation with these sediment records are lacking. To establish new, independent records of the Blake excursion, and link these into a larger Quaternary GITS, we have undertaken 40Ar/39Ar incremental heating and unspiked K-Ar experiments on groundmass from the transitionally magnetized Inzolfato flow on Lipari Island. We also obtained 40Ar/39Ar incremental heating results for a lava flow on Amsterdam Island originally thought to record the Mono Lake excursion and from the transitionally magnetized El Calderon basalt flow, New Mexico that was K-Ar dated by Champion et al. (1988) at 128 ± 66 ka.Unspiked K-Ar ages of four samples from the Inzolfato flow are 102.5 ± 4.7, 101.3 ± 3.3, 97.1 ± 2.6, and 96.8 ± 3.1 ka and thus indistinguishable from one another. 40Ar/39Ar results are more complex, with three samples yielding discordant age spectra. Based on incremental heating data obtained in both the UW-Madison and Gif-sur-Yvette 40Ar/39Ar laboratories, a fourth sample yields six concordant age plateaus and a weighted mean age of 105.2 ± 1.4 ka that we take as the best estimate of time since the flow erupted. Five 40Ar/39Ar incremental heating experiments on the Amsterdam Island lava yield a plateau age of 120 ± 12 ka, whereas ages from two sites in the Calderon flow are 112 ± 23 and 101 ± 14 ka, together giving a weighted mean of 104 ± 12 ka. The age of 120 ± 12 ka from Amsterdam Island, though imprecise, correlates with the Blake excursion. In contrast, the 104–105 ka age obtained from both Lipari and New Mexico indicates that these lavas record a younger period of dynamo instability, most probably associated with the post-Blake excursion. These radioisotopic ages are consistent with the astronomical ages of two paleointensity minima in the PISO-1500 global stack. Our findings indicate that the Blake and post-Blake excursions are both global features of past geodynamo behavior and support the hypothesis that Brunhes chron excursions are temporally clustered into two groups of at least a half-dozen each spanning over 220 to 30 ka and 720 to 520 ka.  相似文献   

17.
Determinations of40Ar/39Ar and U-Th-Pb are reported for three clasts from the Abee (E4) enstatite chondrite, which has been the object of extensive consortium investigations. The clasts give40Ar/39Ar plateau ages and/or maximum ages of 4.5 Gy, whereas two of the clasts give average ages of 4.4 Gy. Within the range of 4.4–4.5 Gy these data do not resolve any possible age differences among the three clasts.206Pb measured in these clasts is only ～1.5–2.5% radiogenic, which leads to relatively large uncertainties in the Pb isochron age and in the207Pb/206Pb model ages. The Pb data indicate that the initial207Pb/206Pb was no more than 0.08±0.07% higher than this ratio in Can?on Diablo troilite. The U-Th-Pb data are consistent with the interpretation that initial formation of these clasts occurred 4.58 Gy ago and that the clasts have since remained closed systems, but are contaminated with terrestrial Pb. The40Ar/39Ar ages could be gas retention ages after clast formation or impact degassing ages. The thermal history of Abee deduced from Ar data appears consistent with that deduced from magnetic data, and suggests that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, and experienced no appreciable subsequent heating.  相似文献   

18.
The Ryoke Metamorphic complex has undergone low‐P/T metamorphism and was intruded by granitic magmas around 100 Ma. Subsequently, the belt was uplifted and exposed by the time deposition of the Izumi Group began. The tectonic history of uplift, such as the timing and processes, are poorly known despite being important for understanding the spatiotemporal evolution of the Ryoke Metamorphic Belt. U–Pb zircon ages from sedimentary rocks in the forearc and backarc basins are useful for constraining uplift and magmatism in the provenance. U–Pb dating of detrital zircons from 12 samples (four sandstones and eight granitic clasts) in the Yuasa–Aridagawa basin, a Cretaceous forearc basin in the Chichibu Belt of Southwest Japan, gave mostly ages of 60–110 Ma. Granitic clasts contained in conglomerate suggest that granitic intrusions predate the formation of Coniacian and Maastrichtian conglomerate. Emplacement ages of granitic bodies originated from granitic clasts in Coniacian conglomerate are (110.2 ±1.3) Ma, (106.1 ±1.8) Ma, (101.8+5.8–3.8) Ma, and (95.3 ±1.4) Ma; for granitic clasts in Maastrichtian conglomerate, (89.6 ±1.8) Ma, (87.3+2.4–1.8) Ma, (85.7 ±1.2) Ma, and (82.7 ±1.2) Ma. The results suggest that detrital zircons in the sandstones were mainly derived from volcanic eruptions contemporaneous with depositional age, and plutonic rocks of the Ryoke Metamorphic Belt. Zircon ages of the granitic clast samples also indicate that uplift in the provenance began after Albian and occurred at least during the Coniacian to Maastrichtian. Our results, together with the difference of provenance between backarc and forearc basins suggest that the southern marginal zone of the Ryoke Metamorphic Belt was uplifted and supplied a large amount of clastic materials to the forearc basins during the Late Cretaceous.  相似文献   

19.
Plagioclase in cataclastic anorthosite 67075 occurs as angular matrix grains and as recrystallized clasts of micro-anorthosite. Olivines are Fe-rich and fall into two compositional groupings. Large grains of pyroxene show exceptionally well-developed exsolution lamellae analogous to those observed in pyroxenes from layered complexes. The low-Ca component in both pigeonites and augites shows varying degrees of inversion to orthopyroxene. The lattices of host and lamellae may deviate slightly (up to 2°) from the ideal orientation. Very slow cooling from magmatic temperatures is required to produce the coarse exsolution textures and inversion features. Augite macrocrystals are distinctly subcalcic indicating crystallization at temperatures around1100 ± 50°C while host-lamellae pairs and small grains in lithic clasts and matrix indicate reequilibration on a micron scale to temperatures less than 800°C. Pyroxene compositions tend to cluster into two groups both of which are among the most Fe-rich reported for highland pyroxenes. Ti and Al contents of pyroxenes are very low and Ti, Cr, and Mn follow well-established magmatic differentiation trends. The high Cr content may reflect low?O2 conditions and/or early crystallization of olivine and plagioclase.The87Sr/86Sr ratios in lunar anorthosites are the lowest reported for any lunar rock. It is likely that anorthosites formed as cumulates during the major differentiation episode which occurred prior to～4.3AE. Recrystallization features are common and39Ar/40Ar ages cluster around 4.0 AE. This may be the result of the intense bombardment prior to 4.0 AE which caused repeated cycles of in-situ fracturing and granulation followed by recrystallization. The low siderophile element content and the inferred slow cooling indicate a plutonic source region (10km) not frequently plumbed by impact events. The Fe-rich silicates indicate crystallization from a melt at an advanced stage of fractionation. However, the low REE abundances are not consistent with late-stage crystallization. Plagioclase apparently crystallized relatively early and was concentrated by flotation and/or convection currents while the mafic minerals crystallized from a fractionated trapped liquid. The chemical, isotopic, and mineralogical data place stringent constraints on the nature of genetically related rocks and the relationship of anorthosites to other members of the ANT suite does not appear to be one ofsimple fractionation. The data presented in this paper are consistent with the Taylor-Jake?model of lunar evolution.  相似文献   

20.
The Kula volcanic field in Western Turkey comprises about 80 cinder cones and associated basaltic lava flows of Quaternary age. Based on geomorphological criteria and K-Ar dating, three eruption phases, β2–β4, were distinguished in previous studies. Human footprints in ash deposits document that the early inhabitants of Anatolia were affected by the volcanic eruptions, but the age of the footprints has been poorly constrained. Here we use 3He and 10Be exposure dating of olivine phenocrysts and quartz-bearing xenoliths to determine the age of the youngest lava flows and cinder cones. In the western part of the volcanic field, two basalt samples from a 15-km-long block lava flow yielded 3He ages of 1.5 ± 0.3 ka and 2.5 ± 0.4 ka, respectively, with the latter being in good agreement with a 10Be age of 2.4 ± 0.3 ka for an augen gneiss xenolith from the same flow. A few kilometers farther north, a metasedimentary xenolith from the top of the cinder cone Çakallar Tepe gave a 10Be age of 11.2 ± 1.1 ka, which dates the last eruption of this cone and also the human footprints in the related ash deposits. In the center of the volcanic field, a basalt sample and a metasedimentary xenolith from another cinder cone gave consistent 3He and 10Be ages of 2.6 ± 0.4 ka and 2.6 ± 0.3 ka, respectively. Two β4 lava flows in the central and eastern part of the volcanic province yielded 3He ages of 3.3 ± 0.4 ka and 0.9 ± 0.2 ka, respectively. Finally, a relatively well-preserved β3 flow gave a 3He age of ∼13 ka. Taken together, our results demonstrate that the penultimate eruption phase β3 in the Kula volcanic field continued until ∼11 ka, whereas the youngest phase β4 started less than four thousand years ago and may continue in the future.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号