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1.
通过对宽甸新生代碱性玄武岩、地幔包体及辉石巨晶的稀有气体同位素组成的分析，认为不同岩性稀有气体含量的差异反映了岩浆作用过程中轻、重稀有气体的分馏特性，较轻的稀有气体(He、Ne)比较重的稀有气体(Kr、xe)具有更高的活动性和不相容性；该地区上地幔源区具有典型的MORB型源区特征，以辉石巨晶为代表；地幔包体的^3He／^4He值较低，可能是地幔流体交代作用造成的；大陆碱性玄武岩具有与大洋玄武岩截然不同的He同位素组成，反映了大陆区地幔岩浆上升过程中受到了陆壳物质混染。地幔源区^40Ar／^36Ar值为350左右，二辉橄榄岩和碱性火山岩的^40Ar／^36Ar值比大气略高，可能有大气组分的混合。部分样品中有^21Ne、^22Ne、^129Xe、^134Xe和^136Xe相对于大气的过剩现象，是核成因造成的。 相似文献
2.
The noble gas isotopic composition and content data of 2 alkali basalts, 3 Iherzolite xenoliths and one clinopyroxene megacryst from the Kuandian region have confirmed the occurrence of a fractionation of noble gases during magmatism. Light noble gases such as He and Ne are high in mobility and appear to be incompatible as compared with heavy ones ( such as Kr and Xe). Therefore, light noble gases are abundant in volcanics, especially in the volcanics with bubbles; lherzolite xenoliths have relatively high heavy noble gases. The clinopyroxene megacryst has the lowest abundance of noble gases, probably due to its high P-T origin. Noble gas isotopic composition of the clinopyroxene megacryst reveals that the mantle source beneath the Kuandian area has an MORB-like reservoir with^3 He/^4He ratio of—10 Ra(Ra: atmospheric^3 He/^4He ratio) and^40 Ar/^36 Ar ratio of 345.6. The Iherzolite xenoliths possess moderate^3 He/^4He ratios of 2.59 -4.53 Ra, reflecting the loss of primary helium during rock deformation or metasomatism caused by enriched mantle fluids during the up-lifting. The alkali volcanics have very low^3 He/^4 He ratios(0.47—0.61 Ra),indicating a contribution of radiogenic^4 He, probably having resulted from crust contamination. Most of the samples have excess^21 Ne and^22 Ne as compared with atmospheric neon, but Kr and Xe isotopic compositions are indistinguishable from atmospheric values within uncertainties with only individual samples having excess^129Xe,^134Xeand^136 Xe. 相似文献
3.
I. M. Villa 《Tectonophysics》1990,180(2-4):369-373
Stepwise heating data on four amphiboles separated from various Zabargad rocks (a dolerite dike, a peridotite, a gneiss and a pyroxenite) invariably show saddle-shaped age spectra. The age information in the spectrum is generally masked by 1–2 nl/g excess 40Ar. The spectrum least affected by excess Ar constrains the emplacement age of the dolerite dike (Z-2137) to be 20 Ma. The greatest concentration of excess Ar is found in the peridotite, implying that the ultramafic rocks were metasomatized by a very 40Ar-rich fluid. A complex history of subsolidus reactions with hydrothermal fluids is suggested by the irregularities in the 37Ar/39Ar spectra. 相似文献
4.
《Comptes Rendus Geoscience》2007,339(14-15):937-945
The origin of the Earth's atmosphere can be constrained by the study of noble gases in oceanic basalts. If it is clear that the mantle is degassed and formed part of the present atmosphere, it has been proposed that an important subduction of atmospheric noble gases in the mantle occurred during Earth's history, altering the primordial signature of the solid Earth. This subduction process has been suggested on the basis of the measurements of light xenon isotopes in CO2 well gases. Moreover, the fact that the 38Ar/36Ar ratio is atmospheric in all oceanic basalts, even for uncontaminated samples (e.g. with high 20Ne/22Ne), may also suggest that a massive subduction of atmospheric argon occurred, if the primitive Earth had a solar-like 38Ar/36Ar. This also implies that the atmosphere suffered a massive gas loss accompanied by mass fractionation (e.g. hydrodynamic escape) after mantle degassing or that a late veneer with an atmospheric composition occurred. Such a hypothesis is explored for rare gases, by developing a model in which degassing and subduction of atmospheric noble gases started ∼4.4 Ga ago. In the model, both radiogenic and non-radiogenic isotopic ratios are used (e.g. 38Ar/36Ar and 40Ar/36Ar; 124Xe/130Xe and 129Xe/130Xe) to constrain the subduction flux and the degassing parameters. It is shown that subduction and massive contamination of the entire mantle is possible, but implies that the 40Ar/36Ar and the 129Xe/130Xe ratios were higher in the past than today, which is not observed in Archean samples. It also implies that the sediments and the altered oceanic crust did not loose their noble gases during subduction or that the contaminated mantle wedge is mixed by the convective mantle. Moreover, such a model has to apply to the oceanic island source, since this later shows the same signature of argon and xenon non-radiogenic isotopic ratios. A scenario where the isotopic compositions of the argon and xenon were settled before or during accretion is therefore preferred to the subduction. 相似文献
5.
Fossil particle tracks and spallation-produced He and Ne in the Kenna ureilite indicate that it existed in space as a small object for 23 m.y. In our study of Kenna, we found no evidence of trapped He or Ne. Large amounts of heavy rare gases occur in Kenna in concentrations typical of ureilites. In a step-wise release of gases, the isotopic compositions of Kr and Xe were found to be constant above 600°C, revealing the presence of a single retentively sited component. The Xe isotopic abundances are characterized by 124:126:128:129:130:131:132:134:136 = 0.471:0.414:8.280:103.61: 16.296:81.92:100:37.70:31.23. This isotopic composition is distinct from AVCC (average carbonaceous chondritic), but similar to compositions known for some time in certain temperature fractions of Renazzo, Murray and Murchison. Kenna-type Xe appears to be one of the several components found in carbonaceous chondrites.
Binz et al. (Geochim. Cosmochim. Acta 39, 1576–1579, 1975) have recently found that many volatile trace elements are strongly depleted in ureilites. Thus, the relatively large amounts of heavy rare gases present in ureilites did not result from a mixture of a volatile-rich component with the ureilite host. It appears that some material rich in carbon and heavy rare gases was incorporated into a differentiated ureilite host. All current hypotheses which purport to explain the origin of trapped gases in meteorites encounter difficulty in accounting for trapped gases in ureilites in a straightforward manner. 相似文献
6.
黄土坡铜锌矿床是东天山卡拉塔格地区一中型海相火山岩型块状硫化物（VHMS）矿床,赋存于早志留世海相火山-沉积岩系中。矿体产于酸性火山角砾岩与凝灰岩之间,主矿体呈厚大的扁豆状,矿石主要呈块状、浸染状、条带状和网脉状构造。本文报道了黄土坡铜锌矿床成矿流体的氦和氩同位素组成,黄铁矿流体包裹体的3He 含量低,介于0.039×10-12 cm3 STP/g～0.64×10-12 cm3 STP/g,n（3He）/n（4He）值为 0.11～0.94 Ra,介于壳源He与幔源He之间。40Ar 含量变化于 3.49×10-8cm3 STP/g～34.25×10-8 cm3 STP/g 之间,n（40Ar）/n（36Ar）介于301.40～425.98 之间。成矿流体中有幔源 He（1.71%～10.97%）和放射性成因 Ar（40Ar*含量为1.96%～30.63%）。结合矿床包裹体及 H-O-S 同位素特征,认为幔源流体和富放射性成因He 和Ar 的海水参与了成矿。 相似文献
7.
对取自赣南地区10个温泉的地热气体进行了气体化学成分及氦、碳、氖同位素组成的分析。该区地热气体可分为CO2型和N2型两种类型。CO2型地热气体分布在赣南东南部地区,主要成分是CO2,占总体积96.47%以上,二氧化碳气体的δ13C值为 -5.50‰～-3.49‰（PDB）,平均为 -4.66‰,为幔源无机成因,其氦同位素组成为1.36～2.27 Ra,具有明显的幔源成因特征,最高约有28.2%的氦源于地幔,其N2-Ar-He关系研究表明,该型地热气体中的氮源于地幔-地壳-大气混合成因。研究揭示该区CO2型地热气体属幔源无机成因气,是地幔脱气作用的产物。N2型地热气体分布在赣南西部地区,N2含量占91.04%以上,其中二氧化碳气体的δ13C值为 -23.7‰～-12.6‰,平均为 -17.82‰,为壳源有机成因,其氦同位素组成为0.06～0.13 Ra,具有明显的壳源放射性成因特征,3He/4He 与 4He/20Ne关系和He-Ar-N2关系研究表明,N2型温泉气主要来源于大气,并有壳源气体的贡献。 相似文献
8.
9.
《Geochimica et cosmochimica acta》1999,63(7-8):1181-1201
Noble gas isotopes of HIMU and EM ocean island basalts from the Cook-Austral and Society Islands were investigated to constrain their origins. Separated olivine and clinopyroxene (cpx) phenocrysts were used for noble gas analyses. Since samples are relatively old, obtained from the oceanic area and showing chemical zoning in cpx phenocrysts, several tests on sample preparation and gas extraction methods were performed. First, by comparing heating and crushing methods, it has been confirmed that the crushing method is suitable to obtain inherent magmatic noble gases without radiogenic and cosmogenic components which were yielded after eruption, especially for He and Ne analyses. Second, noble gas compositions in the core and the rim of cpx phenocrysts were measured to evaluate the zoning effect on noble gases. The result has been that noble gas concentrations and He and Ne isotope ratios are different between them. The enrichment of noble gases in the rim compared to the core is probably due to fractional crystallization. Difference of He and Ne isotope ratios is explained by cosmogenic effect, and isotope ratios of the trapped component seem to be similar between the rim and the core. Third, leaching test reveals no systematic differences in noble gas compositions between leached and unleached samples.3He/4He ratios of HIMU samples in the Cook-Austral Islands are uniform irrespective of phenocryst type (olivine and cpx) and age of samples (10–18 Ma), and lower (average 6.8 RA) than those of the Pacific MORB. On the other hand, 3He/4He of EM samples in the Cook-Austral Islands are similar to MORB values. EM samples in the Society Islands show rather higher 3He/4He than MORB. Ne, Kr and Xe isotope ratios are almost atmospheric within analytical uncertainties. 40Ar/36Ar are not so high as those of MORB. Anomalous noble gas abundance pattern such as He and Ne depletion and Kr and Xe enrichment relative to atmospheric abundances was observed. Furthermore, Ne/Ar and Kr/Ar show correlation with some trace elemental ratios like La/Yb.Lower 3He/4He of HIMU than MORB values requires relatively high time-integrated (U + Th)/3He for the HIMU source, which suggests that the HIMU source was produced from recycled materials which had been once located near the Earth’s surface. Moreover, extreme noble gas abundance pattern and strong correlation of Ne/Ar and Kr/Ar with La/Yb indicate that the HIMU endmember is highly depleted in light noble gases and enriched in heavy noble gases. Such feature is not common to mantle materials and is rather similar to the noble gas abundance patterns of the old oceanic crust and sediment, which supports the model that the HIMU source originates from subducted oceanic crust and/or sediment.If the HIMU source corresponds to the oceanic crust which subducted at 1–2 Ga as suggested by Pb isotope studies, however, the characteristic 3He/4He of HIMU (6.8 RA) would be too high because radiogenic 4He produced by U and Th decay should dramatically decrease 3He/4He. To overcome this problem, the He open system model is introduced which includes the effects of 4He production and diffusion between the HIMU source material and the surrounding mantle. This model favors that the HIMU source resides in the upper mantle, rather than in the lower mantle. Furthermore, this model predicts the thickness of the HIMU source to be in the order of 1 km.In contrast to low and uniform 3He/4He character of HIMU, 3He/4He of EM are rather variable. Entrainment of upper mantle material and/or a less-degassed component are required to explain the observed 3He/4He of EM in the Polynesian area. Participation of the less-degassed component would be related to the “superplume” below the Polynesian region. 相似文献
10.
Thomas J. Bernatowicz 《Contributions to Mineralogy and Petrology》1981,76(1):84-91
This work reports the results of noble gas (Ne, Ar, Kr, Xe) analyses of accidental mantle xenoliths from San Carlos, Arizona. Except for the addition of radiogenic 40Ar and mass fractionation effects, the isotopic structures of these gases are indistinguishable from atmospheric composition. The absence of 129Xe excesses in these rocks may reflect indirect mixing of atmospheric gases with the source region of the xenoliths. The dominant influence on the noble gas abundances in the San Carlos xenoliths appears to have been diffusive gas loss, which may have occurred in a mantle metamorphic event or during contact with the host basanite magma. Evidence is presented for the partitioning of significant amounts of the heavy noble gases into fluid inclusions in the xenolith minerals; the proportion of each gas in the inclusions increases with increasing atomic weight of the gas, possibly reflecting solubility effects. The noble gases are present in greater concentration in pyroxenes than in olivine, similar to the behavior of other incompatible elements. 相似文献
11.
Fabien Pitre 《Geochimica et cosmochimica acta》2010,74(2):531-4243
The concentration and the isotopic ratios of noble gases He, Ne, Ar, Kr and Xe were measured in porewater trapped in shallow sediments of the estuary of the St-Lawrence River, Québec, Canada. The gases are atmospheric in origin but most samples have gas concentrations 1.7-28 times higher than those expected in solution in water at equilibrium with the atmosphere. Elemental fractionation of heavier noble gases Kr and Xe compared to Ar strongly suggests that noble gases were adsorbed on sediments or organic matter and then desorbed into porewaters due to depressurization, as collected samples were brought to the surface. Atmospheric Ar in porewater is used as a reference to measure the N2-fluxes at the water-sediment interface. Ignoring the Ar enrichments observed in porewater could lead to a severe underestimation of the denitrification rate in oceans and estuaries. 相似文献
12.
Noble gas elemental and isotopic abundances were measured in steam from four wells in the Baca geothermal reservoir located in the Valles Caldera, New Mexico. The ratio and noble gas elemental abundances relative to 36Ar are all strongly correlated with 1/36Ar, the inverse of the argon content. Ratios of (α,n)-produced 21Ne1 and radiogenic 40Ar1 to total 4He (dominantly radiogenic) are nearly constant at 2.1 × 10?8 and 0.20, respectively. The ratio covers a restricted range of 3.9 to 4.8 times atmospheric. The high 3He content of the gas indicates the presence of a helium component ultimately derived from the mantle. Kr and Xe isotopic compositions are close to atmospheric; excess 129Xe1 is <0.25% of the total 129Xe.The high degree of linear correlation among the various noble gas results strongly suggests that the Baca reservoir contains two distinct fluids that are produced in varying proportions from individual wells. The noble gases in fluid A (~2900 mg/1 C1) are air-like, but with lighter gases and isotopes preferentially enriched. The fluid A 36Ar content is low, only 13% that of 10°C air-saturated water (ASW). The second fluid, B (~ 1700 mg/1 C1), is the dominant carrier of the radiogenic and mantle-derived gases. The heavier non-radiogenic gases are preferentially enriched in fluid B, and its 36Ar content is very low, only 5–7% ASW. The source of the noble gases in fluid A is tentatively ascribed to leaching of the relatively young (<1.4 m.y.) volcanic Bandelier Tuff. The radiogenic gases and mantle-derived helium in fluid B suggest a deeper source, possibly including gases escaping from a magma. 相似文献
13.
中国天然气中稀有气体丰度和同位素组成 总被引：4，自引：0，他引：4
徐胜 《矿物岩石地球化学通报》1997,(2)
中国天然气中稀有气体丰度和同位素组成徐胜（中国科学院兰州地质研究所，兰州７３００００）关键词中国天然气、稀有气体丰度和同位素、稀有气体来源稀有气体在地质作用过程中的丰度和同位素组成变化几乎不受复杂化学反应的影响，而主要取决于溶解、吸附、吸着和核反应等... 相似文献
14.
D.D. Bogard E.K. Gibson D.R. Moore N.L. Turner R.B. Wilkin 《Geochimica et cosmochimica acta》1973,37(3):547-557
Total carbon determinations on the Haverö, Dingo Pup Donga, and North Haig ureilites yield values of 2.07, 3.17, and 5.58 wt.%, respectively. Haverö and Dingo Pup Donga contain relatively large amounts of trapped Ar, Kr and Xe, which like the carbon content varies with grain size for Haverö. These two meteorites also contain dominant cosmic rayproduced He and Ne, and show 3He exposure ages of ~23 m.y. and ~7 m.y., respectively. North Haig contains much smaller amounts of trapped gases and spallogenic gases, which may result from loss due to terrestrial weathering. The isotopic composition of Xe in five grain size analyses of Haverö and a whole rock analysis of Dingo Pup Donga shows the presence of a major solar-like Xe component. The presence of this solar component adds an additional complication to the concept of forming ureilites from carbonaceous chondrites. 相似文献
15.
南岭中段主要锡矿床He、Ar同位素组成及其意义 总被引：8，自引：0，他引：8
对南岭中段主要锡矿床的黄铁矿等进行了流体包裹体的He、Ar同位素研究。结果表明,成矿流体的40Ar/36Ar≈288～371,3He/4He≈0.09～28.58Ra ,不同地区流体包裹体He、Ar浓度变化较大,而在同一地区40Ar/36Ar、3He/4He值比较一致;40Ar/36Ar值介于大气饱和水(包括大气降水和海水)的同位素组成（40Ar/36Ar≈295.5）和夏威夷热点（40Ar/36Ar=350～360）之间,3He/4He值大大高于大陆地壳特征值（3He/4He≈0.03Ra）,介于地壳特征值与原始地幔特征值（3He/4He≈8.57Ra）和夏威夷热点特征值（3He/4He≈27.14Ra）之间,表明成矿流体与地幔柱的活动有关,为地幔、地壳和大气水的混合产物,以地幔流体为主。这一结果为南岭地区锡多金属矿床为壳幔相互作用的产物及区域地幔柱的存在提供了新证据。 相似文献
16.
We report 3He/4He ratios from 10 peridotite xenoliths considered to represent samples of the uppermost mantle wedge above the downgoing Juan de Fuca Plate. Helium isotopic ratios in all but two of the xenoliths are similar to many arc magmas, roughly 7 Ra (1 Ra=atmospheric value). Based on decoupling of He from Sr, Nd and Os in these samples, similar He ratios in olivines from rims of larger xenoliths, and modeling of helium exchange between xenoliths and magmas, we interpret this ratio as that of helium in the host basalt. 3He/4He ratios as low as 4.2 Ra are found in olivines from the cores of the two largest xenoliths. These results cannot be reasonably explained by interaction with crustal material or post-eruptive ingrowth of 4He, but have been produced by interaction between mantle peridotite and a 4He-rich melt or fluid. Either 4He already present in the subducting oceanic crust has been retained to significant depths below Simcoe and then directly released behind the arc to interact with the mantle wedge, or, more likely, 4He has been produced by decay of U and Th in metasomatized mantle directly above the slab; a He-rich fluid or melt from this source has then ascended and modified the region of mantle represented by the xenoliths. This latter model is supported by estimates of residence time for the Simcoe metasomatic agent from U–Th–Pb isotopic systematics of pyroxenes from the Simcoe peridotites, estimated U and Th concentrations in the source of the fluid or melt, and commonly assumed patterns of helium behavior. This model is also consistent with higher 3He/4He ratios typically measured in arc samples; the portion of sub-arc mantle with such low He isotope ratios may be quite small, but the Simcoe xenoliths record a much larger volumetric contribution of the He-rich metasomatic agent than do arc lavas. 相似文献
17.
苏鲁超高压变质岩区深部流体He-Ar的系统关系：中国大陆科学钻探工程在线流体监测的解析 总被引：3，自引：1，他引：3
中国大陆科学钻探工程在线监测从泥浆中分离出的气体地球化学组成揭示了一段重要的气体异常,从2004年12月24日夜里开始到12月29日晚上结束。从12月10日到24日晚上11点30分的气体的Ar、He和N2基本上沿着趋势A分布,而12月26日早上7点半到29日晚上7点半的数据沿趋势C分布。相对于趋势A,趋势C中的气体含有相对升高的Ar,Ar/He和Ar/N2的平均值分别为3653和0.0142,明显高于空气的比值1800和0.0119。趋势A中的气体的Ar/He和Ar/N2比值分别围绕1851和0.0118变化,其中的Ar/He比值稍微高于空气的比值,但Ar/N2比值近似于空气比值,表明背景地下流体含非常低的Ar,而He和N2主要是大气组分。在趋势A和趋势C之间的数据(时间段B)具有和空气接近的Ar/N2比值,但平均Ar/He比值为3265,明显高于空气比值,反映了该段气体具有相对亏损的He。苏鲁-大别山地区的热年代学研究已经表明云母和角闪石的Ar/Ar的冷却年龄大大地高于磷灰石或锆石的He的冷却年龄,说明Ar在超高压变质地体折返早期就已封闭,而He一直保持开放状态,直到超高压变质岩接近地表。这种Ar和He对温度变化的不同反应,导致大部分的He在超高压岩石折返过程中脱气并释放到空气中,Ar则相对圈闭在固体岩石或封闭的断裂带中。在He-Ar的系统关系上,表现为来自于超高压岩石或断裂带中的流体具有富集Ar、亏损He及升高的Ar/He比值。气体组分从趋势A向趋势C的骤然跳跃,反映了地下流体组分的强烈变化,即具有相对富集Ar的深部流体的贡献大大增强。 相似文献
18.
《International Geology Review》2012,54(12):1051-1057
Most of Earth's volcanism occurs at plate boundaries—mid-ocean ridges and island arcs. Most of the 3He and other noble gases also are expelled from the interior at these locations. A small amount, compared to the global budget, of magma (<10%) and 3He («1%) escapes at mid-plate volcanoes, the socalled “hotspots.” Locations of hotspots are controlled by stresses and cracks, not by deep, narrow mantle upwellings. Most, or all, of Earth's volatile inventory appears to have been brought in by a late veneer. The high 3He content and high He/Ne and He/Ar ratios of mid-ocean ridge basalts, plus the excess 129Xe and 136Xe, suggest that the ridge basalt reservoir is relatively undegassed and that there is no primordial undegassed reservoir. The two prevailing views of the origin of Earth are summarized in the context of this information. 相似文献
19.
Mantle helium in Sacramento basin natural gas wells 总被引：16，自引：0，他引：16
R. J. Poreda P. D. Jenden I. R. Kaplan H. Craig 《Geochimica et cosmochimica acta》1986,50(12):2847-2853
Helium isotope ratios in Sacramento basin natural gas wells show a strong mantle signal. The 3He/4He ratios range from 0.11 times the atmospheric ratio (0.11 RA) in the Rio Vista field to 2.75 RA in the Moon Bend field, indicating that 1% to 34% of the helium is mantle-derived. 3He/4He versus CH4/4He ratios provide evidence of two-component mixing between crustal and magmatic end-members. Extrapolation of the linear regression line to CH4/4He = 0 gives a hypothetical magmatic end-member 3He/4He ratio of 3.84 RA, half the typical mantle ratio. This indicates that the magmatic end-member may actually represent a mixture of mantle and crustal helium. Gases which deviate from the simple two-component mixture can be explained by addition of pure methane, radiogenic 4He, or a high N2-He component with 3He/4He = 0.6 Ra to 1.0 RA. The CH4/3He ratio of the magmatic end-member remains poorly constrained (0 to 3 × 109) and one cannot rule out the possibility that a significant proportion of the methane in some fields may be of deep-earth origin. However, fields with the highest 3He/4He ratios are associated with buried Plio-Pleistocene intrusives which have up-arched sediments to form hydrocarbon traps. The methane in these fields may have been produced by rapid thermal alteration of the intruded sediment. Elsewhere, the methane appears either to have migrated from deeply-buried sediments in the western basin or to have been produced by local microbial activity. 相似文献
20.
Alkaline magmas from the late-Cenozoic Marie Byrd Land Volcanic Province, West Antarctica, have entrained lithospheric xenoliths which vary from spinel lherzolites to supracrustal rocks. Lower crustal xenoliths have been collected from the Executive Committee Range (Mounts Hampton in the north and Mount Sidley in the south) in central Marie Byrd Land, and their petrological characteristics together with preliminary geochemical data are discussed here. Granulite xenoliths include metaigneous gabbros and norites with varying proportions of clinopyroxene, spinel and either olivine or orthopyroxene. Pyroxenites occur together with granulites, which on the basis of their similar mineral assemblage, texture and composition are considered to be related to the granulites. The composition of xenoliths from Mounts Hampton and Sidley differ (e.g. Mount Sidley xenoliths have Mg# 32–80, are relatively LREE enriched and have 87Sr/86Sr of 0.70286–0.70376 and 143Nd/144Nd of 0.512864–0.512870, whereas Mount Hampton xenoliths have Mg# 68–78, are LREE depleted and have 87Sr/86Sr of 0.70420–0.70458 and 143Nd/144Nd of 0.512771–0.512819), defining a major lateral lower crustal discontinuity beneath the Executive Committee Range. Relict igneous textures and low abundances of incompatible elements indicate that the xenoliths initially formed as cumulates rather than as trapped melts. The xenolith suite differs in composition to the host rocks (Mount Sidley VOLCANICS = 87Sr/86Sr of 0.70300–0.70312 and 143Nd/144Nd of 0.512814–0.512907) and cannot be co-genetic with them. They are interpreted here to represent the cumulates of mantle melts that evolved by crystal fractionation at lower crustal depths. 相似文献