| Helium is inert chemically, but with very strong capabilities of diffusion and moving. Theabundance of helium in the Earth is very low, but it distributes widely in various terrestrialmaterials. The helium isotope compositions vary in a wide range, 3He/4He=10-1～10-8. The heliumfrom different source has different isotope composition. For this reason, helium isotope has beenused as an important mean to study the genesis and evolution of meteorite, to trace impact event,to distinguish the terrestrial and extraterrestrial matters; to sort types of mantle, to studyinteraction between crust and mantle, to delineate mantle metasomatism, metallization of mantlefluid, and existance of mantle plume, and to make forecast of earthquake. However the heliumisotope study is still in primary stage in China, and only the work bas been limited in study of gassamples. Analytic method of helium isotope for solid sample has not been established for a longperiod. In order to develop helium isotope study of solid sample, a mass spectrometer of noblegases was introduced from Ukraine to our Institute in 1995. Combining with laboratory project, astudy of analytic method of helium isotope for solid sample was completed, investigations ofhelium isotope were undertaken in many fields. The helium isotope results of nodules in C-CZone, eclogite in Dabie-Sulu region and mantle xenoliths and xenocrysts in Cenozoic basalt ineastern China were reported in this thesis.The huge reserves of submarine polymetallic nodules and cobalt crust make them animportant kind of mineral resource. The relationship between polymetallic nodules and submarinevolcanic-hydrothermal activities, and the relationship between growth rate of nodules andsedimentation rate of sediments were interesting problems. The resolution of this problem will benot only beneficial to the theory on the genesis of nodules, but also practically important for us tofind out the distribution pattern and to delineate rich ore nodules. A systematic comparative studyon helium isotope in nodules and surrounding sediments, their magnetic fractions and bulk isreported in this thesis. The nodules and their surrounding sediments all have extraordinarily high3He/4He ratios. 3He/4He ratios of all nodule and sediment samples are higher than the average levelof MORB, some even higher than that of hot point. The 3He/4He ratios of magnetic fractions areobviously higher than those of bulk. The 3He and 4He concentrations of magnetic fractions are10～100 times higher than those of bulk, quite different with that of terrestrial matters. All datapoints of nodules and sediments sit in or close to the mixing curve between the IDPs and theterrestrial sediments. It is considered that, just as those of pelagic sediments, the anomaloushelium isotope ratios of nodules can be attributed to micrometeorite interplanetary dust particles(IDPs). The growth rates of nodules and sedimentation rates of sediments were estimated based onextraterrestrial 3He flux and concentration in nodules and sediments. The growth rates of nodulesare in rang of 0.3～0.9 mm kyr-1, similar to the sedimentation rates of surrounding sediments. Thechanges of both rates are synchronous. The equivalent accretion rates of nodules and adjacentsediments may give an explanation of the fact that the nodules are always on or close to the surface of sediments.Since the diamond and coesite were found in eclogites of the Dabie Mountain in easternChina, Dabie-Sulu area became a hot point for research ultrahigh-pressure metamorphic rocks inthe world. Many researchers has studied in detail the ultrahigh-pressure metamorphic rocks indifferent parts and obtained some important results. However there are still serious argumentsexisting on some basic problems, such as the protolith of eclogites and the correlation between theeclogites and their country rocks. Based on the helium isotope composition of eclogites, some ofthose problems are discussed in this paper. The 3He/4He ratios of most eclogites from Dabie andSulu terrane range from 0.056 to 0.67Ra; the data points fall into the mixing area of the crust andthe mantle in the 3He-4He diagram. The helium isotope composition of eclogites from Bixilingcomplex has the mantle character, with 3He/4He ratio of 5.6 Ra. The 4He concentration of eclogitesis visibly inverse correlated with theδ18O value of quartz in eclogites from Sulu area. Thoseresults, combining with geological characters, suggest that the protoliths of eclogites may be thebasic-ultrabasic rock bodies or veins intruded into continental crust in primary stage. Stronginteraction and hybridization between the basic-ultrabasic rocks and continental rocks, meteoricwater during the intrusion period led to that the 3He/4He ratios,δ18O values andεNd(0) values ofthe intrusion bodies or veins extremely be reduced, and the bodies or veins have the continentalrock characters. The 3He/4He ratios of eclogites are obviously correlated with the types of theirsurrounding rocks. The changes ofδ18O of eclogites and country rocks are synchronous. Itshows that the eclogites were formed in-situ.Cenozoic basaltic volcanic activity was very strong in many region of eastern China. Therewere many mantle lherzolites xenoliths and huge xenocrysts in these basalts. In this study thehelium isotope composition of the mantle xenoliths and xenocrysts were systemically determinedfirst time. The variations of helium isotopic ratios can be observed in these xenoliths andxenocrysts from different area, or even from same area, indicating that the xenocrysts didn'tdirectly crystallize from basaltic magma, and basaltic magma is not directly related to thesemantle lherzolites xenoliths. The 3He/4He ratios of xenoliths and xenocrysts from most area weresimilar to those of MORB, implicating that these mantle xenoliths and xenocrysts were fromdepleted MORB mantle source. In the Hannuoba area, the 3He/4He ratios of lherzolites xenoliths isextra lower, 1.5×10-7; but the 3He/4He ratios of garnet xenocrysts, is very high 2.1～2.4×10-5.This evidence indicates the occurrence of strong mantle metasomatism in the area. The mantlelherzolites xenoliths with lower 3He/4He ratios may be from upper metasomatic enriched mantleand the garnet xenocrysts may be from below extra-depleted mantle formed by metasomatism. Ina garnet xenocryst of Hannuoba the highest 3He/4He ratios in the Earth samples were found, 9×10-4. |
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