Hebridean tholeiitic magmas: A geochemical study of the Ardnamurchan cone sheets

The Tertiary complex of doleritic cone sheets in Ardnamurchan has been sampled and a geochemical study made on 98 rock specimens by X-ray fluorescence analysis. A range in composition from 48 to 66% (wt.) SiO₂ is associated with, for example, an increase in Rb from 2 to 126 ppm, and a decrease in Ni from 86 to < 1 ppm. R-mode factor analysis indicates element groupings that are most easily related to a history of crystal fractionation beneath the central intrusive complex. No chemical basis can be found that would add support to either the two-centre or the spiral-fracture hypotheses relating to the disposition of the cone sheets. Plots of oxide and normative data show the Hebridean cone-sheet complex to have a tholeiitic character, which is compared with the Hawaiian tholeiitic and Hebridean alkalic series. A transitional nature for the two Hebridean series, compared with the two Hawaiian series, is probably related to derivation from an olivine basalt parent rather than from an olivine tholeiite as in Hawaii.     

Past and future changes to acidified eastern Canadian lakes: A geochemical modeling approach

As SO₂ emissions are being reduced in North America, it has become important to know how rapidly the surface water chemistry of aquatic systems will recover. The authors applied the model of acidification of groundwater in catchments (MAGIC) to 410 acid-sensitive lakes located in a 3000 km east–west gradient in eastern Canada. The goal was to estimate the water chemistry from pre-acidification times, under worst case conditions (mid 1970s) and what it should be in the year 2030 after proposed acid emission reduction levels agreed-to or planned by Canada and the United States are in place. In eastern Canada, large decreases in pH and ANC are shown between pre-acidification and 1975, the year of greatest historical deposition. Current-day conditions are much improved from 1975. Under the most likely future acid deposition reduction scenarios, an improvement of pH and ANC is shown in all the regions from current-day levels, but not to pre-acidification levels. Dissolved Ca levels were considerably higher at the height of acidification than under pristine conditions, but will return to pre-acidification levels at most of the sites by the year 2030. The results also show that under proposed control programs, a large number of sites in eastern Canada will not return to ANC values >40 μeq L⁻¹, thought to be suitable for healthy aquatic communities.     

Physical behavior of the plume source during intermittent eruptions of Hawaiian basalts

The plume surrounding a source region for magma may either compact by elastic or viscous deformation when magma leaves the source region. It is shown that the compaction is elastic. The elastic deformation implies that only a small volume fraction of the melt can leave the source region. On the other hand, the isotopic secular disequilibria show that the fraction melt must be much larger. This discrepancy is solved if the source region is veined and consists of intercalated veins and residuum, with the veins forming about 1% volume of the source region. The approximate width of the source region is estimated to be 10-20 km and its height is 1-2 km. An eruption starts when a dike is formed above the source region because of overpressure of the melt within the source region. During the repose time, the overpressure of the melt in the source region increases because of the replenishment of melt from the plume situated below.     

First report of Lower Permian basalts in South Tibet: tholeiitic magmatism during break-up and incipient opening of Neotethys

The Neo-Tethys Ocean began to form at Early Permian times, when continental flood basalts were emplaced in various areas of the newly-formed Indian passive margin, exposed today in the so-called Tibetan Sedimentary Zone of the Himalaya. Lower Permian mafic volcanic rocks, which have long been known from various Himalayan localities from Kashmir to Arunachal Pradesh, are here for the first time reported to occur also in South Tibet (Bhote Kosi Basalts of the Gyirong County). The basalts unconformably overlie lowermost Permian diamictites, with locally intervening black shales and debris flow deposits, and are followed in turn by chert-bearing quartzarenites and silty to phosphatic marls yielding brachiopods of Roadian–Wordian age. The age of the lavas can thus be bracketed as late Early Permian (post-Sakmarian and pre-Roadian).The geochemistry of these subalkalic tholeiites, akin to MORBs, testifies to their similarity not only with the adjacent Nar-Tsum Spilites of central Nepal, but also with the Panjal Traps and Abor Volcanics of the western and eastern Himalayas respectively. The geochemical signature of Lower Permian volcanic rocks is in fact uniform all along the Himalayan Range, and markedly different from that of basaltic–rhyolitic alkalic products sporadically emplaced during the previous rifting stage. Rift volcanism in the Tethys Himalaya began in the Early Carboniferous and came to an end in Sakmarian times. In the Early Permian, initial submergence of the rift shoulders and sediment starvation were followed by tholeiitic magmatism, which is therefore interpreted as following break-up and incipient sea-floor spreading in the Neotethys Ocean. Roughly contemporaneous emplacement of continental flood basalts of similar geochemical signature along a 2000 km long rift axis would in fact suggest extensive mantle melting at the transition from continental rifting to break-up and opening of the Neotethys between Northern Gondwana and the Peri-Gondwanian blocks.     

Anhydrous melting of peridotite at 0–15 Kb pressure and the genesis of tholeiitic basalts

The anhydrous melting behaviour of two synthetic peridotite compositions has been studied experimentally at temperatures ranging from near the solidus to about 200° C above the solidus within the pressure range 0–15 kb. The peridotite compositions studied are equivalent to Hawaiian pyrolite and a more depleted spinel lherzolite (Tinaquillo peridotite) and in both cases the experimental studies used peridotite –40% olivine compositions. Equilibrium melting results in progressive elimination of phases with increasing temperature. Four main melting fields are recognized; from the solidus these are: olivine (ol)+orthopyroxene (opx)+clinopyroxene (cpx)+Al-rich phase (plagioclase at low pressure, spinel at moderate pressure, garnet at high pressure)+liquid (L); ol+opx+cpx+Cr-spinel+L; ol+opx+Cr-spinel +L: ol±Cr-spinel+L. Microprobe analyses of the residual phases show progressive changes to more refractory compositions with increasing proportion of coexisting melt i.e. increasing Mg/(Mg+Fe) and Cr/(Cr+Al) ratios, decreasing Al₂O₃, CaO in pyroxene.The degree of melting, established by modal analysis, increases rapidly immediately above the solidus (up to 10% melting occurs within 25°–30° C of the solidus), and then increases in roughly linear form with increasing temperature.Equilibrium melt compositions have been calculated by mass balance using the compositions and proportions of residual phases to overcome the problems of iron loss and quench modification of the glass. Compositions from the melting of pyrolite within the spinel peridotite field (i.e. 15 kb) range from alkali olivine basalt (<15% melting) through olivine tholeiite (20–30% melting) and picrite to komatiite (40–60% melting). Melting in the plagioclase peridotite field produces magnesian quartz tholeiite and olivine-poor tholeiite and, at higher degrees of melting (30–40%), basaltic or pyroxenitic komatiite. Melts from Tinaquillo lherzolite are more silica saturated than those from pyrolite for similar degrees of partial melting, and range from olivine tholeiite through tholeiitic picrite to komatiite for melting in the spinel peridotite field.The equilibrium melts are compared with inferred primary magma compositions and integrated with previous melting studies on basalts. The data obtained here and complementary basalt melting studies do not support models of formation of oceanic crust in which the parental magmas of common mid-ocean ridge basalts (MORB) are attributed to segregation from source peridotite at shallow depths ( 25 km) to leave residual harzburgite. Liquids segregating from peridotite at these depths are more silica-rich than common MORB.     

Processes controlling Sr in surface and ground waters of Tertiary tholeiitic flood basalts in Northern Iceland

Strontium concentrations of 253 natural water samples from Skagafjördur, a Tertiary tholeiitic flood basalt region in northern Iceland range between 0.10 and 28 ppb. Surface environments (rivers, lakes, and peat soil waters) include the whole range of observed Sr concentrations whereas the Sr concentrations of ground waters are, in most cases, <3.5 ppb. Concentrations of Sr derived from basalt dissolution (i.e., rock-derived Sr) in waters of rivers and lakes exhibit a near linear correlation with the concentration of rock-derived Ca with a median molar Ca/Sr ratio of 1350. This systematic correlation suggests that Ca and Sr concentrations are controlled by weathering processes, i.e., the extent of dissolution of the basalt. The relative mobility of Sr during weathering in Skagafjördur is approximately half that of Ca, which is consistent with observed relative mobilities of these elements elsewhere in Iceland and in other basaltic regions. Peat soil waters commonly have lower concentrations of Sr and higher Ca concentrations than rivers and lakes, and molar ratios of rock-derived Ca to Sr in peat soil waters exhibit no systematic pattern. In several cases calculated concentrations of rock-derived Sr in peat soil waters yield negative values, suggesting a mineralogic sink for Sr in these waters.The low Sr concentrations in cold and thermal ground waters (<3.5 ppb) suggest mineralogic control over Sr in the ground water systems. Precipitation of secondary Sr minerals such as strontianite and celestite is ruled out as the ground waters are understaturated with respect to these minerals. Ground waters are characterized by high Ca/Sr molar ratios (∼5000 compared to bedrock Ca/Sr ratio of 730) suggesting that Sr is being preferentially incorporated (relative to Ca) into secondary minerals. The secondary minerals present in the bedrock in Skagafjördur that can preferentially incorporate Sr include zeolites, such as heulandite, chabazite, and thomsonite, and smectite. Ion-exchange calculations demonstrate that activities of Sr²⁺ and Ca²⁺ in ground water solutions in Skagafjördur are consistent with ion-exchange equilibria between these waters and heulandite from other Tertiary basalts in Iceland suggesting that this mineral may play an important role in controlling the concentration of Sr in the Skagafjördur ground waters. Incorporation of Sr into calcite cannot explain the observed high Ca/Sr ratios of the Skagafjördur ground waters because calcite, when precipitating, only admits limited amounts of Sr. Aragonite is not considered a likely candidate either because it has only very slight preference for Sr over Ca and ground waters above 40 °C are undersaturated with respect to this phase. However, predicted Sr content of calcite in equilibrium with the Skagafjördur ground waters (0.5-83 ppm Sr) is in good agreement with measured Sr content of this mineral in Tertiary basalts elsewhere in Iceland (<0.1-63 ppm), suggesting that the Skagafjördur ground waters can be used as analogues for Tertiary crustal solutions involved in the zeolite facies metamorphism of the Icelandic crust.     

Some geochemical characteristics of basalts in the South China Sea

Petrochemistry, geochronology, and trace element and REE geochemistry have been studied of some basalts from the South China Sea. The results show that the South China Sea basalts may be divided into two types: alkali basalts and tholeiites. The former is similar to commor. alkali basalts in petrochemistry, REE distribution pattern and trace element geochemistry, and the latter, to Hawaiian tholeiites, belonging to the transition type between alkali basalts and MORB (mid-ocean ridge basalts).     

Pattern recognition in geochemical hydrocarbon exploration: A fuzzy approach

For the Swedish Deep Gas Project some 240 soil samples were collected and analyzed for trace metals and C. The data were determined to not be sufficient as anomalous patterns obtained were merely reflecting underlying crystalline or Palaeozoic bedrock. Any possible patterns related to a deep-seated gas source were completely swamped; in addition, glacial transport also presented a problem in interpretation. Therefore, the ARIADNE method was applied to the data set. ARIADNE is a pattern recognition system designed for use in a variety of exploration applications, ranging from geochemical regional surveys to detailed geophysical well logging. The system's core is a fuzzy classifier that can work both on differences in location and dispersion in variable space, either combined or separately. For unsupervised classification, a preprocessor, called NARCISSOS, is used, which, by using fuzzy principal components analysis, extracts a robust background and an appropriate number of anomalous populations. Mean vectors and covariance matrices of all populations are submitted to the ARIADNE classifier. By taking advantage of different patterns emerging by using mean vectors or variance-covariance matrices when classifying in the variable space, the relative influence of transport (e.g., glacial transport) can be estimated and probable source areas also can be established. When ARIADNE was applied to the Deep Gas Project data, two anomalous populations emerged. One was strongly tied, both geographically and chemically, to the Palaeozoic ring structure circumscribing the target area, and the background reflected general chemical features of granitic bedrocks inside and outside of that structure. The second anomaly, however, was not related to any bedrock composition, but rather to structural phenomena in the bedrock. It did not show any signs of glacial dispersions, as the other anomalous group did, and its chemical signture was more or less in concordance with what is found over oil or gas fields in other more traditional environments.This paper was presented at Emerging Concepts, MGUS-87, Redwood City, California, 13–15 April 1987.     

A geochemical comparison of manganese oxide deposits of the Hawaiian Archipelago and the Deep Sea

Data are presented on the composition of manganese crusts collected within the Hawaiian Exclusive Economic Zone (HEEZ). These crusts were obtained in 80% of the dredge hauls between 800 and 3000 m. Although the chemistries of crusts on the Cenozoic Hawaiian Ridge and an older Cretaceous seamounts south and north of the ridge are quite similar, there are marked differences, both in composition and in mineralogical organization, between shallow and deep crusts. Shallow crusts (<1500 m) are composed of five distinct phases (i.e. silicate, apatite, iron oxide, manganese oxide, and non-associated copper), whereas the deep crusts display no obvious inter-elemental organization based on dendrograms. The mean composition of HEEZ crusts differs from that of Eastern Tropical Pacific nodules in that crusts are comparatively rich in As, Pb, Co, Ce, Fe, and Ti, whereas nodules have comparatively more Cu, Ni, and Sr. Compositional differences between shallow crusts and nodules are discussed in terms of (1) element source; (2) Mn-oxide mineralogy; and (3) redox conditions in the adjacent seawater. It is concluded that the major differences between crust and nodules is due to metal sources; water column for crusts, sediments for nodules. Advective processes within the O₂ minimum also affect the HEEZ crusts.     

Os-Os systematics of Hawaiian picrites revisited: New insights into Os isotopic variations in ocean island basalts

Eighteen picrites (MgO > 13 wt.%) and three related basalts from six Hawaiian volcanoes were analyzed for ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁶Os/¹⁸⁸Os. Variations in these ratios reflect long-term Re/Os and Pt/Os differences in the mantle source regions of these volcanoes. ¹⁸⁷Os/¹⁸⁸Os ratios vary from ∼0.129 to 0.136, consistent with the range defined by previous studies of Hawaiian picrites and basalts. Samples with lower ¹⁸⁷Os/¹⁸⁸Os are mainly from Kea trend volcanoes (Mauna Kea and Kilauea), and the more radiogenic samples are mainly from Loa trend volcanoes (Mauna Loa, Hualalai, Koolau and Loihi). As previously suggested, differences in ¹⁸⁷Os/¹⁸⁸Os between volcanic centers are most consistent with the presence of variable proportions of recycled materials and/or pyroxenitic components in the Hawaiian source.¹⁸⁶Os/¹⁸⁸Os ratios vary from 0.1198332 ± 26 to 0.1198480 ± 20, with some samples having ratios that are significantly higher than current estimates for the ambient upper mantle. Although the range of ¹⁸⁶Os/¹⁸⁸Os for the Hawaiian suite is consistent with that reported by previous studies, the new data reveal significant heterogeneities among picrites from individual volcanoes. The linear correlation between ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁶Os/¹⁸⁸Os reported by a previous study is no longer apparent with the larger dataset. The postulated recycled materials and pyroxenites responsible for the dominant variations in ¹⁸⁷Os/¹⁸⁸Os are likely not responsible for the variations in ¹⁸⁶Os/¹⁸⁸Os. Such materials are typically characterized by both insufficiently high Os concentrations and Pt/Os to account for the ¹⁸⁶Os/¹⁸⁸Os heterogeneities. The lack of correspondence between ¹⁸⁶Os/¹⁸⁸Os variations and the Kea and Loa trends supports this conclusion.The primary cause of ¹⁸⁶Os/¹⁸⁸Os variations are evaluated within the framework of two mixing scenarios: (1) metasomatic transport of Pt and/or ¹⁸⁶Os-rich Os into some portions of the Hawaiian source, and (2) interaction between an isotopically complex plume source with a common, Os- and ¹⁸⁶Os-enriched reservoir (COs). Both scenarios require large scale, selective transport of Pt, Re and/or Os. Current estimates of HSE concentrations in the mantle source of these rocks, however, provide little evidence for either process, so the dominant cause of the ¹⁸⁶Os/¹⁸⁸Os variations remains uncertain.     

The Tethyan plume: geochemical diversity of Middle Permian basalts from the Oman rifted margin

According to palinspastic reconstructions, the Neo-Tethys opening took place during the Permian between the Cimmerian fragments in the north and the Indo-Arabian margin in the south. Igneous remnants of this opening are exposed in Oman within either the Hawasina nappes or the para-autochtonous Arabian platform exposed in the Saih Hatat tectonic window. They consist predominantly of pillowed basaltic flows among which three groups have been distinguished. Group 1 is tholeiitic and characterized by low TiO₂ and incompatible trace element contents, and a large range of Nd_i values. Group 1 basalts are associated with distal sediments and plot near the boundary of or within the MORB field in the Pb–Pb correlation diagrams and between the MORB and Bulk Silica Earth (BSE) fields in Nd_i–(²⁰⁶Pb/²⁰⁴Pb)_i diagram. Group 2 basalts are alkaline and differ from Group 1 ones by their higher TiO₂, La and Nb contents, and lower and more homogeneous Nd_i values (+3 to +5). Group 2 volcanics are similar to alkali basalts from oceanic islands and share with Group 1 similar initial Pb ratios. Group 3 consists of tholeiitic and alkali basalts which are interbedded either with carbonate-platform sediments from the Saih Hatat window or with distal sediments from the Hawasina Nappes. This group differs from Groups 1 and 2 by its low to negative Nd_i (+1.6 to −2). Group 1 likely derived from the mixing of depleted and enriched sources while Group 2 derived exclusively from an enriched source. There is no indication that continental crust was involved in the genesis of both Groups 1 and 2. In contrast, the low to negative Nd_i values of Group 3 suggest that the magmas of this group were contaminated by the Arabian continental crust during their ascent. The geochemical features of the Middle Permian plume-related basalts suggest thus that the basement of the Hawasina basin was not genuine oceanic crust but either the thinned Arabian rifted continental margin or the continent–ocean transition zone of the Neo-Tethys.     

Ages,rare-earth element enrichment,and petrogenesis of tholeiitic and alkalic basalts from Kahoolawe Island,Hawaii

Kahoolawe Island, Hawaii (18×11 km), is a basaltic shield volcano with caldera-filling lavas, seven identified postshield vents, and at least two occurrences of apparent rejuvenated-stage eruptive. We examined 42 samples that represent all stages of Kahoolawe volcano stratigraphy for their petrography, whole-rock major-and trace-element contents, mineral compositions, and K–Ar ages. The two oldest shield samples have an average age of 1.34±0.08 Ma, and four postshield samples (3 are alkalic) average 1.15±0.03 Ma; ages of 1.08 and 0.99 Ma for two additional tholeiitic samples probably are minimum ages. Whole-rock major- and trace-element and mineral compositions of Kahoolawe shield and caldera-fill laves are generally similar to the lavas forming Kilauea and Mauna Loa tholeiitic shields, but in detail, Kahoolawe shield lavas have distinctive compositions. An unusual aspect of many postshield Ka-hoolawe lavas is anomalously high REE and Y abundances (up to 200 ppm La and 175 ppm Y) and negative Ce anomalies. These enrichments reflect surficial processes, where weathering and soil development promoted REE-Y transport at the weathering front. Major element abundances (MgO, 10–6 wt.%) for shield and caldera-fill basalts are consistent with fractionation of ol+px+pl in frequently replenished magma reservoirs. In general, tholeiitic basalts erupted from late vents are higher in SiO₂ than the shield lavas, and temporal differences in parental magma compositions are the likely explanation. Alkalic basalts that erupted from vents are comparable in composition to those at other Hawaiian volcanoes. Trace-element abundance ratios indicate that alkalic basalts represent either relatively lower degrees of melting of the shield source or a distinct source. Apparent rejuvenated-stage basalts (i.e., emplaced after substantial Kahoolawe erosion) are tholeiitic, unlike the rejuvenated-stages at other Hawaiian volcanoes (alkalic). Kahoolawe, like several other Hawaiian volcanoes, has intercalated tholeiitic and alkalic basalts in the postshield stage, but it is the only volcano that appears to have produced tholeiitic rejuvenated-stage lavas.     

Geochemistry of Kauai volcanics and a mixing model for the origin of Hawaiian alkali basalts

A comprehensive model is developed to explain the major, trace element and strontium and neodymium isotopic characteristics of alkali basalts from Hawaii. The model is similar to that of Chen and Frey (1983) in that it requires mixing of a small melt fraction of MORB-source material with another component to generate the alkalic suite of a particular Hawaiian volcano. It differs from the Chen and Frey model in that the other end-member must be different from primitive mantle if it is to be consistent with both trace element and isotopic data. Alkali basalts and tholeiites from Kauai analyzed in this study show a nearly complete transition in Sr and Nd isotopes. There is a relatively well-constrained array on a Nd-Sr isotope correlation plot that can be explained by two-component mixing of Kauai tholeiite magma and a small amount of melt of East Pacific Rise source rock. After corrections are made for fractional crystallization (involving primarily clinopyroxene and olivine), the Sr and Ba concentrations of Kauai lavas plot along mixing curves defined by the above sources, providing positive tests of the mixing hypothesis. Implications of this model are: (1) the main source of Hawaiian shield-building tholeiites is a mixture of subducted crust, primitive mantle and depleted asthenosphere that has been homogenized prior to melting, (2) early alkalic volcanism (as at Loihi seamount) will be characterized by greater isotopic heterogeneity than will late-stage alkali basalt production, and (3) there are two fundamentally distinct types of alkalic lavas erupted towards the end of magmatism at a given Hawaiian volcano. One represents smaller degrees of melting of the same source that generated shield-building tholeiites (Kohala-type); the other derives from the mixed source discussed in this paper (Haleakala-, Kauai-type).     

Plagioclase of Hawaiian tholeiitic and alkalic magma parentages: distinctions based on REE, Sr, Ba, Hf, and Ta

Summary ?To enhance the ability to distinguish tholeiitic from alkalic magma parentages by mineral compositions, I determined trace-element abundances in plagioclase separated from xenolithic gabbros of Mauna Kea volcano. These gabbros have origins in tholeiitic and alkalic magmas of the Hamakua postshield stage of Mauna Kea volcanism. Chondrite-normalized rare-earth element (REE) patterns for plagioclase show that highly calcic plagioclase, ≥ An₇₈, from alkalic magma has greater light-REE/heavy-REE (LREE/HREE) ratios than less calcic plagioclase, An_64–75, from tholeiitic magma (ratios, 22–33 vs < 20), suggesting that higher LREE/HREE ratios are inherent to plagioclase of alkalic magmas. However, with compositional evolution (i.e., to lower An), plagioclase REE patterns are of limited use for distinguishing tholeiitic from alkalic parentage because LREE/HREE ratios within each group increase and overlap in the range of ∼ 20–90. Sr, Ba, Hf, and Ta can also discern parentages as their abundances in plagioclase largely reflect abundances inherent to their parental magmas. The best expressions for identifying parentage use Sr abundances (Sr vs An; vs Ce/Yb; vs Sr/Ce), although Hf, Ba, and Ta abundances vs An and vs Ce/Yb are also useful – the distinctions due to tholeiitic plagioclase having relatively low Sr (∼ 500–1000 ppm), Ba (< 100 ppm), Hf (< 0.10 ppm), and Ta (< 0.05 ppm). These relationships help to distinguish between the effects of differentiation on trace-element abundances in plagioclase and their abundances owed to intrinsic concentrations in their magmas. They create compositional fields for tholeiitic and alkalic parentages that remain graphically separated even though differentiation may have enriched the plagioclase in incompatible elements.

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Zusammenfassung ?Plagioklas aus tholeitischen und alkalischen Magmen von Hawaii: Unterscheidung aufgrund von REE, Sr, Ba, Hf und Ta Um die M?glichkeit der Unterscheidung tholeitischer von alkalischer Magmaherkunft durch Mineralzusammensetzungen zu verbessern, habe ich die Spurenelementverteilung in Plagioklasen, die von xenolithischen Gabbros des Mauna Kea Vulkans abgetrennt wurden, untersucht, Diese Gabbros entstammen tholeitischen und alkalischen Magmen des Hamakua “Post-Schild” Stadiums des Mauna Kea Vulkanismus. Chondritisch normalisierte Seltene Erd (SEE) Verteilungs-Muster für Plagioklase zeigen, dass stark kalzische Plagioklase, > An₇₈, aus alkalischen Magmen h?here leichte SEE/schwere SEE (LSEE/HSEE) Verh?ltnisse zeigen, als weniger kalzische Plagioklase, An_64–75 aus tholeitischem Magma (Verh?ltniszahlen 22–33 gegenüber < 20). Dies weist darauf hin, dass h?here LSEE/HSEE-Verh?ltnisse typisch für Plagioklase aus alkalischen Magmen sind. Im Zuge der Evolution der Zusammensetzungen (d.h. zu niedrigeren An-Werten hin), sind die SEE Verteilungsmuster von Plagioklasen weniger hilfreich um tholeitische von alkalischer Herkunft zu unterscheiden. Dies ist deshalb so, weil die Verh?ltniszahlen innerhalb jeder Gruppe zunehmen und im Bereich von 20–90 überlappen. Sr, Ba, Hf und Ta k?nnen auch dazu dienen, um die Herkunft der Plagioklase zu definieren, da ihre H?ufigkeit gro?teils auf H?ufigkeiten, die für die Ursprungsmagmen typisch sind, zurückgehen. Die besten Herkunft-Parameter sind die Sr H?ufigkeiten (Sr vs An; vs Ce/Yb; vs Sr/Ce), obwohl die H?ufigkeit von Hf, Ba und Ta gegen An und gegen Ce/Yb auch nützlich sind. Diese Unterscheidungen gehen darauf zurück, dass tholeitische Plagioklase relativ niedrige Sr (∼ 500–1000 ppm), Ba (< 500 ppm) Hf (< 0.10 ppm) und Ta (< 0.5 ppm) führen. Diese Beziehungen erleichtern die Unterscheidung zwischen den Auswirkungen der Differenzierung auf die Spurenelement-Verteilungsmuster in Plagioklasen und auf ihre H?ufigkeiten, die auf die intrisischen Konzentrationen in den Ursprungsmagmen zurückgehen. Sie definieren charakteristische Felder für tholeitische und für alkalische Herkunft, die graphisch separiert bleiben, auch wenn die Gehalte der Plagioklase an inkompatiblen Elementen durch Differenzierung zugenommen haben mag.

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Received July 22, 1999;revised version accepted December 7, 1999     

缓变型地球化学灾害:特征、模型和应用

"缓变型地球化学灾害"是通过长期积累而存在于土壤或沉积物中的包括重金属和有机污染物在内的环境污染物,因环境物理化学条件(例如温度、pH值、湿度、有机质含量等)的改变减小了环境容量,某种或某些形态的污染物大量地被重新活化和突然释放出来并造成严重生态和环境损害的灾害现象.这种灾害具有明显的特征,其定量数学模型可较完整地概括出环境系统从"干净"到"污染"再到"灾害"的整个过程,可以用于灾害的风险概率评估、预测、灾害爆发轨迹等方面的研究,为土壤污染防治和灾害预警提供了定量研究工具和可供实际采用的基本手段,对当前国土资源调查中的"生态环境地球化学评价"具有重要的借鉴意义.     

Role of lithosphere-asthenosphere interaction in the genesis of Quaternary alkali and tholeiitic basalts from Datong, western North China Craton

The geochemistry of Quaternary volcanic rocks from Datong provides important constraints on the petrogenesis of continental alkali and tholeiitic basalts and lithospheric evolution in the western North China Craton. Alkali basalts in north Datong have trace element compositions similar to oceanic island basalts (OIB). They show nearly homogenous isotopic compositions (?_Nd = 5.4-6.8 and ⁸⁷Sr / ⁸⁶Sr = 0.7035-0.7037) that resemble the nearby Hannuoba Miocene basalts, indicating that the two lava suites share a similar asthenospheric source. However, Datong basalts have conspicuously lower Al₂O₃ and CaO, higher SiO₂ and HREE contents and Na / Ti ratios, compared to Hannuoba lavas at comparable MgO. This compositional difference is attributable to the combined effect of source difference and temporal decrease in melting depth. The latter reflects Cenozoic lithospheric thinning of the western North China Craton.Tholeiitic basalts in southeast Datong have incompatible element ratios that differ from OIB; they have lower ?_Nd (1.3-3.7) and higher ⁸⁷Sr / ⁸⁶Sr (0.7039-0.7046) compared to alkali basalts. These moderately evolved rocks (MgO < 7%) display unusually high Cr concentrations (> 200 ppm), a nearly flat LREE pattern and a fractionated HREE with the “kink” occurring at Gd. A shallow melting depth (< 60 km), suggested by their Q-normative composition, is in conflict with the residual garnet in the source (> 75 km) as required by REE modeling. This paradox, which is reminiscent of that for Hawaiian tholeiites, can be reconciled if garnet lherzolite melts react with refractory peridotites during which orthopyroxene is dissolved and olivine precipitates. The diagnostic consequence of this melt-rock reaction includes increases in SiO₂ and Cr, decreases in Al₂O₃ and CaO, and formation of “kinked” REE patterns. Involvement of lithospheric mantle in the genesis of Datong tholeiites may be related to the Cenozoic lithospheric thinning/erosion in the western North China Craton. The spatial distribution of Datong alkali and tholeiitic basalts may be related to enhanced extension along the lithospheric boundary between the Western Block of the North China Craton and the Trans-North China Orogen.     

Petrogenetic reconnaissance investigation of mafic sills associated with flood basalts,Mekelle basin,northern Ethiopia: implications for Ni–Cu exploration

Mafic sills of the Mekelle basin were explored on a reconnaissance level for their magmatic sulphide potential. The sills are related to Oligocene flood basalt magmatism in Ethiopia. They intrude Jurassic shales and limestones and crystallized from high-Ti tholeiitic basalt magmas. Three compositional groups were differentiated. Among them GD1 gabbrodolerites (11.6–4% MgO) show evidence of a fractionation controlled magmatic lineage, while GD2 (∼5.2% MgO) and GD3 gabbrodolerites (5.4–3% MgO) show element depletion and enrichment patterns which deviate from the magmatic fractionation trend. Compared to GD1 rocks with MgO values ∼5%, GD2 gabbrodolerites have higher SiO₂ and Na₂O but lower FeO_tot, TiO₂, P₂O₅, Nb, Zr, Y, Cu, and Ni values, while GD3 gabbrodolerites show higher SiO₂, Na₂O, P₂O₅, Ba and Sr and lower FeO_tot, TiO₂, Nb, Zr, Y, Cu, and Ni values. The distinct geochemical signatures of the latter two gabbrodolerite groups are likely caused by conjunct processes of crustal assimilation and fractional removal of silicate/oxide mineral phases. Depletion of chalcophile elements in the contaminated gabbrodolerites indicates also the segregation of Ni and Cu sulphides. However, settling of sulphide minerals occurred prior to sill emplacement probably in a deeper seated magma chamber. The indication of chalcophile element depleted mafic sills, genetically related to a major flood basalt province clearly warrants further exploration in the area.     

Partitioning of Ni between olivine and siliceous eclogite partial melt: experimental constraints on the mantle source of Hawaiian basalts

Olivine is abundant in Earth’s upper mantle and ubiquitous in basaltic lavas, but rarely occurs in eclogite. Partial melts of eclogite are, therefore, not in equilibrium with olivine, and will react with peridotite as they migrate through the upper mantle. If such melts erupt at Earth’s surface, their compositions will be highly modified and they may be olivine-saturated. We investigated experimentally the reaction between olivine and siliceous eclogite partial melt, and determined element partitioning between olivine and the melt produced by this reaction. Our results demonstrate that mixing of reacted eclogite partial melt with primitive basalt is capable of producing the positive correlation between melt SiO₂ content and olivine Ni content observed in some Hawaiian lavas. Experiments were carried out by equilibrating eclogite partial melt or basalt with San Carlos olivine at 1 bar and 1,201–1,350°C. Our results show that eclogite partial melts equilibrated with mantle olivine retain their high SiO₂, low FeO and MgO characteristics. Further, olivine-melt partition coefficients for Ni measured in these experiments are significantly larger than for basalt. Mixing of these melts with primitive Hawaiian tholeiitic lavas results in crystallization of high-Ni olivines similar to those in Makapuu-stage Koolau lavas, even though the mixed magmas have only moderate Ni contents. This results from a hyperbolic increase of the Ni partition coefficient with increasing polymerization of the mixed melt. Note that while eclogite partial melt in contact with peridotite will equilibrate with pyroxene as well as olivine, this will have the effect of buffering the activity of SiO₂ in the reacted melt at a higher level. Therefore, an eclogite partial melt equilibrated with harzburgite will have higher SiO₂ than one equilibrated with dunite, enhancing the effects observed in our experiments. Our results demonstrate that an olivine-free “hybrid” pyroxenite source is not required to explain the presence of high-Ni olivines in Hawaiian lavas and, therefore, indicate that the proportion of eclogite in the Hawaiian plume is less than has been estimated in recent studies.