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1.
We report systematic noble gas variation within a regional scale in an area that is well separated from all hotspots surrounding a remarkable ridge-ridge-ridge type triple junction—the Rodriguez Triple Junction of the Indian Ocean. We applied a newly established technique to select basaltic glass samples from the Mid-Oceanic Ridge Basalt (MORB) suite from the studied area. Samples were selected to create regular spacing along the ridge axis for investigating the magma system beneath the ridge axis. All samples show a typical isotope signature as MORB regardless of moderate contributions of an atmospheric component. Remarkably, uniform isotope ratios were found even for Ne and Ar in each segment. Such ratios had not been recognized in any magmatic system in Mid-Oceanic Ridges. Elemental abundances of samples are controlled both by mixing with the atmospheric component and by degassing. Contribution of atmospheric components is controlled strongly by a high temperature reaction caused by equilibration of dissolved noble gases between magma and seawater. Of the three models of the magma system around the Rodriguez Triple Junction, controlled contamination without mechanical mixing in a magma chamber presents the most likely scenario.  相似文献   

2.
New volatile data (CO2, H2O, He, Ne, and Ar) are presented for 24 submarine basaltic glasses from the Kolbeinsey Ridge, Tjörnes Fracture Zone and Mohns Ridge, North Atlantic. Low CO2 and He contents indicate that magmas were strongly outgassed with the extent of degassing increasing toward the south, as expected from shallower ridge depths. Ne and Ar are significantly more abundant in the southernmost glasses than predicted for degassed melt. The strong atmospheric isotopic signal associated with this excess Ne and Ar suggests syn- or posteruptive contamination by air. Degassing, by itself, cannot generate the large variations in δ13C values of dissolved CO2 or coupled CO2-Ar variations. This suggests that δ13C values were also affected by some other processes, most probably melt-crust interaction. Modelling indicates that degassing had a negligible influence on water owing to its higher solubility in basaltic melt than the other volatiles. Low H2O contents in the glasses reflect melting of a mantle source that is not water-rich relative to the source of N-MORB.Before eruption, Kolbeinsey Ridge melts contained ∼400 ppm CO2 with δ13C of −6‰, 0.1 to 0.35 wt.% H2O, 3He/4He ∼11 RA, and CO2/3He of ∼2 × 109. We model restored volatile characteristics and find homogeneous compositions in the source of Kolbeinsey Ridge magmas. Relative to the MORB-source, He and Ne are mildly fractionated while the 40Ar/36Ar may be low. The 3He/4He ratios in Tjörnes Fracture Zone glasses are slightly higher (13.6 RA) than on Kolbeinsey Ridge, suggesting a greater contribution of Icelandic mantle from the south, but the lack of 3He/4He variation along the Kolbeinsey Ridge is inconsistent with active dispersal of Icelandic mantle beyond the Tjörnes Fracture Zone.  相似文献   

3.
李理  钟大赉  杨长春  赵利 《岩石学报》2016,32(7):2209-2216
幔源CO_2气的形成和分布与不同级别断层早白垩世以来的活动密切相关。郯庐断裂带是研究区最主要的成气断层,拆离断层和变换断层这些地壳断层是次要的成气断层,二者于早白垩世143Ma、124Ma、新生代~43Ma、~24Ma和~8Ma的走滑或伸展活动,以及与之准同时的新生代碱性玄武岩浆活动,控制了幔源CO_2气的分散和聚集。它们与基底断裂、盖层断裂共同组成运移通道,其中拆离滑脱处的低速带和盖层断裂中的顺层断层是重要的水平运移通道。早白垩世古太平洋板块俯冲脱水脱气,产生的幔源CO_2气沿着郯庐断裂带向上分散聚集;新生代以来受控于太平洋板块俯冲方向和速度的改变以及印欧板块碰撞的远程效应,形成幔源CO_2气。与此同时郯庐断裂带切割深度亦逐渐加大,~43Ma碱性岩浆活动亦开始形成幔源CO_2气并主要位于断裂带,24Ma和8Ma(5Ma)为新近纪碱性岩浆活动脱气两个主要形成时期。郯庐断裂带的活动使地幔脱气形成的CO_2沿断层走向向上运移,并在作为重要横向运移通道的拆离断层拆离滑脱处,与因岩浆脱气形成的CO_2汇合,再通过陡倾斜、缓倾斜基底断层、盖层断层的接力传递在浅部聚集成藏。预测郯庐断裂带附近是无机成因油气重要的聚集分布区带。  相似文献   

4.
Here we present the first data on He, Ne, Ar isotopic and elemental composition in fluid phases of tholeiitic chilled glasses from the Bouvet Triple Junction (BTJ). The chilled glasses from several dredging stations situated at different segments of BTJ have been investigated: Spiess Ridge, Mid Atlantic Ridge (MAR) and in a valley of the Southwest Indian Ridge (SWIR). The data allow to distinguish within BTJ three segments characterized by different geochemical behavior of He, Ne and Ar. MAR and Spiess samples contain MORB-like helium and neon while SWIR is characterized by addition of plume type He and Ne. The strong atmospheric contamination is typical of all segments, but for MAR it is less pronounced. The Ne-Ar isotope systematics suggests that the atmospheric component was most probably introduced into the mantle source of the fluids with fragments of oceanic crust/sediments.  相似文献   

5.
《Applied Geochemistry》2001,16(9-10):1033-1039
The CO2 gas pools of Jiyang sag are located along the Gaoqing–Pingnan fault within a region of alkaline basalts. The concentration of CO2 in the gas pools is in the range of 68.85–96.99%. All of the geochemical tracers for the CO2 gas pools support the suggestion that CO2 was mainly derived from mantle degassing. The δ13C values of CO2 in the gas pools are in the range of −5.67–−3.41‰, which are higher than those of organogenic CO2, and near to those of abiogenic CO2. Their 3He/4He ratios are 2.80–4.47×106, i.e. the R/Ra ratios are 2.00–3.19, showing that the Jiyang sag had undergone strong mantle degassing. CO2/3He ratios are 0.59–0.89×109, which are identical to those for N-MORB, indicating that CO2 in these CO2 gas pools was mainly derived from the mantle. Accompanying the intrusion of mantle-derived magma, the mantle-derived CO2 migrated upwards along deep faults and was trapped in advantageous structures forming gas pools.  相似文献   

6.
Fluid inclusions have recorded the history of degassing in basalt. Some fluid inclusions in olivine and pyroxene phenocrysts of basalt were analyzed by micro-thermometry and Raman spectroscopy in this paper. The experimental results showed that many inclusions are present almost in a pure CO2 system. The densities of some CO2 inclusions were computed in terms of Raman spectroscopic characteristics of CO2 Fermi resonance at room temperature. Their densities change over a wide range, but mainly between 0.044 g/cm3 and 0.289 g/cm3. Their micro-thermometric measurements showed that the CO2 inclusions examined reached homogenization between 1145.5℃ and 1265℃ . The mean value of homogenization temperatures of CO2 inclusions in basalts is near 1210℃. The trap pressures (depths) of inclusions were computed with the equation of state and computer program. Distribution of the trap depths makes it know that the degassing of magma can happen over a wide pressure (depth) range, but mainly at the depth of 0.48 km to 3.85 km. This implicates that basalt magma experienced intensive degassing and the CO2 gas reservoir from the basalt magma also may be formed in this range of depths. The results of this study showed that the depth of basalt magma degassing can be forecasted from CO2 fluid inclusions, and it is meaningful for understanding the process of magma degassing and constraining the inorganogenic CO2 gas reservoir.  相似文献   

7.
Noble gas abundances in basaltic glasses from ocean islands (OIBs) are generally lower than those of mid-oceanic ridge basalts (MORBs), contrary to most geodynamic models which usually require that the source of OIBs is less degassed (resulting in higher primordial noble gas abundances) and more trace element enriched (resulting in higher radiogenic noble gas abundances) than the MORB source. Therefore, noble gas abundances in OIBs are often thought to have been reduced by extensive gas loss from the magma before eruption.The extent of magmatic degassing can be tested as it will cause characteristic changes in the composition of the volatiles; notably the 4He/40Ar* ratio (where 40Ar* is 40Ar corrected for atmospheric contamination) will increase in residual volatiles due to the higher solubility of He relative to Ar. The degree of He-Ar fractionation for a given fraction of gas loss depends on the ratio of the solubilities, SHe/SAr, which is sensitive to (among other things) the CO2 and H2O content of the basalt at the time of degassing.From a global database of OIB and MORB glasses, we show that 4He/40Ar* ratios of MORB glasses are broadly consistent with degassing of a magma with an initial 40Ar of ≈1.5 × 10−5 ccSTP/g, i.e., similar to that of the “popping rock.” However, OIB glasses generally have lower 40Ar* concentration for a given 4He/40Ar*. While this would appear to require lower 40Ar* abundances in the undegassed OIB magmas, the higher volatile contents of OIBs will reduce SHe/SAr (relative to MORBs) during degassing. By modeling SHe/SAr in OIBs, it is possible to show that extensive degassing of OIBs can occur without dramatically increasing the 4He/40Ar* ratio. We show that undegassed 40Ar concentrations of OIB magmas were probably similar to those of MORBs.  相似文献   

8.
The chemistry and isotope ratios of He, C (δ13C) and H (δD) of free gases collected in the San Vittorino plain, an intramontane depression of tectonic origin, were determined to shed light on mantle degassing in central Italy. The C isotopic composition of CO2 (δ13C–CO2 −2.0‰ to −3.8‰) and He isotope ratios (R/RA 0.12–0.27) were used to calculate the fraction of CO2 originating from mantle degassing vs. sedimentary sources. The results show that CO2 predominantly (average of 75%) derives from the thermo-metamorphic reaction of limestone. Between 6% and 22% of the CO2 in the samples derives from organic-rich sedimentary sources. The mantle source accounts for 0–6% of the total CO2; however, in two samples, located in proximity to the most important faults of the plain, the mantle accounts for 24% and 42%. The presence of faults and fractures allows upward gas migration from a deep source to the Earth’s surface, not only in the peri-Tyrrhenian sector, as generally reported by studies on natural gas emissions in central Italy, but also in the pre-Apennine and Apennine belts. Isotope ratios of CH4 (δ13C–CH4 −6.1‰ to −22.7‰; δD–CH4 −9‰ to −129‰) show that CH4 does not appear to be related to mantle or magma degassing, but it is the product of thermal degradation of organic matter (i.e. thermogenic origin) and/or the reduction of CO2 (i.e. geothermal origin). Most of the samples appear to be affected by secondary microbial oxidation processes.  相似文献   

9.
We report new stepped heating He, Ar, CO2 and water data on a petrogenetically diverse suite of lavas from the Manus back-arc basin, where a plume component has previously been identified. The aim of this study is to evaluate the superimposed effects of degassing and contamination in order to identify mantle source characteristics. CO2 abundances and carbon isotopes in both the vesicle ([CO2] up to 180ppm; δ13C as low as -33.6 ‰) and glass ([CO2] up to 270ppm; δ13C as low as -34.3 ‰) phases reveal that samples have been modified by varying degrees of degassing. High water concentration samples (back-arc basin basalts (BABB) and arc type samples) show the highest degrees of degassing (i.e. lower δ13C values and lower CO2 contents). The results are modelled for both the glass and vesicle phases using batch and fractional degassing models. Parental melt compositions can be constrained to show the following CO2 concentration trend: arc-type > BABB s.r. (southern rift) > MORB-2, E-MORB, X-BABB (extreme BABB), BABB > MORB-1 and MORB-smt. 4He/40Ar∗ ratios of samples (14.6-1100) are consistent with residual volatiles from a degassed source. Variations in CO2/3He values are likely due to degassing, followed by contamination from a crustal source (either the subducting Solomon Sea Plate or the pre-existing crust through which the lavas erupt), as evidenced by high K2O/TiO2 ratios and low δ13C. The CO2/3He of the Manus plume is best estimated by the MORB-smt and MORB-1 samples at 3.1 ± 0.6 x 109. This value is similar to previous estimates of plume CO2/3He values, which are either equal to or slightly greater than the upper mantle average of 2 x 109.  相似文献   

10.
Serpentinites occur in transverse fracture zones and adjacent areas in the Mid-Atlantic Ridge near 24° and 30° N. In two fracture zones, about 700 km distant from each other, serpentinites show practically the same trend and range of variation in chemical composition. Their CaO content ranges from 2.05 to 0.07% by weight. Serpentinites relatively high in CaO content contain pargasite, whereas those relatively low in CaO do not. Serpentinites relatively high in CaO are chemically similar to high-temperature peridotites which are widely believed to have been derived from the upper mantle. With a decrease in CaO, the Al2O3, TiO2, K2O and FeO contents and the Fe/Mg ratio tend to decrease, whereas the H2O+ content tends to increase. This compositional variation is probably due partly to heterogeneity of uper mantle peridotite from which the serpentinites were derived, and partly to chemical migration during serpentinization. The interior of the Mid-Atlantic Ridge may be mainly made up of serpentinites. Alternatively, the Mid-Atlantic Ridge Serpentinites may have been formed by serpentinization of peridotites that were intruded into fracture zones from a great depth.Lamont-Doherty Geological Observatory Contribution No. 1354.  相似文献   

11.
Carbon and noble gas isotope analyses are reported for bubbling gas samples from the Tengchong volcanic geothermal area near the Indo-Eurasian suture zone. All samples contain a resolvable component of mantle-derived 3He. Occurrence of mantle-derived 3He coincides with surface volcanism. However, 3He occurs over a larger geographic areathan do surface volcanics. δ13C values for CO2 and CH4 vary from -33.4‰ to 1.6 ‰ and from -52.8‰ to -2.8‰, respectively. He and C isotope systematics indicate that CO2 and CH4 in the CO2-rich gases originated predominantly from magmatic component mixed with crustal CO2 produced from carbonate. However, breakdown of organic matter and near-surface processes accounts for the CH4 and CO2 in N2-rich gases. 3He/4He ratio distribution pattern suggests that mantle-derived He and heat sources of high-temperature system in central Tengchong originate from a hidden magma reservoir at subsurface. CO2-rich gases with the highest 3He/4He ratio (5.2 Ra) may be representative of the  相似文献   

12.
《Applied Geochemistry》2001,16(7-8):883-894
Total CO2 output from fumaroles, bubbling and water dissolved gases and soil gases was investigated at Pantelleria Island volcano, Italy. The preliminary results indicate an overall output of 0.39 Mt a−1 of CO2 from the island. The main contribution to the total output was from diffuse soil degassing (about 0.32 Mt a−1), followed by dissolved CO2 (0.034 Mt a−1), focussed soil degassing (0.028 Mt a−1) and bubbling CO2 (0.013 Mt a−1). The contribution of CO2 from fumarole gases was found to be negligible (1.4×10−6 Mt a−1). Carbon-13 values for CO2 coupled with those for associated He in gases from fumaroles and sites of focussed soil degassing clearly rule out any significant organic CO2 component and suggest a common mantle origin for these gas species. The inferred mantle source beneath Pantelleria would seem to have peculiar geochemical characteristics, quite distinct from those of mantle producing MORB but compatible with those of magmatic sources of central Mediterranean and central European volcanoes. These findings indicate that the Pantelleria volcanic complex is a site of active mantle degassing that is worthy of attention for future geochemical surveillance of the island.  相似文献   

13.
Chemical and isotopic compositions have been measured for N2-He-rich bubbling gases discharging from hot springs in the Hainan Island, Southern China. Observed 3He/4He ratios (0.1–1.3 RA) indicate the occurrence of a mantle component throughout the island, which has been highly diluted by a crustal radiogenic 4He component. The occurrence of mantle-derived helium is high in the northern island (12%–16% of total He) and gradually decreases towards southern coast (1%–3% of total He). Such a distribution pattern is most likely controlled by the Pleocene-Quaternary volcanic activities in the northern island and groundwater circulation along the deep major faults. The 40Ar/36Ar and N2/Ar ratios suggest that N2 and Ar of the hot spring gases are mostly meteoric. Although δ13C values of CO2 (–20‰ to –27‰) with low concentrations are consistent with the biogenic origin, the combination of 3He/4He and d13CCO2 suggests a two end-member mixing of mantle and crustal components with CO2/3He ratios of 2×109 and 8×1011, respectively. However, the low CO2/3He ratios (1–22×106) can not be ascribed in terms of the simple mixing but has to be explained by the addition of radiogenic 4He and loss of CO2 by calcite precipitation in the hydrothermal system, which is most likely controlled by the degree of gas-water-rock interaction.  相似文献   

14.
Gas concentrations and isotopic compositions of He and CO2 were determined on free gas samples from ten hot springs of the Rehai geothermal field, Tengchong, China. The results showed that hot-spring CO2 gas, together with He,was derived mainly from the mantle, indicating the accumulation of mantle-derived volatiles beneath the survey area. The δ^13C values of CO2, higher than those of the typical mantle-derived carbon and the isotopic composition of hot-spring-free CO2 in unequilibrium with dissolved CO2, are recognized only in the Rehai geothermal field, suggesting that there seems to be a still-degassing magmatic intrusion at depths, which provides mantle-derived volatiles to the hydrothermal system above. The accumulation of those volatiles has probably played an important role in triggering earthquakes in this region.In addition, the isotopic characteristics of He and C also indicate that the magmatic intrusion seems to have been derived from the MORB source, and could be contaminated by crustal materials during its upwelling through the continental crust.  相似文献   

15.
Vesicles,water, and sulfur in Reykjanes Ridge basalts   总被引：1，自引：0，他引：1  
Dredge hauls of fresh submarine basalt collected from the axis of the Reykjanes Ridge (Mid-Atlantic Ridge) south of Iceland were taken aboard R/ V TRIDENT in 1967 and 1971. The samples show systematic changes as the water depth of collection (and eruption) decreases: radially elongate vesicles and concentric zones of vesicles appear at about 700 m depth and are conspicuous to shallow water; the smoothed volume percent of vesicles increases from 5% at 1000 m, 10% at 700 m, to 16% at 500 m, and the scatter in degree of vesicularity increases in shallower water; specific gravity decreases from 2.7±0.1 at 1000 m to 2.3±0.3 at 100 m. Bulk sulfur content for the outer 2 cm averages 843 ppm up to a depth of 200 m, then drops off rapidly in shallower water owing to degassing. Sulfur content below 200 m is independent of depth (or geographic position), and the melt is apparently saturated with sulfur, but the excess cannot escape the lava unless another vehicle carries it out. Only shallower than 200 m, where intense vesiculation of other gases occurs can excess sulfur be lost from the lava erupting on the sea floor. H2O+110° averages about 0.35 percent and H2O+150° about 0.25 percent, and both apparently decrease in water shallower than 200 m as a result of degassing. H2O+ (below 200 m) decreases with distance from Iceland or increasing depth, presumably as a result of either adsorption of water on the surface of shallower, more vesicular rocks; or more likely due to the presence of the Iceland hot mantle plume supplying undifferentiated primordial material, relative to lavas of the Reykjanes Ridge supplied from the low velocity layer already depleted in volatiles and large lithophile elements. The H2O+110°/S ratio of lava erupting below 200 m water depth ranges from 3 to 5 which is comparable to reliable gas analyses from oceanic basaltic volcanoes. Publication authorized by the Director, U.S. Geological Survey.  相似文献   

16.
A metabasalt dredged at a junction of the median valley with the Atlantis fracture zone (30°01/tN, 42°04/tW) in the Mid-Atlantic Ridge shows complete recrystallization under a metasomatic condition, though the original igneous texture of a coarse-grained basalt is still recognizable. There is strong circumstantial evidence suggesting that this rock is not an ice-rafted erratic, but an authentic Mid-Atlantic Ridge rock. The 40Ar-39Ar age of this sample is 169 m.y. (Jurassic) which should represent the time of recrystallization. The initial value (87Sr/86Sr)O is 0.720, far above the values previously observed in oceanfloor basalts, including both tholeiitic and alkalic rocks (0.701–0.704). Sr with such a high isotopic ratio is considered to have been introduced by metasomatism during metamorphism by a solution coming from a continental mass or masses which were then located very close to the Mid-Atlantic Ridge. The 40Ar−39Ar age of sample AM50 may approximate the time of the commencement of the opening of the Atlantic. All these data support the possible existence of ancient rock masses in the Mid-Atlantic Ridge, as was formerly claimed by Bonatti, Melson and others.  相似文献   

17.
In the Czech-German border region of the Vogtland and NW Bohemia (western Eger rift, Central Europe), chemical and isotopic compositions (C, N, He, Ar) of free gas from a thermal water escape (fluorite mine, Schönbrunn), two mineral springs (“Eisenquelle,” Bad Brambach; “Sprudel III,” Bad Elster) and a mofette (Bublak) located along an ∼40-km long traverse are reported. The gases of Bublak and Bad Brambach are CO2-rich (>99 vol.%) and have δ13C values of −1.95 and −4.29‰, respectively. With distance from the center of CO2 degassing (Bublak) the δ13C values decrease, most likely due to physico-chemical fractionation of CO2 between gaseous and aqueous phases rather than to admixture of organic/biogenic CO2. The δ15N values range between −3.2 and −0.6‰, compared to an upper mantle value of −4.0 ± 1.0‰. The four locations are characterized by 3He/4He ratios decreasing from 5.9 Ra in the center (Bublak) to 0.8 Ra in the periphery (Schönbrunn) and give evidence for mixing of He from a deep-seated magmatic source with a crustal source. The location with the highest 3He/4He ratio (5.9 Ra) is accompanied by the highest 40Ar/36Ar (550). We argue that the nitrogen of the Bublak mofette gas is a mixture of predominantly atmospheric and mantle-derived components, whereas at the other three locations crustal nitrogen may also be present. The Bublak δ15N value of ≈−4.5 ± 1.0‰ represents the first free gas δ15N reference from the European subcontinental mantle (ESCM) and indicates that, in contrast to the 3He/4He ratios, the δ15N values are equal for ESCM and MORB, respectively.  相似文献   

18.
The paper presents data on the composition of a gas phase of underground fluids in the East Carpathian region, including 3He/4He, 40Ar/36Ar, and 4He/20Ne ratios. The argon isotope composition of these gases was used to estimate the fractions of atmospheric Aratm and radiogenic 40Arrad formed in the rocks, N2/Aratm ratio, and to reveal the admixture of nonatmogenic (“excess”) nitrogen in most samples. The CO2 content in gases positively correlates with the fraction of mantle component in fluid helium. At the same time, the CO2 content shows a negative correlation with the total helium (and light 3He enriching mantle derivatives), thus excluding the simultaneous influx of CO2 and helium from a common mantle source in the fluids. A wide spectrum of 3He/4He = R in gases of the region spanning three orders of magnitude confirms the concept of mixing of the crustal and mantle components in the helium. However, even gases with similar R values show a wide scatter of He concentrations. This is mainly caused by the additional influx of other gases: CH4 formed during OM transformation or CO2 released during the thermal metamorphism of carbonate sequences. Correlation of the CH4/3He ratio and the helium isotope composition in the Carpathian gases indicates the crustal origin of hydrocarbons, which formed economic gas pools in the Ciscarpathian Trough and the adjacent part of the Folded Carpathians. Lateral chemical and isotope variations revealed in the underground fluids are related to the tectonic zoning of the region. The helium isotope variations are also consistent with the geodynamic setting of the region (thinning of the crust and lithosphere towards the Pannonian Basin, growth of the background conductive heat flow and corresponding ascent of isotherms). In combination with geothermal data, they reflect specifics of the mantle heat-and-mass flow discharge.  相似文献   

19.
The islands of the Azores archipelago emerge from an oceanic plateau built on lithosphere increasing in age with distance from the Mid-Atlantic Ridge from 10 to 45 Ma. Here, we present the first comprehensive major and trace element and Sr–Nd–Pb isotope data from Santa Maria, the easternmost island of the archipelago, along with published data from the other Azores islands situated much closer to the Mid-Atlantic Ridge axis. We can show that the distinctively more variable and more enriched trace element ratios at Santa Maria combined with a relatively small range in Sr–Nd–Pb isotope ratios are the result of low degrees of partial melting of a common Azores mantle plume source underneath thicker lithosphere. This implies that melt extraction processes and melting dynamics may be able to better preserve the trace element mantle source variability underneath thicker lithosphere. These conclusions may apply widely for oceanic melts erupted on relatively thick lithosphere. In addition, lower Ti/Sm and K/La ratios and SiO2 contents of Santa Maria lavas imply melting of a carbonated peridotite source. Mixing of variable portions of deep small-degree carbonated peridotite melts and shallow volatile-free garnet peridotite could explain the geochemical variability underneath Santa Maria in agreement with the volatile-rich nature of the Azores mantle source. However, Santa Maria is the Azores island where the CO2-rich nature of the mantle source is more evident, reflecting a combination of a smaller extent of partial melting and the positioning at the edge of the tilted Azores mantle plume.  相似文献   

20.
Major element compositions of submarine basalts, quenched glasses, and contained phenocrysts are reported for samples from 25 dredge stations along the Mohns-Knipovich Ridge between the Jan Mayen fracture zone and 77°30N. Most of the basalts collected on the Jan Mayen platform have a subaerial appearance, are nepheline normative, rich in incompatible elements, and have REE-patterns strongly enriched in light-REE. The other basalts (with one exception) are tholeiitic pillow basalts, many of which have fresh quenched glass rims. From the Jan Mayen platform northeastwards the phenocryst assemblage changes from olivine±plagioclase±clinopyroxene±magnetite to olivine +plagioclase±chrome-spinel. This change is accompanied by a progressive decrease in the content of incompatible elements, light-REE enrichments and elevation of the ridge that are similar to those observed south of the Azores and Iceland hotspots. Pillow basalts and glasses collected along the esternmost part of the Mohns Ridge (450 to 675 km east of Jan Mayen) have low K2O, TiO2, and P2O5 contents, light-REE depleted patterns relative to chondrites, and Mg/(Mg+Fe2+) ratios between 0.64 and 0.60. Pillow basalts and glasses from the Knipovich Ridge have similar (Mg/Mg+Fe2+) ratios, but along the entire ridge have slightly higher concentrations of incompatible elements and chondritic to slightly light-REE enriched patterns. The incompatible element enrichment increases slightly northward. Plagioclase phenocrysts show normal and reverse zoning on all parts of the ridge whereas olivines are unzoned or show only weak normal zoning. Olivine-liquid equilibrium temperatures are calculated to be in the range of 1,060–1,206° C with a mean around 1,180° C.Rocks and glasses collected on the Jan Mayen Platform are compositionally similar to Jan Mayen volcanic products, suggesting that off-ridge alkali volcanism on the Jan Mayen Platform is more widespread than so far suspected. There is also evidence to suggest that the alkali basalts from the Jan Mayen Platform are derived from deeper levels and by smaller degrees of partial melting of a mantle significantly more enriched in light-REE and other incompatible elements than are the tholeiitic basalts from the Eastern Mohns and Knipovich Ridge. The possibility of the presence of another hitherto unsuspected enriched mantle region north of 77° 30 N is also briefly considered.It remains uncertain whether geochemical gradients revealed in this study reflect: (1) the dynamics of mixing during mantle advection and magma emplacement into the crust along the Mid-Atlantic Ridge (MAR) spreading axis, (e.g. such as in the mantle plume — large-ion-lithophile element depleted asthenosphere mixing model previously proposed); or (2) a horizontal gradation of the mantle beneath the MAR axis similar to that observed in the overlying crust; or (3) a vertical gradation of the mantle in incompatible elements with their contents increasing with depth and derivations of melts from progressively greater depth towards the Jan Mayen Platform.  相似文献   

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