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1.
We report a comprehensive morphological, gravity and magnetic survey of the oblique- and slow-spreading Reykjanes Ridge near the Iceland mantle plume. The survey extends from 57.9°N to 62.1°N and from the spreading axis to between 30 km (3 Ma) and 100 km (10 Ma) off-axis; it includes 100 km of one arm of a diachronous ‘V-shaped' or ‘chevron' ridge. Observed isochrons are extremely linear and 28° oblique to the spreading normal with no significant offsets. Along-axis there are ubiquitous, en-echelon axial volcanic ridges (AVRs), sub-normal to the spreading direction, with average spacing of 14 km and overlap of about one third of their lengths. Relict AVRs occur off-axis, but are most obvious where there has been least axial faulting, suggesting that elsewhere they are rapidly eroded tectonically. AVRs maintain similar plan views but have reduced heights nearer Iceland. They are flanked by normal faults sub-parallel to the ridge axis, the innermost of which occur slightly closer to the axis towards Iceland, suggesting a gradual reduction of the effective lithospheric thickness there. Generally, the amplitude of faulting decreases towards Iceland. We interpret this pattern of AVRs and faults as the response of the lithosphere to oblique spreading, as suggested by theory and physical modelling. An axial, 10–15 km wide zone of high acoustic backscatter marks the most recent volcanic activity. The zone's width is independent of the presence of a median valley, so axial volcanism is not primarily delimited by median valley walls, but is probably controlled by the lateral distance that the oblique AVRs can propagate into off-axis lithosphere. The mantle Bouguer anomaly (MBA) exhibits little mid- to short-wavelength variation above a few milliGals, and along-axis variations are small compared with other parts of the Mid-Atlantic Ridge. Nevertheless, there are small axial deeps and MBA highs spaced some 130 km along-axis that may represent subdued third-order segment boundaries. They lack coherent off-axis traces and cannot be linked to Oligocene fracture zones on the ridge flanks. The surveyed chevron ridge is morphologically discontinuous, comprising several parallel bands of closely spaced, elevated blocks. These reflect the surrounding tectonic fabric but have higher fault scarps. There is no evidence for off-axis volcanism or greater abundance of seamounts on the chevron. Free-air gravity over it is greater than expected from the observed bathymetry, suggesting compensation via regional rather than pointwise isostasy. Most of the observed variation along the ridge can be ascribed to varying distance from the mantle plume, reflecting changes in mantle temperature and consequently in crustal thickness and lithospheric strength. However, a second-order variation is superimposed. In particular, between 59°30′N and 61°30′N there is a minimum of large-scale faulting and crustal magnetisation, maximum density of seamounts, and maximum axial free-air gravity high. To the north the scale of faulting increases slightly, seamounts are less common, and there is a relative axial free-air low. We interpret the 59°30′N to 61°30′N region as where the latest chevron ridge intersects the Reykjanes Ridge axis, and suggest that the morphological changes that culminate there reflect a local temperature high associated with a transient pulse of high plume output at its apex.  相似文献   

2.
We present the results of a detailed petrological study of a sparsely phyric basalt (MAPCO CH98-DR11) dredged along the Mid-Atlantic Ridge (30°41′N). The sample contains microphenocrysts of olivine that display four different rapid-growth morphologies. Comparison of these morphologies with those obtained in dynamic crystallization experiments allows us to constrain the thermal history of the sample. The dendritic morphology (swallowtail, chain and lattice olivine) is directly related to the final quenching during magma–seawater interaction. In contrast, the three other morphologies, namely the complex polyhedral crystal, the closed hopper and the complex swallowtail morphology result from several cycles of cooling–heating (corresponding to a maximum degree of undercooling of 20–25°C) during crystal growth. These thermal variations occurred before eruption and are interpreted to be the result of turbulent convection in a small magmatic body beneath the ridge. The results suggest that the Mid-Atlantic Ridge is underlain by a mush zone that releases batches of liquid during tectonic segregation. Aphyric basalts are emitted during eruptions controlled by the tectonic activity, whereas phyric basalts correspond to small fractions of magma from the mush zone mobilized by reinjections of primitive magmas.  相似文献   

3.
The new data presented here from a 10–24°N segment of the North Mid-Atlantic Ridge show that this segment is the most depleted of the 10–70°N ridge section. They also show the existence of: (1) a geochemical gradient from the 14°N anomaly to 17°10′N; (2) a very depleted mantle source (the lowest Sr isotopic ratios found so far in the North Atlantic); and (3) a geochemical limit located at about 17°10′N without any obvious relation with any structural feature. The 15°20′N fracture zone does not show any relationship with respect to this gradient. The basalts located north of 17°10′N have very homogeneous features, which allow their characteristics to be averaged (i.e., 87Sr/86Sr= 0.70238 ± 0.00004, (Nb/Zr)N = 0.28 ± 0.1) and they are defined as normal mid-ocean ridge basalts. The basaltic glasses located south of 17°10′N present a wide spectrum of isotopic compositions and extended rare earth element patterns (from depleted to enriched). Despite this, they have a constant K/Nb of 233 ± 9 (1sM, n = 18) whereas this ratio is 344 ± 29 north of 17°10′N. These observations illustrate the strong coherence of behaviour between K and Nb (Ta) during the petrogenic processes involved in the generation of these mid-ocean ridge basalts and also their fractionation during previous mantle processes. Possible interpretations of mixing processes are discussed and sources at the ridge segment scale are favoured. However, when looking in detail, local heterogeneities are still common and can even be traced back off-axis to 115 my.

Placed in the context of the North Atlantic Ridge from 10° to 70°N, the Sr isotopic ratios reveal the Azores superstructure (23–50°N), whereas the trace element ratios (La/Sm-Nb/Zr) trace the second-order structures (33–40°N, 42–48°N) superimposed on the superstructure. This study illustrates the complementarity of information given by certain well chosen trace element ratios on the one hand and by isotopic ratios on the other. Since there is evidence of decoupling between isotopic ratios and/or trace element ratios, it introduces the notion of complementary “chemical memory” as recorded by a given type of trace element ratio or a given type of isotopic ratio  相似文献   


4.
In July 1985 the first high-temperature, hydrothermal vents ever discovered on the MAR were found at 26°N (TAG). A transponder-navigated survey of a 4 × 8 km area was begun when on-board measurements of manganese revealed concentrated hydrothermal plumes in the water column comparable in size and intensity to those found over geothermal fields on the EPR. These plumes also possess anomalously high light scattering properties and anomalously warm potential temperatures. Using relay-transponder navigation with a rosette-CTD system, it was possible to sample bottom waters with temperature anomalies of several decidegrees. Samples collected near vent orifices possessed silica anomalies of up to 17 μM and dissolved manganese anomalies as large as 360 nmol/kg. These manganese and silica anomalies are linearly related and predict aMn/Si ratio of 20 mmol/mol for these MAR vent fluids, a value essentially identical to that measured in 350°C fluid from the EPR at 21°N. The distribution of temperature anomalies in the water column over this area requires the presence of more than one active vent site. A rough estimate of the convective heat loss from this section of the MAR based onθ-z profiles is 1.2 × 108 cal/s which is similar to the heat loss estimated for the vent field on the EPR at 21°N.  相似文献   

5.
The segmentation of the Mid-Atlantic Ridge between 29°N and 31°30′ N during the last 10 Ma was studied. Within our survey area the spreading center is segmented at a scale of 25–100 km by non-transform discontinuities and by the 70 km offset Atlantis Transform. The morphology of the spreading center differs north and south of the Atlantis Transform. The spreading axis between 30°30′N and 31°30′N consists of enéchelon volcanic ridges, located within a rift valley with a regional trend of 040°. South of the transform, the spreading center is associated with a well-defined rift valley trending 015°. Magnetic anomalies and the bathymetric traces left by non-transform discontinuities on the flanks of the Mid-Atlantic Ridge provide a record of the evolution of this slow-spreading center over the last 10 Ma. Migration of non-transform offsets was predominantly to the south, except perhaps in the last 2 Ma. The discontinuity traces and the pattern of crustal thickness variations calculated from gravity data suggest that focused mantle upwelling has been maintained for at least 10 Ma south of 30°30′ N. In contrast, north of 30°30′N, the present segmentation configuration and the mantle upwelling centers inferred from gravity data appear to have been established more recently. The orientation of the bathymetric traces suggests that the migration of non-transform offsets is not controlled by the motion of the ridge axis with respect to the mantle. The evolution of the spreading center and the pattern of segmentation is influenced by relative plate motion changes, and by local processes, perhaps related to the amount of melt delivered to spreading segments. Relative plate motion changes over the last 10 Ma in our survey area have included a decrease in spreading rate from 32 mm a−1 to 24 mm a−1, as well as a clockwise change in spreading direction of 13° between anomalies 5 and 4, followed by a counterclockwise change of 4° between anomaly 4 and the present. Interpretation of magnetic anomalies indicates that there are significant variations in spreading asymmetry and rate within and between segments for a given anomaly time. These differences, as well as variations in crustal thickness inferred from gravity data on the flanks of spreading segments, indicate that magmatic and tectonic activity are, in general, not coordinated between adjacent spreading segments.  相似文献   

6.
Temperature measurements of hydrothermal vent fluids provide an important indicator of the physical and chemical state of mid-ocean ridge crest hydrothermal and magmatic systems. Changes in vent fluid temperature and chemistry can have dramatic effects on biological communities that inhabit these unique ecosystems. In an attempt to understand temporal variability of ridge crest hydrothermal activity as it relates to geological processes at the ridge axis, six high-temperature hydrothermal vents on the East Pacific Rise crest between 9°49′N and 9°51′N were instrumented and sampled repeatedly during five years following a submarine volcanic eruption in 1991. Bio9 vent, located on the floor of the axial trough near 9°50.2′N, has the most complete record of fluid temperatures from 1991 to 1997, including a continuous temperature record of nearly three years (1994–1997). Bio9 vent fluids were 368°C in 1991, increased to an estimated temperature ≥388°C after a second volcanic event in 1992, and thereafter declined over the next 2 years reaching a temperature of 365°C in December 1993. Continuous temperature records and point measurements made by Alvin's thermocouple probe show Bio9 vent fluids were stable for 15 months at 365±1°C, until March 26, 1995. On March 26, an abrupt 7°C increase occurred over a period of eight days at this vent, and a maximum temperature of 372±1°C persisted for 14 days. The vent fluid cooled gradually over 3.5 months to 366±1°C, and for several months at the end of the recording period the temperature increased a few degrees. A continuous record of fluid temperature at this vent between November 1995 and November 1997 shows a 5±1°C increase for the two-year period. The abrupt temperature increase at Bio9 vent, and coincident changes in faunal community structure, and geochemistry of vent fluids from this area suggest that a crustal event occurred, either in the form of a cracking front in the crust or intrusion of a small dike. Based on the results of a microseismicity experiment conducted around the Bio9 vent in 1995 [Sohn et al., Trans. Am. Geophys. Union 78 (1997) F647; Sohn et al., Nature (in press)], and the identification of a small earthquake swarm which occurred on March 22, 1995 we conclude that the temperature anomaly measured at Bio9 four days following the swarm was caused by a cracking front penetrating into hot crustal rocks beneath the vent.  相似文献   

7.
We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02′S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48′S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an  18 km2 area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005−06 at the East Pacific Rise, 9°50′N and reference to global seismic catalogues reveals that a swarm of large (M 4.6−5.6) seismic events was centred on the 5°S segment over a  24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at  3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest.  相似文献   

8.
Zinc stable isotopes in seafloor hydrothermal vent fluids and chimneys   总被引：3，自引：0，他引：3  
Many of the heaviest and lightest natural zinc (Zn) isotope ratios have been discovered in hydrothermal ore deposits. However, the processes responsible for fractionating Zn isotopes in hydrothermal systems are poorly understood. In order to better assess the total range of Zn isotopes in hydrothermal systems and to understand the factors which are responsible for this isotopic fractionation, we have measured Zn isotopes in seafloor hydrothermal fluids from numerous vents at 9–10°N and 21°N on the East Pacific Rise (EPR), the TAG hydrothermal field on the Mid-Atlantic Ridge, and in the Guaymas Basin. Fluid δ66Zn values measured at these sites range from + 0.00‰ to + 1.04‰. Of the many physical and chemical parameters examined, only temperature was found to correlate with fluid δ66Zn values. Lower temperature fluids (< 250 °C) had both heavier and more variable δ66Zn values compared to higher temperature fluids from the same hydrothermal fields. We suggest that subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite (Zn sulfide), and that this process is a primary cause of Zn isotope variation in hydrothermal fluids. Thermodynamic calculations carried out to determine saturation state of sphalerite in the vent fluids support this hypothesis with isotopically heaviest Zn found in fluids that were calculated to be saturated with respect to sphalerite. We have also measured Zn isotopes in chimney sulfides recovered from a high-temperature (383 °C) and a low-temperature (203 °C) vent at 9–10°N on the EPR and, in both cases, found that the δ66Zn of chimney minerals was lighter or similar to the fluid δ66Zn. The first measurements of Zn isotopes in hydrothermal fluids have revealed large variations in hydrothermal fluid δ66Zn, and suggest that subsurface Zn sulfide precipitation is a primary factor in causing variations in fluid δ66Zn. By understanding how chemical processes that occur beneath the seafloor affect hydrothermal fluid δ66Zn, Zn isotopes may be used as a tracer for studying hydrothermal processes.  相似文献   

9.
We have developed techniques to determine238U,234U and232Th concentrations in seawater by isotope dilution mass spectrometry. U measurements are made using a233U236U double spike to correct for instrumental fractionation. Measurements on uranium standards demonstrate that234U/238U ratios can be measured accurately and reproducibly.234U/238U can be measured routinely to ± 5‰ (2σ) for a sample of 5 × 109 atoms of234U (3 × 10−8 g of total U, 10 ml of seawater). Data acquisition time is 1 hour. The small sample size, high precision and short data acquisition time are superior to-counting techniques.238U is measured to ± 2‰ (2σ) for a sample of 8 × 1012 atoms of238U ( 3 × 10−9 g of U, 1 ml of seawater).232Th is measured to ± 20‰ with 3 × 1011232Th atoms (10−10 g232Th, 1 1 of seawater). This small sample size will greatly facilitate investigation of the232Th concentration in the oceans. Using these techniques, we have measured238U,234U and232Th in vertical profiles of unfiltered, acidified seawater from the Atlantic and238U and234U in vertical profiles from the Pacific. Determinations of234U/238U at depths ranging from 0 to 4900 m in the Atlantic (7°44′N, 40°43′W) and the Pacific (14°41′N, 160°01′W) Oceans are the same within experimental error (± 5‰,2σ). The average of these234U/238U measurements is 144 ± 2‰ (2σ) higher than the equilibrium ratio of 5.472 × 10−5. U concentrations, normalized to 35‰ salinity, range from 3.162 to 3.281 ng/g, a range of 3.8%. The average concentration of the Pacific samples (31°4′N, 159°1′W) is 1% higher than that of the Atlantic (7°44′N, 40°43′W and 31°49′N, 64°6′W).232Th concentrations from an Atlantic profile range from 0.092 to 0.145 pg/g. The observed constancy of the234U/238U ratio is consistent with the predicted range of234U/238U using a simple two-☐ model and the residence time of deep water in the ocean determined from14C. The variation in salinity-normalized U concentrations suggests that U may be much more reactive in the marine environment than previously thought.  相似文献   

10.
The Vema Transverse Ridge (VTR) is a prominent, long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the Vema transform at 11° N in the Atlantic. It rises abruptly about 140 km from the axis of the Mid-Atlantic Ridge (MAR) in 10 Myr old crust and runs continuously up to 25 Myr old crust. It reaches over 3 km above the predicted lithospheric thermal contraction level. It is absent in crust younger than 10 Myr; thus, the uplift of the VTR must have ended roughly 10 Ma. The VTR is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere that was generated at an 80 km long MAR segment. Based on satellite gravimetry imagery this MAR segment was born roughly 50 Ma and increased its length at an average rate of 1.6 mm/yr. Multibeam data show that the MAR-parallel sea floor fabric south of the VTR shifts its orientation by 5° to 10° clockwise in 11–12 Myr old crust, indicating a change at that time of the orientation of the MAR axis and of the position of the Euler rotation pole. This change caused extension normal to the transform, followed between 12 and 10 Ma by flexure of the edge of the lithospheric slab, uplift of the VTR at a rate of 2 to 4 mm/yr, and exposure of a lithospheric section (Vema Lithospheric Section or VLS) at the northern edge of the slab, parallel to the Vema transform. Ages of pelagic carbonates encrusting ultramafic rocks sampled at the base of the VLS at different distances from the MAR axis suggest that the entire VTR rose vertically as a single block within the active transform offset. A 50 km long portion of the crest of the VTR rose above sea level, subsided, was truncated at sea level and covered by a carbonate platform. Subaerial and submarine erosion has gradually removed material from the top of the VTR and has modified its slopes. Spreading half rate of the crust south of the transform decreased from 17.2 mm/yr between 26 and 19 Ma to 16.9 mm/yr between 19 and 10 Ma, to 13.6 mm/yr from 10 Ma to present. The slowing down of spreading occurred close in time to the change in ridge/transform geometry, suggesting that the two events are related. A numerical model relates lithospheric flexure to extension normal to the transform, suggesting that the extent of the uplift depends on the thickness of the brittle layer, consistent with the observed greater uplift of the older lithosphere along the VTR.  相似文献   

11.
In the Seventh cruise of R/V “Professor Logatchev” anomalies of natural electric field (EF), Eh and pS were discovered using a towed instrument package (RIFT) at 14°45′N on the MAR (Logatchev hydrothermal field). The anomalous zone (AZ) is situated close (10–35 m) to two low-temperature venting areas of degrading sulphides and a black smoker (Irina-Microsmoke) forming a distinct buoyant plume. Over or close to the main area of high-temperature venting situated to the south-east from the AZ, no EF or Eh anomalies were observed. According to the results of Mir dives the highly mineralised solutions from smoking craters at the main mound mostly form non-buoyant plumes (reverse-plumes). The buoyant plume structure shows the differentiation of the electrical and Eh fields within the plume. Maxima of the EF, Eh and EH2S anomalies were revealed in the lower part (15 m) of the plume. The negative redox potential plume coupled with a sulphide anomaly is more localized in comparison with the EF. This observation indicates a distinct change in the composition of buoyant plume water, which may be due to the formation and fallout of early formed Fe sulphide particles soon after venting.  相似文献   

12.
Twenty stations were established in the near-shore regions of South Fujian Shoal (116°10′–119°00′ E, 21°20′–24°10′ N) on summer and winter cruises during the period from August 1997 and February to March 1998. The distribution pattern of marine bacterial β-glucosidase activity (β-GlcA) has been investigated by using fluorogenic model substrate (FMS) technique in order to have better understanding of the β-GlcA, as well as its relation to marine bacterial biomass, productivity and environmental factors in Taiwan strait. The results showed that: (1) In summer, the average of β-GlcA at the Southern stations of Taiwan strait was 1.94 nmol/1 h. While in winter, the average of β-GlcA at the Northern stations was 0.86 nmol/1 h and the range of variation (0.34–1.89 nmol/1 h) was much more narrow than that in summer (0.31–8.1 nmol/1 h). (2) According to the carbon conversion factor, the β-GlcA was 0.14 and 0.062 ugc/1 h in summer and winter respectively. These β-GlcA values were higher than the bacterial production of the two seasons respectively. (3) The β-GlcA gradually rises from offshore water to near-shore water. (4) The correlation between the β-GlcA and the bacterial secondary production was not so obvious. (5) The correlation between the section distributions, daily varying of the β-GlcA and the bacterial production was not obvious. (6) In the surface water, the distribution character of free-state β-GlcA from bacteria was equal to that of the total β-GlcA in the whole sea area.  相似文献   

13.
Despite a spreading rate of 65–70 km Ma−1, the East Scotia Ridge has, along most of its length, a form typically associated with slower rates of sea floor spreading. This may be a consequence of cooler than normal mantle upwelling, which could be a feature of back-arc spreading. At the northern end of the ridge, recently acquired sonar data show a complex, rapidly evolving pattern of extension within 100 km of the South Sandwich Trench. New ridge segments appear to be nucleating at or near the boundary between the South American and Scotia Sea plates and propagating southwards, supplanting older segments. The most prominent of these, north of 56°30′S, has been propagating at a rate of approximately 60 km Ma−1 for at least 1 Ma, and displays a morphology unique on this plate boundary. A 40 km long axial high exists at the centre of this segment, forming one of the shallowest sections of the East Scotia Ridge. Beneath it, seismic reflection profiles reveal an axial magma chamber, or AMC, reflector, similar to those observed beneath the East Pacific Rise and Valu Fa Ridge. Simple calculations indicate the existence here of a narrow (<1 km wide) body of melt at a depth of approximately 3 km beneath the sea floor. From the topographic and seismic data, we deduce that a localised mantle melting anomaly lies beneath this segment. Rates of spreading in the east Scotia Sea show little variation along axis. Hence, the changes in melt supply are related to the unique tectonic setting, in which the South American plate is tearing to the east, perhaps allowing mantle flow around the end of the subducting slab. Volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melting observed. A southward mantle flow may have existed beneath the axis of the East Scotia Ridge throughout its history.  相似文献   

14.
Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manifestations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Channel rift zone, south of the African–European collision plate boundary. We find that Pantelleria fluids contain a CO2–He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a main thermal (100°C) aquifer of mixed marine–meteoric water. The measured 3He/4He ratios and δ13C of both the free gases (4.5–7.3 Ra and −5.8 to −4.2‰, respectively) and dissolved helium and carbon in waters (1.0–6.3 Ra and −7.1 to −0.9‰), together with their covariation with the He/CO2 ratio, constrain a 3He/4He ratio of 7.3±0.1 Ra and a δ13C of ca. −4‰ for the magmatic end-member. These latter are best preserved in fluids emanating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir. Outside the caldera, the magmatic component is more affected by air dilution and, at a few sites, by mixing with either organic carbon and/or radiogenic 4He leached from the U–Th-rich trachytic host rocks of the aquifer. Pantelleria magmatic end-member is richer in 3He and has a lower (closer to MORB) δ13C than all fluids yet analyzed in volcanic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9±0.2 Ra, δ13C=−3±1‰). This observation is consistent with a south to north increasing imprint of subducted crustal material in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of the African–Eurasian plate collision boundary. Our results for Pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or ‘less contaminated’, 3He-richer mantle source beneath the Sicily Channel rift zone. The lower than MORB 3He/4He ratio but higher than MORB CO2/3He ratio of Pantelleria volatile end-member are compatible with petro-geochemical evidence that this mantle source includes an upwelling HIMU–EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle.  相似文献   

15.
Seismic reflection and refraction data acquired on four transects spanning the Southeast Greenland rifted margin and Greenland–Iceland Ridge (GIR) provide new constraints on mantle thermal structure and melting processes during continental breakup in the North Atlantic. Maximum igneous crustal thickness varies along the margin from >30 km in the near-hotspot zone (<500 km from the hotspot track) to 18 km in the distal zone (500–1100 km). Magmatic productivity on summed conjugate margins of the North Atlantic decreases through time from 1800±300 to 600±50 km3/km/Ma in the near-hotspot zone and from 700±200 to 300±50 km3/km/Ma in the distal zone. Comparison of our data with the British/Faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist. Joint consideration of crustal thickness and mean crustal seismic velocity suggests that along-margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature. The thermal anomaly (ΔT) at breakup was modest (100–125°C), varied little along the margin, and transient. Data along the GIR indicate that the potential temperature anomaly (125±50°C) and upwelling ratio (4 times passive) of the Iceland hotspot have remained roughly constant since 56 Ma. Our results are consistent with a plume–impact model, in which (1) a plume of radius 300 km and ΔT of 125°C impacted the margin around 61 Ma and delivered warm material to distal portions of the margin; (2) at breakup (56 Ma), the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin; and (3) by 45 Ma, both the remaining plume head and the distal warm layer were exhausted, with excess magmatism thereafter largely confined to a narrow (<200 km radius) zone immediately above the Iceland plume stem. Alternatively, the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre-existing thermal anomaly unrelated to the plume.  相似文献   

16.
Water contents have been measured in basaltic glasses from submarine and subglacial eruption sites along the Reykjanes Ridge and Iceland, respectively, in order to evaluate the hypothesis of Schilling et al. [Phil. Trans. R. Soc. London A 56 (1980) 147-178] that hot spots are also wet spots. Having erupted under pressure the water contents measured in these samples are potentially unaffected by degassing. After correcting these water contents for the effects of crystallisation (to give H2O(8) values) they indicate that the concentration of water in the source regions increases from 165 ppm at the southern end of the Reykjanes Ridge to between 620 and 920 ppm beneath Iceland. This suggests that Iceland is a wet spot and the H2O(8) values indicate that its influence on basalt compositions increases northwards along the Reykjanes Ridge from ∼61°N (650 km from the plume centre) towards Iceland. The existence of wetter Icelandic source regions have important implications for mantle melting, as enrichments of this magnitude depress the mantle solidus, increasing the degree of melting at a given temperature. Therefore the enhanced rates of volcanism on Iceland may be a result of wetter sources in addition to a thermal anomaly beneath Iceland.  相似文献   

17.
Successions of Lower to lower Middle Cambrian, Upper Permian to Upper Triassic and Lower Tertiary carbonates and arenites have been sampled in five sections, representative of the three main segments of the Mianwali reentrant in the (Trans-Indus) Salt Range (northern Pakistan), i.e.: the southern Khisor Range, the northern Surghar Range and the western Salt Range. Comparison of primary and secondary magnetization directions with the Indian APWP demonstrates the secondary origin of the Mianwali reentrant and shows a pattern of rotations which varies in sense and magnitude along the reentrant with the main structural trends. Data from the Trans-Indus and western Salt Range and published Early Cambrian, Early Permian and Late Tertiary palaeomagnetic results from the southern Salt Range and the Potwar Plateau show that the Hazara Arc underwent a 20–45° counterclockwise rotation relative to the Indian Shield. A contrasting clockwise rotation over about 45° has recently been established for thrust sheets in the opposing eastern limb of the Western Himalayan Syntaxis, i.e. for the Panjal Nappe [1] and the Riasi thrust sheet [2]. These palaeomagnetically established rotations conform with the about 75° azimuthal change in structural trend along the Syntaxis, and support Crawford's [3] suggestion that the Salt Range was originally in line with the northwestern Himalaya. The Salt Range front prograded and moved southwards as part of the Hazara Arc thrust sheet, detached from basement along the evaporitic Salt Range Formation. The Mianwali reentrant originated through obstruction of the southwards advancing thrust sheet by moulding around basement topography of the northwest oriented Sarghoda Ridge.  相似文献   

18.
Basalts from young seamounts situated within 6.8 m.y. of the East Pacific Rise, between 9° and 14°N latitude, display significant variations in 143Nd/144Nd (0.51295–0.51321), 87Sr/86Sr (0.7025–0.7031), and(La/Sm)N (0.415–3.270). Nd and Sr isotope ratios are anti-correlated and form a trend roughly parallel to the “mantle array” on a143Nd/144Nd vs.87Sr/86Sr variation diagram. Nd and Sr isotope ratios display negative and positive correlations, respectively, with(La/Sm)N. The geochemical variations observed at the seamounts are nearly as great or greater than those observed over several hundred kilometers of the Reykjanes Ridge, or at the islands of Iceland or Hawaii.

Samples from one particular seamount, Seamount 6, display nearly the entire observed range of chemical variations, offering an ideal opportunity to constrain the nature of heterogeneities in the source mantle. Systematics indicative of magma mixing are recognized when major elements, trace elements, trace element ratios, and isotope ratios are compared with each other in all possible permutations. The source materials required to produce the end-member magmas are: (1) a typical MORB-source-depleted peridotite; and (2) a relatively enriched material which may represent ancient mantle segregations of basaltic melt, incompletely mixed remnants of subducted ocean crust, or metasomatized peridotite such as that found at St. Paul's Rocks or Zabargad Island. Due to the proximity of the seamounts to the East Pacific Rise (EPR), the source materials are thought to comprise an intimate mixture in the mantle immediately underlying the seamounts and the adjacent EPR. Lavas erupted at the ridge axis display a small range of isotopic and incompatible trace element compositions because the large degrees of melting and presence of magma chambers tend to average the chemical characteristics of large volumes of mantle.

If the postulated mantle materials, with large magnitude, small-scale heterogeneities, are ubiquitous in the upper mantle, chemical variations in basalts ranging from MOR tholeiites to island alkali basalts may reflect sampling differences rather than changes in bulk mantle chemistry.  相似文献   


19.
Since 1938, untreated copper mine tailings of Potrerillos and El Salvador have been disposed into the sea at Chañaral, Chile (26° 21′ Lat. S., 70° 42′ Long. W.). Over 220×106 t of sediments have been dumped. This pollution has caused drastic geomorphological changes in the c. 16 km contaminated area. The occurrence of new artificial beaches and modification of coastal contours are reported. Sandy beach macrofauna monitoring (1975–1982) demonstrated a progressive lowering of density and biomass in those communities affected by copper mine tailings.  相似文献   

20.
A longitudinal seismic reflection profile along the east flank of Reykjanes Ridge, from Charlie fracture zone to the vicinity of Iceland, has important implications both for bottom water movement and for hypotheses of crustal generation at the axis of the mid-oceanic ridge. In this paper bottom water movement is considered. Between 52°N and 57°N Reykjanes Ridge is cut by about 12 fractures whose trend, inferred from other data, is approximately east-west. North of 57° there is little or no indication of east-west fracturing. Fracture valley bottoms are typically 1 km below the surrounding basement level; sediment fills are about 0.5 km; present bottoms are 2.1 to 2.8 km below sea level. Depositional asymmetry is apparent in 9 cases, 7 of which have the deepest and generally least reflective bottom at the northern edge. This suggests predominately west-flowing bottom currents, carrying Norwegian Sea overflow water through the fracture valleys, a result consistent with previously published data.  相似文献   

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