Distributions of hydrothermal activity along the Mid-Atlantic Ridge: interplay of magmatic and tectonic controls     

C. R. German  L. M. Parson 《Earth and Planetary Science Letters》1998,160(3-4):327-341

Hydrothermal activity has been investigated along three different sections of the slow-spreading Mid-Atlantic Ridge (MAR): 11°20′–30°N, 36–38°N and the Reykjanes Ridge, 57°45′–63°06′N. When considered in total, the incidence of venting along these three sections of the MAR compares well with the predictions of a model in which frequency of venting is linearly related to ridge-crest spreading-rate. At the scale of individual study areas, however, departure from the model is observed by up to an order of magnitude. Venting is anomalously rare along the Reykjanes Ridge but anomalously abundant along the MAR 36–38°N. Whilst such variability may be within the error of the linear spreading-rate model, we note that the interplay between magmatic and tectonic processes also differs between the three study areas. In the case of the Reykjanes Ridge we propose that the low incidence of venting reported may reflect a limitation of the sampling/investigative strategy because the style of venting which predominates may not give rise to conventional black-smoker hydrothermal plumes. Along the oblique and broadly segmented MAR 36–38°N, we propose that vigorous hydrothermal venting in broad segment-end non-transform discontinuities may be focussed along deeply penetrating active faults with the requisite heat supply being supported through some combination of along-axis magmatic intrusions and thermal release associated with the serpentinisation of crustal peridotites.  相似文献   

Geochemical morphology of the North Mid-Atlantic Ridge, 10°–24°N: Trace element-isotope complementarity     

Laure Dosso  Henri Bougault  Jean-Louis Joron 《Earth and Planetary Science Letters》1993,120(3-4):443-462

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 (1s_M, 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   

The Mid-Atlantic Ridge between 29°N and 31°30′N in the last 10 Ma     

J.-C. Sempr  P. Blondel  A. Briais  T. Fujiwara  L. Gli  N. Isezaki  J.E. Pariso  L. Parson  P. Patriat  C. Rommevaux 《Earth and Planetary Science Letters》1995,130(1-4):45-55

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   

Spinels in Mid-Atlantic Ridge basalts: Chemistry and occurrence     

Haraldur Sigurdsson  J.-G. Schilling 《Earth and Planetary Science Letters》1976,29(1):7-20

Three groups of spinels have been identified in dredged basalts from the Mid-Atlantic Ridge in the Azores region (30–40°N): (1) magnesiochromites with 0.4–0.5 Cr/(Cr + Al) are most common and characteristic of olivine tholeiites of the region; (2) titaniferous magnesiochromites are found in an olivine basalt with alkali affinities, of local occurrence and evolved in relatively high fugacity of oxygen; (3) chromian spinels with 0.23 Cr/(Cr + Al) occur in unusual high-Al picrites of local occurrence and possible high-pressure origin. Spinels are restricted in occurrence to the least fractionated lavas, with FeO^*/FeO^* + MgO ratio less than 0.575 and with Cr content greater than 350 ppm. A close relationship between Al content of liquidus spinel and Al content of magma has been observed for basaltic types. High-Al spinels deviating from this relationship, such as those found in picritic lavas from the Mid-Atlantic Ridge, may have crystallized at high pressure. The use of spinels as geobarometers in magmas of a restricted compositional range seems a promising prospect. There is no evidence of systematic variation in spinel chemistry of occurrence along the Mid-Atlantic Ridge, such as could be related to different mantle sources of the basalts, plume versus non-plume or binary mantle mixing.  相似文献   

Mantle thermal structure and active upwelling during continental breakup in the North Atlantic   总被引：4，自引：0，他引：4  

W.Steven Holbrook  H.C. Larsen  J. Korenaga  T. Dahl-Jensen  I.D. Reid  P.B. Kelemen  J.R. Hopper  G.M. Kent  D. Lizarralde  S. Bernstein  R.S. Detrick 《Earth and Planetary Science Letters》2001,190(3-4):251-266

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   

Granoblastic olivine aggregates in magnesian chondrules: Planetesimal fragments or thermally annealed solar nebula condensates?     

Scott A. Whattam  Roger H. Hewins  Bosmat A. Cohen  Nicholas C. Seaton  David J. Prior 《Earth and Planetary Science Letters》2008,269(1-2):200-211

Granoblastic olivine aggregates (GOA) have been discovered in some Type I magnesian chondrules within carbonaceous chondrites by Libourel and Krot [Libourel, G., Krot, A.N., 2007. Evidence for the presence of planetesimal material among the precursors of magnesian chondrules of nebular origin. Earth Planet. Sci. Lett. 254, 1–8], who proposed an origin from pre-existing planetesimals. Amoeboid olivine aggregates (AOA), generally considered as aggregates of solar nebula condensates and found within similar carbonaceous chondrites, display similar equilibrium texture, though on a finer scale. For these reasons, we conducted experiments to determine if annealing of olivine required time scales appropriate to planetesimal or nebular heating. Pressed < 43 µm and < 63 µm San Carlos olivine powder (Fo_88.4) was isothermally heated at temperatures ranging from 1350–1550 °C for 1–100 h. The 100 h runs yield olivine aggregates with well-developed granoblastic texture at all temperatures, manifest as a network of randomly-oriented and sutured olivine grains with 120° triple junctions. Individual olivine grains are 4–6 sided and polygonal by 1450 °C and equigranular texture is developed at high temperature (1500–1550 °C). Melting of olivine commences at 1450–1500 °C and aids in ‘ripening’ and suturing (grain coarsening and grain boundary migration). Textural equilibrium is clearly met at 1550 °C. A planetesimal origin cannot be ruled out; however, the experimental evidence reveals that granoblastic texture can be reproduced in an interval not inconsistent with heating times for nebular objects. GOA may have experienced higher degrees of thermal processing than the finer-grained AOA. If the precursors were the same, grain coarsening would have to be accompanied by modification to bulk and isotopic compositions. However, the precursors could have been olivine condensates formed later than AOA. Annealing may have been a widespread process operating in the primordial solar nebula responsible for thermal processing and formation of GOA prior to their incorporation into chondrules.  相似文献   

Tectonic pattern of the Azores spreading centre and triple junction     

Roger Searle 《Earth and Planetary Science Letters》1980,51(2):415-434

The major tectonic elements of the Azores triple junction have been mapped using long-range side-scan sonar. The data enable the Mid-Atlantic Ridge axis to be located with a precision of a few kilometres. Major faults and other tectonic and volcanic elements of the ridge maintain their regional trend of 010° to 020° past the triple junction area. There is no oblique spreading, and only minor transform offsets of the Mid-Atlantic Ridge occur here. The main effect of the triple junction or Azores hot spot is to diminish the amplitude of the median valley to 200 m or less. There is no axial high: a topographic high seen on several profiles is located to the east of the Mid-Atlantic Ridge spreading axis and does not appear to have any fundamental significance.The third arm of the triple junction includes the Azores srreading centre which appears to have developed as a series of en echelon rifted basins (the Terceira Rift) extending from Formigas Trough at 36.8°N, 24.5°W to a point near 39.3°N, 28.8°W. There are indications that recent activity in the spreading centre may be concentrated in a series of ridges which flank the older rifted basins. Until recently the northwest end of the Terceira Rift was connected to the Mid-Atlantic Ridge axis either directly at an RRR junction, or via a transform fault. The triple junction has probably moved south during the last 6 Ma to a positin on the Mid-Atlantic Ridge near 38.7°N.Initiation of the Azores spreading centre may have occurred during the 36 Ma B.P. rearrangement of poles, with an RFF triple junction north from the East Azores fracture zone to the North Azores fracture zone and transferring a wedge of European plate to the African plate.The tectonic elements revealed by this study are in good agreement with inferred earthquake mechanisms and with the RM2 plate tectonic model of Minster and Jordan, but east-west motion between North America and Africa does not seem to be compatible with the other motions at the triple junction unless it is of very recent (2>3 Ma) origin.  相似文献   

Rift Valley in the Atlantic Ocean near 36°50′N: petrology and geochemistry of basaltic rocks     

H. Bougault  R. Hekinian 《Earth and Planetary Science Letters》1974,24(2):249-261

Dredged rocks from an area of about 15 km² within the inner floor and on the adjacent walls of the Rift Valley were collected. Based on petrographic and chemical data, four types of basaltic rocks were recognized: (1) picritic basalts with olivine xenocrysts, TiO₂ < 0.6%, K₂O < 0.1%, (2) olivine basalts with olivine megacrysts, TiO₂ = 0.8–1.5%,K₂O = 0.1–0.2%, (3) highly phyric and moderately phyric plagioclase basalts with megacrystic plagioclase, TiO₂ < 1.3%, K₂O < 0.3%, and (4) pyroxene basalts with pyroxene > plagioclase, TiO₂ = 0.8–1%,K₂O = 0.2–0.4%. The Cr and Ni having high partition coefficients show different variation trends for each type of rock and their values decrease continuously as crystallization proceeds within each type of basalt. It is speculated that two different magmas have given rise to the above-mentioned rocks. One has yielded the picritic basalts and subsequently the olivine basalts after a separation of the olivine cumulates; the other gave rise to the plagioclase basalts.  相似文献   

The Reykjanes Ridge: structure and tectonics of a hot-spot-influenced, slow-spreading ridge, from multibeam bathymetry, gravity and magnetic investigations     

R.C Searle  J.A Keeton  R.B Owens  R.S White  R Mecklenburgh  B Parsons  S.M Lee   《Earth and Planetary Science Letters》1998,160(3-4):463-478

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   

Spinel peridotite xenoliths in the West Carpathian late cenozoic alkali basalts and their tectonic significance     

D. Hovorka  P. Fejdi 《Bulletin of Volcanology》1980,43(1):95-106

In the Late Cenozoic West Carpatian alkali olivine basalts spinel peridotite xenoliths have been found. Their mineral composition corresponds to those found in this type of xenoliths in alkali basalts throughout the world (Mg-rich olivine, Cr-diopside, clinopyroxene, spinel). For the studied West Carpathian alkali olivine basalts kaersutite amphibole and clinopyroxene magacrysts are characteristic. The presence of the xenoliths under consideration in the boundary zone between Tatra and Pannonian blocks is the consequence of tectonic conditions (thickness of the Crust) in the area mentioned as distinct from the other West Carpathian areas of alkali olivine basalt occurrences.  相似文献   

Plagioclase ultraphyric basalts in Iceland: the mush of the rift   总被引：1，自引：0，他引：1  

H. Hansen  K. Grnvold 《Journal of Volcanology and Geothermal Research》2000,98(1-4)

Glassy, plagioclase ultraphyric basalts from six locations in Iceland have bimodal phenocryst size distributions where microphenocrysts (ol+plg±cpx±mt) are in equilibrium with the matrix glass, but macrophenocrysts (ol+plg±cpx) are too primitive to be so. Matrix glass compositions are similar to those of other rift zone glasses from Iceland, and oxygen isotope variations suggest they interacted with the Icelandic crust. A lack of negative Eu-anomalies in matrix glasses precludes large amounts of plagioclase crystallisation from their parental liquids. Compositions of glass inclusions in plagioclase and olivine macrophenocrysts indicate that parental magma compositions of the macrophenocryst assemblage are similar to those of primitive, Icelandic rift zone glasses. Temperatures for plagioclase macrophenocryst crystallisation obtained from Linkam^® heating stage experiments, and from glass inclusion compositions corrected for post-entrapment crystallisation, give temperatures up to 1260°C, corresponding to crystallisation at middle to deep crustal levels. Temperature differences of less than 100°C between plagioclase-hosted glass inclusions before and after post-entrapment plagioclase crystallisation show that the macrophenocrysts must have been kept at elevated temperatures prior to incorporation in their present host magmas. We suggest that the macrophenocrysts of the plagioclase ultraphyric basalts accumulated in crystal mush bodies underneath the rift zone and were picked up by their present hosts during a rifting event with increased magma supply from the mantle.  相似文献   

Single-domain and superparamagnetic titanomagnetite with variable Ti content in young ocean-floor basalts: No evidence for rapid alteration     

Weiming Zhou  Rob Van der Voo  Donald R. Peacor 《Earth and Planetary Science Letters》1997,150(3-4):353-362

High resolution electron microscope studies have been carried out on ‘zero-age’ (New Flow) basalts from the Juan de Fuca Ridge and on young (< 20 ka) basalts from the axis of the East Pacific Rise at 12°N. Such data lead to characterization of the magnetic minerals, especially those of smaller grain size, which have been hypothesized by Kent and Gee to have undergone grain size-dependent alteration. In addition to larger titanomagnetite grains, abundant submicrometer titanomagnetite has been observed in globules within a glassy matrix. These grains, likely to be single-domain (SD) or superparamagnetic, are associated with apatite, uncommon pyrrhotite and residual glass. The submicrometer titanomagnetite grains have a wide compositional range (0 < x < 0.8), where x is the fraction of ulvöspinel component, whereas the larger, multi-domain (MD)-sized titanomagnetite grains have a narrow composition range of approximately x = 0.6. This variability in Ti content provides a ready explanation for the thermal rock magnetic properties observed by Kent and Gee and eliminates the need to invoke extremely rapid (< 20 ka) alteration of these young basalts.  相似文献   

Petrology, geochemistry and tectonic implications of volcanics dredged from the intersection of the Yap and Mariana trenches     

A.J. Crawford  L. Beccaluva  G. Serri  J. Dostal 《Earth and Planetary Science Letters》1986,80(3-4):265-280

Rocks dredged from the forearc very close to the intersection of the Yap and Mariana trenches include a suite of highly depleted arc tholeiites, and several samples of transitional to slightly alkaline basalt. The tholeiites range from magnesian quartz tholeiites with 0.46–0.6% TiO₂, to andesites with up to 62% SiO₂ and 8.2% FeO^*. All show pronounced LREE depletion and have very low contents of Ba and Sr. They are postulated to have been produced by partial melting of upper mantle peridotite residual after MORB extraction, following influx of hydrous fluids from the subducted slab. While these fluids were responsible for small enrichments in Ba, K, Rb and Sr in melts generated, LREE were not involved in the metasomatism, and the strong LREE depletion probably reflects the unmodified, depleted source peridotite.

The second lava suite includes slightly Ne-normative, Ti-augite-bearing basalts with convex-upward REE patterns, showing slight LREE depletion ((La/Sm)_N = 0.76). The chemical features of these basalts support affinities with basalts erupted during the earliest stages of backarc basin opening. A KAr age on one sample(7.8 ± 1.3m.y.) is in good agreement with the initial opening of the Mariana Trough.

The tectonic significance of the dredged arc tholeiite suite is less obvious. A KAr age of10.8 ± 0.4 My on one andesite, and the occurrence of similar lavas in dredges from at least 300 km along the length of the Yap arc, suggest that subduction was occurring beneath the Yap arc in the Late Miocene, after overthrusting of the Yap greenschist allochthon, and while calc-alkaline arc magmatism was occurring further north on the West Mariana Ridge. We suggest that the depleted arc tholeiites in dredge 1438 were generated by abnormally shallow melting of upper mantle beneath the Yap forearc following subduction beneath this area of young, hot Sorol Trough crust. These arc tholeiites represent a magma type transitional between more typical arc tholeiites (e.g. Tongan) and high-Mg andesites and boninites.  相似文献   

Sea-floor spreading: The central anomaly magnetization high     

Kim D. Klitgord 《Earth and Planetary Science Letters》1976,29(1):201-209

Indications of a narrow region of high magnetization within the central magnetic anomaly on some mid-ocean ridges are found on near-bottom and sea surface magnetic profiles. This zone, which probably represents the most recent extrusions onto the ocean floor, is similar to the narrow region of high magnetization found on the Mid-Atlantic Ridge at 45°N with a suite of dredge samples. This narrow region is probably the result of the initial high magnetization of pillow basalts when they are extruded onto the ocean floor and the subsequent rapid oxidation of the outer variolitic zone of the pillows. The large-amplitude, short-wavelength (<15 km) magnetic anomaly found within the central anomaly over both slow- and fast-spreading ridges is produced by this narrow magnetization high. This magnetic anomaly can be used to locate the region of most recent extrusions on most ridges. The absence of this short-wavelength anomaly on some ridges may reflect the episodicity with which basalts are extruded onto the ocean floor.  相似文献   

He diffusion in olivine   总被引：1，自引：0，他引：1  

S.R. Hart 《Earth and Planetary Science Letters》1984,70(2):297-302

Helium diffusion in olivine (dunite xenolith) has been measured in the temperature range 1180–1460°C; a linear Arrhenius function was obtained with an activation energy of 120₋₂₇⁺³² kcal/mole, and a pre-exponential factor (D₀) = 2.2 × 10⁸ cm²/s.

Diffusion mechanisms are not a viable means of degassing He from the mantle. Olivine phenocrysts can be expected to retain previously trapped He, during cooling in extrusive basalts, provided the flow units are thinner than 50 m; xenoliths will retain mantle He signatures only if magma transport times are less than 50 years, or if the He fugacity in the magma is high enough to prevent xenolith degassing. The lower oceanic crust is probably substantially degassed of He. Trapped He will be qualitatively retained in quenched submarine basalt glass only if the cooling rate is faster than 5 × 10¹⁴°C/m.y.; glass at several centimeters depth in a basalt flow (near the spherulite zone) will have cooling rates lower than this, so He loss may be significant in many basalt glass samples.  相似文献   

Major element chemistry of Galapagos Rift Zone magmas and their phenocrysts     

Martin R. Fisk  A.E. Bence  J.-G. Schilling 《Earth and Planetary Science Letters》1982,61(1):171-189

Basalts dredged from the Galapagos Rift Zone between 85°W and 100°W were analyzed by electron microprobe to determine the chemistry of the glass exteriors and included phenocrysts, microphenocrysts and quench minerals. The basalts come from both “normal” mid-ocean ridge segments and from ridge segments that cross the Galapagos Platform. The basalts fall into two chemical and geographical groups. Group A basalts come from outside the central region of the Galapagos Platform (i.e., outside 89–92.5°W) and are chemically similar to basalts from “normal” ocean ridge segments. Group B basalts come from the center of the Galapagos Platform (89–92.5°W) and are enriched in incompatible elements like “plume-influenced” basalts from the Mid-Atlantic Ridge. The spinel, olivine, plagioclase and clinopyroxene phenocrysts in both groups of basalts are low-pressure, equilibrium phases, but the chemical difference among basalts from within each group indicates high-pressure fractional crystallization is also responsible for the chemical evolution of some of these basalts. Presently, no crystallization or partial melting model can relate the chemistry of the two groups of basalts and the compositional influence of a large-ion-lithophile elements and water-rich mantle beneath the Galapagos Platform is a viable alternate hypothesis. The eruption temperature of magmas from the “normal” ridge segments, as determined by olivine-liquid thermometry is 1217±10°C, suggesting steady-state conditions but on the Galapagos Platform the eruption temperatures are lower and more variable than on the “normal” ridge (1186°C±30°C) suggesting a more complex plumbing system and the absence of a steady-state magma chamber beneath the platform.  相似文献   

Helium isotope geochemistry of mid-ocean ridge basalts from the South Atlantic     

David W. Graham  William J. Jenkins  Jean-Guy Schilling  Geoffrey Thompson  Mark D. Kurz  Susan E. Humphris 《Earth and Planetary Science Letters》1992,110(1-4):133-147

We report new helium isotope results for 49 basalt glass samples from the Mid-Atlantic Ridge between 1°N and 47°S.³He/⁴He in South Atlantic mid-ocean ridge basalts (MORB) varies between 6.5 and 9.0 R_A (R_A is the atmospheric ratio of1.39 × 10⁻⁶), encompassing the range of previously reported values for MORB erupted away from high³He/⁴He hotspots such as Iceland. He, Sr and Pb isotopes show systematic relationships along the ridge axis. The ridge axis is segmented with respect to geochemical variations, and local spike-like anomalies in³He/⁴He, Pb and Sr isotopes, and trace element ratios such as(La/Sm)_N are prevalent at the latitudes of the islands of St. Helena, Tristan da Cunha and Gough to the east of the ridge. The isotope systematics are consistent with injection beneath the ridge of mantle “blobs” enriched in radiogenic He, Pb and Sr, derived from off-axis hotspot sources. The variability in³He/⁴He along the ridge can be used to refine the hotspot source-migrating-ridge sink model.

MORB from the 2–7°S segment are systematically the least radiogenic samples found along the mid-ocean ridge system to date. Here the depleted mantle source is characterized by⁸⁷Sr/⁸⁶Sr of 0.7022, Pb isotopes close to the geochron and with²⁰⁶Pb/²⁰⁴Pb of 17.7, and³He/⁴He of 8.6–8.9 R_A. The “background contamination” of the subridge mantle, by radiogenic helium derived from off-ridge hotspots, displays a maximum between 20 and 24°S. The HePb and HeSr isotope relations along the ridge indicate that the³He/⁴He ratios are lower for the hotspot sources of St. Helena, Tristan da Cunha and Gough than for the MORB source, consistent with direct measurements of³He/⁴He ratios in the island lavas. Details of the HeSrPb isotope systematics between 12 and 22°S are consistent with early, widespread dispersion of the St. Helena plume into the asthenosphere, probably during flattening of the plume head beneath the thick lithosphere prior to continental breakup. The geographical variation in theHe/Pbratio deduced from the isotope systematics suggests only minor degassing of the plume during this stage. Subsequently, it appears that the plume component reaching the mid-Atlantic ridge was partially outgassed of He during off-ridge hotspot volcanism and related melting activity.

Overall, the similar behavior of He and Pb isotopes along the ridge indicates that the respective mantle sources have evolved under conditions which produced related He and Pb isotope variations.  相似文献   

Magma dynamics, crystallization, and chemical differentiation of the 1959 Kilauea Iki lava lake, Hawaii, revisited     

A. Mark Jellinek  Ross C. Kerr   《Journal of Volcanology and Geothermal Research》2001,110(3-4)

Using constraints from an extensive database of geological and geochemical observations along with results from fluid mechanical studies of convection in magma chambers, we identify the main physical processes at work during the solidification of the 1959 Kilauea Iki lava lakes. In turn, we investigate their quantitative influence on the crystallization and chemical differentiation of the magma, and on the development of the internal structure of the lava lake. In contrast to previous studies, vigorous stirring in the magma, driven predominately by the descent of dense crystal-laden thermal plumes from the roof solidification front and the ascent of buoyant compositional plumes due to the in situ growth of olivine crystals at the floor, is predicted to have been an inevitable consequence of very strong cooling at the roof and floor. The flow is expected to have caused extensive but imperfect mixing over most of the cooling history of the magma, producing minor compositional stratification at the roof and thermal stratification at the floor. The efficient stirring of the large roof cooling is expected to have resulted in significant internal nucleation of olivine crystals, which ultimately settled to the floor. Additional forcing due to either crystal sedimentation or the ascent of gas bubbles is not expected to have increased significantly the amount of mixing. In addition to convection in the magma, circulation driven by the convection of buoyant interstitial melt in highly permeable crystal-melt mushes forming the roof and the floor of the lava lake is envisaged to have produced a net upward flow of evolved magma from the floor during solidification. In the floor zone, mush convection may have caused the formation of axisymmetric chimneys through which evolved magma drained from deep within the floor into the overlying magma and potentially the roof. We hypothesize that the highly evolved, pipe-like ‘vertical olivine-rich bodies’ (VORBs) [Bull. Volcanol. 43 (1980) 675] observed in the floor zone, of the lake are fossil chimneys. In the roof zone, buoyant residual liquid both produced at the roof solidification front and gained from the floor as a result of incomplete convective mixing is envisaged to have percolated or ‘leaked‘ into the overlying highly-permeable cumulate, displacing less buoyant interstitial melt downward. The results from Rayleigh fractionation-type models formulated using boundary conditions based on a quantitative understanding of the convection in the magma indicate that most of the incompatible element variation over the height of the lake can be explained as a consequence of a combination of crystal settling and the extensive but imperfect convective mixing of buoyant residual liquid released from the floor solidification front. The remaining chemical variation is understood in terms of the additional influences of mush convection in the roof and floor on the vertical distribution of incompatible elements. Although cooling was concentrated at the roof of the lake, the floor zone is found to be thicker than the roof zone, implying that it grew more quickly. The large growth rate of the floor is explained as a consequence of a combination of the substantial sedimentation of olivine crystals and more rapid in situ crystallization due to both a higher liquidus temperature and enhanced cooling resulting from imperfect thermal and chemical mixing.  相似文献   

The effects of the geothermal gradient on amino acid racemization     

Barry J. Katz  Christopher G.A. Harrison  Eugene H. Man 《Earth and Planetary Science Letters》1979,44(2):279-286

The “second region” of the kinetic curve for the racemization (epimerization) of isoleucine in foraminifera is defined from previously published data corrected for the thermal history experienced by the samples. These kinetic parameters are applied to racemization data from Deep Sea Drilling Project Sites 332A and 333 from the Deep Drill Valley, Mid-Atlantic Ridge, and Site 148 from the Aves Ridge, in order to determine whether there are any observable effects due to the geothermal gradient. The data for Site 148 clearly show an increase of temperature with depth. The data for the two sites of the Mid-Atlantic Ridge do not show this clear relationship. When bottom water temperature variations are taken into consideration, the effects of the geothermal gradient become apparent. Since the degree of racemization is dependent upon both age and temperature, a knowledge of the age of a sample places constraints upon its thermal history, and hence on the heat flow at the location since deposition of the sample. The crude heat flow models thus developed are compatible with present geologic and geophysical information. It appears probable that detailed heat flow models may be developed by improving analytical precision.  相似文献   

Transition from platemargin to intraplate environment: Geochemistry of basalts in Paleogene Liaohe basin,northeastern China   总被引：3，自引：0，他引：3  

WU Changzhi  GU Lianxing  REN Zuowei    ZHANG Zunzhong  CHEN Zhenyan & ZHAO Ming . State Key Laboratory of Mineral Deposit Research 《中国科学D辑(英文版)》2005,48(12):2069-2080

Mesozoic and Cenozoic volcanic rocks are widely distributed in the circum-Pacific area of eastern China. These rocks have long been genetically linked to westward subduction of the paleo-Pacific oceanic plate to the eastern Asia continent[1,2]. Research in re-cent years[3―6] has attained conclusions that a simple paleo-Pacific subduction model does not work well in interpreting all the volcanic rocks in eastern China, although some of them could be attributed to circum-Pacific interaction …  相似文献