摘要:

We present a thermodynamically self‐consistent model of plate and buoyancy driven flow and melt generation beneath mid‐ocean ridges. Mantle flow is driven by a rigid lithosphere and by buoyancy forces resulting from melting, depletion, and melt extraction. Melt is generated using the solidus of Kinzler and Grove (1992a). Constant viscosity models without melt buoyancy forces show that no significant narrowing of the melting region or pressure gradient focusing of the melt is achieved. Temperature‐dependent viscosity models show that pressure gradients in a high viscosity lithosphere are insufficient for focusing melt to the ridge axis. Constant viscosity models with melt buoyancy forces show that significant narrowing of the melting region is possible at slow spreading rates but not at fast spreading rates. Melt buoyancy forces cause the crustal thickness to decrease with spreading rate. Aggregate melts are similar to those required to form mid‐ocean ridge basalt (MORB) but are too depleted in incompatible elements. They are also insensitive to spreading rate and melt region shape but are sensitive melting rate distribution. Model residuum trends are similar to those in abyssal peridotites and imply that abyssal peridotites result from partial melting and not refertilization. Massif peridotites appear to result from refertilization of harzburgite with MORB primary melt. Vertically integrated melts show very similar trends to data from the off‐axis Lamont seamounts on the flanks of the East Pacific Rise. This implies the melting region at fast spreading ridges is at least 80–

ISSN:0148-0227    

DOI:10.1029/93JB01831

年代:1993

数据来源: WILEY

摘要:

Bathymetry and gravity data obtained during a detailed Hydrosweep survey of the southern East Pacific Rise from 7°S to 9°S are used to investigate isostasy at the axis of a fast spreading mid‐ocean ridge. In particular, we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160‐km‐long 7°12'S‐8°38'S ridge segment. The crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km. The summit is broad and flat, and an axial summit caldera is present for the entire length of the ridge segment. However, the width and cross‐sectional shape of the crestal high vary systematically along the axis. It is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment. The ridge crest is marked by a free‐air gravity anomaly high about 15–20 km wide with an amplitude of 10–15 mGal relative to the ridge flanks. Mantle Bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment. The axial mantle Bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross‐sectional area of the crestal bathymetric high. The effects of cooling and subsidence away from the axis were removed from the bathymetry and free‐air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest. The residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis. The lithosphere was treated as a broken plate, either with a constant flexural rigidity or an effective elastic thicknessTewhich grows at a rate proportional to the square root of distance from the axis. The best fitting values ofTefor the constant rigidity case are in the range of 0.3–0.6 km. For the growing plate model,Teincreases at a rate of 0.2–0.3 km½. The gravity constrains the mass deficiency to extend to a depth of 20–30 km for both lithospheric models. We interpret this low‐density material as a region of partial melt feeding magma to the ridge axis. The best fitting density anomalies imply that a 4–9% melt fraction is present beneath the crestal high. Upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis. The mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment. It thus appears that differences between the along‐axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three‐dimensional, focused upwelling at slow spreading ridges to two‐dimensional sheetlike upwelling at fast‐spreading ridges. Rather, the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of

ISSN:0148-0227    

DOI:10.1029/93JB01551

年代:1993

数据来源: WILEY

摘要:

We report new major element, trace element, isotope ratio, and geochronological data on the Galápagos Archipelago. Magmas erupted from the large western volcanos are generally moderately fractionated tholeiites of uniform composition; those erupted on other islands are compositionally diverse, ranging from tholeiites to picritic basanitoids. While these volcanos do not form a strictly linear age progressive chain, the ages of the oldest dated flows on any given volcano do form a reasonable progression from youngest in the west to oldest in the east, consistent with motion of the Nazca plate with respect to the fixed hotspot reference frame. Isotope ratios in the Galápagos display a considerable range, from values typical of mid‐ocean ridge basalt on Genovesa (87Sr/86Sr: 0.70259, ϵNd: +9.4,206pb/204Pb: 18 44), to typical oceanic island values on Floreana (87Sr/86Sr: 0.70366, ϵNd: +5.2,206pb/204Pb: 20.0). La/SmNranges from 0.45 to 6.7; other incompatible element abundances and ratios show comparable ranges. Isotope and incompatible element ratios define a horseshoe pattern with the most depleted signatures in the center of the Galápagos Archipelago and the more enriched signatures on the eastern, northern, and southern periphery. These isotope and incompatible element patterns appear to reflect thermal entrainment of asthenosphere by the Galápagos plume as it experiences velocity shear in the uppermost asthenosphere. Both north‐south heterogeneity within the plume itself and regional variations in degree and depth of melting also affect magma compositions. Rare earth systematics indicate that melting beneath the Galápagos begins in the garnet peridotite stability field, except beneath the southern islands, where melting may occur entirely in the spinel peridotite stability field. The greatest degree of melting occurs beneath the central western volcanos and decreases both to the east and to the north and south. Si8.0, Fe8.0, and Na8.0values are generally consistent with these inferences. This suggests that interaction between the plume and surrounding asthenosphere results in significant cooling of the plume. Superimposed on this thermal pattern produced by plume‐asthenosphere interaction is a tendency for melting to be less extensive and to occur at shallower depths to the south, presumably reflecting a decrease in ambient asthenospheric temperatures away from the Galápagos Spr

ISSN:0148-0227    

DOI:10.1029/93JB02018

年代:1993

数据来源: WILEY

摘要:

Analysis of sea surface gravity data along six profiles collected during the Crossgrain 2 and EW9103 expeditions to the Marquesas Islands demonstrates that the elastic plate thicknessTeis remarkably uniform, with a value of 18±2 km over the entire chain. Given the 50 Ma age of the Marquesan lithosphere at the time of loading, thisTevalue agrees with the empirical relationship between elastic plate thickness and plate age for hotspot volcanoes in the North Pacific, Atlantic, and Indian oceans. Using more limited gravity data from the Society chain, we find that the best fitting elastic plate thickness there is resolvably higher, 23±2 km. In both cases, our values forTeare larger than those derived from analysis of satellite altimetry profiles. We attribute the lowTevalues derived from satellite altimetry to inaccuracies in available gridded bathymetric databases for this part of the world and the assumption of a high value for the density of the seafloor. Although our shipboard data sample only two French Polynesian island chains, the results suggest that the amount of elastic plate thinning over the South Pacific Superswell may be less than previously proposed, thus eliminating conflict between estimates of the thermal structure of the Superswell lithosphere as derived from elastic plate thickness and that derived from heat flow, depth anomalies, and Love wave dispersio

ISSN:0148-0227    

DOI:10.1029/93JB01720

年代:1993

数据来源: WILEY

摘要:

Fluid inclusions in a suite of gabbro, quartz‐breccia, and metabasalt samples recovered from the MARK area on the Mid‐Atlantic Ridge are the product of a complex hydrothermal history involving late stage magmatic fluids at temperatures>700°C and penetration by modified seawater at 300–400°C. The evolution of volatiles during the early stages of solidification and cooling of magma bodies near the ridge‐transform intersection is marked by exsolution of a CO2fluid, entrapped within primary inclusions in fluorapatites. Attendant with progressive melt fractionation, residual evolved melts reached water saturation, and locally, supercritical CO2+H2O+NaCl±Fe brines (>50 wt % NaCl) and cogenetic H2O+CO2‐rich vapors (1–2 wt % NaCl) were exsolved as immiscible phases. Concomitant or subsequent fracturing, perhaps in response to volatile exsolution from the melts, allowed migration of these fluids along microfracture networks at>700°C. Trondhjemitic‐hosted inclusions, which homogenize by halite dissolution, indicate that the last fluids exsolved from the melts may have been 35–40 wt % brines. The transition from magmatic to seawater‐dominated hydrothermal conditions in the gabbros is marked by initial penetration of lower salinity fluids (1–7 wt % NaCl) at temperatures in excess of 400°C, with the general cessation of fluid flow occurring at minimum temperatures of ≈ 250°C. The relative enrichment and depletion of NaCl with respect to seawater in these fluids may record supercritical phases separation of seawater or boiling of hydrothermal fluids enriched in NaCl. Migration along microfracture networks of CH4‐rich, 350°C fluids, may reflect deeper seated hydrothermal processes involving hydration of underlying mantle material in response to fluid flow along deeply penetrating fault systems. In shallow crustal rocks, circulation of seawater‐derived fluids occurred at temperatures up to 400°C, with subsequent collapse of the active hydrothermal system at minimum temperatures of 200–250°C. In fault‐related upflow zones, multiple hydrothermal pulses involving 180–340°C and 3.5–10 wt % NaCl fluids, pervasively altered bounding wall rocks, forming chlorite‐rich, pyrite‐ and chalcopyrite‐bearing breccias. At shallow crustal depths, fluids reached temperatures of 150–300°C and contained salinities of 3.8–6.9 wt % NaCl. Following collapse of the axial‐related hydrothermal system, the plutonic and shallow crustal rocks were uplifted and emplaced as allocthonous blocks attend

ISSN:0148-0227    

DOI:10.1029/93JB01432

年代:1993

数据来源: WILEY

摘要:

An extensive suite of hydrothermally altered gabbros was recovered byAlvinand dredging from the Mid‐Atlantic Ridge, in the vicinity of the eastern ridge‐transform intersection (RTI) of the Kane Fracture Zone (MARK), where tectonic extension has provided a window into the lower crustal component of hydrothermal cells. Four alteration types are distinguished on the basis of metamorphic assemblage, mineral composition, and deformation textures. A conceptual model is presented that places temperature ‐ fluid/rock ratio relationships, in conjunction with styles of deformation, into a spatial and temporal framework. Exsolution of late stage magmatic fluids in the vicinity of the magma‐hydrothermal interface at temperatures>700°C marks the onset of fluid‐rock interaction in the MARK gabbros (Kelley et al., this issue), with subsequent alteration involving seawater‐derived fluids being strongly influenced by deformation mechanisms. In zones of ductile shear, hydration was initiated at temperatures between 550° and 700°C and moderate fluid/rock ratios. Elsewhere in the lower crust, the onset of seawater penetration occurred at temperatures of 400°–550°C and was facilitated by brittle fracturing. Early vein networks indicate very low initial fluid/rock ratios and are interpreted as being related to the solidification of plutons. Later fractures indicate higher water/rock ratios and may be related to the downward propagation of shallow fault systems. Throughout the crust, fluid‐rock interaction ceased at temperatures 180°–300°C. Subsequent cataclastic deformation associated with the unroofing and emplacement of crustal blocks in the RTI massif produced moderate fluid/rock ratios within localized zones. Hydrothermal alteration in the crustal section exposed at MARK was initiated at lower temperatures, and water/rock interaction proceeded to lower temperatures than many other plutonic suites. These differences must reflect the temporal and spatial evolution of magmatic and tectonic extension that is particular to each

ISSN:0148-0227    

DOI:10.1029/93JB01717

年代:1993

数据来源: WILEY

摘要:

A compilation of published exit temperatures and salinities for seafloor hydrothermal fluids, along with fluid inclusion data on oceanic and ophiolitic rocks, shows that ridge axes hydrothermal systems are generally characterized by maximum temperatures around 300°–360°C, thus clearly being in the one‐phase field. They overlie a lower system marked by the generation of highly saline brines at high temperatures. The brines may have originated either when seawater approached the top of the magma chamber or more probably as exsolution products of the differentiated melts that typify the roof of oceanic magma chambers. In ophiolites, the brines typically occur within the transition zone between the sheeted‐dike complex and the plutonic sequence, i.e., on top of magma chambers. This transition zone, relatively well known from ophiolite studies (Oman, Troodos, Bay of Islands, Josephine), is characterized by mutually intrusive relationships between gabbro, plagiogranite, and dikes and xenoliths of altered diabase within plagiogranite and gabbro. In fast to medium spreading ophiolites (Oman) it is the locus of a major downward decrease in the density of hydrothermal veins and alteration. The vein system is strongly anisotropic and shows a well‐marked preferred vertical along‐strike orientation. The underlying cumulates are almost unaltered and exhibit only a diffuse net of amphibole veins, while the overlying sheeted‐dike complex is densely veined and pervasively altered. This zone is marked by major gradients in the physical parameters over short distances: temperature (several degrees Celsius per meter), pressure (several hundred kilopascals), transition from brittle to ductile. In steady state systems, direct interactions between the magma chamber and the hydrothermal system are restricted to this zone, characterized by exsolution of magmatic volatiles into the hydrothermal system and the incorporation of hydrothermal fluids into the magma chamber by digestion of altered roof rocks. Complications to this steady state model resulting from a non‐steady state magma chamber (e.g., Josephine ophiolite) are mainly the development of late stage faulting and more pronounced retrogres

ISSN:0148-0227    

DOI:10.1029/93JB01822

年代:1993

数据来源: WILEY

摘要:

This paper develops a general framework for the analysis of the moments greater than 2 of a topographic field. This framework uses “iterated” expectation to reduce a statistical moment function to component parts involving the vertical (disjoint) moment of the same order, lower moments, and two‐point conditional expectations. In this way it is possible to isolate the unique informational contribution of each moment. Use of this framework necessitates a “bootstrap” or perturbation method, where lower moments are modeled first and then are used as constraints in the modeling of higher moments. Functional modeling of any moment is thus reducible to characterization of the disjoint moment (e.g., skewness, kurtosis) and the two‐point conditional expectation. In this paper, I demonstrate how it is possible to “design” a statistical model most sensitive to specific non‐Gaussian topographic characteristics by solving for the two‐point conditional expectation under an invertable mapping between Gaussian and non‐Gaussian fields of interest. Mappings of this sort are useful both for the fact that they can be intuitive descriptions of non‐Gaussian characteristics and for their utility in generating non‐Gaussian synthetic topography. The primary intent of this methodology is to forge a link between physical topographic characteristics, the information we want to know, and statistical moments, our tool for quantitatively measuring topographic fields. In addition, mapping models can be used to calculate the skewness and kurtosis (or higher moments) of topographic slopes directly. The applicability of these methods is demonstrated for mapping models which create vertical and lateral asymmetry and peakine

ISSN:0148-0227    

DOI:10.1029/93JB01092

年代:1993

数据来源: WILEY

摘要:

Hydraulic piston coring operations at Deep Sea Drilling Project site 522 in the South Atlantic retrieved an unusually continuous section of late Eocene to late Oligocene pelagic sediments, which we sampled at 3–4 cm intervals (∼3–5 kyr). Natural remanent magnetization demagnetization studies indicate a well‐behaved remanence. Various rock magnetic procedures strongly suggest the magnetic carrier is dominated by pseudo‐single domain magnetite appropriate for recording relative intensity variations of the paleomagnetic field. Nine zones of unusually low relative paleointensity were identified within the 2 my Chron C12R interval. Seven can be typified by a ∼20–40 kyr interval of low field intensity accompanied by apparently random, low‐amplitude, short‐duration directional fluctuations. The other two are of approximately equal duration and intensity but exhibit an orderly progression of directional changes that result in well‐defined virtual geomagnetic pole (VGP) paths confined along a preferred meridian of ∼70–90°W longitude. We propose that both styles occur when the main dipole term diminishes significantly but that the former result when undiminished “normal” secular variation is continuous during the period of low axial dipole moment. We propose that the other two lows in relative paleointensity, along with one reversal record, reflect a field structure of low axial dipole moment dominated by a low‐degree nonzonal spherical harmonic term. Alternatively, the confined VGP paths could be an artifact of heavy remanence smoothing between nonantipodal, semistable transitional geomagnetic pole positions. Geographical control of VGP paths, particularly along ∼70–90° W longitude, has recently been noted for much younger reversals. The site 522 record may indicate that the underlying cause of this phenomenon was present at 32 Ma. We compare our C12R record of paleointensity lows with C12R marine magnetic anomaly “tiny wiggles”. These data appear to indicate that C12R tiny wiggles resulted from periods of low geomagnetic field intensity that were sometime

ISSN:0148-0227    

DOI:10.1029/93JB02019

年代:1993

数据来源: WILEY

摘要:

Recent 1980–1986 Mount St. Helens dacites contain the phenocryst assemblage, plagioclase, amphibole, low‐Ca pyroxene, magnetite, ilmenite, and rare high‐Ca pyroxene, which indicates that they all originated from an 8 km deep reservoir at 900°±20°C withXH2O= 0.67 in fluid according to experimental data. Iron‐titanium oxide phenocryst compositions indicate that all post May 18 dacitic magmas erupted at 900°±20°C except for the final lava extrusion in October 1986; the magma reservoir may have cooled to 866°C by October 1986. Amphiboles in the post May 18, 1980, magma contain one or more amphibole populations characterized by reaction rims of different thicknesses. The development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir. Constant P and T and isothermal decompression experiments show that during a 900°C constant rate decompression from 8 km to the surface, no reaction rim develops on amphibole in 4 days, a 10‐μm rim develops in 10 days, and a 35‐μm rim develops in 20 days. These experimental data and histograms of rim widths in 1980–1986 Mount St. Helens dacites show that post May 18 eruptions are composed in large part of magma represented by a population of thin‐rimmed amphiboles, magma which ascended from the deep (8 km) reservoir in 6 to 10 days. The remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 μm, magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly (millimeter scale) into the erupting magma. The mixing in a viscous, slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population. The experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post May 18 dacites of about 15–30 m/h for magma represented by the thick‐rimmed amphiboles and from 35 to 50 m/hr for magma represented by the thin‐rimmed crystals. An ascent rate of>66 m/h is indicated for the May 18, 1980, eruption, which contains amphiboles with no reaction rims. The volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath Mount St. Helens in 1981–1982 was equivalent to a cyli

ISSN:0148-0227    

DOI:10.1029/93JB01613

年代:1993

数据来源: WILEY