首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 23 毫秒
1.
Rare earths (RE) in basalts erupted within the rift of the Mid-Atlantic Ridge show a progressive change from light-RE enriched to depleted patterns from the Azores Platform (40°N) down to 33°30′N. South, the pattern remains light-RE depleted as along other “normal ridge” segments. A progressive increase in chemical variability of the basalts towards the Azores is also noted.The latitudinal RE profile and corresponding ΣFeO/ΣFeO + MgO variations, together, indicate that the origin of these basalts cannot be accounted for simply by considering variable extents of partial melting of a single mantle source and subsequent fractional crystallization during the ascent of the magmas. These two processes produce only second-order effects on the RE patterns. The data requires the presence of a distinct, light-RE richer, mantle source beneath the Azores Platform relative to that of south of 33°30′N and an intermediate zone where both mantle types mix. The relative contribution of the Azores mantle source to the mix appears to decrease fairly regularly southward along the ridge and becomes negligible at 33°30′N. Increasing chemical variability of the basalts towards the Azores is probably caused by correspondingly larger extent of fractional crystallization at shallow depth, and/or greater variability in the extent of partial melting, apparently subsequent to, and superimposed on the mixing of the two mantle sources.The combined morphological, geophysical and RE evidence along the profile are consistent with a model suggesting upwelling of a major blob (plume) under the Azores Plateau; and reveal the present extent of the blob's overflow and mixing with the asthenosphere depleted in large ionic lithophile trace elements. The influence of the Azores blob is geochemically detectable up to 1000 km southwestward beneath the ridge axis.  相似文献   

2.
IPOD Leg 49 recovered basalts from 9 holes at 7 sites along 3 transects across the Mid-Atlantic Ridge: 63°N (Reykjanes), 45°N and 36°N (FAMOUS area). This has provided further information on the nature of mantle heterogeneity in the North Atlantic by enabling studies to be made of the variation of basalt composition with depth and with time near critical areas (Iceland and the Azores) where deep mantle plumes are thought to exist. Over 150 samples have been analysed for up to 40 major and trace elements and the results used to place constraints on the petrogenesis of the erupted basalts and hence on the geochemical nature of their source regions.It is apparent that few of the recovered basalts have the geochemical characteristics of typical “depleted” midocean ridge basalts (MORB). An unusually wide range of basalt compositions may be erupted at a single site: the range of rare earth patterns within the short section cored at Site 413, for instance, encompasses the total variation of REE patterns previously reported from the FAMOUS area. Nevertheless it is possible to account for most of the compositional variation at a single site by partial melting processes (including dynamic melting) and fractional crystallization. Partial melting mechanisms seem to be the dominant processes relating basalt compositions, particularly at 36°N and 45°N, suggesting that long-lived sub-axial magma chambers may not be a consistent feature of the slow-spreading Mid-Atlantic Ridge.Comparisons of basalts erupted at the same ridge segment for periods of the order of 35 m.y. (now lying along the same mantle flow line) do show some significant inter-site differences in Rb/Sr, Ce/Yb,87Sr/86Sr, etc., which cannot be accounted for by fractionation mechanisms and which must reflect heterogeneities in the mantle source. However when hygromagmatophile (HYG) trace element levels and ratios are considered, it is the constancy or consistency of these HYG ratios which is the more remarkable, implying that the mantle source feeding a particular ridge segment was uniform with respect to these elements for periods of the order of 35 m.y. and probably since the opening of the Atlantic. Yet these HYG element ratios at 63°N are very different from those at 45°N and 36°N and significantly different from the values at 22°N and in “MORB”.The observed variations are difficult to reconcile with current concepts of mantle plumes and binary mixing models. The mantle is certainly heterogeneous, but there is not simply an “enriched” and a “depleted” source, but rather a range of sources heterogeneous on different scales for different elements — to an extent and volume depending on previous depletion/enrichment events. HYG element ratios offer the best method of defining compositionally different mantle segments since they are little modified by the fractionation processes associated with basalt generation.  相似文献   

3.
The Western Mediterranean Jurassic ophiolites contain abundant volcanic rocks that resemble modern ocean floor basalts. In this paper we report analyses of rare earth elements (REE), transition elements (Ni, Co, Cr, V, Sc) and high field strength elements (Ti, P, Y, Zr, Ta, Hf) for metabasalts from representative ophiolite outcrops in the Eastern Alps, Corsica and the Northern Apennines (Liguria).The chemical characteristics of the metabasalts range from “normal” to “transitional” mid-ocean ridge basalt (MORB). Most chemical variation in the metabasalts from the different areas can be explained by low-pressure fractional crystallization, by differences in degree of partial melting, and by minor chemical heterogeneities of the source, but the “transitional” MORB characteristics of some metabasalts from Corsica (Balagne) might reflect formation from a source with different mineral and chemical composition. The estimated REE pattern of the source of the Liguria-type metabasalts corresponds to the pattern for certain Ligurian ultramafic rocks, which might therefore represent the residue from extraction of some ophiolitic lavas.While the Liguria-type ophiolitic metabasalts might represent products of a “normal”, for instance, oceanic ridge, the “transitional” metabasalts might be lavas erupted, during the early stages of opening of a small ocean basin, or along the continental margin of a larger ocean basin.  相似文献   

4.
Five separate exposures of oceanic basalts were dredged in the vicinity of the Peru-Chile Trench between 9° and 27°S latitude. Each dredge is dominated by abundant pillow basalts. Approximately ten of the most unaltered, glassy and fine-grained samples were selected for detailed chemical and petrographic analyses from each dredge area. All basalts recovered in the Peru-Chile Trench are olivine and quartz-normative tholeiites that are believed to have formed at the now extinct Galapagos Rise 30–50 m.y. ago. Detailed chemical analyses of the basalts, including major and selected trace and rare earth elements, indicate that considerable compositional variability exists both within each of the dredged areas as well as between areas. Most of the inherent chemical variability observed within particular basement sections appears consistent with the concept of temporal evolution of magma bodies at a former spreading center by shallow-level fractional crystallization involving primarily plagioclase and olivine. In contrast, important chemical differences between the dredged areas suggest compositional heterogeneities in the mantle source regions. Our results indicate that although shallow-level fractionation has brought about large changes in composition of basalts in each area, compositional trends are distinct and appear to reflect original mantle-derived compositional differences.  相似文献   

5.
Field observations in the Oman ophiolite and petrological data are used to constrain a model of melt segregation at the top of the mantle beneath an oceanic spreading centre. Foliations and lineations in outcrops of mantle-derived peridotites oriented at high angle relative to the crust–mantle boundary have been interpreted as the footprint of a former axial asthenospheric convective upwelling several kilometers in cross-section that reached Moho levels. Basaltic melts migrating through this upwelling reacted with their host harzburgites and suffered fractional crystallization. The mantle–crust transition zone at the top of the upwelling is characterized by an very thick (about 400 m) dunite layer whose detailed structure and composition point to the development by compaction of a former “mantle mush”. The more important structures (in terms of volume of crystallization products) found in the underlying harzburgites are dunitic–troctolitic horizons a few meters thick and of lateral extent reaching 1 km and more. They crystallized at high temperature (>1190 °C) from melts similar to mid-ocean ridge basalts (MORB). These are called “sills” because they are sub-parallel to the crust–mantle boundary, but they can present a moderate dip (15° to 20° at most) relative to this paleo-horizontal surface. These observations have motivated the modelling of melt segregation by compaction within the crystallization domain inside the top convective boundary layer of the mantle upwelling. Two original inputs to the modelling are considered here: (i) the slope of the iso-curves of melt concentration due to the progressive cooling of the mantle in the boundary layer away from the axis of the rising convective flow; (ii) the reduction in permeability caused by the crystallization of the inter-granular melt. Modelling shows that a unique condition is required to generate the troctolite sills and the thick dunite layer nested at the top of the Maqsad diapir: namely a dramatic drop of the interstitial melt concentration at the top of the mantle. Besides, the model developed here allows to scale the time, volume and velocity of the melt segregation.  相似文献   

6.
The unusual petrological diversity of abyssal lavas erupted along some segments of the Galapagos spreading center is a direct consequence of the propagation (elongation) of these segments into older oceanic crust. With increasing distance behind propagating rift tips, relatively unfractionated MORB erupted close to the tips are joined first by FeTi basalts (bimodal assemblage) and then by a wide range of basaltic and siliceous lavas. Further behind propagating rift tips, this broad range diminishes again, approaching the narrow compositional range of adjacent normal ridge segments.These compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre-existing plate. We envisage this evolution as proceeding from small, isolated, ephemeral magma chambers through increasing numbers of larger, increasingly interconnected chambers to the steady-state buffered system of a normal ridge. Throughout this evolution, magma supply rates gradually increase and cooling rates of crustal magma bodies decrease. High degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved.At other propagating and non-propagating ridge-transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas. These effects may be evaluated provided a number of tectonic variables including transform length, spreading and propagation rates are taken into account.  相似文献   

7.
Consideration of geochemical data from basalts formed near major Atlantic and Pacific transform faults reveals two significant sets of observations. First, compared to basalts formed far from the transform, basalts near the ridge/transform intersection have, for the same MgO contents, higher abundances of TiO2 and other incompatible elements, higher La/Sm and La/Yb ratios, and often higher FeO. These enrichments are distinct from and occur in addition to the more variable and fractionated compositions which have been frequently noted [10–13]. Modeling of this “transform fault effect” using data from the Tamayo/EPR intersection suggests the chemical systematics are caused by decreasing extents of melting as the transform is approached.Second, there are chemical discontinuities in the major element, trace element and isotopic chemistry of basalts across many transforms. These “transform discontinuities” occur in normal ocean crust as well as around hot spots.Consideration of the melting zone in the mantle suggests that the transform fault effect is a natural consequence of the ridge/transform plate boundary. The melting zone beneath a ridge segment must terminate across the transform, leading to lower extents of melting at the transform edge. The surface manifestation of the change in the melting zone may be affected by the age of the transform offset, the spreading rate, the transform spacing and the interaction of mantle flow with the local thermal structure; it may be obscured by episodic magma chamber processes and mantle heterogeneity.The significance of transform discontinuities depends on whether they persist with age. If they do not, then temporally variable crust-forming processes may produce changes along a flow line similar to those at zero age across a transform. If, on the other hand, a discontinuity persists with age, then the transform may be related to a fundamental discontinuity in the underlying mantle. Long-lived transform discontinuities would have profound implications for the nature of plate motions, mantle convection and mantle heterogeneity.  相似文献   

8.
The intersection of the Juan de Fuca ridge and Blanco fracture zone is characterized by unusually high amplitude magnetic anomalies (over 1500 nT) which appear to be associated with a body roughly 50 km in length and 20 km in width aligned along the fracture zone. Simple three-dimensional magnetic models indicate that this anomaly is probably caused by a highly magnetized block of material situated in the western end of the Blanco fracture zone near its intersection with the Juan de Fuca ridge. Rock magnetization studies of tholeiitic basalts dredged from this area confirm the presence of highly magnetized basalts near the ridge crest/transform fault intersection. These tholeiitic basalts are enriched in iron and titanium relative to “normal” oceanic tholeiites, apparently the result of extensive shallow fractionation involving olivine, plagioclase, and clinopyroxene. Magnetic model studies indicate that an average thickness of no more than 500 m of these iron-rich basalts is necessary to produce the observed anomaly pattern. Comparison of these basalts with samples previously dredged from the Juan de Fuca ridge crest suggests that these Fe-rich, highly magnetized basalts probably “leaked” out of the southernmost portion of the Juan de Fuca ridge.  相似文献   

9.
A detailed submersible investigation of a 20-km segment of the East Pacific Rise near 12°50′N between the Orozco and Clipperton fracture zones has resulted in the localization of 24 active hydrothermal vent fields and over 80 sites of sulfide accumulations. The active vents range from low-temperature vents characterized by exotic benthic communities to high-temperature “black smokers” and the deposition of polymetallic sulfides. The study is based upon a combination of fine scale topography obtained using the SEABEAM sonar system on N/O “Jean Charcot”, camera lowerings along the axis using the RAIE vehicle, and 32 dives by the submersible “Cyana” operating from N/O “Le Suroit”. The observations made between the Orozco and Clipperton fracture zones show topographic highs situated along the strike of the accreting plate segment separated by a small ridge offset at 11°49′N. This offset divides this portion of the ridge into two separate spreading segments each of which has a primary topographic high along strike. Secondary highs are associated with each segment of the ridge separated by either small offsets (or relay zones) or in some cases, zones where spreading centers overlap. Dives made on the tops of both primary highs (12°50′N and 11°30′N) confirm the presence inferred from previous surface work of high-temperature vent fields while one reconnaissance dive (14°20′N) near the Orozco fracture zone/ridge axis intersection reveals the absence of any hydrothermal activity in the present or recent past. The vast majority of vent fields investigated were found at the topographic high near 12°50′N, are associated with the most recent period of volcanism, and are confined to lava ponds situated within the axial graben.  相似文献   

10.
Post-glacial tholeiitic basalts from the western Reykjanes Peninsula range from picrite basalts (oldest) to olivine tholeiites to tholeiites (youngest). In this sequence there are large systematic variations in rare earth element (REE) abundances (La/Sm normalized to chondrites ranges from 0.33 in the picrite basalts to 1.25 in the fissure tholeiites) and corresponding variations in 143Nd/144Nd (0.51317 in the picrite basalts to 0.51299 in the fissure tholeiites). The large viaration in 143Nd/144Nd, more than one-third the total range observed in most ocean islands and mid-ocean ridge basalts (MORB), is accompanied by only a small variation in 87Sr/86Sr (0.7031–0.7032). These 87Sr/86Sr ratios are within the range of other Icelandic tholeiites, and distinct from those of MORB.We conclude that the mantle beneath the Reykjanes Peninsula is heterogeneous with respect to relative REE abundances and 143Nd/144Nd ratios. On a time-averaged basis all parts of this mantle show evidence of relative depletion in light REE. Though parts of this mantle have REE abundances and Nd isotope ratios similar to the mantle source of “normal” MORB, 87Sr/86Sr is distinctly higher. Unlike previous studies we find no evidence for chondritic relative REE abundances in the mantle beneath the Reykjanes Peninsula; in fact, the data require significant chemical heterogeneity in the hypothesized mantle plume beneath Iceland, as well as lateral mantle heterogeneity from the Reykjanes Ridge to the Reykjanes Peninsula. The compositional range of the Reykjanes Peninsula basalts is consistent with mixing of magmas produced by different degrees of melting in different parts of the heterogeneous mantle source beneath the Reykjanes Peninsula.  相似文献   

11.
Basalts from seamounts within the Nazca Plate representing intraplate volcanism, and the East Pacific Ridge between 19°S and 2°N have similar light rare earth depleted abundance patterns. Both intraplate and ridge basalts appear to have been derived from the low-velocity layer apparently depleted in large lithophile elements (DLVL). Nepheline-normative basalts and ferrobasalts occasionally occurring on the East Pacific Rise are shown to have also been derived from the same DLVL source. Furthermore, the rare-earth pattern similarity of nepheline-normative and tholeiitic basalts from the East Pacific Rise is best explained by distinct, pressure induced, conditions of partial melting of the DLVL source; whereas total rare-earth pattern enrichment and relative europium depletion of the ferrobasalts are consistent with shallow depth fractional crystallization during ascent.  相似文献   

12.
An en echelon suite of four fracture zones, trending approximately N40°E, has been discovered during a survey of the Southwest Indian Ocean Ridge between Bouvet Island and 14°E. The largest of these fracture zones, the Islas Orcadas and Shaka, are less than 30 km wide, have more than 3 km of vertical relief, and are respectively 100 and 200 km in length. The morphology of these and the Bouvet and Prince Edward fracture zones have been used to compute a pole for the relative motion between Africa and Antarctica. This pole, at 4°S and 32°W, is within the range of previously computed pole positions.Ridge basalts were dredged at three separate locations: at the Conrad fracture zone near 55°40′S and 3°51′W, at the Islas Orcadas fracture zone near 54°5′S and 6°4′E, and at the ridge crest near 11°E. In addition, samples from a probable upper mantle intrusion were recovered from one wall of the Islas Orcadas fracture zone. The opposite wall was very different consisting entirely of normal mid-ocean ridge basalt.  相似文献   

13.
Data from the hot springs at the Galapagos spreading center (T = 3–13°C) show depletions of the exiting waters in Cu, Ni, Cd, Se, Cr and U relative to ambient seawater. Manganese is strongly enriched. Iron shows highly variable behavior between vent fields but is in general low. The data confirm the occurrence of extensive subsurface mixing between the primary high-temperature, acid, reducing hydrothermal fluids and “groundwater”. The composition of the latter is indistinguishable from that of the free water column adjacent to the ridge axis. The final solutions are on the boundary between those forming MnO2 crusts and those producing iron-manganese rich sediments. The suite of metal rich deposits observed at ridge crests — Mn-O, Fe-Mn-O, Fe-S — can be explained as the manifestation of the degree of subsurface mixing, decreasing from 100 : <1 to <1 : 1 across the series (assuming an end-member temperature of 350°C).  相似文献   

14.
A correlary of sea floor spreading is that the production rate of ocean ridge basalts exceeds that of all other volcanic rocks on the earth combined. Basalts of the ocean ridges bring with them a continuous record in space and time of the chemical characteristics of the underlying mantle. The chemical record is once removed, due to chemical fractionation during partial melting. Chemical fractionations can be evaluated by assuming that peridotite melting has proceeded to an olivine-orthopyroxene stage, in which case the ratios of a number of magmaphile elements in the extracted melt closely match the ratios in the mantle. Comparison of ocean ridge basalts and chondritic meteorites reveals systematic patterns of element fractionation, and what is probably a double depletion in some elements. The first depletion is in volatile elements and is due to high accretion temperatures of a large percentage of the earth from the solar nebula. The second depletion is in the largest, most highly charged lithophile elements (“incompatible elements”), probably because the mantle source of the basalts was melted previously, and the melt, enriched in these elements, was removed. Migration of melt relative to solid under ocean ridges and oceanic plates, element fractionation at subduction zones, and fractional melting of amphibolite in the Precambrian are possible mechanisms for depleting the mantle in incompatible elements. Ratios of transition metals in the mantle source of ocean ridge basalts are close to chondritic, and contrast to the extreme depletion of refractory siderophile elements, the reason for which remains uncertain. Variation of ocean ridge basalt chemistry along the length of the ridge has been correlated with ridge elevation. Thus chemically anomalous ridge segments up to 1000 km long appear to broadly coincide with regions of high magma production (plumes, hot spots). Basalt heterogeneity at a single location indicates mantle heterogeneity on a smaller scale. Variation of ocean ridge basalt chemistry with time has not been established, in fact, criteria for recognizing old oceanic crust in ophiolite terrains are currently under debate. The similarity of rare earth element patterns in basalt from ocean ridges, back-arc basins, some young island arcs, and some continental flood basalts illustrates the dangers of tectonic labeling by rare earth element pattern.  相似文献   

15.
In the course of studying the Deccan Trap Hows around Igatpuri (latitudes 19°38′ and 19°45′: longitudes 73°30′ and 73°42′), picrite-basalts, not hitherto reported from this area, have been found occurring associated with basaltic flows. Thirty-eight flows, of a total thickness of 2200 feet, have been delimited. Of these, 8 flows may be termed picrite-basalts with a thickness varying from 25 to 75 feet. A feature of these basic flows is the abundance of olivine phenocrysts, with a complete absence of pyroxene phenocrysts in two of the flows. Two flows may be termed oceanite, two ankaramite, while four flows have phenocrysts of olivine, pyroxene and lelspar of An 65–70 %. In the oceanite flows the olivine phenocrysts constitute 20 to 30 per cent of the rock. They are mostly fresh, but some are altered to iddingsite. As regards the basaltic flows, about half have scanty olivine phenocrysts, the remainder being devoid of olivine. Eight are coarse enough to be termed olivine-dolerites. In the picrite-basalts the pyroxene phenocrysts have an optic axial angle of from 55° to 60°. In the basaltic flows the angle varies from 45° to 52°. The olivine phenocrysts of the picrite-basalts are highly magnesian, whereas those of the basalts are more ferruginous, as determined by optical methods. West’s view that the origin of these picrite-basalts is due to differentiation by crystal settling followed by freezing and extrusion, seems to be supported by this study.  相似文献   

16.
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   

17.
Petrological, volcanological and geochronological data collected at Mathews Tuya together provide constraints on paleoclimate conditions during formation of the edifice. The basaltic tuya was produced via Pleistocene glaciovolcanism in northern British Columbia, Canada, and is located within the Tuya volcanic field (59.195°N/130.434°W), which is part of the northern Cordilleran volcanic province (NCVP). The edifice comprises a variety of lithofacies, including columnar-jointed lava, pillow lava, massive dikes, and volcaniclastic rocks. Collectively these deposits record the transition from an explosive subaqueous to an effusive subaerial eruption environment dominated by Pleistocene ice. As is typical for tuyas, the volcaniclastic facies record multiple fragmentation processes including explosive, quench and mechanical fragmentation. All samples from Mathews Tuya are olivine-plagioclase porphyritic alkali olivine basalts. They are mineralogically and geochemically similar to nearby glaciovolcanic centers from the southeastern part of the Tuya volcanic field (e.g., Ash Mountain, South Tuya, Tuya Butte) as well as the dominant NCVP rock type. Crystallization scenarios calculated with MELTS account for variations between whole rock and glass compositions via low pressure fractionation. The presence of olivine microphenocrysts and the absence of pyroxene phenocrysts constrain initial crystallization pressures to less than 0.6 GPa. The eruption of Mathews Tuya occurred between 0.718 ± 0.054 Ma and 0.742 ± 0.081 Ma based on 40Ar/39Ar geochronology (weighted mean age of 0.730 Ma). The age determinations provide the first firm documentation for large (>700 m thick), pre-Fraser/Wisconsin glaciers in north-central British Columbia ~0.730 Ma, and correlate in age with glaciovolcanic deposits in Russia (e.g., Komatsu et al. Geomorph 88: 352-366, 2007) and with marine isotopic evidence for large global ice volumes ~0.730 Ma.  相似文献   

18.
The November 2002 eruption of Piton de la Fournaise in the Indian Ocean was typical of the activity of the volcano from 1999 to 2006 in terms of duration and volume of magma ejected. The first magma erupted was a basaltic liquid with a small proportion of olivine phenocrysts (Fo81) that contain small numbers of melt inclusions. In subsequent flows, olivine crystals were more abundant and richer in Mg (Fo83–84). These crystals contain numerous melt and fluid inclusions, healed fractures, and dislocation features such as kink bands. The major element composition of melt inclusions in this later olivine (Fo83–84) is out of equilibrium with that of its host as a result of extensive post-entrapment crystallization and Fe2+ loss by diffusion during cooling. Melt inclusions in Fo81 olivine are also chemically out of equilibrium with their hosts but to a lesser degree. Using olivine–melt geothermometry, we determined that melt inclusions in Fo81 olivine were trapped at lower temperature (1,182 ± 1°C) than inclusions in Fo83–84 olivine (1,199–1,227°C). This methodology was also used to estimate eruption temperatures. The November 2002 melt inclusion compositions suggest that they were at temperatures between 1,070°C and 1,133°C immediately before eruption and quenching. This relatively wide temperature range may reflect the fact that most of the melt inclusions were from olivine in lava samples and therefore likely underwent minor but variable amounts of post-eruptive crystallization and Fe2+ loss by diffusion due to their relatively slow cooling on the surface. In contrast, melt inclusions in tephra samples from past major eruptions yielded a narrower range of higher eruption temperatures (1,163–1,181°C). The melt inclusion data presented here and in earlier publications are consistent with a model of magma recharge from depth during major eruptions, followed by storage, cooling, and crystallization at shallow levels prior to expulsion during events similar in magnitude to the relatively small November 2002 eruption.  相似文献   

19.
Compositional heterogeneity (56–64 wt% SiO2 whole-rock) in samples of tephra and lava from the 1986 eruption of Augustine Volcano, Alaska, raises questions about the physical nature of magma storage and interaction beneath this young and frequently active volcano. To determine conditions of magma storage and evolutionary histories of compositionally distinct magmas, we investigate physical and chemical characteristics of andesitic and dacitic magmas feeding the 1986 eruption. We calculate equilibrium temperatures and oxygen fugacities from Fe-Ti oxide compositions and find a continuous range in temperature from 877 to 947°C and high oxygen fugacities (ΔNNO=1–2) for all magmas. Melt inclusions in pyroxene phenocrysts analyzed by Fourier-transform infrared spectroscopy and electron probe microanalysis are dacitic to rhyolitic and have water contents ranging from <1 to ∼7 wt%. Matrix glass compositions are rhyolitic and remarkably similar (∼75.9–76.6 wt% SiO2) in all samples. All samples have ∼25% phenocrysts, but lower-silica samples have much higher microlite contents than higher-silica samples. Continuous ranges in temperature and whole-rock composition, as well as linear trends in Harker diagrams and disequilibrium mineral textures, indicate that the 1986 magmas are the product of mixing between dacitic magma and a hotter, more mafic magma. The dacitic endmember is probably residual magma from the previous (1976) eruption of Augustine, and we interpret the mafic endmember to have been intruded from depth. Mixing appears to have continued as magmas ascended towards the vent. We suggest that the physical structure of the magma storage system beneath Augustine contributed to the sustained compositional heterogeneity of this eruption, which is best explained by magma storage and interaction in a vertically extensive system of interconnected dikes rather than a single coherent magma chamber and/or conduit. The typically short repose period (∼10 years) between Augustine's recent eruptive pulses may also inhibit homogenization, as short repose periods and chemically heterogeneous magmas are observed at several volcanoes in the Cook Inlet region of Alaska.  相似文献   

20.
A suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the Mount Adams volcanic field has been analyzed for U–Th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis. All of the lavas have (230Th/238U) > 1 and span a large range in (230Th/232Th) ratios, and most basalts have higher (230Th/232Th) ratios than andesites and dacites. Several of the lavas contain antecrysts (crystals of pre-existing material), yet internal U–Th mineral isochrons from six of seven lavas are indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to eruption. One isochron gave a crystallization age that is ~ 20–25 ka older than its corresponding eruptive age, and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in the crust. Much of the eruptive volume since 350 ka consists of lavas that have small to moderate 230Th excesses (2–16%), which are likely inherited from melting of a garnet-bearing intraplate (“OIB-like”) mantle source. Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas interacted with young, mafic lower crust, as indicated by 187Os/188Os ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os–Th isotope variations suggest that unusually large 230Th excesses (25–48%) and high 187Os/188Os ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level ‘cryptic’ processes has been decoupled from shallow-level crystallization.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号