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1.
In a global examination of mid-ocean ridge basalt (MORB) glasscompositions, we find that Na8–Fe8–depth variationsdo not support modeling of MORBs as aggregates of melt compositionsgenerated over a large range of temperature and pressure. However,the Na8–Fe8 variations are consistent with the compositionalsystematics of solidus melts in the plagioclase–spinellherzolite transition in the CaO–MgO–Al2O3–SiO2–Na2O–FeO(CMASNF) system. For natural compositions, the P–T rangefor melt extraction is estimated to be 1·2–1·5GPa and 1250–1280°C. This P–T range is a closematch with the maximum P–T conditions for explosive pressure-releasevaporization of carbonate-bearing melts. It is proposed thatfracturing of the lithosphere induces explosive formation andescape of CO2 vapor. This provides the vehicle for extractionof MORBs at a relatively uniform T and P. The upper portionof the CO2-bearing and slightly melted seismic low-velocityzone flows toward the ridge, rises at the ridge axis to lower-lithospheredepths, melts much more extensively during this rise, and releasesMORB melts to the surface driven by explosively escaping CO2vapor. The residue and overlying crust produced by this meltingthen migrate away from the ridge axis as new oceanic lithosphere.The entire process of oceanic lithosphere creation involvesonly the upper 140 km. When lithospheric stresses shift fractureformation to other localities, escape of CO2 ceases, the vehiclefor transporting melt to the surface disappears, and ridgesdie. Inverse correlations of Na8 vs Fe8 for MORB glasses areexplained by mantle heterogeneity, and positive variations superimposedon the inverse variations are consistent with progressive extractionof melts from short, ascending melting columns. The uniformlylow temperatures of MORB extraction are not consistent withthe existence of hot plumes on or close to ocean ridges. Inthis modeling, the southern Atlantic mantle from Bouvet to about26°N is relatively homogeneous, whereas the Atlantic mantlenorth of about 26°N shows significant long-range heterogeneity.The mantle between the Charlie Gibbs and Jan Mayen fracturezones is strongly enriched in FeO/MgO, perhaps by a trappedfragment of basaltic crust. Iceland is explained as the productof this enrichment, not a hot plume. The East Pacific Rise,Galapagos Ridge, Gorda Ridge, and Juan de Fuca Ridge samplemantle that is heterogeneous over short distances. The mantlebeneath the Red Sea is enriched in FeO/MgO relative to the mantlebeneath the northern Indian Ocean. 相似文献
2.
Dean C Presnall Gudmundur H GudfinnssonMichael J Walter 《Geochimica et cosmochimica acta》2002,66(12):2073-2090
We propose a model for the generation of average MORBs based on phase relations in the CaO-MgO-Al2O3-SiO2-CO2 system at pressures from 3 to 7 GPa and in the CaO-MgO-Al2O3-SiO2-Na2O-FeO (CMASNF) system at pressures from ∼0.9 to 1.5 GPa. The MELT seismic tomography (Forsyth et al., 2000) across the East Pacific Rise shows the largest amount of melt centered at ∼30-km depth and lesser amounts at greater depths. An average mantle adiabat with a model-system potential temperature (Tp) of 1310°C is used that is consistent with this result. In the mantle, additional minor components would lower solidus temperatures ∼50°C, which would lower Tp of the adiabat for average MORBs to ∼1260°C. The model involves generation of carbonatitic melts and melts that are transitional between carbonatite and kimberlite at very small melt fractions (<0.2%) in the low-velocity zone at pressures of ∼2.6 to 7 GPa in the CMAS-CO2 system, roughly the pressure range of the PREM low-velocity zone. These small-volume, low-viscosity melts are mixed with much larger volumes of basaltic melt generated at the plagioclase-spinel lherzolite transition in the pressure range of ∼0.9 to 1.5 GPa.In this model, solidus phase relations in the pressure range of the plagioclase-spinel lherzolite transition strongly, but not totally, control the major-element characteristics of MORBs. Although the plagioclase-spinel lherzolite transition suppresses isentropic decompression melting in the CMAS system, this effect does not occur in the topologically different and petrologically more realistic CMASNF system. On the basis of the absence of plagioclase from most abyssal peridotites, which are the presumed residues of MORB generation, we calculate melt productivity during polybaric fractional melting in the plagioclase-spinel lherzolite transition interval at exhaustion of plagioclase in the residue. In the CMASN system, these calculations indicate that the total melt productivity is ∼24%, which is adequate to produce the oceanic crust. The residual mineral proportions from this calculation closely match those of average abyssal peridotites.Melts generated in the plagioclase-spinel lherzolite transition are compositionally distinct from all MORB glasses, but do not have a significant fractional crystallization trend controlled by olivine alone. They reach the composition field of erupted MORBs mainly by crystallization of both plagioclase and olivine, with initial crystallization of either one of these phases rapidly joined by the other. This is consistent with phenocryst assemblages and experimental studies of the most primitive MORBs, which do not show an olivine-controlled fractionation trend. The model is most robust for the eastern Pacific, where an adiabat with a Tp of ∼1260°C is supported by the MELT seismic data and where the global inverse correlation of (FeO)8 with (Na2O)8 is weak. Average MORBs worldwide also are well modeled. A heterogeneous mantle consisting of peridotite of varying degrees of major-element depletion combined with phase-equilibrium controls in the plagioclase-spinel lherzolite transition interval would produce the form of the global correlations at a constant Tp, which suggests a modest range of Tp along ridges. Phase-composition data for the CMASNF system are presently not adequate for quantitative calculation of (FeO)8-(Na2O)8-(CaO/Al2O3)8 systematics in terms of this model. The near absence of basalts in the central portion of the Gakkel Ridge suggests a lower bound for Tp along ridges of ∼1240°C, a potential temperature just low enough to miss the solidus for basalt production at ∼0.9 GPa. An upper bound for Tp is poorly constrained, but the complete absence of picritic glasses in Iceland and the global ridge system suggests an upper bound of ∼1400°C. In contrast to some previous models for MORB generation that emphasize large potential temperature variations in a relatively homogeneous peridotitic mantle, our model emphasizes modest potential temperature variations in a peridotitic mantle that shows varying degrees of heterogeneity. Calculations indicate that melt productivity changes from 0 to 24% for a change in Tp from 1240 to 1260°C, effectively producing a rapid increase to full crustal thickness or decrease to none as ridges appear and disappear. 相似文献
3.
Some workers have held that mid-ocean ridge basalts are fractionated from high pressure (15–30 kbar) picritic primary magmas
whereas others have favored primary magmas generated at about 10 kbar with compositions close to those of mid-ocean ridge
basalts. Of critical significance are presumed differences in composition between experimentally determined primary magmas
and the least fractionated mid-ocean ridge basalts. To evaluate the significance of these differences, all based on electron
microprobe analyses, we consider three sources of uncertainty: (1) analytical uncertainties for a single microprobe laboratory,
(2) systematic interlaboratory analytical differences, and (3) real variations in the possible compositions of primary magmas
that can be produced from a peridotite source at a given pressure. The first source of error is surprisingly large and can
account for a substantial part of the total variation of normative quartz (hypersthene calculated as equivalent olivine and
quartz) in FAMOUS basalts. The second is not as serious but remains undetermined for many laboratories. The third is potentially
the largest but is not yet fully documented. The least fractionated FA-MOUS basalts have high mg numbers (70–73) compatible with derivation from the mantle by direct partial melting with little or no subsequent fractional
crystallization. Because of the wide range of normative quartz content in these basalts, it appears necessary to consider
them as representatives of multiple parental magmas. When all the sources of uncertainty are taken into account, we conclude
that the experimental data by various investigators are all fairly consistent and favor derivation of the least fractionated
mid-ocean ridge basalts by at most only a small amount of fractional crystallization from primary magmas having a wide range
of normative quartz content and generated over a range of pressures from about 7–11 kbar.
Contribution No. 420, Department of Geosciences, The University of Texas at Dallas 相似文献
4.
5.
D.C. Presnall 《Physics of the Earth and Planetary Interiors》1980,23(2):103-111
For a lherzolite mantle with about 0.1 wt.-percent CO2 or less, and a CO2/H2O mole ratio greater than about one, the mantle solidus curve in P-T space will have two important low-temperature regions, one centered at about 9 kbar (30 km depth) and another beginning at about 28 kbar (90 km depth). It is argued that the depth of generation of primary tholeiitic magmas beneath ridge crests is about 9 kbar, and that the geotherm changes from an adiabatic gradient at greater pressures to a strongly superadiabatic gradient at lesser pressures. Such a ridge geotherm would intersect the solidus at two separate depth intervals corresponding to the two low-temperature regions on the solidus. With increasing age and cooling of the lithosphere, the shallow partial melt zone would pinch out and the thickness of the deep partial melt zone would decrease. With increasing depth in a mature oceanic lithosphere, the rock types would consist of depleted harzburgite from directly beneath the crust to about 30 km depth, fertile spinel lherzolite from about 30 km to 50–60 km, and fertile garnet lherzolite from about 50–60 km to the top of the deep partial melt zone at about 90 km. 相似文献
6.
In studies of iron silicate liquids under reducing conditions at 1 atm pressure, iron losses from the melt can be minimized by suspending the liquid as a drop from a short segment of fine Pt wire. For a basaltic composition at 1275°C and oxygen fugacities appropriate to terrestrial magmas, iron losses from the melt are less than 0.5 wt.% of the amount present for run times less than about 20 hr. 相似文献
7.
A model is proposed for the origin of hot spots that depends on the existence of major-element heterogeneities in the mantle. Generation of basaltic crust at spreading centers produces a layer of residual peridotite ～20–25 km thick directly beneath the crust which is depleted in Fe/Mg, TiO2, CaO, Al2O3, Na2O and K2O, and which has a slightly lower density than undepleted peridotite beneath it. Upon recycling of this depleted peridotite back into the deep mantle at subduction zones, it becomes gravitationally unstable, and tends to rise as diapirs through undepleted peridotite. For a density contrast of 0.05 g cm?3, a diapir 60 km in diameter would rise at roughly 8 cm y?1, and could transport enough heat to the base of the lithosphere to cause melting and volcanism at the surface. Hot spots are thus viewed as a passive consequence of mantle convection and fractionation at spreading centers rather than a plate-driving force.It is suggested that depleted diapirs exist with varying amounts of depletion, diameters, upward velocities and source volumes. Such variations could explain the occurrence of hot spots with widely varying lifetimes and rates of lava production. For highly depleted diapirs with very low Fe/Mg, the diapir would act as a heat source and the asthenosphere and lower lithosphere drifting across the diapir would serve as the source region of magmas erupted at the surface. For mildly depleted diapirs with Fe/Mg only slightly less than in normal undepleted mantle, the diapir could provide not only the source of heat but also most or all of the source material for the erupted magmas. The model is consistent with isotopic data that require two separate and ancient source regions for mid-ocean ridge and oceanic island basalts. The source for mid-ocean ridge basalts is considered to be material upwelling at spreading centers from the deep mantle. This material forms the oceanic lithosphere. Oceanic island basalts are considered to be derived from varying mixtures of sublithospheric and lower lithospheric material and the rising diapir itself. 相似文献
8.
D. C. Presnall Selena A. Dixon James R. Dixon T. H. O'Donnell N. L. Brenner R. L. Schrock D. W. Dycus 《Contributions to Mineralogy and Petrology》1978,66(2):203-220
In the system CaO-MgO-Al2O3-SiO2, the tetrahedron CaMgSi2O6(di)-Mg2SiO4(fo)-SiO2-CaAl2 SiO6(CaTs) forms a simplified basalt tetrahedron, and within this tetrahedron, the plane di-fo-CaAl2Si2O8(an) separates simplified tholeiitic from alkalic basalts. Liquidus phase relations on this join have been studied at 1 atm and at 7, 10, 15, and 20 kbar. The temperature maximum on the 1 atm isobaric quaternary univariant line along which forsterite, diopside, anorthite, and liquid are in equilibrium lies to the SiO2-rich side of the join di-fo-an. The isobaric quaternary invariant point at which forsterite, diopside, anorthite, spinel, and liquid are in equilibrium passes, with increasing pressure, from the silica-poor to the silica-rich side of the join di-fo-an, which causes the piercing points on this join to change from forsterite+diopside+anorthite+liquid and forsterite +spinel+anorthite+liquid below 5 kbar to forsterite +diopside+spinel+liquid and diopside +spinel+anorthite+liquid above 5 kbar. As pressure increases, the forsterite and anorthite fields contract and the diopside and corundum fields expand. The anorthite primary phase field disappears entirely from the join di-fo-an between 15 and 20 kbar. Below about 4 kbar, the join di-fo-an represents, in simplified form, a thermal divide between alkalic and tholeiitic basalts. From about 4 to at least 12 kbar, alkalic basalts can produce tholeiitic basalts by fractional crystallization, and at pressures above about 12 kbar, it is possible for alkalic basalt to be produced from oceanite by crystallization of both olivine and orthopyroxene. If alkalic basalts are primary melts from a lherzolite mantle, they must be produced at high pressures, probably greater than about 12 kbar.Department of Geosciences, University of Texas at Dallas Contribution No. 327. Hawaii Institute of Geophysics Contribution No. 814. 相似文献
9.
We have experimentally determined the solidus position of model lherzolite in the system CaO-MgO-Al2O3-SiO2-CO2 (CMAS.CO2) from 3 to 7 GPa by locating isobaric invariant points where liquid coexists with olivine, orthopyroxene, clinopyroxene,
garnet and carbonate. The intersection of two subsolidus reactions at the solidus involving carbonate generates two invariant
points, I1A and I2A, which mark the transition from CO2-bearing to dolomite-bearing and dolomite-bearing to magnesite-bearing lherzolite respectively. In CMAS.CO2, we find I1A at 2.6 GPa/1230 °C and I2A at 4.8 GPa/1320 °C. The variation of all phase compositions along the solidus has also been determined. In the pressure range
investigated, solidus melts are carbonatitic with SiO2 contents of <6 wt%, CO2 contents of ˜45 wt%, and Ca/(Ca+Mg) ratios that range from 0.59 (3 GPa) to 0.45 (7 GPa); compositionally they resemble natural
magnesiocarbonatites. Volcanic magnesiocarbonatites may well be an example of the eruption of such melts directly from their
mantle source region as evidenced by their diatremic style of activity and lack of associated silicate magmas. Our data in
the CMAS.CO2 system show that in a carbonate-bearing mantle, solidus and near-solidus melts will be CO2-rich and silica poor. The widespread evidence for the presence of CO2 in both the oceanic and continental upper mantle implies that such low degree SiO2-poor carbonatitic melts are common in the mantle, despite the rarity of carbonatites themselves at the Earth's surface.
Received: 9 April 1997 / Accepted: 25 November 1997 相似文献
10.
Liquidus phase relationships determined on the join anorthite-forsterite-quartz at 20 kbar show primary phase fields for quartz (q), forsterite (fo), enstatite (en), spinel (sp), anorthite (an), sapphirine (sa), and corundum (cor). Increasing pressure causes (1) thefo andan primary phase fields to contract, (2) theen, q, andcor fields to expand, (3) thefo-en boundary line to move away from the Q apex, (4) theen-q boundary line to move also away from the Q apex but by a smaller amount, and (5) a primary phase field forsa to appear at a pressure between 10 and 20 kbar. Seven liquidus piercing points at 20 kbar have been located as follows:
| Crystalline phases 相似文献
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