We propose a model for the generation of average MORBs based on phase relations in the CaO-MgO-Al
2O
3-SiO
2-CO
2 system at pressures from 3 to 7 GPa and in the CaO-MgO-Al
2O
3-SiO
2-Na
2O-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 (T
p) 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 T
p 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-CO
2 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 T
p of ∼1260°C is supported by the MELT seismic data and where the global inverse correlation of (FeO)
8 with (Na
2O)
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 T
p, which suggests a modest range of T
p along ridges. Phase-composition data for the CMASNF system are presently not adequate for quantitative calculation of (FeO)
8-(Na
2O)
8-(CaO/Al
2O
3)
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 T
p 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 T
p 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 T
p from 1240 to 1260°C, effectively producing a rapid increase to full crustal thickness or decrease to none as ridges appear and disappear.
The compositions of liquids in equilibrium with lherzolite over a range of temperatures (1380–1505°C) above the carbonated lherzolite solidus have been determined in the system CaO–MgO–Al2O3–SiO2–CO2 ...at 6 GPa. Melt compositions show systematic variation with temperature from carbonatitic (Mg/Ca ratio 1; 5 wt % SiO2) at the solidus (1380°C) through intermediary compositions to kimberlitic (Mg/Ca ratios >2; >25 wt % SiO2) 70–100°C above the solidus. For melting of model lherzolite with a CO2 content of 0.15 wt %, this continuous change in melt composition from carbonatitic to kimberlitic takes place in the melting range 0–1%. Our data are thus consistent with an origin for group 1B kimberlites by low-degree partial melting of carbonated, garnet lherzolite at pressures of at least <10 GPa. Furthermore, the observed carbonatite–kimberlite continuum in melt compositions supports petrogenetic links between carbonatites and kimberlites in the mantle source region by small variations in the melt fraction. Carbonatites are associated with kimberlites in mobile belts adjacent to cratons, such as in the Sarfartoq region in west Greenland. Here, a continuum of rock compositions that range from kimberlite through ultramafic lamprophyres to dolomitic carbonatites is in very good agreement with the experimental data at 6 GPa, consistent with the variations in magma compositions in the Sarfartoq region being produced mainly by variations in the amount of melting at the source. Our data suggest that a similar origin may apply to other carbonatite–kimberlite–ultramafic alkaline rock associations.
The focus of this study is a suite of garnet-bearing mantle xenoliths from Oahu, Hawaii. Clinopyroxene, olivine, and garnet constitute the bulk of the xenoliths, and orthopyroxene is present in small ...amounts. Clinopyroxene has exsolved orthopyroxene, spinel, and garnet. Many xenoliths also contain spinel-cored garnets. Olivine, clinopyroxene, and garnet are in major element chemical equilibrium with each other; large, discrete orthopyroxene does not appear to be in major-element chemical equilibrium with the other minerals. Multiple compositions of orthopyroxene occur in individual xenoliths. The new data do not support the existing hypothesis that all the xenoliths formed at 1ċ 6–2ċ2 GPa, and that the spinel-cored garnets formed as a consequence of almost isobaric subsolidus cooling of a spinel-bearing assemblage. The lack of olivine or pyroxenes in the spinel–garnet reaction zones and the embayed outline of spinel grains inside garnet suggest that the spinel-cored garnets grew in the presence of a melt. The origin of these xenoliths is interpreted on the basis of liquidus phase relations in the tholeiitic and slightly silica-poor portion of the CaO–MgO–Al2O3–SiO2 (CMAS) system at pressures from 3ċ0 to 5ċ0 GPa. The phase relations suggest crystallization from slightly silica-poor melts (or transitional basaltic melts) in the depth range ∼110–150 km beneath Oahu. This depth estimate puts the formation of these xenoliths in the asthenosphere. On the basis of this study it is proposed that the pyroxenite xenoliths are high-pressure cumulates related to polybaric magma fractionation in the asthenosphere, thus making Oahu the only locality among the oceanic regions where such deep magmatic fractional crystallization processes have been recognized.
Phase equilibrium data pertaining to melting of simplified carbonated peridotite in the systems CaO-MgO-SiO2-CO2 and CaO-MgO-Al2O3-SiO2-CO2 at pressures of 10-26 GPa, corresponding to ~300-750 km ...depths in the Earth, are presented. In both the studied systems, liquid compositions, with changing crystalline phase assemblage, are carbonatitic throughout the studied pressure range. In the system CMS-CO2, melting phase relations are isobarically invariant; liquid is in equilibrium with forsterite + clinoenstatite + clinopyroxene + magnesite, forsterite + majorite + clinopyroxene + magnesite, wadsleyite + majorite + clinopyroxene + magnesite, ringwoodite + majorite + calcium-silicate perovskite + magnesite, magnesium-silicate perovskite + periclase + calcium-silicate perovskite + magnesite at 12, 14, 16, 20, and 26 GPa, respectively. In the system CMAS-CO2, a phase assemblage consisting of forsterite + orthopyroxene + clinopyroxene + magnesite + garnet + melt from 10 to 14 GPa is isobarically invariant. However, owing to the disappearance of orthopyroxene at pressures greater than 14 GPa, from 16 and up to at least 26 GPa, the solidus of simplified carbonated peridotite spans a divariant surface in pressure-temperature space. The liquid coexists with wadsleyite + clinopyroxene + garnet + magnesite, ringwoodite + calcium-silicate perovskite + garnet + magnesite, and magnesium-silicate perovskite + periclase + calcium-silicate perovskite + magnesite at 16, 20, and 26 GPa, respectively. A curious, and as yet unexplained, feature of our study is an abrupt drop in the solidus temperature between 14 and 16 GPa that causes a small amount of melting of carbonated mantle in the Transition Zone of the Earth. In the systems CMS-CO2 and CMAS-CO2 liquid compositions at 16 and 20 GPa are highly calcic bona fide carbonatites; however, these liquids revert to being magnesiocarbonatites at 10-14 and 26 GPa. In the system CMS-CO2, at 16 GPa we locate an isobaric invariant point consisting of wadsleyite + clinopyroxene + anhydrous B + magnesite + melt. The presence of anhydrous B at 16 GPa and 1475°C is interesting, as it lies outside the composition space of the mantle peridotite analog we have studied. However, despite the presence of two highly magnesian silicate crystalline phases, wadsleyite and anhydrous B, at 16 GPa and 1475°C, the liquid composition remains calcic with molar Ca-number Ca/(Ca + Mg) × 100 of about 63. The melting reactions at 16 and 20 GPa (with or without anhydrous B) show that lime-bearing crystalline silicates play a fairly large part in generating and controlling the composition of the liquids. At 16 GPa, in the system CMS-CO2, we also report an experimental run at 1575°C, in which liquid coexists with only wadsleyite and majorite. The liquid composition is less calcic (Ca-number 54) than that for other runs at lower temperatures, but is still more calcic than liquids at 10-14 and 26 GPa in both the studied systems. At present, the likely cause for these changes in the reported phase relations is not known. For normally assumed mantle temperatures, melting in the Transition Zone of the Earth, owing to the presence of carbonate, is probably unavoidable. The depth range of the drop in the carbonated peridotite solidus closely matches that of commonly observed low seismic velocities at ~400-600 km depth in the Earth.
We develop a model for oceanic volcanism that involves fracturing of the seismic lithosphere to access melts from the partly melted seismic low-velocity zone. Data on global seismic shear-wave ...velocities are combined with major-element compositions of global mid-ocean ridge basalt glasses, Hawaiian basalt glasses, and phase relations in the CaO-MgO-Al2O3-SiO2-CO2 and CaO-MgO-Al2O3-SiO2-Na2O-FeO systems at pressures from 1 atm to 6 GPa. We use these data to constrain the pressure-temperature conditions for melt extraction at Hawaii and mid-ocean ridges (including Iceland), and to evaluate the existence of hot mantle plumes. In the low-velocity zone, the maximum reduction and maximum anisotropy of seismic shear-wave velocity (maximum melt fraction) occurs at a depth of ∼140-150 km for crustal ages >∼50 Ma, and a depth of ∼65 km at the East Pacific Rise axis. Seismic data indicate a smooth depth transition between these extremes. Experimental phase-equilibrium data, when combined with natural glass compositions, show that pressure-temperature conditions of tholeiitic melt extraction at Hawaii (∼4-5 GPa, 1450-1500°C) and the global oceanic ridge system (∼1·2-1·5 GPa, 1250-1280°C) are an excellent match for the two depth ranges of maximum melting indicated by seismic shear-wave data. At Hawaii, magmas escape to the surface along a fracture system that extends through the lithosphere into the low-velocity zone. This allows eruption of progressively deeper melts from the low-velocity zone, which exist at equilibrium along a normal geotherm. No significant decompression melting occurs. This produces the characteristic sequence at each volcano of initial low-volume alkalic magmas, then voluminous tholeiitic magmas that show low-pressure olivine-controlled crystallization, and final low-volume alkalic magmas from extreme depths. At the East Pacific Rise, the more shallow depth of magma extraction is caused by a perturbed ridge geotherm that grazes the lherzolite solidus within the thermal boundary layer. This results in an absence of olivine-controlled crystallization. Hawaii is not a hot plume. Instead, it shows magmas characteristic of normal mature-ocean thermal conditions in the low-velocity zone. We find no evidence of anomalously high temperatures of magma extraction and no role for hot mantle plumes anywhere in the ocean basins.
We have experimentally determined the solidus position of model lherzolite in the system CaO-MgO-AI2O3-SiO2-CO2 (CMAS.C02) 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.
Origin of the Oceanic Lithosphere Presnall, Dean C.; Gudfinnsson, Gudmundur H.
Journal of petrology,
04/2008, Letnik:
49, Številka:
4
Journal Article
Recenzirano
Odprti dostop
In a global examination of mid-ocean ridge basalt (MORB) glass compositions, we find that Na8–Fe8–depth variations do not support modeling of MORBs as aggregates of melt compositions generated over a ...large range of temperature and pressure. However, the Na8–Fe8 variations are consistent with the compositional systematics of solidus melts in the plagioclase–spinel lherzolite transition in the CaO–MgO–Al2O3–SiO2–Na2O–FeO (CMASNF) system. For natural compositions, the P–T range for melt extraction is estimated to be ∼1·2–1·5 GPa and ∼1250–1280°C. This P–T range is a close match with the maximum P–T conditions for explosive pressure-release vaporization of carbonate-bearing melts. It is proposed that fracturing of the lithosphere induces explosive formation and escape of CO2 vapor. This provides the vehicle for extraction of MORBs at a relatively uniform T and P. The upper portion of the CO2-bearing and slightly melted seismic low-velocity zone flows toward the ridge, rises at the ridge axis to lower-lithosphere depths, melts much more extensively during this rise, and releases MORB melts to the surface driven by explosively escaping CO2 vapor. The residue and overlying crust produced by this melting then migrate away from the ridge axis as new oceanic lithosphere. The entire process of oceanic lithosphere creation involves only the upper ∼140 km. When lithospheric stresses shift fracture formation to other localities, escape of CO2 ceases, the vehicle for transporting melt to the surface disappears, and ridges die. Inverse correlations of Na8 vs Fe8 for MORB glasses are explained by mantle heterogeneity, and positive variations superimposed on the inverse variations are consistent with progressive extraction of melts from short, ascending melting columns. The uniformly low temperatures of MORB extraction are not consistent with the existence of hot plumes on or close to ocean ridges. In this modeling, the southern Atlantic mantle from Bouvet to about 26°N is relatively homogeneous, whereas the Atlantic mantle north of about 26°N shows significant long-range heterogeneity. The mantle between the Charlie Gibbs and Jan Mayen fracture zones is strongly enriched in FeO/MgO, perhaps by a trapped fragment of basaltic crust. Iceland is explained as the product of this enrichment, not a hot plume. The East Pacific Rise, Galapagos Ridge, Gorda Ridge, and Juan de Fuca Ridge sample mantle that is heterogeneous over short distances. The mantle beneath the Red Sea is enriched in FeO/MgO relative to the mantle beneath the northern Indian Ocean.
Phase equilibrium data from the CaO‐MgO‐Al2O3‐SiO2, CaO‐MgO‐Al2O3‐SiO2‐Na2O, and CaO‐MgO‐Al2O3‐SiO2‐FeO systems on the melting behavior of plagioclase, spinel, and garnet lherzolite, are used to ...determine the molar partition coefficient of MgO between olivine and melt, DMgO*ol/liq, as a function of temperature, pressure, and composition. The data sets cover the pressure range 0.1 MPa to 5 GPa and temperatures of 1225°–1830°C. It is shown that the trend of DMgO*ol/liq is little affected by the transitions between different lherzolite assemblages and that one empirical equation, which has reciprocal temperature (in Kelvin) and mole fraction of NaO0.5 in the melt as independent variables, describes the variation of DMgO*ol/liq in pressure, temperature, and composition space and has the form lnDMgO*ol/liq = (4723/T) + 2.566CNaO05*liq − 1.729. This expression is tested by using it to calculate the temperatures of melting experiments containing olivine‐saturated basalt and picrite melts and is shown to yield good results for multiply saturated melts, consistent with the fact that the observed simple trend of DMgO*ol/liq is dictated by low‐variance phase relations. With fewer phases present, the equation systematically predicts temperatures that are too high. If the value of KDFe2+/Mgol/liq (KD) is known, the value of DMgO*ol/liq can be calculated from DMgO*ol/liq = 0.6667/(CFeO*liqKD + CMgO*liq), and hence the mole fractions of MgO, FeO, and NaO0.5 can be used to calculate the equilibrium temperature of multiply saturated olivine‐bearing melts. This geothermometer is mainly applicable to primitive tholeiitic and mildly alkalic basalt and picrite melts, with small amounts of volatiles only, in the 1150°–1800°C temperature range.
We present a method for calculating quantitative melting reactions in systems with multiple solid solutions that accounts for changes in the mass proportions of phases between two points at different ...temperatures along a melting curve. This method can be applied to any data set that defines the phase proportions along a melting curve. The method yields the net change in mass proportion of all phases for the chosen melting interval, and gives an average reaction for the melting path. Instantaneous melting reactions can be approximated closely by choosing sufficiently small melting intervals. As an application of the method, reactions for melting of model upper mantle peridotite are calculated using data from the system CaO-MgO-Al
2O
3-SiO
2-Na
2O (CMASN) over the pressure interval 0.7 – 3.5 GPa. Throughout almost this entire pressure range, melting of model lherzolite involves the crystallization of one or more solid phases, and is analogous to melting at a peritectic invariant point. In addition, we show that melting reactions for small melting intervals (< 5%) along the solidus of mantle peridotite are significantly different from those calculated for large melting intervals. For large melting intervals (> 10%), reaction stoichiometries calculated in CMASN are usually in good agreement with those available for melting of natural peridotite. The coefficients of melting reactions calculated from this method can be used in equations that describe the behavior of trace elements during melting. We compare results from near-fractional melting models using (1) melting reactions and rock modes from CMASN, and (2) constant reactions representative of those used in the literature. In modeling trace element abundances in melt, significant differences arise for some elements at low degrees of melting (< 10%). In modeling element abundances in the residue, differences increase with increase in degree of melting. Reactions calculated along the model lherzolite solidus in CMASN are the only ones available at present for small degrees of melting so we recommend them for accurate trace element modeling of natural lherzolite.
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