Generation of Mid-ocean Ridge Tholeiites

Presnall, D. C.; Dixon, J. R.; O'Donnell, T. H.; Dixons, S. A.

doi:10.1093/petrology/20.1.3

Cited by 351 publications

(159 citation statements)

References 41 publications

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“…Experiments were conducted along the quartz-fayalite-magnetite buffer, so that iron is mainly present as FeO in the silicate melt. Comparing these data with phase equilibria in the CaO-MgO-Al 2 O 3 -SiO 2 (CMAS) simplified system (Andersen, 1915;Osborn and Tait, 1952;Kushiro, 1972;Presnall et al, 1978Presnall et al, , 1979Longhi and Boudreau, 1980;Sen and Presnall, 1984;Liu and Presnall, 1990) allows us to constrain the phase relations for the entire plausible range of Mg-number of lavas that might be present on the surface of Mercury. At the high Mg-numbers represented in the Mercury and CMAS compositions, the low-Ca pyroxene stability fields along the olivine liquidus surface are protoenstatite, followed by orthoenstatite and then pigeonite (Longhi and Pan, 1988).…”

Section: Experiments: Goals and Methodsmentioning

confidence: 99%

Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy

Charlier

Grove

Zuber

2013

Earth and Planetary Science Letters

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a b s t r a c tMeasurements of major element ratios obtained by the MESSENGER spacecraft using x-ray fluorescence spectra are used to calculate absolute element abundances of lavas at the surface of Mercury. We discuss calculation methods and assumptions that take into account the distribution of major elements between silicate, metal, and sulfide components and the potential occurrence of sulfide minerals under reduced conditions. These first compositional data, which represent large areas of mixed highreflectance volcanic plains and low-reflectance materials and do not include the northern volcanic plains, share common silica-and magnesium-rich characteristics. They are most similar to terrestrial volcanic rocks known as basaltic komatiites. Two compositional groups are distinguished by the presence or absence of a clinopyroxene component. Melting experiments at one atmosphere on the average compositions of each of the two groups constrain the potential mineralogy at Mercury's surface, which should be dominated by orthopyroxene (protoenstatite and orthoenstatite), plagioclase, minor olivine if any, clinopyroxene (augite), and tridymite. The two compositional groups cannot be related to each other by any fractional crystallization process, suggesting differentiated source compositions for the two components and implying multi-stage differentiation and remelting processes for Mercury. Comparison with high-pressure phase equilibria supports partial melting at pressure o10 kbar, in agreement with last equilibration of the melts close to the crust-mantle boundary with two different mantle lithologies (harzburgite and lherzolite). Magma ocean crystallization followed by adiabatic decompression of mantle layers during cumulate overturn and/or convection would have produced adequate conditions to explain surface compositions. The surface of Mercury is not an unmodified quenched crust of primordial bulk planetary composition. Ultramafic lavas from Mercury have high liquidus temperatures (1450-1350 1C) and very low viscosities, in accordance with the eruption style characterized by flooding of pre-existing impact craters by lava and absence of central volcanoes.

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Section: Experiments: Goals and Methodsmentioning

confidence: 99%

Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy

Charlier

Grove

Zuber

2013

Earth and Planetary Science Letters

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“…In principle, the solidus should change slope as the mantle assemblage changes from plagioclase-to spinel-to garnetlherzolite with increasing pressure [e.g., Presnall et al, 1979]. These differences, however, are not obvious in the existing experimental data on natural systems.…”

Section: The Melting Functionmentioning

confidence: 99%

“…These experiments span a range of pressures of up to 60 kbar, and a range of compositions including MORBs [Grove and Bryan, 1983;Grove et al, 1990;Bender et a/., 1978;Walker et al, 1979;Tormey et al, 1987], highly alkaline basalts [Sack et al, 1987], lunar basalts [Grover et al, 1980] andesites [Grove et al, 1982], komatiites [Agee and Walker, 1990] and the ironand alkali-free CMAS system [Presnall et al, 1979]. Thus we have included an even wider range of compositions and pressures than are expected to occur in the melting regime.…”

Section: Appendix B: Olivine Liquid Partitioning As a Function Of Temmentioning

confidence: 99%

Petrological Systematics of Mid-Ocean Ridge Basalts: Constraints on Melt Generation Beneath Ocean Ridges

Langmuir

Klein

Plank

2013

Geophysical Monograph Series

671

390

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Mid-ocean ridge basalts (MORB) are a consequence of pressure-release melting beneath ocean ridges, and contain much information concerning melt formation, melt migration and heterogeneity within the upper mantle. MORB major element chemical systematics can be divided into global and local aspects, once they have been corrected for low pressure fractionation and interlaboratory biases. Regional average compositions for ridges unaffected by hot spots ("normal" ridges) can be used to define the global correlations among normalized Na20, FeO, TiO2 and Si02 contents, CaO/Al203 ratios, axial depth and crustal thickness. Back-arc basins show similar correlations, but are offset to lower FeO and TiO2 contents. Some hot spots, such as the Azores and Galapagos, disrupt the systematics of nearby ridges and have the opposite relationships between FeO, Na 20 and depth over distances of 1000 km.Local variations in basalt chemistry from slow-and fast-spreading ridges are distinct from one another. On slow-spreading ridges, correlations among the elements cross the global vector of variability at a high angle. On the fast-spreading East Pacific Rise (EPR), correlations among the elements are distinct from both global and slow-spreading compositional vectors, and involve two components of variation. Spreading rate does not control the global correlations, but influences the standard deviations of axial depth, crustal thickness, and MgO contents of basalts.Global correlations are not found in very incompatible trace elements, even for samples far from hot spots. Moderately compatible trace elements for normal ridges, however, correlate with the major elements. Trace element systematics are significantly different for the EPR and the mid-Atlantic Ridge (MAR). Normal portions of the MAR are very depleted in REE, with little variability; hot spots cause large long wavelength variations in REE abundances. Normal EPR basalts are significantly more enriched than MAR basalts from normal ridges, and still more enriched basalts can erupt sporadically along the entire length of the EPR. This leads to very different histograms of distribution for the data sets as a whole, and a very different distribution of chemistry along strike for the two ridges. Despite these differences, the mean Ce/Sm ratios from the two ridges are identical.Existing methods for calculating the major element compositions of mantle melts [Klein and Langmuir, 1987;McKenzie and Bickle, 1988;Niu and Batiza, 1991] are critically examined. New quantitative methods for mantle melting and high pressure fractionation are developed to evaluate the chemical consequences of melting and fractionation processes and mantle heterogeneity. The new methods rely on new equations for partition coefficients for the major elements between mantle minerals and melts. The melting calculations can be used to investigate the chemical compositions produced by small extents of melting or high pressures of melting that cannot yet be determined experimentally. Application of the new models to the ...

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“…An adiabatic diapir may consequently cross the solidus as it rises, thereby undergoing partial or complete fusion (Figure 1). Since the adiabatic gradient of a wholly molten system is solely a function of the EOS of the liquid and that of a partially molten system is a strong function of the liquid EOS, our komatiitic liquid EOS Talalhashi, 1986] with ,fie~ds showing conditions under which kornatiite (Wei et al, 1990], MORB (as primitive melt) (Presnall et al, 1979;McKenzie and Biclde, 1988], and picrite (also possible MORB parent) (Green et al, 1979;Elthon and Scaife, 1984] could have been extracted from their source regions. Adiabatic paths for ascending mantle source material, assuming single stage melting, are indicated by light lines.…”

Section: Adiabatic Gradientsmentioning

confidence: 99%

“…To the extent that modem magmas approaching komatiites are uncommon (although some paleogene "picritic" magmas get close to ''komatiitic" in composition [Gansser et al, 1979;Echeverria, 1982]), this suggests to us that models invoking low degrees of partial melting, including those involving pseodoinvariant melting, are unlikely to be correct. Figure 1 compares the conditions that have been suggested for the formation of komatiitic liquids to those that have been suggested for the genesis of primary magmas parental to modem MORBs (Presnall et al [1979] versus Green et al [1979] and Elthon and Scarfe [1984]). Note that we have restricted our discussion to a KLB-1 model for mantle source material and that the quantitative details will vary with the phase equilibria of the potential source.…”

Section: 854mentioning

confidence: 99%

The equation of state of a molten komatiite: 2. Application to komatiite petrogenesis and the Hadean Mantle

Miller¹,

Stolper²,

Ahrens³

1991

J. Geophys. Res.

129

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New experimental data for the equation of state of a komatiitic liquid were used to model adiabatic melting in a peridotitic mantle. If komatiites are formed by > 30% partial melting of a peridotitic mantle, then komatiites generated by adiabatic melting come from source regions that began their unmelted ascent in the lower transition zone (:::500-670 km) or the lower mantle (>670 km). The great depth of incipient melting implied by this model suggests that komatiitic liquids may form in a pressure regime where they are denser than their coexisting crystals, possibly the bulk mantle. Although komatiitic magmas are thought to separate from residual crystals in the mantle at a temperature :::200"C greater than modem mid-()cean ridge basalts (MORBs), their ultimate sources are predicted to be diapirs that, if adiabatically decompressed from initially solid mantle, were more than 70Cf'C hotter than the sources of MORBs and were derived from great depth. We also studied the evolution of an initially molten mantle, i.e., a magma ocean. Our model considers the thermal structure of the magma ocean, density constraints on crystal segregation, and approximate phase relationships for a nominally chondritic mantle. Crystallization will begin at the core-mantle boundary. Perovskite buoyancy at >70 GPa may lead to a compositionally stratified lower mantle with iron-enriched magnesiowiistite content increasing with depth. Large convective velocities in the magma ocean would prohibit crystal-liquid fractionation by settling or flotation until quiescent boundary layers form. Such boundary layers could form when the crystal content of the magma reaches a critical value (near 44 vol %) and, in the late stages of crystallization, around unmelted blocks of foundered protocrust. Matrix compaction and diapirism could also lead to fractionation effects. The upper mantle could be depleted or enriched in perovskite components relative to the bulk mantle. Olivine neutral buoyancy may lead to the formation of a dunite septum in the upper mantle, partitioning the ocean into upper and lower reservoirs, but this septum must be permeable.

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Generation of Mid-ocean Ridge Tholeiites

Cited by 351 publications

References 41 publications

Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy

Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy

Petrological Systematics of Mid-Ocean Ridge Basalts: Constraints on Melt Generation Beneath Ocean Ridges

The equation of state of a molten komatiite: 2. Application to komatiite petrogenesis and the Hadean Mantle

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