Peer-reviewed International publications
The role of pyroxenite in basalt genesis: Melt-PX, a melting parameterization for mantle pyroxenites between 0.9 and 5 GPa
Sarah Lambart, Michael B. Baker, Edward M. Stolper - 2016
Geochemical and isotopic data suggest that the source regions of oceanic basalts may contain pyroxenite in addition to peridotite. In order to incorporate the wide range of compositions and melting behaviors of pyroxenites into mantle melting models, we have developed a new parameterization, Melt-PX, which predicts near-solidus temperatures and extents of melting as a function of temperature and pressure for mantle pyroxenites. We used 183 high-pressure experiments (25 compositions; 0.9–5 GPa; 1150–1675°C) to constrain a model of melt fraction vs. temperature from 5% melting up to the disappearance of clinopyroxene for pyroxenites as a function of pressure, temperature, and bulk composition. When applied to the global set of experimental data, our model reproduces the experimental F-values with a standard error of estimate of 13% absolute; temperatures at which the pyroxenite is 5% molten are reproduced with a standard error of estimate of 30°C over a temperature range of ~500°C and a pressure range of ~4 GPa. In conjunction with parameterizations of peridotite melting, Melt-PX can be used to model the partial melting of multi-lithologic mantle sources—including the effects of varying the composition and the modal proportion of pyroxenite in such source regions. Examples of such applications include calculations of isentropic decompression melting of a mixed peridotite + pyroxenite mantle; these show that, although the potential temperature of the upwelling mantle plays an important role in defining the extent of magma production, the composition and mass fraction of the pyroxenite also exert strong controls.▲ top
Experimental derivation of nepheline syenite and phonolite liquids by partial melting of upper mantle peridotites
Didier Laporte, Sarah Lambart, Pierre Schiano, Luisa Ottolini - 2014
Piston-cylinder experiments were performed to characterize the composition of liquids formed at very low degrees of melting of two fertile lherzolite compositions with 430 ppm and 910 ppm K2O at 1 and 1.3 GPa. We used the microdike technique (Laporte D. et al., 2004. Contrib. Mineral. Petrol. 146: 463-484) to extract the liquid phase from the partially molten peridotite, allowing us to analyze liquid compositions at degrees of melting F down to 0.9 %. At 1.3 GPa, the liquid is in equilibrium with olivine + orthopyroxene + clinopyroxene + spinel in all the experiments; at 1 GPa, plagioclase is present in addition to these four mineral phases up to about 5 % of melting (T = 1240 °C). Important variations of liquid compositions are observed with decreasing temperature, including strong increases in SiO2, Na2O, K2O, and Al2O3 concentrations, and decreases in MgO, FeO, and CaO concentrations. The most extreme liquid compositions are phonolites with 57 % SiO2, 20-22 % Al2O3, Na2O + K2O up to 14 %, and concentrations of MgO, FeO, and CaO as low as 2-3 %. Reversal experiments confirm that low degree melts of a fertile lherzolite have phonolitic compositions, and pMELTS calculations show that the amount of phonolite liquid generated increases from 0.3 % in a source with 100 ppm K2O to more than 3 % in a source with 2000 ppm K2O. The enrichment in silica and alkalis with decreasing melt fraction results in major changes in melt structure and polymerization, which have important consequences for the partitioning of minor and trace elements. Thus Ti4+ in our experiments, and by analogy other highly charged cations and rare earth elements, become more compatible near the peridotite solidus. The generation of phonolite liquids by low degree partial melting of a fertile peridotite brings a strong support to the hypothesis that some phonolitic lavas or their plutonic equivalents (nepheline syenites) are produced directly by partial melting of mantle peridotites. The circulation of peridotite low-degree melts into the lithospheric mantle may be responsible for a special kind of metasomatism characterized by Si- and K-enrichment. If they are unable to escape by porous flow, low-degree melts will ultimately be trapped inside neighbouring olivine grains and give rise to the silica- and alkali-rich glass inclusions found in peridotite xenoliths.▲ top
Quantifying lithological variability in the mantle
Oliver Shorttle, John Maclennan, Sarah Lambart - 2014
We present a method that can be used to estimate the amount of recycled material present in the source region of mid-ocean ridge basalts by combining three key constraints: (1) the melting behaviour of the lithologies identified to be present in a mantle source, (2) the overall volume of melt production, and (3) the proportion of melt production attributable to melting of each lithology.
These constraints are unified in a three-lithology melting model containing lherzolite, pyroxenite and harzburgite, representative products of mantle differentiation, to quantify their abundance in igneous source regions.
As a case study we apply this method to Iceland, a location with sufficient geochemical and geophysical data to meet the required observational constraints. We find that to generate the 20 km of igneous crustal thickness at Iceland coasts, with 30 ± 10% of the crust produced from melting a pyroxenitic lithology, requires an excess mantle potential temperature (ΔTp) of ∼ 130°C (Tp ∼ 1460°C) and a source consisting of at least 5% recycled basalt. Therefore, even with lithological heterogeneity the mantle beneath Iceland requires a significant excess temperature to match geophysical and geochemical observations: lithological variation alone is not viable. Determining a unique source solution is only possible if mantle potential temperature is known precisely and independently, otherwise a family of possible lithology mixtures is obtained across the range of viable ΔTp. For Iceland this uncertainty in ΔTp means that the mantle could be > 20% harzburgitic if ΔTp > = 150°C (Tp > = 1480°C).
Markers of the pyroxenite contribution on the major-element compositions of oceanic basalts: Review of the experimental constraints
Sarah Lambart, Didier Laporte, Pierre Schiano - 2013
Based on previous and new results on partial melting experiments of pyroxenites at high pressure, we attempt to identify the major element signature of pyroxenite partial melts and to evaluate to what extent this signature can be transmitted to the basalts erupted at oceanic islands and mid-ocean ridges. Although peridotite is the dominant source lithology in the Earth's upper mantle, the ubiquity of pyroxenites in mantle xenoliths and in ultramafic massifs, and the isotopic and trace elements variability of oceanic basalts suggest that these lithologies could significantly contribute to the generation of basaltic magmas. The question is how and to which degree the melting of pyroxenites can impact the major-element composition of oceanic basalts. The review of experimental phase equilibria of pyroxenites shows that the thermal divide, defined by the aluminous pyroxene plane, separates silica-excess pyroxenites (SE pyroxenites) on the right side and silica-deficient pyroxenites (SD pyroxenites) on the left side. It therefore controls the melting phase relations of pyroxenites at high pressure but, the pressure at which the thermal divide becomes effective, depends on the bulk composition; partial melt compositions of pyroxenites are strongly influenced by non-CMAS elements (especially FeO, TiO2, Na2O and K2O) and show a progressive transition from the liquids derived from the most silica-deficient compositions to the liquids derived from the most silica-excess compositions.
Another important aspect for the identification of source lithology is that, at identical pressure and temperature conditions, many pyroxenites produce melts that are quite similar to peridotite-derived melts, making difficult the determination of the presence of pyroxenite in the source regions of oceanic basalts; only pyroxenites able to produce melts with low SiO2 and high FeO contents can be identified on the basis of the major-element compositions of basalts. In the case of oceanic island basalts, a high CaO/Al2O3 ratio can also reveal the presence of pyroxenite in the source-regions. Experimental and thermodynamical observations also suggest that the interactions between pyroxenite-derived melts and host peridotites play a crucial role in the genesis of oceanic basalts by generating a wide range of pyroxenites in the upper mantle: partial melting of such secondary pyroxenites is able to reproduce the features of primitive basalts, especially their high MgO contents, and to transmit, at least in some cases, the major-element signature of the original pyroxenite melt to the oceanic basalts. At last, we highlight that the very silica depleted compositions (SiO2 > 42 wt%) and high TiO2 contents of some OIBs seem to require the contribution of fluids (CO2 or H2O) through melting of either carbonated lithologies (peridotite or pyroxenite) or amphibole-rich veins.
Fate of pyroxenite-derived melts in the peridotitic mantle: Thermodynamic and experimental constraints
Sarah Lambart, Didier Laporte, Ariel Provost, Pierre Schiano - 2012
We performed a thermodynamic and experimental study on the fate of pyroxenite-derived melts during their migration through the peridotitic mantle. We used a simplified model of interaction, where peridotite is impregnated by and then equilibrated with a finite amount of pyroxenite-derived liquid. We considered two pyroxenite compositions and three contexts of pyroxenitic melt impregnation: (i) in a subsolidus lithospheric mantle, (ii) beneath a mid-ocean ridge (MOR) in a subsolidus asthenospheric mantle at high pressure, and (iii) beneath MOR in a partially molten asthenospheric mantle. Calculations were performed with pMELTS at constant pressure and temperature with a melt-rock ratio varying from 0 to 1. Concurrently, a series of impregnation experiments was performed at 1 and 1.5 GPa to reproduce the final stages of calculations where the magma-rock ratio is 1.
Incoming melt and host rocks react differently according to melt composition and the physical state of the surrounding mantle. Whereas clinopyroxene (Cpx) is systematically a reaction product, the role of olivine (Ol) and orthopyroxene (Opx) depends on incoming melt silica activity aSiO2: if it is lower than the silica activity a0SiO2 of a melt saturated in Ol and Opx at the same pressure P and temperature T, Opx is dissolved and Ol precipitates, and conversely if aSiO2 gt; a0SiO2 (see figure). Such contrasted reactions between pyroxenitic melts and peridotitic mantle may generate a large range of new lithological heterogeneities (wehrlite, websterite, clinopyroxenite) in the upper mantle. Also, our study shows that the ability of pyroxenite-derived melts to migrate through the mantle depends on the melting degree of surrounding peridotite: the reaction of these melts with a subsolidus mantle results in a strong melt consumption (40-100%) and large Cpx production (with some spinel or garnet, depending on P). This is expected to drastically decrease the system permeability and the capacity of pyroxenite-derived melts to infiltrate neighbouring rocks. On the contrary, melt migration to the surface should be possible if the surrounding mantle is partially melted: though liquid reactivity varies with composition, melt consumption is then restricted to less than 20%. Hence, magma/rock interactions can have a significant impact on the dynamics of melting and magma migration and should not be neglected when modelling the partial melting of heterogeneous mantle.
An experimental study of pyroxenite partial melts at 1 and 1.5 GPa: Implications for the major element composition of Mid-Ocean Ridge Basalts
Sarah Lambart, Didier Laporte, Pierre Schiano - 2009
To better assess the potential role of pyroxenites in basalt generation at mid-ocean ridges, we performed partial melting experiments on two natural websterites and one clinopyroxenite representative of worldwide pyroxenites. The experiments were conducted at 1 and 1.5 GPa in a piston-cylinder apparatus; the microdike technique was used to separate the liquid fromthe solid phases and to obtain reliable glass analyses even at low degrees of melting. Contrasted melting behaviors were observed depending on the phase proportions at the solidus, especially the abundance of orthopyroxene. (1) If orthopyroxene is abundant, the main melting reaction is similar to the melting reaction in peridotites (clinopyroxene+orthopyroxene±spinel=liquid+olivine), and the liquids are similar to peridotite-derived melts for most major elements. (2) In the absence of orthopyroxene, the mainmelting reaction is clinopyroxene+spinel=liquid+olivine, yielding liquids that are strongly depleted in SiO2 in comparison to peridotite-derived melts. This low-SiO2 content can be associated with a high FeO content, a combination usually ascribed to a high average pressure of melting (of a peridotitic source).
Because of their higher melt productivities and lower solidus temperatures, 5wt.% of pyroxenites in a heterogeneous mantle may contribute up to 40wt.% of the total melt production. (1) In some cases, pyroxenitederived melts differ strongly from peridotite partial melts, leading to a distinct pyroxenite signature in the average melt (lower alkali and TiO2 contents, lower SiO2, higher FeO and/or lower Mg#). The classical criteria used to select primitive mantle-derived magmas (melt inclusions hosted into highMg# olivine orMORB glasses with Mg# ≥67) or to track down enriched mantle sources (MORB glasses with high incompatible element contents) must be considered with caution, otherwise melts carrying a pyroxenite signature may be eliminated. (2) In general, however, the major-element signature of pyroxenites should be hardly detectable in the average melt because of the similarity of most pyroxenite-derived melts with peridotite partial melts. This similarity may explain why MORB have relatively uniform major-element compositions, but may have variable trace element and/or isotopic compositions.
Keywords: experimental petrology, pyroxenite, partial melting, primitive MORB▲ top
An experimental study of focused magma transport and basalt-peridotite interactions beneath mid-ocean ridges: implications for the generation of primitive MORB compositions
Sarah Lambart, Didier Laporte, Pierre Schiano - 2009
We performed experiments in a piston-cylinder apparatus to determine the effects of focused magma transport into highly permeable channels beneath midocean ridges on: (1) the chemical composition of the ascending basalt; and (2) the proportions and compositions of solid phases in the surrounding mantle. In our experiments, magma focusing was supposed to occur instantaneously at a pressure of 1.25 GPa. We first determined the equilibrium melt composition of a fertile mantle (FM) at 1.25 GPa-1310°C; this composition was then synthesised as a gel and added in various proportions to peridotite FM to simulate focusing factors X equal to 3 and 6 (X = 3 means that the total mass of liquid in the system increased by a factor of 3 due to focusing). Peridotite FM and the two basalt-enriched compositions were equilibrated at 1 GPa-1290°C; 0.75 GPa-1270°C; 0.5 GPa-1250°C, to monitor the evolution of phase proportions and compositions during adiabatic decompression melting. Our main results may be summarised as follows: (1) magma focusing induces major changes of the coefficients of the decompression melting reaction, in particular, a major increase of the rate of opx consumption, which lead to complete exhaustion of orthopyroxene (and clinopyroxene) and the formation of a dunitic residue.
A focusing factor of ≈4 (that is, a magma/rock ratios equal to ≈0.26) is sufficient to produce a dunite at 0.5 GPa. (2) Liquids in equilibrium with olivine (±spinel) at low pressure (0.5 GPa) have lower SiO2 concentrations, and higher concentrations in MgO, FeO, and incompatible elements (Na2O, K2O, TiO2) than liquids produced by decompression melting of the fertile mantle, and plot in the primitive MORB field in the olivinesilicadiopsideplagioclase tetrahedron. Our study confirms that there is a genetic relationship between focused magma transport, dunite bodies in the upper mantle, and the generation of primitive MORBs.
Keywords: Dunite, Peridotite, Partial melting, Focused magma transport, Primitive MORB, Magma/rock interactions▲ top