Geochemichal Perspectives Letters
Volume 4, pages 7-12, 12 July 2017

No direct contribution of recycled crust in Icelandic basalts

Sarah Lambart

Abstract:
Using Melt-PX to model the decompression melting of a heterogeneous mantle, I investigated the role of major-element composition of the lithologies present in the source on magmatic productivity, and trace element and isotopic melt compositions, independently of the bulk mantle composition. My calculations demonstrate that the volume of magma produced is not significantly affected by the nature of the lithological heterogeneity, but depends on the bulk mantle composition. However, an isochemical bulk mantle can produce contrasting trace element and isotopic melt compositions depending on the major-element compositions of the lithologies present in the source. Results show that the observed crust thickness of the Icelandic rift zones is consistent with about 10 % of recycled crust in the source, but also demonstrate there is no need to involve the contribution of melts derived from a recycled basalt component to explain the compositional variability of the Icelandic basalts in rift zones, and rather advance the contribution of olivine-bearing hybrid lithologies formed by solid-state reactions between the recycled crust and the peridotite.


Representation of the melting column in the three configurations. Colors show the lithologies that are partially melting at a given pressure.


Journal of Geophysical Research - Solid Earth
Volume 121 (8), pages 5708–5735, 18 August 2016

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

Abstract:
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.


Percentage of natural pyroxenite compositions [Lambart et al., 2009a and references therein] that have calculated T5% (Melt-PX) > T5%,peridotite [Katz et al., 2003] as a function of mantle potential temperature (TP) (see text for details). Green and red vertical bands denote the TP ranges for MORB and OIB, respectively, [Courtier et al., 2007]; values at the boundaries of the bands denote pyroxenite percentages with T5% > T5%,peridotite.


Earth and Planetary Science Letters
Volume 404, pages 319-331, 15 October 2014

Experimental derivation of nepheline syenite and phonolite liquids by partial melting of upper mantle peridotites

Didier Laporte, Sarah Lambart, Pierre Schiano, Luisa Ottolini

Abstract:
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.


(a) SEM-Backscattered electron microphotograph of sample MBK3 (1.3 GPa-1200 °C) showing a microdike filled with quenched melt (“glass”) into the graphite container (black). (b) Composition of partial melts of fertile peridotites between 1 and 1.5 GPa plotted in the total alkali-silica diagram. Our partial melting experiments at 1.3 GPa are shown by the red circles (open circles: MBK+; solid circles: MBK); the open green squares are for the experiments with MBK at 1 GPa. The open red circle with a central dot is reversal experiment MBK+13: partial melting experiment MBK+6 and its reversal MBK+13 are surrounded by the red dashed line. The small black crosses correspond to fertile peridotite MM3 at 1 GPa; the diamonds correspond to a K2O-free analogue of MM3 at 1 and 1.5 GPa (open and solid diamonds, respectively). The solid triangle is the near-solidus melt of MORB-pyrolite MPY at 1.5 GPa. The blue symbols correspond to Heldburg phonolite and to the glass analyzed in the crystallization experiment CrysPho2 (crystallization of a composition equivalent to partial melt MBK+6 at 1 GPa-1150 °C). The star labeled “DGA-40” corresponds to a trachyte melt in equilibrium with ol + opx + cpx at 1.2 GPa and 1150 °C. Melt compositions computed by pMELTS for MBK+ at 1.3 GPa are shown by the blue curve (the theoretical trend is shifted to the left of the experimental data because of a slight underestimation of SiO2 by pMELTS). The grid corresponds to the chemical classification of volcanic rocks, with the main fields of interest labeled as follows: (0) Phonotephrite; (1) Tephriphonolite; (2) Phonolite; (3) Trachyte; (4) Trachyandesite; (5) Basaltic trachyandesite; (6) Trachybasalt; (7) Basalt; (8) Basaltic andesite.


Earth and Planetary Science Letters
Volume 395, pages 24-40, 1 June 2014

Quantifying lithological variability in the mantle

Oliver Shorttle, John Maclennan, Sarah Lambart

Abstract:
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).


Ternary diagrams combining crustal thickness (tc) and geochemical constraints (Fpxmelt) to identify the allowable lithology combinations beneath Iceland. Each apex of the ternary represents an endmember lithology in the mantle: lherzolite (lz), harzburgite (hz) and pyroxenite (px, for (a) the KG1 composition from Kogiso et al., 1998, for (b) the G2 composition from Pertermann and Hirschmann, 2003). The three endmember lithologies are mechanically mixed in variable proportions and three-lithology melting calculations performed to fill in the ternary space for Fpxmelt and tc. Background colours in these diagrams correspond to Fpxmelt determined for model runs with Tp = 1480 °C. The dashed white line marks the lithology combinations melting to produce a tc = 20 km, the solid white line and grey shaded region mark the lithology combinations reproducing the observed Fpxmelt. The point of intersection of the solid and dashed white lines is the lithology mixture able to match both crustal thickness and geochemical constraints (in (a) this is lz71hz17px12, in (b) lz70hz22px8).


Lithos
Volume 160-161, pages 14-36, February 2013

Invited Review Article

Markers of the pyroxenite contribution on the major-element compositions of oceanic basalts: Review of the experimental constraints

Sarah Lambart, Didier Laporte, Pierre Schiano

Abstract:
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.


(a) Iron versus MgO contents (in wt.%) of natural pyroxenites (small grey circles) and hornblendites (pink stars) from xenoliths and alpine-­type massifs compared to starting compositions in experimental studies (large circles). Pyroxenites M5‐40 and M7‐16 used as starting materials in this study are represented by the red and purple circles, respectively. Also shown are the fields of mantle peridotites (green area) and MORB matrix glasses (yellow area). (b)Total iron contents versus SiO2 contents for MORB glasses with MgO ≥ 9 wt.%. Red symbols represent MORB glasses with Mg# ≥ 67. The orange rectangle represents the compositional area of px­‐MORBs (with both lower SiO2 and higher FeOT than MORB glasses with Mg# ≥ 67), good candidates to carry the signature of SiO2-­poor and FeO-­rich pyroxenites, such as M7-­16.


Journal of Petrology
Volume 53(3), pages 451-476, March 2013

Fate of pyroxenite-derived melts in the peridotitic mantle: Thermodynamic and experimental constraints

Sarah Lambart, Didier Laporte, Ariel Provost, Pierre Schiano

Abstract:
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.


Sketch of MORB petrogenesis in the case of a heterogeneous mantle composed of pyroxenite veins (stippled, folded layers at the bottom) in a peridotite matrix. Processes acting at a given depth are listed in the boxes on the right: melting processes in red boxes, and peridotite-pyroxenite interactions in yellow boxes. The outcome of each process for the chemical and mineralogical evolution of mantle is summarized in the left column. The grey triangle is the melting zone: in its lower part, only pyroxenites are partially molten; in its upper part (hatched area), both pyroxenites and peridotites are partially molten. Black arrows above the peridotite solidus mark the onset of penetrative porous flow; white arrows represent melt focusing into high-permeability channels.


Earth and Planetary Science Letters
Volume 288(1-2), pages 335-347, 30 October 2009

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

Abstract:
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.


Melt compositions in pyroxenites M5–103, M5–40, and M7–16 at 1 GPa plotted as a function of temperature.
The temperature interval of 1245–1305 °C represented by yellow boxes corresponds to the range of temperatures (at 1 GPa) of a mantle undergoing adiabatic decompression melting (assuming potential temperatures in the range 1280–1400 °C, McKenzie and Bickle (1988)). Symbols are as follows: diamonds, M5–103; triangles, M5–40; squares, M7–16. Green fields correspond to liquids produced by peridotites PHN1611, MM3, DMM1 and Depma. Error bars (1σ on oxide concentrations; ±5 °C on temperature) are shown in the bottom right corner of each diagram.

Main result from this study: (1) Most of pyroxenites produce liquids that are similar to peridotite-derived melts for most major elements (SiO2, Al2O3, CaO, MgO, and FeO). This may explain why MORB have relatively uniform major-element compositions, but may have variable trace element and isotopic compositions. (2) Pyroxenite melts are not enriched (and may even be depleted) in incompatible elements (Na2O, TiO2 and K2O) in comparison to peridotite-derived melts. Therefore, the concentrations of these elements cannot be used as markers of pyroxenites in MORB mantle sources. (3) Some pyroxenites yield melts with a distinct signature, such as a low-SiO2 content and/or a high FeO content, two features usually ascribed to a high average pressure of melting (of a peridotitic source).


Contributions to Mineralogy and Petrology
Volume 157(4), pages 429-451, April 2009

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

Abstract:
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 olivine–silica–diopside–plagioclase 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.


Simplfied model of decompression melting, magma focusing and transport beneath mid-ocean ridges
(1) Adiabatic decompression (TP = 1350°C)
(2) A single event of magma focusing is assumed to occur at P = 1.25 GPa and T = 1310°C, when the degree of melting is close to 10%: the total mass of partial melt in a "channel" is multiplied by a factor Ω.
(3) Adiabatic decompression from 1.25 GPa to 0.5 GPa.
(4) At 0.5 GPa, liquid is extracted from the solid matrix to form dykes.
(5) Rapid ascent of magmas in dykes to a shallow chamber beneath the mid ocean ridge.
Pictures are backscattered electron micrographs illustrating the textures and phase assemblages in the three experimental series before and after the focusing event. Scale bars: 10 μm


 
 
 

Dr Sarah Lambart
School of Earth and Ocean Sciences, Cardiff University
1.37, Main Building
Park Place, Cardiff, CF10 3AT, United Kingdom