News!

Earth and Planetary Science Letters:

Experimental derivation of nepheline syenite and phonolite liquids by partial melting of upper mantle peridotites
Didier Laporte, Sarah Lambart, Pierre Schiano, Luisa Ottolinie

AGU Fall Meeting 2014:

#1. A coupled geochemical and geodynamical approach for mantle melting beneath Hawaii
Sarah Lambart, Peter B. Kelemen

#2. Experimental investigation of the pressure of crystallization
Heather M. Savage, Sarah Lambart, Peter B. Kelemen, Theodore A. Koczynski

About me

2010 - Ph.D - Blaise Pascal University, Clermont-Ferrand, France
2010-2013 - Postdoctoral Associate - California Institute of Technology, Pasadena CA
Since 2013 - Lamont Postdoctoral Fellow - Columbia University, Palisades NY

My primary research is focused on the understanding of mantle melting and basalt genesis processes. This part of my research is articulated on the role of mantle heterogeneities and magma-rock interactions in the genesis of basalt. I discuss this topic via an experimental approach coupled to thermodynamical modeling.
Since I joined the Lamont-Doherty Earth Observatory, I also work on the experimental investigation of the reactive cracking process during fluid-rock interactions. Reaction-driven cracking could be important in geological capture and storage of CO2 as it is essential for in situ mineral carbonation methods to maintain or enhance permeability and reactive surface area.

Parameterization of melting for mantle pyroxenites

Decompression melting occurs in oceanic (mid-ocean ridges and mantle plumes) and continental (rifts and LIPs) settings and is considered as the most important process of magma generation on Earth. The decompression melting of a peridotitic and homogeneous mantle is correctly constraint by models taking into account major and trace element compositions of mantle source and physical parameters as the mantle temperatures and the processes of magma transport. However, it is also well established that, at least, some of chemical variation seen in oceanic and continental volcanism is the result of a variable contribution of partial melts from pyroxenite in the mantle source [1].Models of the melting of such mixtures require knowledge of the relationships between melt fraction, temperature, pressure, and bulk composition for both peridotites and pyroxenites. While various parameterizations are available to model the melting behavior of peridotites, none yet exist to model pyroxenite melting.
However, this last decade, pyroxenites have been the subject of numerous experimental studies. These works help for the understanding of melting behavior of these rocks and provide a database of major element compositions of pyroxenite-derived melts.
At Caltech, I use data from the literature to build a parameterization, Melt-PX, that, while remaining mathematically simple, succeeds in capturing the important features of the behavior of pyroxenites melting. Coupled with a parameterization on peridotite [3], my work permits calculations of how multilithologic mantle sources melt during adiabatic decompression, including the effects of varying the composition and the modal proportion of pyroxenite in such source regions.
I presented these results during the American Geophysical Unions in December 2011 and 2012. A part of these results have been used by Shorttle et al. (2014) to determine the proportion of recycled basalt in the magma source of Iceland. The final model and its applications have been presented at the 6th Orogenic Lherzolite Conference in May 2014 in May 2014 and will be submitted soon to EPSL.
Recently, I used Melt-PX to propose a new geochemically and geophysically consistent model of heterogeneous plume beneath Hawaii. The results will be presented at the American Geophysical Union in December 2014 and will be submitted to the special volume of Journal of Petrology - Thematic Issue: 6th Orogenic Lherzolite Conference.

[1] Hofmann & White (1982), EPSL 57:421-436; [2] Ghiorso et al. (2002), G³ 3(5); [3] Katz et al. (2003), G3 4(9)

Reactive cracking investigation

The observation that anthropogenic CO2 can influence global warming has focused attention on carbon capture and storage. One proposed option for carbon sequestration is to increase the natural process that consists in the conversion of CO2 gas to stable, solid carbonate minerals in peridotites that have been tectonically exposed. Kelemen and coworker propose that peridotites may be able to capture and store billions of tons of CO2 per year. This could make a significant difference in the overall CO2 budget of the planet until alternative energy sources replace global fossil fuel use. But this option needs the development of new engineered methods emulating the natural process (reactive cracking) to create dense fracture networks.
I’m conducting experiments to map out the limits of reaction-driven cracking as a function of confining pressure, temperature, reaction rate, volume change and available chemical potential energy. Preliminary results have been presented at the American Geophysical Union in December 2013 and new results will be presented at the American Geophysical Union in December 2014

Tracer of mantle heterogeneity

Several aspects of MORB composition, including variations in abundance of highly incompatible elements [1] and in radiogenic-isotope ratio [2] cannot easily be explained with a homogeneous source. In order to understand the role of these lithologies for MORB petrogenesis, experiments are necessary. Nevertheless, there are few studies on this rock-type at pressures shallower than 2 GPa and no work was devoted to the influence of this rocks-type in the major element composition of MORB.
My PhD work was devoted to experimental determination partial melting behavior of three natural pyroxenites between 1 and 2.5 GPa. To analyze the composition of liquids in equilibrium with mineral phases, I used the ‘‘microdike’’ technique developed by Laporte et al. [3], which consists of extracting small volumes of liquid into fractures of the graphite container that formed during experiments.
For the range of temperatures beneath mid-ocean ridge, compositions of melts from most of pyroxenites are close melt from peridotite. On the contrary, liquids from peridotite and pyroxenites would have contrasted isotopic and trace element signatures. This could explain why MORBs are rather homogeneous in terms of major elements but heterogeneous in terms of isotope and trace elements.
Otherwise, 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 [3]. Thus, MORB with high FeO and low SiO2 contents may reflect the participation of a pyroxenite component in their source rather than a higher pressure of melting.
Hence the classical criteria used to select primitive mantle-derived magmas (e.g., MORB 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.
This work has been published in 2009 in Earth and Planetary Sciences Letter. I also presented these results during the American Geophysical Union in December 2008. I have been also invited by Lithos to write a review article on the role of pyroxenites in the major-element compositions of oceanic basalts.

Interaction between pyroxenitic melt and peridotite

Interactions between pyroxenites and/or pyroxenite-derived melts and the surrounding peridotites might have a significant role on melt extraction dynamics and, ultimately, on the preservation of a pyroxenite signature in aggregated melts erupted at the Earth's surface. Here, we try to evaluate the fate of melts from pyroxenitic sources during their transport through the peridotite mantle as functions of their composition, P-T conditions and the physical state (subsolidus vs. partially molten) of surrounding mantle. In order to model these interactions, we use a simplified model of interaction, where peridotite is impregnated by a finite amount of pyroxenite-derived liquid modelled with pMELTS. Concurrently, we perform impregnation experiments in a piston-cylinder apparatus to test the validity of the calculations.
Results show that Cpx is systematically produced whereas the Ol and Opx behaviors depend on incoming melt silica activity: if it is lower than the silica activity of a melt saturated in Ol and Opx at the same pressure P and temperatureT,Opx is dissolved and Ol precipitates, and conversely. This has strong implications on the ability of pyroxenite-derived melts to migrate through the mantle. Indeed, Opx dissolution and Ol precipitation facilitates the transport of melt in the system by increasing the porosity and permeability of mantle rocks. Inversely, Opx crystallisation at the expense of Ol may slow down or even stop magma ascent by porous flow. Moreover, the ability of melts to migrate also depends on the melting degree of surrounding peridotite: the reaction with a subsolidus mantle results in a strong melt consumption resulting to a drastic decrease of the system permeability. On the contrary, melt migration to the surface should be possible if the surrounding mantle is partially melted. We illustrate these results in a model for MORB genesis taking into account the implications of melt-peridotite interactions for melt transport and mantle lithological diversity, and the role of pyroxenites for magma genesis at MORs.
This work has been published in 2012 in Journal of Petrology.

Magma focusing

The transport of basaltic melts beneath oceanic spreading centers occurs predominantly in high porosity dunitic channels, resulting from the complete dissolution of pyroxenes in peridotites mantle [1].
The formation of dunitic assemblages is due to magma focusing that is, to the circulation of important melt volumes in these channels. Many experiments were run in a piston-cylinder apparatus to simulate magma focusing processes beneath oceanic spreading centers, and to study the reaction between peridotites and basalt, as function of pressure and magma/rock ratio. Then, results were compared with pMELTS predictions.
We proposed a primitive MORBs formation model, implying: 1) partial melting of lherzolitic mantle at depth, 2) magma focusing involving the formation of dunitic channels and liquids evolution toward primitive MORBs composition by interaction with surrounding dunites, 3) destabilization of high-porosity channels to form dykes is possible at high degrees of melting. Thus, our study confirmed the necessity of dunites formation beneath the oceanic ridges and strengthens the hypothesis that high-porosity channels constitute the main mean of magma transport at oceanic spreading centers.
This work is the basis of my research project. It is focused on the genesis of oceanic basalts and combines the experimental data with numerical modeling. It has been published in 2009 in Contributions to Mineralogy and Petrology. I also presented these results during the European Geophysical Union in April 2007.

[1] Kelemen et al. (1995), Nature 375:747-753

Compositional variability of Mauna Kea shield lavas

It has been recently shown that the compositional diversity of lavas collected during the second phase of the drilling project (HDSP-2) varies with depth and that samples from the deepest part of the core have the highest variability [1].
In 2013 I supervised Valérie Payré, a first year graduate student from the ENS (École Normale Supérieure, Paris - France) to work on samples collected in this deepest part (3098-3506 mbsl) recovered during Phase-2 (2003-2007) of the HSDP-2 project [2] in order to better constrain the origin of the compositional variability of these lavas.
She conducted the petrographic and geochemical study of glasses using an electron microprobe and thermodynamic modeling using MELTS software [3] in order to determine the conditions of formation of these lavas (pressure of fractionation, water content, oxygen fugacity).
All degassed samples follow a fractionation trend at 5-10 bars. Most of the undegassed (and partially degassed high-SiO2) glasses can be reproduced by simultaneous depressurization and cooling (i.e. from 70 to 25 bars – ΔT = 20°C).
Surprisingly, the results also reveal the presence of degassed samples in deep part of the drill. The preferred model to explain this observation is mixing of a relatively volatile-rich, undegassed component with magmas that experienced low pressure degassing (in the conduit or at shallow depth) during which substantial volatile amounts were lost, as previously suggested by Dixon et al. [3]. To confirm this assumption, water and carbon dioxide analyzes have to be performed on these samples.

[1] Rhodes et al. (2012), G3 13(3); [1] Stolper et al. (2009), Scientific Drilling 7:4-14; [1] Ghiorso & Sack (1995), CMP 119:197-212; Dixon et al. (1991)