COFUND Fellow - School of Earth and Ocean Sciences - Cardiff University, UK
Research Associate - Earth and Planetary Sciences - UC DAVIS, CA
Adjunct Associate Research Scientist - Lamont-Doherty Earth Observatory - Columbia University, NY


2015-2016 Visiting Assistant Professor - Earth and Planetary Sciences - UC DAVIS, CA
2013-2015 - Lamont Postdoctoral Fellow - Columbia University, NY
2010-2013 - Postdoctoral Associate - California Institute of Technology, CA
2010 - Ph.D - Blaise Pascal University, Clermont-Ferrand - France

My primary research focuses on the chemistry of Earth and planetary interiors with a particular emphasis on the role of magma generation and magma and fluid - interaction on the differentiation and chemical evolution of terrestrial planets.
I combine experimental techniques and thermodynamical modeling to investigate a range of various processes from magma focusing in the asthenosphere to reactive-cracking in the crust.

Upcoming Events!

Publication:

New paper accepted in Geochemical Perspectives Letters. More info soon...

CUROP Project:

Matthew Cook, third year student at Cardiff U., will join me this summer to work on melt-rock reaction in the oceanic crust for his CUROP project.

Visit to Vrije Universiteit Amsterdam:

I will be visiting V.U. this summer and fall and will collaborate with Janne Koornneef to perform high spatial resolution isotopic analyses on lower crust cumulate rocks.

Goldschmidt 2017:

Invited talk in the session 05a: "MORB Petrogenesis: From Mantle Partial Melting to Fractional Crystallization"

News!

Funding:

Second year of funding for my NSF project has been approved!

Visit to IODP Core repository:

MARUM Bremen, May 14th to 19th: collecting samples from the Atlantis Massif (IODP 304/305)

GGRiP Meeting:

Bristol, April 3rd and 4th, GGRiP Meeting 2017

Seminar:

University of Utah, Salt Lake City, USA - February 23rd, 2017

Women in volcanology:

Our message for girls

Seminar:

CRPG, Nancy, France - February 6th, 2017

Prifysgol Caerdydd:

I'm excited to announce that I just started (January 2017) a new position as a COFUND fellow in the School of Earth and Ocean Sciences at Cardiff University. I will use a combination of experimental and analytical techniques to reconstruct the evolution of the oceanic crust.

Highlight & Spotlight:

My latest article featured in the Editors' Highlight of Journal of Geophysical Research - Solid Earth. And also in the Research Spolight of EOS.

Seminar:

Sept. 19th, 2016: Invited research presentation at the Department of Geological Sciences and Engineering, University Of Nevada, Reno

Funding:

I just got confirmation from NSF: My project on the effect of the melting regime for quantifying the mantle heterogeneities has been funded!

Publication:

New paper accepted in Journal of Geophysical Research - Solid Earth
Information on how to use Melt-PX can be found here


Mantle2MORB

MORB, having formed by mantle melting, retain many characteristics of their mantle source. Hence, MORB has traditionally been the primary tool to map geochemical variations and melting processes of the upper mantle. However, although the mantle has long been established to be compositionally heterogeneous, MORB display relatively homogeneous compositions. This has been suggested to be the result of magma mixing during their travel from the mantle source to the surface. Oceanic cumulates are believed to be the first crystallisation products of MORB once they enter the crust. Hence, they should record the full compositional range of melt compositions delivered to the crust. If so, their analysis would reveal all or part of the mantle heterogeneity hidden in MORB. Furthermore, their compositions record the crustal evolution of MORB, and hence offer the opportunity to reconstruct the processes that control the MORB trace element evolution. At Cardiff, using a combination of high pressure experiments and high spatial resolution isotopic analyses, I will look at these two components to determine both the extent to which MORB reflect its mantle source, and the effects of crustal processing. To pursue this project, I obtained a MSCA-COFUND fellowship for a duration of three years.


Melting regime and mantle heterogeneity

Thanks to numerous experimental studies, parameterizations are available to model the melting behavior of peridotite and pyroxenite compositions that are thought to be present in the mantle. Based on these parameterizations, numerous studies have attempted to estimate the proportion of pyroxenites in magmatic sources. However, while these parameterizations are mostly based on batch melting experiments, oceanic basalts are likely to be formed by near fractional melting rather than batch melting. My preliminary calculations show that using a batch melting regime instead of a continuous regime might yield to a significant underestimation of the amount of recycled material in the mantle. A higher proportion of the recylcled material in the mantle has important geodynamical implications in terms of mantle composition and temperature. I'm conducting incremental melting experiments to simulate a continuous melting regime of pyroxenite. Because melt extraction during generation of MORB and OIB is likely to occur via continuous melting rather than batch melting, these new calibrations will allow me to better quantify the proportion of recycled material in the mantle source of the most abundant magmas on Earth.
I recently been funded by NSF to pursue this project and I have been invited to present preliminary results at the Goldschmidt conference 2017 - session 5a, in Paris.


Parameterization of melting for mantle pyroxenites

A variety of data requires that the mantle source for basaltic magmatism is heterogeneous. 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.
I used experimental data from the literature to build an empiric model, 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 [1], 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. One major result of this study has been to show that a significant portion of mantle pyroxenites can start to melt after the peridotite along an adiabatic path.
The final model: Melt-PX, is published in Journal of Geophysical Research - Solid Earth, and can be downloaded here. A previous version of the model has also been used by Shorttle et al. (2014) to determine the proportion of recycled basalt in the magma source of Iceland. The exact nature of the mantle heterogeneity beneath Iceland is discussed in a paper submitted in Geochemical Perspective Letters.
This study has been featured in AGU Editor's Highlights and selected to be featured in AGU Research spotlight.

[1] 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.
We performed experiments to understand the effect of the confining pressure on the pressure of crystallization and on the reactive-cracking processes. Preliminary results have been presented at the American Geophysical Union in December 2014 and at the ACEME in June 2015. Our study has been submitted for publication. More information soon...


Tracer of mantle heterogeneity

Several aspects of MORB compositions, including variations in abundance of highly incompatible elements and in radiogenic-isotope ratio 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 Didier Laporte, 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. 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); [2] Stolper et al. (2009), Scientific Drilling 7:4-14; [3] Ghiorso & Sack (1995), CMP 119:197-212; Dixon et al. (1991)


 
 
 

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