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Article: Melting in the Earth's deep upper mantle caused by carbon dioxide Authors: Dasgupta, R;Hirschmann, MM Journal: NATURE, 440 (7084): 659-662 MAR 30 2006 Addresses: Univ Minnesota, Dept Geol & Geophys, 30 Pillsbury Dr SE, Minneapolis, MN 55455 USA. Univ Minnesota, Dept Geol & Geophys, Minneapolis, MN 55455 USA.

Rajdeep Dasgupta talks with ScienceWatch.com and answers a few questions about this month's Fast Moving Fronts paper in the field of Geosciences.

  Why do you think your paper is highly cited?

Partial melting is the most important process by which the Earth's interior continues to chemically differentiate and releases deep volatiles to the exosphere. The key parameter that controls partial melting in the Earth's mantle is the location of the solidus of Earth's dominant mantle rock (peridotite) or the depth of first melting for the Earth's mantle.

Before this study, the depth of first melting of the Earth's mantle was thought to be a much shallower (60-70 km deep). This paper, for the first time, demonstrated that in the presence of trace amount of carbon dioxide, natural mantle composition starts to melt as deep as 300 km or deeper.

This new result implies that a much larger mass of the Earth's interior actually undergoes chemical differentiation per billion years (Earth is ~4.6 billion years old) and liberation of trace and volatile elements (e.g., CO2, noble gases) from the interior to the exosphere is a much more efficient process than previously conceived.

Figure 1:

"Cartoon showing the framework of melting and mantle flow beneath mid-ocean ridges...."

View larger image & complete description in tab below.

The finding of the study has implications for a range of sub-disciplines in geosciences including seismology, geochemistry, mineral physics, petrology, and even evolution of atmosphere, climate, and biosphere through time.

  Does it describe a new discovery, methodology, or synthesis of knowledge?

The paper describes a new discovery and a first convincing demonstration of a very deep melting in the present-day interior of the planet. The paper presents new data in the field of high pressure-temperature experimental petrology (studying the behavior of rocks and minerals through laboratory experiments).

  Would you summarize the significance of your paper in layman's terms?

Volcanic activity is responsible for bringing molten rocks and releasing water vapor and CO2 from the Earth's interior to the atmosphere. This process affects Earth's climate over geologic time. The process in the Earth's mantle that feeds the volcanoes with magma is partial melting and a critical question is how deep in the Earth's mantle partial melting may commence?

In our Nature paper we showed, based on high-pressure laboratory experiments, that in the presence of even a trace quantity of CO2 (the Earth's mantle contains around tenth to hundredth of a weight percent of CO2), the partial melting in the mantle commences as deep as 300-350 km (a factor of five deeper than previously thought).

This result suggests that degassing of the Earth's mantle by partial melting is a much more efficient process. The results also implied that the geophysical properties of the Earth's deep interior can be affected by the presence of trace amount of magma, generated under the influence of CO2.

  How did you become involved in this research, and how would you describe the particular challenges, setbacks, and successes that you've encountered along the way?

This study was part of my doctoral dissertation work. One of the major challenges was to conduct high-pressure experiments that generate less than 1 mm-cubed size samples with only a very small quantity of CO2.

One of the key hurdles included preparation of experimental samples for electron microscopy work and particularly preserving fragile carbonate minerals or quenched melts during mechanical polishing. Another challenge was to detect trace amount of carbonate-rich quenched melt (magma) in experimental samples.

I tackled these problems by polishing the samples without the use of any liquid lubrication, which tends to soften the labile carbonates and by studying the samples with the aid of high resolution, field emission gun microscopes.

  Where do you see your research leading in the future?

The influence of volatiles and fluids in the Earth's interior processes and the feedback between the deep Earth processes and the surface environment continues to be the guiding theme of my research.

My current and future research will continue to explore the process of magma genesis in the interior of planets and the role of such a process in the chemical evolution of the interior and the exosphere.

One key aspect is constraining the effects of water, CO2, and halogens on the conditions of melting and magma compositions in the planetary interiors. Another aspect for future research is the fractionation of volatiles during the early differentiation of the Earth and gaining insight into the deep volatile cycles of a young Earth.

  Do you foresee any social or political implications for your research?

The questions that drive my research are curiosity-driven. The research is of fundamental nature and relates to the natural evolution of the Earth as a planet through time.

The aspect of my research that likely has societal impact is the one that constrains the relative budget and fluxes of trace (often greenhouse) gases between the atmosphere and interior of the Earth. Thus my research has implication for long-term habitability of the Earth.

Dr. Rajdeep Dasgupta
Assistant Professor
Department of Earth Science
Rice University
Houston, TX, USA

KEYWORDS: MELTING, EARTH, DEEP UPPER MANTLE, CARBON DIOXIDE, EAST PACIFIC RISE, SEISMIC ANISOTROPY, PHASE RELATIONS, MIDOCEAN RIDGE, OCEANIC CRUST, LOW VELOCITY, CO2, SOLIDUS, GPA.

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• July 2010 - Fast Moving Fronts
• Rajdeep Dasgupta on High Pressure-Temperature Experimental Petrology