Author Topic: Electrical Volcanoes - carbonatites and mid ocean ridges  (Read 9437 times)

electrobleme

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Electrical Volcanoes - carbonatites and mid ocean ridges
« on: September 07, 2009, 00:49:16 »
Why is the mantle of the Earth electrically conductive?

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Researchers from INSU-CNRS, working with chemists at a CNRS research unit, have explained that the high conductivity of the Earth’s upper mantle is due to molten carbonates. They demonstrated the very high conductivity of this form of carbon.

Appearing in the 28 November issue of Science, their work has revealed the high carbon content of the interior of the upper mantle. This composition can be directly linked to the quantity of carbon dioxide produced by 80% of volcanoes. This result is important for quantifying the carbon cycle, which contributes significantly to the greenhouse effect.

Geologists have long claimed that significant amounts of carbon have been present in the Earth’s mantle for thousands of years. Up until now, there was very little direct proof of this hypothesis, and samples from the surface of the mantle contained only very small quantities of carbon. Also, for the last thirty years, scientists have been unable to explain the conductivity of the mantle, which is crossed by natural electrical currents at depths of 70 to 350 km, even though olivine, one of the main mineral components of the upper mantle, is completely isolating.

To explain these phenomena, researchers from the Institut des Sciences de la Terre d'Orléans (ISTO, CNRS / Université de Tours / Université d'Orléans) looked into liquid carbonates, one of the most stable forms of carbon within the mantle, along with graphite and diamond. The Masai volcano is Tanzania is the only place in the world where these carbonates can be observed. Elsewhere, the carbonates are dissolved in basalts and emitted into the atmosphere in gaseous form, as CO2.

Based on lab measurements at CNRS’s CEMHTI, the researchers established the high conductivity of molten carbonates. Their conductivity is 1000 times higher than that of basalt, which was previously thought to be the only potential conductor in the mantle. Fabrice Gaillard and his team have shown that the conductivity of the Earth’s mantle is a result of the presence of small amounts of molten carbonates between chunks of solid rock.

This work shows that the electrical characteristics of the asthenosphere, the conductive part of the upper mantle, are directly connected to the amount of carbonate in the layer. The work also points to varying carbon distribution according to the regions and depth of the mantle. The researchers calculated that the amount of carbon present as liquid carbonate directly within the asthenosphere is between 0.003 and 0.025%, which seems low but makes it possible to explain the amounts of CO2 emitted into the atmosphere by 80% of volcanoes. This nonetheless represents a reservoir of carbon integrated into the mantle which is higher than that present on the surface of the earth. These results are unmatched in helping to quantify the carbon cycle, which plays a major role in the greenhouse effect. Indeed, the CO2 emitted by volcanic activity had never before been evaluated at the source (at the level of the mantle).

The presence of molten carbonates in the asthenosphere certainly has major implications on the viscosity of this region of the mantle, which participates in the sliding of tectonic plates, a phenomenon we know little about. The behavior of liquid carbonates in solids and potential effects on viscosity remain to be studied. Everything seems to indicate that the asthenosphere contains only oxidated forms of carbon (carbonates), and not carbon in its reduced solid form (diamond).

Diamond formation remains mysterious, but researchers are guessing that diamonds form from liquid carbonates at the base of the lithosphere, below the asthenosphere. Enfin, the electrical measurements of the team on liquid carbonates are of interest to the field of clean energy production, as they can be used as electrolytes in high temperature
batteries (eg. lithium carbonate).

This work was funded through a Young Researcher ANR project led by Fabrice Gaillard. He hopes to continue the work on liquid electrolytes through another ANR project and to therefore clarify these new hypotheses.
Why Is The Earth’s Mantle Conductive? - ScienceDaily


« Last Edit: September 07, 2009, 12:49:33 by electrobleme »

electrobleme

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Electrical Volcanoes - Carbonatite Melts and Electrical Conductivity
« Reply #1 on: September 07, 2009, 00:52:43 »

Carbonatite Melts and Electrical Conductivity in the Asthenosphere

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Electrically conductive regions in Earth's mantle have been interpreted to reflect the presence of either silicate melt or water dissolved in olivine. On the basis of laboratory measurements, we show that molten carbonates have electrical conductivities that are three orders of magnitude higher than those of molten silicate and five orders of magnitude higher than those of hydrated olivine. High conductivities in the asthenosphere probably indicate the presence of small amounts of carbonate melt in peridotite and can therefore be interpreted in terms of carbon concentration in the upper mantle. We show that the conductivity of the oceanic asthenosphere can be explained by 0.1 volume percent of carbonatite melts on average, which agrees with the carbon dioxide content of mid-ocean ridge basalts.
Carbonatite Melts and Electrical Conductivity in the Asthenosphere - sciencemag.org - Abstract


electrobleme

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Oldoinyo Lengai volcano - no silica but 50+ percent carbonate minerals

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Scientists studying the world's most unusual volcano have discovered the reason behind its unique carbon-based lavas. The new geochemical analyses reveals that an extremely small degree of partial melting of typical minerals in the earth's upper mantle is the source of the rare carbon-derived lava erupting from Tanzania's Oldoinyo Lengai volcano.

Although carbon-based lavas, known as carbonatites, are found throughout history, the Oldoinyo Lengai volcano, located in the East African Rift in northern Tanzania, is the only place on Earth where they are actively erupting. The lava expelled from the volcano is highly unusual in that it contains almost no silica and greater than 50 percent carbonate minerals. Typically lavas contain high levels of silica, which increases their melting point to above 900°C (1652°F). The lavas of Oldoinyo Lengai volcano erupt as a liquid at approximately 540°C (1004°F). This low silica content gives rise to the extremely fluid lavas, which resembles motor oil when they flow.

A team of scientists from University of New Mexico, Scripps Institution of Oceanography at UC San Diego and Centre de Recherches Petrographiques et Geochimiques in Nancy, France, report new findings of volcanic gas emissions in a paper published in the May 7 issue of the journal Nature.

"The chemistry and isotopic composition of the gases reveal that the CO2 is directly sourced from the upper mantle below the East African Rift," said David Hilton, professor of geochemistry at Scripps Institution of Oceanography at UC San Diego and coauthor of the paper. "These mantle gases allow us to infer the carbon content of the upper mantle that is producing the carbonatites to be around 300 parts per million, a concentration that is virtually identical to that measured below mid-ocean ridges."

Mid-ocean ridges are underwater mountain ranges where the seafloor is spreading due to tectonic plates moving away from one another. Rift valleys, such as the one where Oldoinyo Lengai volcano is located, and mid-ocean ridges are considered to be distinct tectonic regions. However, this study has shown that their chemistries are identical, which led the scientists to suggest that the carbon contents of their mantle sources were not different but due to partial melting of typical minerals located in the earth's mantle.

"Since the volcano was under magma pressure during the eruption, we were able to collect pristine samples of the volcanic gases, with minimal air contamination," said Tobias Fischer, volcanologist at the University of New Mexico. The pristine samples collected during a 2005 eruption offered the scientists a deeper look at the processes taking place in the earth's upper mantle.

The geochemical analyses, some of which were conducted at Hilton's geochemical lab at Scripps Oceanography, revealed that magma from the upper mantle below both the oceans and continents is a uniform and well-mixed reservoir of "typical" volcanic gases such as carbon dioxide, nitrogen, argon and helium.

The lava expelled from the volcano is highly unusual in that it contains almost no silica and greater than 50 percent carbonate minerals. Typically lavas contain high levels of silica, which increases their melting point to above 900°C (1652°F). The lavas of Oldoinyo Lengai volcano are comprised of carbonatites, which erupts as a liquid at approximately 540°C (1004°F). This low silica content gives rise to the extremely fluid lavas, which resembles motor oil when they flow.

"These finding are significant because it shows that these extremely bizarre lavas and their parent magmas, nephelinites, were produced by melting of a typical upper mantle mineral assemblage without an extreme carbon content in the magma source," said geochemist Bernard Marty at the Centre de Recherches Petrographiques et Geochimiques in Nancy, France. "Rather, in order to make carbonatite lavas, all you need is a very low melt fraction of 0.3 percent or less."

Oldoinyo Lengai, like all volcanoes, emits carbon dioxide into the atmosphere as a gas. However, Lengai's magma is unusual in that it also contains high sodium contents. About one percent of the mantle-derived carbon emitted from Lengai goes into the carbonatite melt with the remainder being emitted into the atmosphere as CO2 gas. The CO2 released into the atmosphere by volcanoes worldwide is a small fraction when compared to man-made emissions.
World's Most Unusual Volcano: Origin Of Carbon-based Lavas Revealed - Science Daily .com


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Oldoinyo Lengai Volcano, northern Tanzania - sodium carbonatite lavas
« Reply #3 on: September 07, 2009, 12:54:54 »

Oldoinyo Lengai Volcano - carbonatites and incandescence

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Oldoinyo Lengai Volcano (2962m) is the only volcano to erupt sodium carbonatite lavas in historical times.  These lavas have a significantly lower melting point (around 500'C) than “normal” silicate lavas (around 1200'C) and their weak incandescence can only be observed at night.  Documented activity is characterized by short explosive eruptions of predominantly silicate ashes which leave a funnel-shaped crater, followed after an eruptive pause by a phase of intracrater activity involving intermittent effusion of carbonatite (soda) lavas.  The current effusive phase started in 1983 and has completely filled the crater formed by the 1967 explosive eruption

Oldoinyo Lengai volcano consists of various types of peralkaline (Na2O- and K2O-rich) silicate lavas. Two main structural units are recognized. The remains of the initial structure, "Lengai I" form the south flank and much of the base of the volcano and account for 60% of its volume. "Lengai II" is more recent and formed in the scar left behind by a major flank collapse of the N flank of Lengai I about 10000 years ago. Flank collapses feature in the history of many volcanoes and are dealt with in more detail in the sections on Stromboli and Augustine volcanoes. Both "Lengai I and Lengai II" are formed primarily from pyroclastic deposits, suggesting mainly explosive activity during the cone-building phases. Natroarbonatites, which form less than 5% of the structure (mainly in and around the N crater), appear to be a recent feature of Lengai activity. Lengai I is made of phonolite (14-17% alkaline). Lengai II is predominantly nephelinite (15-21% alkali). These lavas contain between 53 and 43% silicate, with a gradually decreasing trend during evolution of the structure (Klaudius and Keller, 2006 (Lithos 91:173-190)). Although these lavas show a gradual increase in alkalinity and a gradual decrease in silica, they are far removed from natrocarbonatite lavas which have over 40% alkali content and usually less than 0.5% silica.

Most lavas contain 40-80% silicate, whereas carbonatites usually contain significantly under 10%. Carbonatites have over 50% volume of carbonate materials. The most commonly found carbonatite deposits are rich in calcite (CaCO3). Deposits of natrocarbonatites are extremely rare, although this may reflect the fact that the sodium and potassium carbonate minerals nyerereite and gregoryite making up much of their composition are rapidly weathered. At Lengai, as the erupted anhydrous natrocarbonatite cools on the surface, hydration rapidly occurs, changing the colour of the material from dark grey to an off-white colour usually within a matter of days. The process can be observed particularly well when raindrops fall onto fresh natrocarbonatite deposits each leaving lighter marks on the surface. Hydration and subsequent alteration by chemical reactions and leaching out of certain constituents eventually results in a weak material which can be crushed to powder between ones fingers. The rapid weathering has the effect that sites of fresh lava emission can be easily recognized as darker areas on the crater floor.

Less than 400 carbonatite deposits are known and only few of these represent eruption of carbonatite lavas at the earths surface. The formation of carbonatites is thought to result from differentiation of mixed magmas as they cool on approaching the earths surface. As magma temperatures fall, silicate minerals crystallize, increasing relative levels of carbonates in the melt. The carbonate-rich magma eventually seperates from the silicate magma and can be erupted as carbonatite lava if it reaches the surface. The process of natrocarbonatite formation at Lengai can possibly explained by natrocarbonatite seperating from the Lengai II-forming combeite and wollastonite bearing nephelinite after combeite crystallization (Dawson 1998 (J. Petrology 39:2077-2094)). The processes occurring in the magma chamber are thought to also account for the predominance of carbon dioxide in the gases emitted by the volcano.

A pocket of natrocarbonatite coexists with nephelenitic melts in Lengai's magma chamber(s). This can be deduced from eruption of mixed material during larger eruptions. For example, the tephra from the minor 1993 eruption was predominantly silicate with inclusion of small globules of natrocarbonatite. All larger explosive eruptions have involved mixed silicate-natrocarbonatite tephras with varying relative compositions. Explosive eruptions of Lengai have been documented in 1917, 1940-41 and 1966-67. Tephra deposits surrounding Lengai suggest that significantly larger eruptions have occurred every several hundred years (last about 450 years ago) before historical records began.

Minor intracrater eruptions of natrocarbonatites can take several forms, reminiscent of activity at other volcanoes. Lava flows can be observed, as can lava fountaining or strombolian activities or flank failures of cones. The observation that one cone erupted an A'a flow shortly after which another erupted a Pahoehoe flow (personal observation 2004) is interesting , since the difference could not be accounted for by the topology of the area of emplacement or by an obviously different flow rate and could result from differences in temperature or composition of lava in individual cones within the crater. Such variance in types of products produced by intracrater activity is repeatedly observed.
Oldoinyo Lengai Volcano, northern Tanzania - photovolcanica .com