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Tracing Mantle Oxidation

Sunday, 2. August 2009, 08:29:29

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The chemical composition of the Earth's mantle varies with tectonic setting. Material from volcanoes near subduction zones is more oxidized than near divergent plate boundaries.
In a study on “Water and the Oxidation State of Subduction Zone Magmas” Kelley and Cottrell analysed samples from three different tectonic settings: ridges, back-arc basins, and arcs.
(Illustration from Kelley and Cottrell 2009)
Quote from Hirchmann’s “perspectives” article relating to the paper:

“Oxidizing mantle rocks and magmas. Subduction of oceanic lithosphere carries oxidized surface rocks into Earth’s interior. These rocks, including sediments and hydrothermally altered basalts, are rich in water, which is released into the overlying mantle wedge, as indicated by the region in brown. This process initiates melting in the mantle wedge, which in turn leads to formation of volcanoes in island arcs such as Japan and Indonesia. Regions where silicate melt is present are shown schematically in red. Kelley and Cottrell show that the subducted, volatile-rich geochemical component found in island arc volcanoes is also associated with oxidation, strongly suggesting that the fluids added from the subducted lithosphere to the mantle wedge are rich in an oxidizing agent such as ferric or sulfate ions. The mantle wedge is dragged into the deeper mantle by viscous coupling to the subducted lithosphere (curved arrow).”



(See http://www.sciencemag.org/cgi/content/full/sci;325/5940/545 )

The observations indicate a direct link between mass transfer from the subducted plate and oxidation of the mantle wedge.





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Abiotic Oil?

Tuesday, 28. July 2009, 07:19:35

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There is widespread evidence that petroleum originates from biological processes. Whether hydrocarbons (oil and gas) can also be produced from abiogenic precursor molecules under the high-pressure, high-temperature conditions characteristic of the upper mantle remains a disputed question.

A hypothesis that oil can be created by non-biological mechanisms originated in Russian and Ukrainian scientific circles in the 1950s. Put briefly, it proposed that petroleum forms deep in the Earth’s mantle under extremely high pressure and temperature through a reaction between carbonates, iron oxides and water. This process goes on continuously, and the petroleum migrates upwards through the lithosphere. At issue is the formation of complex hydrocarbons. There has never been any doubt that simple hydrocarbons such as methane can be formed by inorganic processes.

If true it was speculated it might be possible to find oil deep under the Siljan Impact Crater (Sweden), and they drilled here in the 1980’s and 1990’s, among other things in the hope to find oil. The hypothesis went that (inorganic) methane (gas) migrates upwards from the mantle and transforms into oil in the upper crust in igneous rocks (like granite). The Siljan ring is a crater of granitic rocks overlain by soil formed by a large meteorite impact 360 million years ago. The impact was postulated to have created fractures at great depth through which gas and oil would have been able to migrate. Although a little bit of oil was found, the drilling was more or less a failure – as far as oil and gas is concerned. Many scientists (though not everybody) thought that that was the final end of the abiotic oil story.

Apparently the dream has been awakened again.

As I said Scientists have debated for years whether some of our oil and gas (hydrocarbons) could also have been created deeper in the Earth and formed without organic matter. Now for the first time, according to a study published in the July 26, advanced on-line issue of Nature Geoscience, scientists have found that ethane and heavier hydrocarbons can be synthesised under the pressure-temperature conditions of the upper mantle —the layer of Earth under the crust and on top of the core (red in the figure below).


Click on image to enlarge!

Using a diamond anvil cell and a laser heat source, scientists first subjected methane to pressures exceeding 20 thousand times the atmospheric pressure at sea level and temperatures ranging from 704°C to over 1 227 °C. These conditions mimic those found 65 to 150 km deep inside the Earth. The methane reacted and formed ethane, propane, butane, molecular hydrogen, and graphite. The scientists then subjected ethane to the same conditions and it produced methane. The transformations suggest heavier hydrocarbons could exist deep down. The reversibility implies that the synthesis of saturated hydrocarbons is thermodynamically controlled and does not require organic matter.

The results from the study seem to support the suggestion that hydrocarbons heavier than methane can be produced by abiogenic processes in the upper mantle.

Reference:
Kolesnikov et al.
Methane-derived hydrocarbons produced under upper-mantle conditions
Nature Geoscience
Published online: 26 July 2009
doi:10.1038/ngeo591

• http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo591.html
• http://www.ciw.edu/news/hydrocarbons_deep_earth
• http://www.scientificblogging.com/news_articles/peak_oil_not_if_deep_earth_hydrocarbon_theory_true
• http://www.npd.no/English/Aktuelt/Nyheter/22.5.2007+Fossilt+drivstoff+uten+fossiler.htm?print=true
• http://www.physorg.com/news167835116.html
• http://www.eurekalert.org/pub_releases/2009-07/ci-hit072409.php
• http://www.eurekalert.org/multimedia/pub/15568.php?from=141472
• http://www.sciencecodex.com/deep_earth_hydrocarbon_discovery_oil_and_methane_without_organic_matter
• http://www.planetthoughts.org/?pg=pt/Whole&qid=2982&src=rss
• http://www.sciencedaily.com/releases/2009/07/090726150843.htm
• http://www.energy-daily.com/reports/Hydrocarbons_In_The_Deep_Earth_999.html
• http://www.abc.net.au/science/articles/2009/07/28/2638484.htm

In Danish:
• http://ing.dk/artikel/100215-olieproduktion-i-laboratorier-nye-forsoeg-vaekker-forskernes-haab?utm_medium=rss&utm_campaign=nyheder



PS of 28 July 2009:
Glenn Reynolds files the story under “THINGS I’D LIKE TO BE TRUE” at http://pajamasmedia.com/instapundit/82538/

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The Tails of Two Plumes

Monday, 9. February 2009, 14:54:31

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Hans Christian Andersen once wrote a fairy tale about a little feather that grew into five hens (It's Quite True!). Mantle plumes may sound as another fairy tale - but is it?

The concept of mantle plumes was advanced in the 1970s to explain intra-plate or hot spot volcanism erupting far from any plate boundary. The concept is still being heavily discussed. Do mantle plumes exist? Where and how do they form? Are all, none, or some hotspot volcanoes associated with mantle plumes? What do they look like, anyway?

First a very brief description of the concept. Mantle plumes are plume-like upwelling currents of hot material from the core-mantle boundary (the D“ layer) - or maybe higher up from the junction of the upper and lower mantle - that finally erupts as hotspot volcanoes or flood basalts.

And what do they look like? One idea is a cavity plume with a large spherical head and a thin trailing tail. Another idea is a diapir plume with a thick tail.

The conditions required for the formation of these two types of plumes are different, and it may seem unlikely that both could coexist in a chemically more or less homogeneous mantle. But on the other hand maybe not so if they are formed in the layer just above the core-mantle boundary, also known as the D” layer (pronounced "dee double prime"). In recent years a better understanding of this layer is accumulating. The core-mantle boundary region is chemically heterogeneous, particularly where mantle plumes are thought to originate. The thickness of the layer also varies a lot from place to place - probably from 0 and 300 km thick.

According to a study just published in the GSA journal Geology (with FREE access!) under the title Tails of two plume types in one mantle the variable thickness of this layer at the base of the mantle can lead to the coexistence of the two mentioned distinct plume types.

The form of mantle plumes is governed by viscosity. A locally thick chemical layer leads to small viscosity variation instabilities and hence to diapir plumes. The diapir plume is characterised by a cylindrical stem with a diameter twice the thickness of the thermal boundary layer capped by a head only slightly larger in radius than the stem. A locally thin chemical layer allows for large viscosity variations across the active portion of the lower mantle thermal boundary layer and, hence, for cavity plume formation. The thin tail reflects the thickness of the lowest viscosity active part of the thermal boundary layer (i.e. the velocity boundary layer), which feeds the upwelling plume.

The authors expect weak hotspots (such as the Azores) to be associated with diapir plumes and strong hotspots (such as the Hawaii) to be associated with cavity plumes.

Reference:
Lenardic and Jellinek:
Tails of two plume types in one mantle
Geology 2009;37;127-130
doi:10.1130/G25229A.1

• http://geology.gsapubs.org/cgi/content/abstract/37/2/127?rss=1


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Core-Mantle Boundary

Thursday, 13. November 2008, 16:08:49

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Can slabs of oceanic crust descend as deep as to the core-mantle boundary? Is cold mid-ocean-ridge basalt dense enough to sink so deep into the mantle?

Do mantle plumes exist? And if so, can they well up from as deep as the core-mantle boundary?

These and other core-mantle boundary related questions are occupying many earth scientists. They just want to know.

Most of our earlier knowledge of the Earth's interior came from studying seismic waves (after earthquakes). It became clear that the boundary between the core and the mantle was something special. S-waves (secondary wave or shear wave) stopped. They cannot propagate through liquids. Seismic velocity is linked to the density of the medium through which the waves travel. P-wave (primary wave) velocity in general increases with increasing density - In liquids, however, the speed will be less. The core is much denser than the mantle.

When it was discovered that a thin layer directly above the core-mantle boundary had some mysterious properties it had to get a name consistent with the naming of the earth’s layers in the middle of last century. It is now still referred to as the D" ("D double-prime" or "D prime prime"). The D" name originates from the mathematician Keith Bullen's designations for the Earth's layers. His system was to label each layer alphabetically, A through G, with the crust as 'A' and the inner core as 'G'. In his 1942 publication of his model, the entire lower mantle was the D layer. In 1950, Bullen found his "D" layer to actually be two different layers. The upper part of the D layer, about 1800 km thick, was renamed D’ (D prime) and the lower part (the bottom 200 km) was named D" (pronounced "dee double prime"). A layer that produces strange seismic properties.

Research into the mysteries of the D“ layer has advanced spectacularly since 2004, when Japanese researchers found that high temperatures and pressures like those existing in the D” layer transform perovskite, the major mineral in Earth's mantle. The publication in the journal Science of Post-Perovskite Phase Transition in MgSiO3 is seen as a turning point. Since then many papers on this topic have followed. The latest I have seen is Radiative conductivity in the Earth's lower mantle by Goncharov et al. in today’s (13 November 2008) issue of Nature.

The discovery of post-perovskovite has profound implications for the chemical, thermal, and dynamical structure of the lowermost mantle (the D” region). Several major seismological characteristics of the D” region can now be explained by the presence of post-perovskite, and the specific properties of the phase transition provide the first direct constraints on absolute temperature and temperature gradients in the lowermost mantle. A discussion of the current understanding of the core–mantle boundary region can be found in Discovery of Post-Perovskite and New Views on the Core–Mantle Boundary Region by Kei Hirose and Thorne Lay in Elements of June 2008 (DOI: 10.2113/GSELEMENTS.4.3.183 - start downloading of the full article as large pdf-file by clicking here).

The Mg2+ site in post-perovskite is smaller than in perovskite, resulting in a volume reduction of 1.0–1.5%. Unlike perovskite, the post-perovskite phase has a layered structure of the SiO6 octahedra, which may lead to a large contrast in some properties with perovskite.


Scenario for the D” region. The D” seismic discontinuity is caused by the perovskite (Pv) to post-perovskite (PPv) phase transition. Post-perovskite may transform back to perovskite in the bottom thermal boundary layer with a steep temperature gradient. The large low-shear-velocity provinces (LLSVP) underneath upwellings (forming plumes) possibly represent large accumulations of dense MORB-enriched materials. The solid residue formed by partial melting in the ultralow-velocity zone (ULVZ - thin reddish zone on image) might also be involved in upwelling plumes. Such thin (20-40 km) ULVZ-layers are mainly (but not only) found under the Pacific Ocean and under Africa.

• http://www.eurekalert.org/pub_releases/2008-11/ci-eht111008.php
• http://www.cems.umn.edu/research/wentzcovitch/highlights/science_now_040324.htm
• http://olivine.ethz.ch/~artem/NewMinerals.html
• http://www.ucsc.edu/news_events/press_releases/text.asp?pid=978
• http://www.nature.com/ngeo/journal/v1/n1/full/ngeo.2007.44.html (full paper in HTML)

For my (increasing number of) Scandinavian readers there is review in Norwegian at Geoportalen - http://www.geoportalen.no/planetenjorden/jordensindre/revolusjon/ . The article here by Reidar G. Trønnes, Natural History Museum, Universitty of Oslo, on the Earth’s Interior is somewhat easier to consume, and is well illustrated.

Maybe I ought to add that some of the topics discussed in the (review) papers are still controversial.




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Heterogenous Mantle and Hotspots

Tuesday, 29. January 2008, 09:16:13

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Thermally driven upwellings in the mantle - so-called mantle plumes - are often envisioned to originate at the core-mantle boundary, from which they rise to create hot-spot volcanism. Clear geochemical signatures of the outer core in hot-spot lavas would prove the existence of deep-rooted mantle plumes, with important implications for large-scale mantle dynamics, and for decades mantle geochemists have tried to find such clear isotopic or chemical signatures. Different isotopes have been used in such studies. In my post on Volcanoes and Isotopes I concentrated on Helium isotopes. The platinum-osmium (Pt-Os) and rhenium-osmium (Re-Os) isotopic systems have, however, played a dominant role in these studies.

Earth's mantle was strongly depleted in osmium, platinum, and rhenium during core formation, because these elements tend to move into metallic phases. If the osmium concentration is much higher in the liquid outer core than in the surrounding solid mantle, mixing a little bit of outer-core material back into the mantle at the core-mantle boundary will change the local osmium isotopic composition to resemble that of the outer core. And if the osmium isotopic composition of the outer core is very different from that of the upper mantle, it might be detected in lavas brought to Earth's surface.

Osmium isotopic analyses of mantle-derived materials has, however, showed that the upper mantle is very heterogeneous. According to a report by Luguet et al. published in Science of 25 January 2008 (Enriched Pt-Re-Os Isotope Systematics in Plume Lavas Explained by Metasomatic Sulfides) there is no longer a need to invoke an outer-core input to explain the osmium isotopic compositions of mantle-derived materials. Their results may radically change the basis on which osmium isotopic compositions from mantle-derived materials are evaluated. That there is evidence for a very high degree of geochemical heterogeneity in the upper mantle is not only true for the osmium isotopic systems; it seems to be generally true and has important consequences for explaining the origin of isotopic "anomalies" in mantle-derived materials. The minerals in question are highly mobile in the mantle.

Geochemical heterogeneity is introduced into the mantle, for example, by subduction of sediments, oceanic crust, and lithosphere, and by melt extraction. Other intramantle mixing processes can also contribute to the creation of a range of geochemical components in the upper mantle. All these processes redistribute (fractionate) major and trace elements among different minerals, fluids, and melts, which in turn allows different components to evolve along divergent isotopic trajectories.

With an upper mantle as heterogeneous as the data by Luguet et al. suggest, it is difficult to imagine that isotopic signatures in oceanic basalts can be uniquely tied to the outer core. The debate about the existence and possible origins of deep-rooted mantle plumes will most likely have to be settled with geophysical methods.

With such a heterogenous mantle I think that we are still left with two fundamental questions. We can follow cold dense sinking subducted crust for a few hundred km down into the mantle, but what really happens with this material at greater depth (in the lower mantle)? And where (and how) do mantle plumes (if any) arise? Further speculations over these two questions may lead to theories like the one I wrote about yesterday in my post on Tectonic Plate Recycling.

If you want my personal opinion (but who cares), I still believe in mantle plumes, but find the theory I described yesterday too far fetched. Further studies are needed - as we so often read in the conclusions of (geological) scientific papers.

• http://www.sciencemag.org/cgi/content/full/319/5862/418
• http://www.sciencemag.org/cgi/content/full/319/5862/453





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Tectonic Plate Recycling

Monday, 28. January 2008, 09:49:48

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I had just started writing a new post concerning a paper that could change our view on mantle plumes, when I became aware of a weird piece in Der Spiegel. The post on which I began will now have to wait till tomorrow, but in some ways the two articles are related.

On January 25 2008 Spiegel Online International brought an article titled Where Continents Go To Die - A New Look into the Center of the Earth.

Let me start with saying that I don’t believe in this theory (yet), but find it worth discussing. So here we go.

Old, cold plates are pushed down into the Earth's mantle on the continental edges, where they collect large amounts of iron. Weighted down by the iron, the plates sink farther and farther into the Earth's mantle. There, at a depth of 2,900 km, they settle into "plate graveyards”. Heat and pressure in the depths trigger chemical processes, causing the plates to deposit their load of heavy elements. Once liberated of this burden (a few hundreds of millions years later), they become lighter than their surroundings, causing them to rise and as mantle plumes they make their way toward the surface (at hot spots etc.). Well, that is the biggest convection cell that I have ever heard of, and I must stress that this is only a (new) theory, that in no way has been proven by facts, as far as I know.

More about the mantle plume bit in my next post here.


• http://www.spiegel.de/international/world/0%2C1518%2C531023%2C00.html
• http://www.spiegel.de/international/world/0,1518,grossbild-1078154-531023,00.html




PS:
  1. In another context (science journalists/journalism) Chris (goodSchist) had an indirect comment to this post over at Clastic Detritus (Response#2).
  2. There is a dedicated mantle plume website at http://www.mantleplumes.org/ with links to lots of (free) papers on mantle plumes.



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Soft Mantle

Saturday, 26. January 2008, 15:46:59

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Everything on Earth can be explained in terms of 4 states/phases of matter - solid, liquid, gas, and plasma. In this sense the mantle is solid. From the way seismic waves travel through the Earth we know that the mantle is solid. P waves will travel and refract through both fluid and solid materials. S waves, however, cannot travel through fluids. S waves travel through the mantle, but not through the outer core, so the mantle is solid.

Chris at Highly Allochthonous had an excellent post on the solid state of the mantle titled Annoying misconceptions in Geology. But how solid is it after all. It is often described as plastic - the plastic mantle. Other relevant words may be elastic or soft. I find it a telling fact, that you don’t get earthquakes beneath something like 700 km, so down there the mantle cannot be that rigid.

According to an article published in Science of 25 January 2008 laboratory experiments suggest that the lower mantle is softer than previously thought. The lower mantle extends from about 660 km to 2900 km into Earth and sits atop the liquid outer core. Knowledge of this deep and inaccessible region is derived largely from seismic data. Pressures and temperatures are so brutal there that materials are changed into forms that don’t exist in rocks at the planet’s surface and must be studied under carefully controlled conditions in the laboratory. The pressures range from 230,000 times the atmospheric pressure at sea level (23 GPa), to 1.35 million times sea-level pressure (135 GPa). And the heat is equally extreme - from about 1,500° to 3,700° Celsius.

Changes in the electronic configuration of iron at these pressures and temperatures seem to alter the elastic behaviour, making the lower mantle "softer" than previously estimated. The results suggest that scientists may have to go back to the drawing board to model this region of the Earth.

Crowhurst et al. Elasticity of (Mg,Fe)O Through the Spin Transition of Iron in the Lower Mantle http://www.sciencemag.org/cgi/content/abstract/319/5862/451
• http://www.eurekalert.org/pub_releases/2008-01/ci-eg012208.php
• http://www.scientificblogging.com/news_releases/earth_getting_soft_in_the_middle
• http://www.livescience.com/environment/080124-soft-middle.html





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