March 8, 2011

The Mantle MOJO

Posted by Evelyn Mervine

Mixing of classic mantle reservoirs in tetrahedron space, Figure 2 from Hart et al. (1992). Click to view larger.

If I ever name a mantle reservoir, I am going to call it the MOJO. Why? Because I would love to give a geochemistry talk where I discuss such-and-such volcanic rock having a mantle MOJO signature. The mantle MOJO will be the sexiest of mantle reservoirs. All geochemists will hope their rocks have a little MOJO in them. For those of you who are not familiar with meaning of the slang word “mojo,” I recommend watching the Austin Powers move “The Spy Who Shagged Me” or consulting the trusty urban dictionary

My only problem in naming this mantle reservoir is, of course, coming up with a good definition of the acronym. Well, and actually finding some evidence for the proposed reservoir. But really, this is such a great mantle reservoir acronym that I’m sure we can find a place for it in geochemistry.

Here are some ideas:
MOJO = Mantle Original Juvenile Order
MOJO = MOderately reJuvenated Orb
MOJO = Mantle Overly Juvenated Offshoot

Honestly, I’m not happy with any of these acronyms. Any better proposals?

Those of you who are not mantle isotope geochemists may be wondering: what is a mantle reservoir? A mantle reservoir is basically a region of the Earth’s mantle with a specific geochemical (generally isotopic) fingerprint. The classical model for mantle reservoirs is that they are broad regions of the mantle that have specific geochemical fingerprints– they are enriched in one element or depleted in another element– because of their origin and geochemical history. Because geologists cannot directly sample the Earth’s mantle, they use geochemical fingerprints of mantle sources found in rocks at the surface to infer the composition of the inaccessible mantle.

To explain this further, I will quote a paper I wrote back in undergrad:

“One of the methods modern scientists use to learn about the chemical composition of the Earth’s interior is the analysis of isotopic compositions in rocks on Earth’s surface. Geochemical analysis of radiogenic Hf and Nd [and other] isotope ratios in erupted lavas is a powerful tool for examining the processes governing the geochemical evolution of magma deep within the Earth’s mantle. Isotope compositions and their ratios are minimally affected by the immense physical stresses imposed at great depths in the Earth’s interior, the processes of magma upwelling to the crust, and the final eruption at the Earth’s surface. Therefore, these analyses can be used to infer the compositions of the igneous source material as well as how these sources mixed and melted to form the rocks on the surface. Combined with major and trace element data, Hf and Nd isotope ratios can even be used to determine the degree of melting that occurred.”

As geochemists collect more and more isotopic data, they are realizing that the mantle reservoirs are not as simple as first proposed, mostly back in the 1980s by Stan Hart and other geochemists. Like many scientific models, the model of the mantle reservoir is one that seemed simple and straightforward for many years but which now requires modification and re-analysis now that scientists have much more data and understanding. The mantle seems to be really, really heterogeneous- there are probably not just a few large mantle reservoirs but rather many small mantle reservoirs. Perhaps in the near future, geochemists will talk about a source having the general geochemical “flavor” of one of the classic mantle reservoirs rather than originating from a specific, somewhat magical mantle source. Oh! I have a sudden inspirational idea. Maybe geochemists can say that a source has the “mojo” of a classic mantle reservoir.

I am currently writing a paper about evidence for extremely small-scale mantle heterogeneity in Iceland. One of these days (hopefully soon!) I’ll actually finish and submit the paper. I’ll be sure to blog about the paper once it’s published, but until the paper is out I won’t talk about the results in detail.

Mantle end-members in Nd-Pb isotope space. Data from oceanic basalts is plotted with the general locations of the end-members marked. Data compiled by Stracke et al. (2003). Click to view larger.

For those of you who are not familiar with them, these are the classic mantle reservoirs:

-EMI: End-member I 
Low Nd, Pb, & Hf, intermediate Sr isotope values
Originally, EMI was thought to be subcontinental lithosphere. However, many geochemists now believe that EMI is better represented by pelagic (deep-sea) sediment that has been recyled into the mantle.

-EMII: End-member II
High Pb & Sr, intermediate Hf, and low Nd isotope values
Most geochemists agree that EMII is subducted continental material.

-HIMU: High “mu” or μ
Very high Pb, intermediate Nd, Hf, & Sr isotope values
Mostly defined as a source with high 206Pb/204Pb. Since 206Pb decays from 238U, this means that the source had high U/Pb (relative to “normal” mantle). Note that μ = 206Pb/204Pb in geochemistry speak.
There are various ideas about what can produce high U/Pb in the mantle. One option is subducted oceanic crust that became enriched in U/Pb during subduction. Other proposed origins of the HIMU signature are old, U-rich ocean crust (perhaps enriched because the ocean used to have more U?) and delaminated continental lithosphere.

-DMM: Depleted MORB Mantle
High Nd & Hf, low Pb & Sr isotope values.
Note that there is a nested acronym within an acronym here. MORB = Mid-Ocean Ridge Basalt.
DMM is “regular” mantle that has been depleted by the earlier extraction of the enriched continental crust. Perhaps I’ll blog about the geochemical relationships among continental crust, oceanic crust, and mantle another day. For now, just know that DMM is pretty much your run-of-the-mill mantle.

-FOZO: Focused Zone
Low Sr, high Nd & Sr isotopes.
If the FOZO exists, it probably is in the deep mantle, near the core-mantle boundary, and is a sort of ultimate source material for mantle plumes.
Elements discussed in this post:
Hf- Hafnium
Nd- Neodymium
Pb- Lead
U- Uranium
Sr- Strontium

Dickin, Alan. 2005. Radiogenic Isotope Geology. Cambridge: Cambridge University Press.
Hart et al., 1992. Mantle plumes and entrainment: isotopic evidence. Science, vol. 256: 517-520.
Stracke et al., 2003. Theistareykir Revisited. G3, vol. 4, no. 2.