Project Mohole

 Unlocking the secret history of the earth
      - the story of an early attempt at offshore drilling of the ocean crust.
      - and a really cool project name. 

http://en.wikipedia.org/wiki/Project_Mohole

video
http://www.youtube.com/watch?v=y5_AxsT292o

Willard Bascom. Director of the Moho project. 1959.

Starbursts and snowballs

when stars explode, earth gets cold

Many folks agree that the planet suffered a small number of drastic cooling events (snowball earth); at least one between 2.2-2.4 billion years ago and another 770 to 550 million years ago, but their triggers are a matter of debate. Katoaka et al. (2013) suggest that the events could be driven by encounters between Earth and cosmic dust and rays originating from stellar explosions (nebulae). They use the ages of stars, and star clusters, to deduce a causal effect between the death (and subsequent growth) of nearby stars in the Milky Way galaxy and periods snowball Earth. They also suggest that some of the major mass extinctions in the Phanerozoic could also be related to encounters with remnants from supernovae.

Image from Katoaka et al., showing interaction of stellar particles with two of the Earth’s protective shields (geomagnetic field, and ozone layer). The heliosphere provides further protection.

Reference, Kataoka, R., Ebisuzaki, T., Miyahara, H., Maruyama, S., 2013, Snowball Earth events driven by starbursts of the Milky Way Galaxy, New Astronomy, 21, 50-62.

Archean crustal growth

Has continental crust always formed the same way?

In an introduction to new research published in Geology, Ali Polat (2013) summarizes some of the controversies related to the timing and processes of the formation of Earth’s continental crust. An important questions is whether crustal growth process were different (non-uniformitarian) during the Archean. Many folk suggest that about half of the Earth’s continental crust was emplaced by the end of the Archean (2.5 billion years ago), citing isotopic values that imply the mantle was strongly depleted in incompatible elements at this time. High Nb/Th and Nb/U ratios of Archean komatiites further supports this hypothesis (Th and U are incompatible and concentrate in oceanic crust during partial melting of the mantle). Another question of consequence of this manner concerns the origin of tonalite-trondhjemite-granodiorite (TTG) intrusive suites, which may comprise up to 80% of remaining Archean crust. TTGs are characterized by strongly fractionated REE patterns, typically attributed to residual garnet and are often thought to be derived by melting oceanic crust as it subducts into the mantle. The Earth was much hotter in the Archean, and capable of this process, unlike the planet's later years. TTGs may not be exclusive to slab melting, however, as recent research of Greenland rocks by Nagel et al., 2013 supports an origin by melting thickened oceanic crust, on the upper plate of an island arc. Adam et al. (2013) reach a similar conclusion for Archean rocks in Canada. Key to the geochemical distinction between the two models is the evidence of water (bound in amphibole) in many TTGs, which would be present beneath an island arc crust, but absent in the dehydrated subducting slab. Upper plate melting is a process that continues today, and is consistent with uniformitarian models of crustal formation.

Figure 1 of Polat (2013) showing hydrous melting of mafic lower crust as the key producer of Archean TTGs, as opposed to dry melting of subducted oceanic crust. The image was modified after Davidson and Arculus, 2006)

Source: Polat, A., 2013, Growth of Archean continental crust in oceanic island arcs, Geology, v. 40, p. 383-384.

Nagel, T.J., Hoffmann, J.E., and Münker, C., 2012, Generation of Eoarchean tonalitic-trondhjemitic-granodioritics from thickened mafic arc crust: Geology, v. 40, p. 375–378, doi:10.1130/G32729.1.

Adam, J., Rushmer, T., O’Neil, J., and Francis, D., 2012, Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the Earth’s early continental crust: Geology, v. 40, p. 363–366, doi:10.1130/G32623.1.

Davidson, J.P., and Arculus, R.J., 2006, The significance of Phanerozoic arc magmatism in generating continental crust, in Brown, M., and Rushmer, T.,eds., Evolution and Differentiation of the Continental Crust: New York, Cam- bridge University Press, p. 135–172.

Sierra Nevada uplift

geologic controversy - Rise of the Sierra Nevada

The Sierra Nevada range of eastern California is one the most famous ranges in the world, but the timing and processes that led to its elevated topography is a major question mark. Christopher Henry (2009) summarizes this controversy and explains that most researchers once considered Sierran uplift to have mostly occurred during the last 10 million years, but more recent studies argue for much earlier uplift in the Mesozoic. The issue is related to the proposed “Nevadoplano” of DeCelles (2004), who argued that the present day Great Basin region (mostly Nevada) was a high plateau during the Mesozoic, much like the modern day Altiplano of the central Andes. The Sierra Nevada region would have formed the flank of this great plateau. Mulch et al. (2006) and Cassel et al. (2009) supports an early rise model with stable isotope evidence that suggest the Sierra Nevada was near its current elevation by at least the Eocene and Oligocene. Major batholithic volcanism also occurred at this time and is consistent with thickened crust. In contrast, Ducea and Saleeby (1996) propose the Sierra Nevada was uplifted in the late Cenozoic following the removal of a dense eclogitic root; citing thermobarometry of mantle xenoliths. Both sides seem to have support look for more work to be done on this issue.

Figure 1 of Henry (2009) showing modern day topography overlain by flow direction of ancient rivers.

Sources

Henry, C. D., 2009, Uplift of the Sierra Nevada, California, Geology,

Cassel, E.J., Graham, S.A., and Chamberlain, C.P., 2009, Cenozoic tectonic and topographic evolution of the northern Sierra Nevada, California, through stable isotope paleoaltimetry in volcanic glass: Geology, v. 37, p. 547–550.   

DeCelles, P.G., 2004, Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western U.S: American Journal of Science, v. 304, p. 105–168, doi: 10.2475/ ajs.304.2.105.

Ducea, M.N., and Saleeby, J.B., 1996, Buoyancy sources for a large, unrooted mountain range, the Sierra Nevada, California: Evidence from xenolith thermobarometry: Journal of Geo- physical Research, v. 101, p. 8229–8244, doi: 10.1029/95JB03452.

Mulch, A., Graham, S.A., and Chamberlain, C.P., 2006, Hydrogen isotopes in Eocene river gravels and paleoelevation of the Sierra Nevada: Science, v. 313, p. 87–89. 

viscous death in subduction zones

Fun geology term: viscous death

What is controlling water concentrations in arc magmas? 

Water is the weirdest and most important molecule there is. Besides sustaining life on our planet, water affects every part of the evolution of an arc volcano. Water's presence in the deep mantle dramatically lowers its melting temperature, resulting in magmatism in an otherwise cold subduction zone. Water also lowers the viscosity of magmas allowing them to rise, and later expands as a gas fueling explosive eruptions. Despite its importance to the transfer of heat and matter throughout nearly every system on our planet, little is known about the filtering effects of subduction zone water. This is changing with the rise of improved measurements for water concentrations in magmatic inclusions, tiny bits of volcanic glass that were trapped as liquids in growing crystals (usually olivine), prior to degassing and eruption. The new data seem to point to a surprisingly narrow range of water concentrations within each arc, and a fairly uniform global average of about 4 weight percent.

This uniformity implies a process that regulates the water concentration. Plank et al., 2103 suggest two hypotheses.
1) Arc magmas stall at around the same depth (~6 km) beneath the Earth’s surface water is saturated within the melt and melt inclusions form at relatively constant water concentrations. Such uniformity in upper crustal process, however, seems unlikely.

2) Water concentration strongly controls the amount of melting in the mantle, maintaining a general proportion of water to melt: more water = more melting.

The answer will carry important implications on the amount of water that is transferred from the mantle to the surface, and how much magma ultimately gets stuck and never erupts. For example, magmas with more water leads to an earlier crystallization of amphibole, which subsequently lowers the water content of the remaining melt. Lower amounts of water increase the melt viscosity and also raises the melting temperature, bringing on further crystallization. Thus magmas with more water may actually never erupt, but rather succumb to a viscous death. This leads to the counterintuitive idea that plutonic rocks are actually wetter than volcanic rocks. 

Figure 4 from Plank et al. (2013) showing the fairly constant water concentrations of arc magmas. 

Source:

Terry Plank, Katherine A. Kelley, Mindy M. Zimmer, Erik H. Hauri, Paul J. Wallace, Why do mafic arc magmas contain ∼4 wt% water on average?, Earth and Planetary Science Letters, Volume 364, 15 February 2013, Pages 168-179, ISSN 0012-821X, (http://www.sciencedirect.com/science/article/pii/S0012821X1200670X)

Earth's plate tessellation

Fun geologic process of the day: The numbers and sizes of Earth’s tectonic plates are controlled by styles of mantle convection

Earth’s two largest tectonic plates (Pacific and Africa) are on exact opposite sides of the globe (antipodes) and it is perhaps not a coincidence. The numbers and sizes of all of the earth’s plates aren’t random either, but instead are controlled by movements in the mantle. Morra et al. (2013) analyzed the position and size of earth’s plates over the last 200 million years and suggest the systematic tessellation (the gap-free arrangement of tiles, or plates) of Earth’s surface is a consequence of mantle convection styles that alternate between being subduction driven (top-down) and deep mantle driven (bottom-up).

Source: Gabriele Morra, Maria Seton, Leonardo Quevedo, R. Dietmar Müller, Organization of the tectonic plates in the last 200 Myr, Earth and Planetary Science Letters, Volume 373, 1 July 2013, Pages 93-101, ISSN 0012-821X,(http://www.sciencedirect.com/science/article/pii/S0012821X13002021).

Petit spots

Fun geology term for the day: petit spot
 

Hirano et al. (2013) use the term to “petit spot” to describe young (~8 Ma to 50 ka), small (<1 km³) basaltic eruptions on the downgoing plate just before it enters the Japan trench. As the lavas are on the wrong side of the trench for subduction related volcanism, and thousands of km from a divergent margin, the authors suggest some unusual explanations for their presence: bending (flexure) of the downgoing plate just before it arrived to the trench created fractures which allowed asthenospheric melts to rise to the surface. High ⁴⁰Ar/³⁶Ar ratios are attributed to an upper mantle source. The problem of why the asthenosphere melted in the first place is not delved into too much, although they suggest that these petit spots provide evidence for the long-debated hypothesis that the low velocity zone in the asthenosphere is a reservoir of long-lived partial melts.

 

Source: Volcanism in Response to Plate Flexure, 2006, Naoto Hirano, Eiichi Takahashi, Junji Yamamoto, Natsue Abe, Stephanie P. Ingle, Ichiro Kaneoka, Takafumi Hirata, Jun-Ichi Kimura, Teruaki Ishii, Yujiro Ogawa, Shiki Machida, Kiyoshi Suyehiro, Science 313, 1426 (2006); DOI: 10.1126/science.1128235

Unexplained volcanism: Saudi Arabia

The Arabian Peninsula has been diverging from Africa for the past 30 million years, following the arrival of the Afar hot spot. Decompression melting has been providing the lavas that create new oceanic crust in the opening Red Sea, but a suite of volcanoes in western Saudi Arabia that have been erupting for the past 12 million years is probably too far to the east of the divergent margin to be directly related. What then is causing this volcanism? (check out western Saudi Arabia in Google maps to see the impressive black lava surrounded by the vast desert landscape). The Afar hot spot would be a good candidate, but it is also thought to be a bit too far to the south, near the diverging triple junction. Perhaps lateral flow of the hot spot plume is driving the melting, or maybe a separate plume directly beneath Saudi is involved. Bob Duncan and Abdullah Al-Amri (2013) set out to address this question by studying Harrat Lunayyir volcanic field, which last erupted about 1000 years ago and occasionally experiences seismic activity suggesting that it is still active. Their 40Ar/39Ar age determinations show that the field is very young (less than 600 thousand years) with lavas characterized by intraplate geochemistry (perhaps ruling out the divergent margin or other extension related melting). The lack of age progression, along with isotopic considerations, also seems to rule out lateral flow from the Afar plume. More study is necessary to understand the cause of volcanism this region.

Source: Robert A. Duncan, Abdullah M. Al-Amri, Timing and composition of volcanic activity at Harrat Lunayyir, western Saudi Arabia, Journal of Volcanology and Geothermal Research, Volume 260, 15 June 2013, Pages 103-116, ISSN 0377-0273, (http://www.sciencedirect.com/science/article/pii/S0377027313001479).

orogeny and volcanism in Tibet

Fun geologic relationship of the day: orogenic plateaus and volcanic triggers

The processes occurring beneath the world’s great plateaus such as Tibet and the central Andes are difficult to decipher. How the deep crust and mantle lithosphere are behaving, and how they interact with the underlying lithosphere has major implications for how the plateaus grow and why they contain widespread volcanism. In one of the first major studies of volcanism in Tibetan, Turner et al. (1996) interpreted highly potassic lavas to indicate melting of biotite-rich, garnet bearing mantle lithosphere that had been metasomatized (enriched in volatiles and hydrous fluids) during earlier (pre-collisional) periods of subduction. Evidence includes negative Nb-Ta and Ti anomalies, fractionated LREE/HREE, high K contents, and isotopic considerations. Turner et al. (1996) suggest that the hydrated mantle lithosphere most likely melted due to temperature increases related to convective removal of its lower regions, as proposed by England and Houseman (1988, 1989). In this model, the collision between India and Eurasia (beginning ~52 million years ago) thickened the cold, relatively dense lower mantle lithosphere until it detached and was replaced by warm, buoyant asthenosphere. This influx of asthenosphere then raised the plateau, and led to gravitational collapse of its edges. Turner et al. proposed that the newly exposed (undetached) mantle lithosphere heated up and melted, resulting in the plateau volcanism. They further suggest that the age of the lavas (< 13 Ma) puts a time stamp on the removal process, as shown in this figure. 

 Figure 13 of Turner et al. (1996).

The timing of plateau uplift has major implications concerning the evolution of our atmosphere and the life it harbors, as a rising plateau may lead to increased global cooling. However, it is very difficult to pin down the dates of plateau uplift (it does not necessarily correlate with crustal thickening alone) and is the focus of numerous studies.  

An alternatives to the convecting thinning / mantle lithosphere melting model discussed in this post was addressed on this blog earlier, and el volcán tranquilo will, no doubt, return to this controversy repeatedly.  

References

Turner, S., Arnaud, N., Liu, J., Rogers, N., Hawkesworth, C.,Harris, N., Kelley, S., Van Calsteren, P. & Deng, W. (1996). Post-collision, shoshonitic volcanism on the Tibetan plateau: implications for convective thinning of the lithosphere and the source of ocean island basalts. Journal of Petrology 27(1), 45--71.

Molnar, P., P. England, and J. Martinod, Mantle dynamics, the uplift of the Tibetan Plateau, and the Indian monsoon, Reviews of Geophysics, 31, 357-396, 1993.

England, P. C. & Houseman, G. A.; 1988. The mechanics of the Tibetan plateau. Philosophical Transactions of the Royal Society of London A326, 301-319.

England, P. C. & Houseman, G. A., 1989. Extension during continental convergence, with application to the Tibetan Plateau. Journal of Geophysical Research 94, 17561-17579.

hidden plate tectonics

--> Fun geology term for the day - hidden plate tectonics

The Tibetan plateau is the largest topographic feature on Earth and is related to the ongoing collision between India and Eurasia that began some 55 million years ago. Construction of the plateau began earlier with the accretion of ocean island crustal blocks to the Eurasian continent in the Jurassic, although the extreme topography likely wasn’t created until the Cenozoic following the collision with India (see my earlier post about the debate over the timing of uplift). How and when the plateau crust became thickened and elevated is a major debate in geology. Tapponnier et al. (2001) suggest that the collision with India resulted in intracontinental subduction (“hidden plate tectonics”) of mantle lithosphere of the previously accreted terranes.

Image from figure 3 from Tapponier et al. (2001). The authors caution that the proposed continental subduction has not yet been imaged geophysically. 

Tapponier et al. (2001) suggest that the terranes act as coherent blocks, contradicting other ideas of plateau growth that suggest the Tibetan mantle lithosphere behaves more fluidly. They challenge an earlier hypothesis put forth by mainly by Philip England and Gregory Houseman of a “soft Tibet” that involves: a) the entire lithosphere thickening as a viscous sheet, b) eventual removal and sinking of this dense lithosphere into the asthenosphere, c) subsequent buoyant rise, and extension of the  plateau.

Image from figure 9 of Molnar et al. (1993) showing convective removal of the Tibetan lithosphere.

Another simple model of lithospheric removal beneath Tibet. From Harrison et al., 1992

The debate often centers on determining of the timing of plateau thickening vs. uplift and in the interpretation of the Late Miocene to recent faulting that appears to be forcing the plateau crust eastward. Tapponier et al. (2001) suggest that the observed extension by normal faulting due to gravitational collapse is negligible, and that the lateral extension of the plateau is largely occurring along strike slip faults, which are more in line with the continental subduction hypothesis. How the plateau is deforming has strong implications on the causes for the volumetrically minor, but widespread potassic volcanism on the plateau. Tapponnier et al. further argue that the post-collisional volcanism is localized in three different belts that line up well with the continental subduction model, and not so much with wholesale removal of the lithospheric mantle. How continental subduction promotes melting is not addressed in this paper, but later researchers do and will be discussed here shortly.    

Citations: 

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T. M. Harrison, P. Copeland, W. S. F. Kidd, A. Yin, Science 255, 1663 (1992).

Molnar, P., P. England, and J. Martinod, Mantle dynamics, the uplift of the Tibetan Plateau, and the Indian monsoon, Reviews of Geophysics, 31, 357-396, 1993.

Tapponnier, P., Xu, Z.Q., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G., and Yang, J.S., 2001, Oblique stepwise rise and growth of the Tibet plateau: Science, v. 294, p. 1671–1677.

slab suction

-->Fun geology term for the day: slab suction (bonus term: slab penetration).

There are two giant plateaus on our planet: one in Tibet, the other in the central Andes of South America. Despite sharing roughly similar crustal thicknesses and extreme average elevations, the two orogens (mountain building zones) have drastically different plate tectonic settings: continental collision between India and Eurasia in Tibet and oceanic subduction beneath the South American continent. A new hypothesis described by Faccenna et al. (2013), however, suggests that these two mountainous regions are ultimately formed by the same process: slab suction. The idea begins with the observation that the two plateaus are situated above regions of cold dense material that are thought to be sinking into the deep mantle. These two sinking regions are said to be the downward return of giant convection cells involving the entire (upper and lower) mantle. Both of these convection cells share the large, upwelling column (“superswell)” of hot, deep mantle beneath Africa. 

The downwelling beneath each orogen creates a suction that generates forces on the overriding plates dragging India into Eurasia and the Nazca plate into South America. Without this suction, Faccenna et al. (2013) argue that the other forces driving plate movement (slab pull, ridge push, and plume push) would be insufficient to support the large-scale mountain building. The non-suction forces may be more important in driving upper mantle convection cells and smaller-scale mountain building. The full mantle convection cells may ultimately be driven by the suction generated by deep-sinking subducted lithospheric plates that accumulated at the base of the lower mantle, before reaching a critical mass and penetrating the lower mantle at ~ 65-55 Ma (Tibet) and ~45 Ma (Andes).

Slab suction dragging plates together. From figure 7. Faccenna et al. 2013
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Citation: Faccenna, C., T. W. Becker, C. P. Conrad, and L. Husson, 2013, Mountain building and mantle dynamics, Tectonics, 32, 80–93, doi:10.1029/2012TC003176.

Tibetan uplift controversy

Fun geology controversy of the day - did uplift of the Tibetan plateau predate continental collision?

Most textbooks will tell you that the Himalayas and the Tibetan plateau, which sits greater than 5 km above sea level, began to form some 50 million years ago when the Indian continent began its slow but persistent collision with Eurasia. The uplift of the plateau is thought to be a major player in the long-term global cooling throughout most of the Cenozoic (66 – 0 million years ago). Quite a few geologists, however, want to throw a wrench in this nice little paradigm. They suggest that the plateau was already elevated in the early Cretaceous, when it was much warmer and dinosaurs were still running around doing their thing. England and Searle (1987) suggested that this early uplift could have occurred in a non-collisional, subduction zone setting similar to that responsible for the rise of the central Andean plateau in South America. Murphy et al. (1997) used geologic mapping and chronological constraints to suggest that the majority of Tibetan uplift occurred during the Cretaceous due to collision of an ocean island tectonic block with the continent. A recent study by Hetzel et al. (2011), however, suggests that these earlier studies document evidence only for Cretaceous crustal shortening (squeezing), and not necessarily a sustained elevated plateau. They interpret thermochronologic and cosmogenic nuclide data to indicate that a low-elevation fluvial system was established by 50 million years ago, when the Indian collision began. They incorporate the early-rise model into a comprehensive model where the Tibetan plateau rose slowly in the Cretaceous, but erosion kept elevations relatively low. They argue that the majority of Tibetan uplift occurred rapidly to its current elevation between 50 and 35 million years ago following continental collision, just like the books say it happened.

 

From figure 2. Heltzel et al., 2013

Sources

England, P., and Searle, M., 1986, The Cretaceous-Tertiary deformation of the Lhasa block and its implications for crustal thickening in Tibet: Tectonics, v. 5, p. 1–14.

Hetzel, R., Dunkl, I., Haider, V., Strobl, M., von Eynatten, H., Ding, L., and Frei, D., 2011, Peneplain formation in southern Tibet predates the India-Asia collision and plateau uplift: Geology, v. 39, p. 983–986.

Murphy, M.A., Yin, A., Harrison, T.M., Dürr, S.B., Chen, Z., and four others, 1997, Did the Indo-Asian collision alone create the Tibetan plateau? Geology, v. 25 (8), p. 719-722. 

crystal reaming

Fun geology term of the day: crystal reaming

Recent research at the Aucanquilcha volcanic complex in northern Chile supports the idea that many volcanic eruptions do not simply represent a single batch of magma that rises to the surface, but a mixture of components that have stalled for various amounts of time in the Earth’s crust. Barry A. Walker Jr. and coauthors from Oregon State University introduce the term crystal reaming to illustrate this process. Crystal reaming refers to the process where the latest batch of hot, mafic magma incorporates numerous crystals from previous magmatic episodes as it passes its way through the crust to the surface. In this way, many plutonic bodies may be accumulated over time as magmas periodically pass through, carrying away some crystals, and leaving others behind. 

From figure 14. Walker et al. (2012).

 

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Source: Walker, B.A, Klemetti, E.W., Grunder, A.L.., Dilles, J.H., Tepley, F.J., Giles, D, 2012, Crystal reaming during the assembly, maturation, and waning of an eleven-million-year crustal magma cycle: thermobarometry of the Aucanquilcha Volcanic Cluster, Contributions to Mineralogy and Petrology, v. 165 (4), p. 663-682. http://link.springer.com/article/10.1007%2Fs00410-012-0829-2

divergent double subduction

Fun geology term of the day: divergent double subduction

After writing my previous post about double subduction, I did a quick search for the term to see where else it is used. I found this cool article published in Geology in 1997. Soesoo et al. (1997) describe a process called divergent double subduction (see figure below) where subduction of an oceanic plate occurs on both sides until the two overriding plates meet and the remaining plate sinks into the mantle. Soesoo and coauthors argue that such a process will produce quite a bit of volcanism in at least two processes: 1) decompression melting of asthenosphere as it rises to take the place of the sinking plate, and 2) flux melting driven by dehydration of the sinking plate to greater pressures. The remnants of divergent double subduction is suggested to be found in the abundant Paleozoic magmatism of the Lachlan fold belt in southeastern Australia.

 Image from figure 1 of Soesoo et al., 1997

Citation: Soesoo, I., Bons, P., Gray, D.,  and Foster, D., 1997, Divergent double subduction: Tectonic and petrologic consequences, Geology, v. 25, p. 755-758

double subduction

Fun geology term of the day: double subduction

Plate tectonics get weird in the western pacific. They get weird in the eastern pacific too. Pretty much everywhere, actually. But the western Pacific has its own type of weird. Double subduction for starters. Nakamura and Iwamori (2013) use the term double subduction to describe where one plate is subducting beneath another subducting plate. In this case beneath the south of Japan, the Pacific plate is subducting from the east, while the Philippine plate is subducting from the southeast and overlaps the Pacific plate (see map below). To add further confusion, the Pacific plate is also subducting beneath the Philippine plate further to the east. As you might guess, this weird configuration results in some weird volcanoes. Nakamura and Iwamori (Nakamura and Iwamori, 2013) investigate a subset of these lavas that contain the controversial adakite geochemical signature. Adakites once described magmas that were derived from melting of the subducting slab, but now they describe any magma that has the composition originally attributed to melts of the subducting slab. In this case, Nakamura and Iwamori (2013) eschew plate melting and argue for more-or-less normal subduction-induced melting of the asthenosphere, but with a heightened fluid flux from the two subducting slabs and deep residual garnet. 

Image from figure 1 of Nakamura and Iwamori (2013). Note teeth denote subduction plate boundary with the arrows pointing in the direction of the downgoing slab. 

Source:  Nakamura, H., and Iwamori, H., 2013, Generation of adakites in a cold subduction zone due to double subducting plates, Contributions to Mineralogy and Petrology, v. 165 (6), p. 1107-1134. 

hello world

Hey there world. Just getting started here. That's me and Mt. Shasta in northern California.