1. The Earth IV: Subduction Zones
Lecture 44

2. How were the continents created?
So far, we have talked about the when, but not the how.
Today, there are 3 main mechanisms by which new crust is created:
Rifting (e.g., African and Rio Grande Rifts) and divergent plate boundaries (North Atlantic Tertiary Province is an example).
Flood basalt events and associated underplating by basaltic magma associated with mantle plumes (and accretion of oceanic plateaus)- the Deccan of India is an example..
Subduction zones, for example the Andes, the Pacific Northwest, the Alaska Peninsula and the Aleutians.
Which is most important?
Of course, geochemistry provides the essential clue.

3. Schematic cross section of convergent, collisional, and extensional plate boundaries associated with supercontinent cycle showing estimated amounts (in km3 yr−1) of continental addition (numbers in blue above Earth surface) and removal (numbers in red below...
Schematic cross section of convergent, collisional, and extensional plate boundaries associated with supercontinent cycle showing estimated amounts (in km3 yr−1) of continental addition (numbers in blue above Earth surface) and removal (numbers in red below surface). Data are from Scholl and von Huene (2007, 2009). The volume of continental crust added through time via juvenile magma addition is approximately compensated by the return of continental and island-arc crust to the mantle, implying that there is no net growth of continental crust at the present day. Data compilations from Clift et al. (2009) and Stern (2011) calculated values of crustal recycling that are greater than the volume of juvenile crustal addition, requiring a decrease in present-day continental volumes. MOR—mid-ocean ridge.
P.A. Cawood et al. Geological Society of America Bulletin 2012;125:14-32

4. The Clue Negative Nb-Ta anomalies in continent
Concentration/Primitive Mantle
Negative Nb-Ta anomalies in continent
crust also present in arc volcanics
From Rudnick (2017)

5. Subduction Zones and Subduction Zone Processes
From Plank (2017)

6. Major Elements in Arc Magmas
Magmas found in island arcs & continental margins are predominantly andesitic.
It is unlikely that andesite is the principle magma produced in arcs. The lower parts of arc volcanic edifices may be basaltic.
Andesite cannot be produced by partial melting of the mantle, except at shallow depth under high water pressure. Most arcs sit about 100 km above the Benioff zone, and magmas may be generated close to this depth.
A safer bet is that the primary magma is actually basaltic, of which andesites are fractional crystallization products.
In major element composition, island arc volcanics (IAV) are not much different from other volcanic rocks. Compared with MORB, the major difference is perhaps simply that siliceous compositions are much more common among the island arc volcanics. Most IAV are silica-saturated or oversaturated; silica undersaturated magmas (alkali basalts) are rare.

7. Tholeiitic & Calc-Alkaline Trends
Two principle evolutionary trends: tholeiitic and calc-alkaline:
Reflecting differences in pressure, water content, and oxidation state.
Early crystallization of oxides limits iron enrichment in the calc-alkaline trend.
Suppression of plagioclase crystallization also plays a role.
Numerous factors may contribute to this:
Extent and depth of melting
Thickness of the arc crust
Water content of parent

8. Controls on Magma Composition
Plank and Langmuir argued that crustal thickness determines the height of the mantle column available for melting.
Most island arc volcanoes are located above the point where the subducting lithosphere reaches a depth of 100–120 km. This suggests that melting begins at a relatively constant depth in all island arcs.
If this is so, then the distance over which mantle can rise and undergo decompressional melting will be less if the arc crust is thick, leading to smaller extents of melting beneath arcs with thick crust, and higher Na6.0 and Ca6.0 in the parental magmas.

9. Rare Earth in IAV
Island arc volcanics (IAV) are typically, but not uniformly, somewhat LRE-enriched.
Some show relative middle rare earth depletion, probably a result of amphibole fractionation.
Negative Ce-anomalies occur in some lavas. Ce, normally in the III valence state, can be in the IV valence state under oxidizing conditions at the Earth’s surface. The Ce-anomalies suggest an inherited sedimentary component in these lavas.

10. Sedimentary Component
Plank and Langmuir found they could relate the degree of enrichment of most incompatible elements to the sediment flux of that element. For example, the Ba/Na and Th/Na ratios (after correction for fractional crystallization) correlate strongly with the La and Th sediment fluxes.
Different arcs are enriched to different degrees in these elements: for example, the Lesser Antilles arc has moderate Th/Na ratios but low Ba/Na ratios. The difference appears to be due to the difference in the sediment flux.

11. Sr & Nd Isotope Ratios
Sr and Nd isotope ratios span a large range, overlapping a bit with MORB and extensively with OIB. There is, however, a tendency to plot to higher 87Sr/86Sr for a given εNd than the ‘mantle array. This suggests an inherited component of altered oceanic crust.

12. Pb Isotope Ratios
Pb isotope ratios in IAV tend to define arrays that overlap MORB at one end and marine sediments at the other - indicating an inherited sedimentary component.

13. Genesis of Subduction Zone Magmas
Arc magmas are produced primarily within the ‘mantle wedge’ overlying the subducting slab. The evidence for this is as follows:
Primary arc magmas differ only slightly in major element chemistry from oceanic basalts (the typical andesitic composition results from fractional crystallization). Thus IAV are partial melts of peridotite rather than subducted basalt or sediment.
Radiogenic isotopic and trace element systematics generally allow only a small fraction of sediment (generally a few percent or less) to be present in arc magma sources. Relatively high 3He/4He ratios in arc lavas confirm a mantle source.
REE patterns of IAV are consistent with partial melting of peridotite, not of eclogite (high pressure basalt). Because the heavy rare earths partition strongly into garnet, melts of eclogite should show steep rare earth patterns, with low concentrations of the heavy rare earths. (Rare high-magnesium andesites, or “adakites” with steep rare earth patterns may represent exceptions to this rule.) It is possible that such “slab melts” were more common several billion years ago.

14. Initial Dewatering
 As oceanic material enters the subduction zone,  increasing P and T produces compaction and dehydration of the sediments. This and clay minerals transformations to illite Li, B, Cs and Rb are liberated into a water-rich fluid that emerges in the forearc.
From Plank (2017)

15. Fluids to Melts
At higher temperatures, fluids evolve from breakdown of lawsonite, amphibole, and mica are more solute rich, containing Si, Al, and alkalis. If flushed with water-rich fluids, the top of the plate may melt at 700 C and 3.
At pressures higher than 5 GPa, sediments no longer melt at a discrete solidus but instead evolve “supercritical” liquids that range from from dilute aqueous liquids to water-rich silicate melts.
fluid
melt
From Plank (2017)

16. Melting and the Role of Water
This brings us to what is perhaps the most fundamental question: why does melting occur at all in an area where cold lithosphere is descending?
The answer is water. Water lowers the solidus of rock and leads to enhanced melting at any given temperature compared with dry conditions; water released by the subducting slab migrates into the overlying hotter mantle wedge where it induces melting.
Under water-saturated conditions, the peridotite solidus is depressed by hundreds of degrees compared with the “dry solidus”. At 1.5 GPa (50 km depth), peridotite begins to melt at over 400˚C cooler temperatures than under “dry” conditions. The effect is even larger at higher pressure.
At pressures above ~2 GPa, ilmenite & chlorite are stable at and above the solidus. Nb and Ta strongly partition into ilmenite. Thus the characteristic Nb-Ta depletion of island arc lavas and the continental crust may be due to residual ilmenite present during the initial states of melting deep within the arc.
Curved dashed lines are the chlorite + ilmenite- and amphibole-out curves. Straight dashed lines illustrate the progressive replacement of spinel with garnet. The broad stippled arrow shows the path the melts take in T-P space as they rise through the mantle wedge.

17. Role of Dehydration
If IAV magmas are not melts of the slab, how do they acquire the geochemical signature or “flavor” of subducting oceanic crust and sediment?
Dehydration and migration of the evolved hydrous fluid has long been suspected as the primary means by which the subducting slab influences the composition of IAV magmas.
Ba/La ratios in Marianas arc lavas plot systematically above a mixing line between MORB and sediment subducting beneath the arc (ODP Hole 801) on a plot of Ba/La vs. La/Sm. The same is true of Pb/Ce vs. Th/Nb ratios.
Elliott et al. (1997) concluded that both a hydrous fluid and a silicate melt were involved in transport of the sediment component. They proposed that the melt was a hydrous partial melt of the subducted sediments. Melting is necessary to account for the fractionation between Th and Nb, neither of which is particularly soluble in aqueous fluids. Furthermore, lavas with the highest Th/Nb also show the greatest light rare earth enrichment. Subsequent dehydration and melting experiments confirmed the need for melting to transport elements such as Th into the magma genesis zone of the mantle wedge.

18. Magma Genesis in Subduction Zones
Subducting sediment and hydrothermally altered oceanic crust carry water and incompatible elements into the mantle.
Compression during the early phases of subduction drives off much of the unbound water occupying pores and veins in the subduction lithosphere. This water sometimes emerges as “seeps” in accretionary prisms.
The subducting lithosphere is metamorphosed as it encounters higher T and P, with water-rich minerals progressively replaced by water-poor ones and anhydrous ones.
The water released in these reactions rises into the overlying mantle wedge. The wedge immediately above the subducting slab has an inverted thermal gradient. 10 km above the slab, temperatures approach 1000˚C, well above the wet solidus and melting begins. These initial melts may contain as much as 28% water, but as they rise, continued melting progressively dilutes the water content.
Work over the past couple of decades has produced evidence directly relating water content to melting in subduction zones: the smallest extents of melting (about 5%) occur in H2O-poor sources and give rise to incompatible element-rich basalts, while the highest extents (over 20%) give rise to H2O-rich and incompatible element-poor basalts.

19. Building Continents

20. Continents & Subduction Zones
Concentration/Primitive Mantle
From Rudnick (2017)

21. Refining the Continental Crust
Conundrum: nearly all mantle-derived magmas are mafic (basaltic) and are poorer in SiO2 and generally richer in MgO and FeO than the continental crust.
If the continental crust has been produced by partial melting of the mantle, why then is it not basaltic in composition, as is the oceanic crust? Four possibilities:
Magmas have already evolved, by fractional crystallization to andesitic composition by the time they cross the crust–mantle boundary (the Moho). The complementary mafic cumulates are left behind in the upper mantle. This idea is not supported by observation.
Lower crustal floundering, or delamination, may occur when continental crust is thickened in compressional environments, such as convergent plate boundaries. when the lower crust is transformed into eclogite. This process would preferentially remove the mafic part of the crust, leaving a residual crust that consequently becomes more silicic. A related process is subduction erosion. Lower crust is more likely to be removed by subduction erosion than upper crust.
Preferential loss of Mg and Ca from continents by weathering and erosion. Mg is then taken up by the oceanic crust during hydrothermal alteration; Ca is precipitated as carbonate sediment. Both are returned to the mantle by subduction.
Under hotter conditions of the Archean, melting of subducting oceanic crust may have been much more common, giving rise to silicic melting, particularly the trondhjemite, tonalite, and granodiorite (TTG) suites common to the Archean. However, Taylor and McLennan’s estimate of Archean crustal composition is slightly more mafic that their estimate of present composition, which is inconsistent with this idea. ✔ ✓