Growth of the Continental Crust Lecture 48

Age of the Crust The oceanic crust is ephemeral; its mean age is 60 Ma and, with the exception of possible Permian age crust preserved in the Eastern Mediterranean, it is nowhere older than about 167Ma. In contrast, the continental crust is much older. How old? o When did production of continental crust begin? o Has its production been constant through time? o Is continental crust destroyed?

The Acasta Gneisses The oldest crustal rocks, Acasta gneisses in Canada’s Northwest Territory have U-Pb zircon ages of 3.98 Ga. These oldest rocks define the end of the Hadean and beginning of the Archean eons. One zircon from the Acasta gneisses has a 4.20±0.06 Ga core surrounded by a rim with an age of 3.8 to 3.9 Ga indicating these gneisses formed from even older, Hadean crustal protoliths.

Jack Hills Zircons Even older zircon cores, with ages of up to 4.4 Ga, have been found in mid- to late-Archean quartzite (metamorphosed conglomerate) from the Jack Hills of western Australia. The meta­sedimentary conglomerate itself is thought to be about 3 billion years old. Jack Hills

Jack Hills Zircons The zircons have REE patterns indicating that they originally crystallized in a granitic magma. Furthermore, the zircons have δ 18 O that varies from +4.8 to +8, indicating that magma interacted with water as it cooled.

Nuvvuagittuq Greenstone Belt An apparent 146 Sm- 142 Nd isochron suggests an age of 4388 Ma for rocks mafic amphibolites from the Nuvvuagittuq Greenstone Belt of Labrador. However, 147 Sm- 143 Nd and U- Pb zircon ages are only 3.8 Ga. Whether this is the age of these rocks or simply a mixing line when an Eoarchean magma assimilated Hadean crust is highly controversial. Either way, however, these data provide evidence of very early crust and early differentiation of the Earth.

Hadean Crust It is clear now that the process of forming continental crust began in the Hadean (the time prior to 4 Ga from which no rocks survive). Essentially none of this Hadean crust survives. Indeed, even very little early Archean crust survives. How much Hadean/Early Archean crust was created is very much debated.

Growth through time The rate of crustal growth through time is still not known. Armstrong argued the the mass of continental crust has been virtually constant since the Hadean(curve E) because the rate of creation has been matched by the rate of destruction. This idea cannot be ruled out. The idea of young crust (curve A - Hurley & Rand) can be ruled out. Studies of detrital zircons show that surviving crust has been produced in pulses, perhaps related to cycles of supercontinent construction and breakup.

Sm-Nd Model Ages and Crustal Growth Because there is little fractionation of Sm from Nd in the crust, Sm-Nd model ages provide a means of “seeing through” metamorphic ages and deducing the time regions of the continents were first created.

Growth of the Western US Bennett and DePaolo used Sm-Nd model ages to define broad regions of crust creation in the western US. Ages become progressively younger away from the Wyoming craton, suggesting the crust was built progressively outward.

Growth of the Western US ε Nd indicates the initial continental blocks themselves were mixtures of mantle- derived magmas and older crustal material. Young granitic plutons in the southwestern US have been produced by remelting of the older continental protoliths.

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: o Rifting (e.g., African and Rio Grande Rifts) and divergent plate boundaries (North Atlantic Tertiary Province is an example). o Flood basalt events and associated underplating by basaltic magma associated with mantle plumes (and accretion of oceanic plateaus). o Subduction zones, for example the Andes, the Pacific Northwest, the Alaska Peninsula and the Aleutians. Which is most important? o Of course, geochemistry provides the essential clue.

The Clue

Subduction Zones and Subduction Zone Processes

There are clear similarities between the continental crust and the island arc lava: both exhibit incompatible element enrichment, negative Nb-Ta anomalies, and positive Pb anomalies. It is also true that at present, most new additions to crust occur in subduction zones. Let’s consider the geochemistry of subduction zone volcanism in a bit more detail.

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.

Two Magma Series Two principal magma series are recognized, one called tholeiitic, the other called calc-alkaline. There are two principal differences between these rock series. o First, tholeiites differentiate initially toward higher Fe and tend to maintain higher Fe/Mg than the calc- alkaline lavas. This reflects the suppression of plagioclase crystallization as a result of (1) higher pressure and (2) higher water content of the magma. o The second difference is that calc-alkaline magmas are richer in alkalis (K 2 O and Na 2 O) than tholeiites. Indeed, the calc-alkaline magmas are defined as those that have Na 2 O + K 2 O ≈ CaO, whereas tholeiites have Na 2 O + K 2 O < CaO. Kay et al. found that for the Aleutians occurrence of these 2 series related to tectonic environment. Tholeiites occur in extensional environments within the arc where magmas ascend relatively rapidly and undergo fractional crystallization at low pressure. Calc- alkaline lavas tend to occur in compressional environments where they cannot so readily ascend, and undergo crystallization at greater depth. Fe MgAlkalis: K 2 O+Na 2 O

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 Na 6.0 and Ca 6.0 in the parental magmas.

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.

Alkali, Alkaline Earth Enrichment Another feature of IAV is enrichment in the alkali and alkaline earth trace elements, Rb, Cs, Sr, and Ba. This is apparent in the Ba/La vs La/Sm plot.

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 87 Sr/ 86 Sr for a given ε Nd than the ‘mantle array. This suggests an inherited component of altered oceanic crust.

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.