Time Scale Age of Oldest Rocks: 4.0 Ga 2.5 Ga EoArchean NeoArchean

In the Beginning… Global distribution of Archean rocks in modern continents. Known (red), suspected (pink). Areas with rocks or zircons older than 3.6 billion years are labeled by name. Stephen Hui Museum Univ. of Hong Kong

Acasta Gneisses: Oldest Known Rocks Age of the Acasta gneiss, zircon dated at 4.03 Ga. Photo: Echos of Earth; Sue Baugh & Lynn Martinelli

Remnant of Hadean Crust

Nuvvaugittuq Amphibolites

Nuvvaugittuq 146 Sm/ 142 Nd Isochron O’Neil et al. (2008) O’Neil et al. (2012) and Roth et al. (2013) reported 142Nd deficits in rocks form the Nuvvaugittuq greenstone belt. In a subset of these rocks, the cummingtonite amphibolites, 142 Nd/ 144 Nd correlates with Sm/Nd forming a psuedo- isochron, implying 146 Sm/ 144 Sm of , which in turn implies rocks from 97 Ma after start of solar system (i.e., at 4.36 Ga). 147 Sm/ 143 Nd isochrons and zircon ages are ≤3.8 Ga. Highly controversial, but most likely explanation is the correlation is a mixing line formed by assimilation of pre-existing Hadean crust by Eoarchean magmas. Regardless, provides evidence of Hadean crust.

Jack Hills Zircons Photo: Echos of Earth; Sue Baugh & Lynn Martinelli

Jack Hills Zircons The Jack Hills zircons occur in a 3 Ga meta-sedimentary conglomerate, surround by the Narryer gneisses ( Ga). Most zircons Ga but a few are >4Ga. All have complex histories. Hf isotope ratios measured on these zircons have negative ε Hf(t), indicating crustal source. Hf isotope evolution projects back as far as 4.46 Ga (depending on what Lu/Hf you assume).

Other Ancient Zircons Some zircons in the Acasta gneiss have cores as old as 4.2 Ga (not shown). Others project back suggesting other areas of Hadean crust.

182 W Anomalies 182 W anomalies have now been reported in a number of Archean rocks. 182 W anomalies initially interpreted as material into which a late accretionary (undifferentiated chondritic and therefore siderophile element-rich) material had not been mixed in. However, no negative anomalies (indicative of the veneer) have been found. Samples with +ive ε W have normal platinum-group element abundances. Now seems this phenomenon, although indicative of early Earth heterogeneity, is not related to a late accretionary veneer.

142 Nd Anomalies 142Nd, both positive and negative, anomalies have been reported in a number of Archean rocks. Indicative of early terrestrial differentiation and formation of Hadean crust. Anomalies disappear by the end of the Archean. Debate about the nature of mantle convection in the Archean: Plate tectonics? Stagnant-lid tectonics?

Pb Isotope Evidence of Early Earth Differentiation Data from Neoarchean Abitibi Greenstone belt of 2.7 Ga age plot near a ‘paleo’-2.7 Ga Geochron – suggesting differentiation event in very early Earth history.

Subsequent Growth of Continents How do we date continents? Do radiometric ages necessarily record formation of continental crust, or simply resetting of chronometers? Work of Hurley and Rand (1969) based on Rb-Sr dating underestimated age of North America.

Rates of Continent Growth

Mechanisms of Crustal Growth Subduction zone volcanism Accretion of island arcs Plume-related volcanism Flood basalts Accretion of Oceanic Plateaus Underplating Rift-related volcanism

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... P.A. Cawood et al. Geological Society of America Bulletin 2012;125:14-32 ©2013 by Geological Society of America

Super Continents & Continental Growth Zircon age populations appear to correlate with supercontinent formation. Some (Condie, Hawkesworth) argue this measure preservation of juvenile crust in orogens, rather than variable production rates. Cawood (2013) et al GSA Bull.

The volumes of magma generated (blue line), and their likely preservation potential (red line) based on relations outlined in Figure 8, vary through the three stages associated with the convergence, assembly, and breakup of a supercontinent. P.A. Cawood et al. Geological Society of America Bulletin 2012;125:14-32 ©2013 by Geological Society of America

(A) Distribution of Hf model ages in 1376 detrital and inherited zircons sampled worldwide, from which O isotopes have been measured (from Dhuime et al., 2012, and references therein). P.A. Cawood et al. Geological Society of America Bulletin 2012;125:14-32 ©2013 by Geological Society of America

Nd Model Ages & Evolution of Southwestern US

Vishnu Schist of Grand Canyon Photo: Echos of Earth; Sue Baugh & Lynn Martinelli

Crustal Residence Times Bennett and DePaolo recognized three Precambrian provinces in the southwestern US based on depleted mantle model ages. Actual rocks are often much younger. Pattern suggests assembly from inside out – but it’s a bit more complicated.

Initial Epsilons Examining initial epsilon values, its likely that many magmas were mixtures of pre-existing crust and mantle-derived magmas. All three provinces probably formed between 1.8 and 1.65 Ga, but incorporating variable amounts of pre- existing crust. Most recent magmatism appears to simply remelt much older crust.

Hf in Lachlan Fold Belt zircons Hf and O isotopes in two granite suites from SE Australia suggest mixing between sedimentary protolith and mantle-derived magma in one case and lower crustal gabbros in the other.

Age Spectra of Lachlan Zircons Age spectra of the detrital zircons from Lachlan Fold Belt metasediments dominated by peaks at 450–600 Ma and 0.9–1.2 Ga. Hf model ages of zircons much older, with peaks ~1.7–1.9 and ~2.9–3.1 Ga. The most mantle- like O isotope ratios tend to have the older ages Zircons with the lower δ 18 O, and older Hf model ages represent reworking deeper crustal material that had never interacted with water at low temperatures.

Isotopic Composition of the Continental Crust

Approaches Broad Sampling Composites Sediments Shale Composites are hybrid Rivers Including their sediments

Sr & Nd Isotope Composition of Rivers & Sediments

Lower Crustal Terrains Lower Continental Crust Sediments & Rivers sample only the upper most crust Doesn’t representatively sample lower crust. Lower crust samples come from two sources: tectonically emplaced terrains and xenoliths OBOB

Pb Isotopes of Lower Crustal Terrains

Pb Isotopes in Lower Crustal Xenoliths

Terrestrial Pb Mass Balance

Pb Isotope Constraints on Crustal Th/U

Sm/Nd in the Continental Crust Sm/Nd in the continental crust shows a fairly restricted distribution. 143 Nd/ 144 Nd shows a wider distribution – a result of varying crustal age.

143 Nd/ 144 Nd and Sm/Nd in the Crust

Estimating Crustal Growth from Crustal Residence Times of Sediments In principle, examining the relationship between stratigraphic and Nd crustal residence times should tell us whether the rate of continental growth is increasing or decreasing. Cannibalistic nature of the sedimentary cycle (sediments are more likely to be eroded than other rocks) complicates things (line B). Could be consistent with Armstrong constant continental mass with decreasing recycling rate.

Isotope Systematics of Mixing We need to understand this before turning to Subduction Zones

Mixing Plots with 2 Ratios A general mixing equation is: If end members are desig­nated 1 and 2 and have ratios (q/Q ) 1 and (p/P) 1, and (q/Q ) 2 and (p/P) 2 respectively, then The curvature of the mixing line will depend on the ratio r:

Mixing Plots of Ratios In most instances, the denominator in an isotope ratio is linearly proportional to the elemental concentration Consequently r is related to the ratio of concentrations in the end- members: Plot of an isotope ratio vs. inverse of concentration will be linear for mixing. Note that an isochron is a ratio-ratio plot with the same denominator in both ratios. Hence mixing curves on such plots are linear. Plotting ratio against inverse concentration can distinguish isochrons from mixing lines.

Pb isotopes in subduction zone volcanics Pb isotope ratios plot on steeper Pb-Pb slopes than oceanic basalts and overlap sedimentary fields. Dick Armstrong noted this in 1971 and argued that subducted sediments contributed to subduction zone magmas.

Island Arcs and Adjacent Sediments

Sr-Nd Isotope Systematics of Subduction Zone Magmas

Lesser Antilles

Isotopic Composition of Sediments adjacent Lesser Antilles

10 Be in arc lavas 10 Be (t 1/2 = 1.6 Ma) produced by spallation in the atmosphere, washed out by rain, adsorbed on particles. 10 Be found in several arc magmas, but not in other kinds of magmas. Clear evidence of sediment subduction Not present in arcs with accretionary prisms such as the Lesser Antilles and Sunda.

Th Isotopes in Subduction Zones Subduction zone magmas typically have U-Th isotope systematics indicating U-enrichment over Th. Inconsistent with partition coefficients. Likely a result of preferential transport of soluble U 6+ into magma genesis zone from (somewhat oxidized) subducting slab.

Subduction Zone Mass Balance