P. Jonathan Patchett

The decay constant of 176Lu determined from Lu-Hf and U-Pb isotope systematics of terrestrial Precambrian high-temperature mafic intrusions

Abstract At present, there is large uncertainty in the decay constant of 176Lu needed for Lu–Hf isotopic studies. We have determined λ176Lu by cross-calibration of U–Pb and Lu–Hf isotopic systems on mineral fractions from the Proterozoic Karlshamn and Sorkka dolerites in Sweden and Finland. The dolerites crystallized at shallow depths from homogeneous, high-temperature magmas, carry olivine that is nearly 100% unaltered, and show no signs of post-magmatic isotopic disturbance. The Lu and Hf isotopic compositions of plagioclase, olivine, pyroxene, apatite, ilmenite and baddeleyite were determined by multicollector-inductively coupled plasma mass spectrometry (MC-ICPMS). Calibrating the Lu–Hf results against baddeleyite U–Pb dates of 954.1±1.2 and 1256.2±1.4 Ma for the dolerites yields a mean λ176Lu of 1.867±0.008×10−11 year−1. The pristine character of the rocks and the agreement of the λ176Lu values with those from other terrestrial data sets [E. Scherer et al., Science 293 (2001) 683–687] suggest that the true value of λ176Lu lies between 1.86 and 1.87×10−11 year−1. Calibration experiments on extraterrestrial samples give significantly higher (4–6%) values, a discrepancy that may be due to plotting of non-cogenetic samples on the same Lu–Hf isochron diagram, or may have other, as yet undetermined, causes. The result of this study also indicates that the Lu–Hf method is capable for dating the crystallization of mafic rocks. The high 176Lu/177Hf ratio in apatite suggests that intrusive ages can be determined at a precision of a few million years or better.

Behavior of hafnium and neodymium isotopes in the crust: Constraints from Precambrian crustally derived granites

Due to the large partition coefficients of garnet for Lu compared to Sm, Nd, and Hf, garnet residual from crustal melting events has a large potential for retaining Lu and, over time, producing high 176Hf177Hf reservoirs in the lower crust. Therefore, melts derived from such garnet-bearing residual assemblages may be high in 176Hf177Hf relative to 143Nd144Nd. In order to determine the extent of potential hafnium isotopic heterogeneities in the lower crust from residual garnet, and to detail coupled HfNd isotopic behavior in older crustal rocks in general, we have determined the hafnium and neodymium isotopic compositions of thirty-two Precambrian granites and rhyolites from diverse suites of crustally derived magmatic suites. The majority of these rocks are known, from major-element and neodymium isotopic evidence, to have been derived from older (>300 m.y.) crust. Modeling of LuHf and SmNd partitioning during melting events in the lower crust indicates that anomalously high 176Hf177Hf compositions should develop within 300–400 m.y. in residual assemblages provided melting was at least 25–30% and 10% or more garnet was left in the residue. In spite of the diverse older crustal sources for the granites and rhyolites in this study, and the potential for garnet to be present in their source regions, none of the granitoids have anomalously high initial 176Hf177Hf compositions: all samples (with one exception) have initial Hf and Nd compositions that plot within a ±8 ϵHf unit wide band of the reference line (ϵHf = 2ϵNd + 2) for juvenile crust. The absence of high initial 176Hf177Hf compositions in these granites and rhyolites most likely implies that either (1) garnet is not a common residual phase in the lower crust or (2) garnet-bearing restite is not easily incorporated into later melts or (3) not enough time was available to develop anomalous Hf compositions.

Nd isotopic ages of crust formation and metamorphism in the Precambrian of eastern and southern Mexico

Sm-Nd ages for garnets in the three Precambrian exposures of eastern and southern Mexico demonstrate that they belong to the Grenville tectonothermal event. The Sm-Nd garnet ages of 0.95 Ga for the Oaxacan Complex and 0.90 Ga for the Huiznopala Gneiss, Molango and the Novillo Gneiss, Ciudad Victoria, are postdated 75 Ma by Rb-Sr ages on biotites. Both sets of data document a cooling history following Grenville metamorphism at or before 1.0 Ga ago. Our garnet data are consistent with a blocking temperature for Sm-Nd in that mineral around 600° C suggested by Humphries and Cliff (1982).The three Precambrian occurrences have Nd chemical ages of separation from depleted mantle (TDM) grouped in the range 1.40–1.60 Ga. This may result from derivation of the rocks from actual crustal protoliths which had been separated from the mantle 0.5 Ga before the Grenville Orogeny. It is much more likely, however, that crustal materials of 1.7 Ga or older age were mixed with mantle-derived products during Grenville events to produce intermediate TDM ages andɛNd values around zero 1.0 Ga ago.

The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes

Neodymium (Nd) isotopes from the Himalayan fold-thrust belt and its associated foreland basin deposits are useful for distinguishing between Himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen. Neodymium isotopic data from the Himalayan fold-thrust belt in Nepal (n=35) reveal that the Lesser Himalayan zone consistently has a more negative ϵNd(0) value than the Greater and Tibetan Himalayan zones. Our data show the average ϵNd(0) value in the Lesser Himalayan zone is −21.5, whereas the Greater and Tibetan Himalayan zones have an average ϵNd(0) value of −16. These consistently distinct values throughout Nepal enable the use of Nd isotopes as a technique for distinguishing between Lesser Himalayan zone and Greater Himalayan zone rock. The less negative ϵNd(0) values of the Greater Himalayan rocks support the idea that the Greater Himalayan zone is not Indian basement, but rather a terrane that accreted onto India during Early Paleozoic time. Neodymium isotopic data from Eocene through Pliocene foreland basin deposits (n=34) show that sediment provenance has been dominated by Greater and Tibetan Himalayan detritus across Nepal. The ϵNd(T) values in the synorogenic rocks in western and central Nepal generally show an up-section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones. At ∼10 Ma in Khutia Khola and ∼11 Ma in Surai Khola, a shift in ϵNd(T) values from −16 to −18 marks the erosional breaching of a large duplex in the northern part of the Lesser Himalayan zone. This shift is not seen in eastern Nepal, where the ϵNd(T) values remain close to −16 throughout Miocene time because there has been less erosional unroofing in this region.

Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution

The 176Lu-176Hf isotope method and its applications in earth sciences are discussed. Greater fractionation of Lu/Hf than Sm/Nd in planetary magmatic processes makes 176Hf177Hf a powerful geochemical tracer. In general, proportional variations of 176Hf177Hf exceed those of 143Ndl44Nd by factors of 1.5–3 in terrestrial and lunar materials. Lu-Hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time, and this is the principal importance of Lu-Hf. New data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in 176Hf177Hf-87Sr86Sr and 143Ndl44Nd-87Sr86Sr diagrams, 176Hf177Hf and 143Nd144Nd display a single, linear isotopic variation in the suboceanic mantle. These discordant 87Sr86Sr relationships may allow, with the acquisition of further Hf-Nd-Sr isotopic data, a distinction between processes such as mantle metasomatism, influence of seawater-altered material in the magma source, or recycling of sediments into the mantle. In order to evaluate the Hf-Nd isotopic correlation in terms of mantle fractionation history, there is a need for measurements of Hf distribution coefficients between silicate minerals and liquids, and specifically for a knowledge of Hf behavior in relation to rareearth elements. For studying ancient terrestrial Hf isotopic variations, the best quality Hf isotope data are obtained from granitoid rocks or zircons. New data show that very U-Pb discordant zircons may have upwardly-biased 176Hf177Hf, but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial 176Hf177Hf. Until the controls on addition of radiogenic Hf to zircon are understood, combined zircon-whole rock studies are recommended. Lu-Hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples, but because it offers little advantage over existing methods, is unlikely to find wide application in pure chronological studies.

Nd isotopes and tectonics of 1.9-1.7 Ga crustal genesis

Abstract The origin of 1.9-1.7 Ga crust in Europe, Greenland and North America can be evaluated using Nd and other isotopic information. Terrains stabilized 1.9-1.7 Ga ago make up 74% of the pre-1.6 Ga continental area, but interpretation of Nd data shows that only ca. 50% of this is new mantle-derived crust, while the other half is recycled or reactivated Archean material. These terrains can be divided into (1) those with > 90% reactivated Archean crust, concentrated in northern Greenland and Canada, whose origin is enigmatic, and (2) those with > 80% newly-differentiated material, containing only a limited Archean contribution, probably in the form of recycled sediments. The new terrains occur principally in a wide zone from Arizona through Colorado, Michigan, South Greenland, Sweden, Finland to the western U.S.S.R., and they border the present southern margin of the Wyoming, Superior, North Atlantic and Kola Archean blocks. The new terrains contain a high proportion of volcanic and plutonic rocks resembling those of present-day island arcs and continental margins, and they aomost certainly represent a major subduction-related mantle-to-crust differentiation during 1.9-1.7 Ga. Igneous activity was complex in pattern from area to area, with sequential accretion of volcano-plutonic belts outwards from Archean continents. The overall crust production rate from the northern continents in the period 1.9-1.7 Ga was ∼ 1.2 km 3 /a, which is very slightly greater than the total Phanerozoic island-arc accretion rate. Thus Phanerozoic or only slightly higher rates of subduction can explain the 1.9-1.7 Ga new terrains only if most subduction-related igneous activity in the 1.9-1.7 Ga world was concentrated in the Arizona-Finland zone. Interpretation of limited geological information available from other continents suggests that this was not the case, and that a crust production rate around double the present one prevailed 1.9-1.7 Ga ago. The results document major crustal growth during Proterozoic time.

Proterozoic and Phanerozoic basement terranes of Mexico from Nd isotopic studies

Nd isotopic data were collected on Precambrian crystalline rocks exposed in northern, eastern, and southern Mexico, as well as from lower crustal xenoliths from central Mexico, in order to constrain the age and character of the Mexican basement. The data indicate that basement belonging to the Grenville (1.0 Ga) tectonothermal event extends from Los Filtros, in Chihuahua, northern Mexico, to Oaxaca, in southern Mexico. These rocks all have average Nd crustal residence times (TDM ages) in the range 1.60 to 1.35 Ga. We infer that this results from mixing average 1.9 Ga or older recycled continental crust with 70% to 90% newly derived mantle-crustal material during the Grenville orogeny. To the west of the Precambrian, the basement contains large amounts of Phanerozoic (probably Paleozoic) crust, identified from lower crustal xenoliths with TDM ages less than 1.0 Ga. The crust represented by these xenoliths may have been emplaced as suspect terranes in Mesozoic Cordilleran events. Alternatively, the apparent Paleozoic crust that underlies parts of central Mexico may connect to the Paleozoic metamorphic Acatlan complex in southern Mexico, and together they would constitute a continuation of the Appalachian-Caledonian orogenic belt through Mexico. Our data do not preclude either of these two models.

Isotopic and structural constraints on the location of the Main Central thrust in the Annapurna Range, central Nepal Himalaya

Five isotope-enhanced geologic transects in the southern Annapurna Range of central Nepal elucidate structural geometries near the Main Central thrust. Whole-rock Nd isotopes and U-Pb ages of detrital zircons unambiguously distinguish Greater Himalayan (hanging wall) and Lesser Himalayan (footwall) metasedimentary rocks. ϵ Nd (0) values for lower Lesser Himalayan rocks typically range from −20 to −26, whereas Greater Himalayan rocks usually have ϵ Nd (0) values of −19 to −12. Lower Lesser Himalayan rocks yield detrital zircons with an age peak at ca. 1880 Ma and no detrital zircons younger than ca. 1550 Ma. In contrast, Greater Himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca. 1050 Ma and no detrital zircons younger than ca. 600 Ma. The protolith boundary between Greater and Lesser Himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology. Field and microstructural observations indicate the presence of a top-to-the-south ductile shear zone superimposed on this boundary, confirming this shear zone as the Main Central thrust. No evidence exists for large-scale structural mixing of Greater and Lesser Himalayan rocks along the Main Central thrust in the Annapurna Range.

Implications for the evolution of continental crust from Hf isotope systematics of Archean detrital zircons

Abstract The fractionation of zircons by sedimentary processes into continental margin sandstone deposits results in a biased preservation of pre-existing continental crust in the form of zircon in those sequences. This provides a unique opportunity to distinguish between the contrasting theories of gradual growth versus constant volume of continental crust over geologic time through Hf isotope ratios of detrital zircons. sol 176 Hf 177 Hf ratios were determined for detrital zircon fractions from 2.0–2.5, 2.6–3.0, and pre-3.0 Ga old sandstones from the Canadian Shield, North Atlantic, Wyoming, and Kaapvaal Cratons. Proterozoic quartzites have 2.39 to 2.84 Ga Hf T(CHUR) ages, indicative of a large expanse of late Archean crust exposed to erosion at the time of deposition. The late Archean (2.6–3.0 Ga) sequences appear to be dominated by zircon populations of late Archean age. Hf model ages are less than 3.0 Ga and ϵ(Hf) values are positive or slightly negative at the time of deposition for most of the Malene, Canadian Shield, Wyoming, and upper portions of the Kaapvaal sequences. Exceptions include basal samples of the Pongola (3.32 Ga), Dominion (3.11 Ga), and Witwatersrand (3.13 Ga), an arkose from Michigan (3.20 Ga), and one Malene sample (2.96 Ga), all of which either unconformably overlie or are closely associated with pre-3.0 Ga crust. Nd data for shales from the same sequences in the Canadian Shield and Kaapvaal sequences mimic the Hf results. Hf model ages, from pre-3.0 Ga old strata (Upernavik of Labrador and quartzites from Montana), range from 3.0 to 3.5 Ga and are broadly consistent with ages of coexisting volcanics or intrusives, suggesting little inheritance of significantly older material. The data strongly suggest inheritance of pre-3.0 Ga zircons only in areas where pre-3.0 Ga old crust exists today, and imply that the quantity of continental crust prior to 3.0 Ga ago was not much greater in extent than the pre-3.0 Ga crust exposed today. Small amounts of continental crust prior to 3.0 Ga ago and rapid addition of continental crust between 2.5 and 3.0 Ga ago are consistent with the gradual growth of continental crust, and argue against no-growth histories.

Hf-Nd isotopic evolution of the lower crust

We report Hf isotopic data for over 50 well studied lower crustal samples from three Proterozoic and Phanerozoic regions in southwest Europe, eastern Australia and southern Mexico. We use these data to characterize the Lu–Hf isotopic composition of the lower crust and, in combination with existing Sm–Nd data, to constrain coupled Hf–Nd isotopic behavior and evolution within this reservoir. Although most of these samples have present-day parent/daughter (p/d) ratios consistent with Hf–Nd evolution within the terrestrial Hf–Nd array, some samples have divergent p/d ratios that would evolve out of the terrestrial array in 1 Ga or less. The present-day 176Hf/177Hf and 143Nd/144Nd isotopic compositions of all samples, with one lone exception, plot within the terrestrial array. This indicates that (1) some present-day p/d ratios may be a relatively recently acquired characteristic through magmatic or metamorphic processes not related to the time-integrated Sm/Nd and Lu/Hf ratios of their sources, and/or (2) the Lu/Hf and Sm/Nd p/d variations exist on a small hand-size scale but not necessarily on a larger scale. The lower crust, from this initial data set, is broadly similar to the upper crust in terms of both its present-day p/d values and time-integrated Lu–Hf and Sm–Nd evolution. As a result, the lower crust appears to have a Hf and Nd isotopic composition similar to that of all other crust and mantle reservoirs so far characterized.