Jeffrey D. Vervoort

Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time

Abstract The covariant behavior of Lu-Hf and Sm-Nd isotopes during most magmatic processes has long been recognized, but the details of this behavior in the depleted mantle reservoir have not been adequately examined. We report new whole-rock Hf and Nd isotope data for 1) juvenile, mantle-derived rocks, mid-Archean to Mesozoic in age, and 2) early Archean gneisses from West Greenland. Hf and Nd isotopic compositions of the juvenile rocks are well correlated, with the best fit corresponding to the equation e Hf = 1.40 e Nd + 2.1, and is similar to the collective Hf-Nd correlation for terrestrial samples of e Hf = 1.36 e Nd + 3.0. The early Archean Greenland gneisses, in contrast, have an extreme range in e Nd values ( − 4.4 to + 4.2; Bennett et al., 1993 ) that is not mirrored by the Hf isotopic system. The e Hf values for these rocks are consistently positive and have much less variation (0 to + 3.4) than their e Nd counterparts. The information from the Hf isotopic compositions of the West Greenland gneisses portrays an early Archean mantle that is relatively isotopically homogeneous at 3.8 to 3.6 Ga and moderately depleted in incompatible elements. There is no evidence that any of these gneisses have been derived from an enriched reservoir. The Hf isotopic data are in stark contrast to the Nd isotopic record and strongly imply that the picture of extreme initial isotopic heterogeneity indicated by Nd isotopes is not a real feature of the West Greenland gneisses but is rather an artifact produced by disturbances in the Sm-Nd isotope system of these rocks. Although Hf and Nd isotopic data do not uniquely constrain either the nature of the earliest crust or the timing of crustal growth, the most probable candidate for the enriched reservoir complementary to the depleted mantle in the pre-4.0 Ga Earth is a mafic, oceanic-type crust. In order to explain the predominantly positive e Hf and e Nd values for the early Archean rocks, this crust must have had a short residence time at the surface of the Earth before returning to the mantle where it was isolated from mixing with the depleted mantle for several hundred million years. The following period from 3.5 to 2.7 Ga may mark a transition during which this early formed mafic crust was mixed progressively back into the depleted mantle reservoir. While a present-day volume of continental crust at 4.0 Ga cannot be excluded on isotopic grounds, we find such a scenario unlikely based on the lack of direct isotopic and physical evidence for its existence. An important aspect of crustal growth and evolution, therefore, may be the transformation of the enriched reservoir from being predominantly mafic in the early Earth to becoming progressively more sialic through time.

RELATIONSHIPS BETWEEN LU-HF AND SM-ND ISOTOPIC SYSTEMS IN THE GLOBAL SEDIMENTARY SYSTEM

Abstract We report new Hf (and Nd) data for more than 100 sedimentary samples, recent to Archean in age, from a wide range of depositional environments. These data document the behavior of Lu–Hf and Sm–Nd isotopic systems in the global sedimentary system. In conjunction with existing data for mantle-derived rocks, we now have reasonable constraints on coupled Hf–Nd isotopic behavior in the crust and mantle. Lu/Hf and Hf isotopic compositions are strongly fractionated between muds and sands in passive margin sediments due to concentration of low Lu/Hf, low 176Hf/177Hf, Hf-rich zircons in mature sands. In active margin settings, Lu–Hf fractionation due to the `zircon effect is minor due to the less weathered and more juvenile character of the sediments. Nd isotopic compositions are not highly fractionated by sedimentary sorting because heavy minerals, also rich in REEs, do not fractionate Sm–Nd efficiently. The lack of a large and systematic fractionation at active margins means that no significant Hf–Nd decoupling occurs here. This is important because sediments at active margins are the most likely to be recycled to the mantle. Hf–Nd isotopic data for all terrestrial samples fall along a single coherent trend (eHf=1.36eNd+2.95) which we call the `terrestrial array. This array is composed of two complementary components: a mantle array (eHf=1.33eNd+3.19, defined by all oceanic basalts; and a crustal array (eHf=1.34eNd+2.82), defined by sediments, continental basalts, granitoids, and juvenile crustal rocks. The similarity of the crustal and mantle arrays indicates that no large-scale Hf–Nd decoupling occurs between the crust and mantle. The coherency of the terrestrial Hf–Nd array implies mixing within the mantle, due to stirring processes, and also within the crust, due to homogenization by collective sedimentary processes. In addition, tight Hf–Nd covariation may also imply that efficient crust to mantle recycling has modulated isotopic correlation in the silicate Earth. All Hf–Nd arrays, including the terrestrial array, lie significantly above (2–3 eHf units) the BSE (bulk silicate Earth) reference. This would appear to require a hidden reservoir in the Earth, heretofore unsampled, to balance the Hf–Nd isotopic composition of the terrestrial array. However, the discrepancy between the terrestrial array and BSE may simply be due to differences in the way the CHUR (chondritic uniform reference) values were determined for the Lu–Hf and Sm–Nd isotope systems.

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 Hfue5f8Nd 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. n nModeling 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.

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.

A Hf‐Nd isotopic correlation in ferromanganese nodules

The 176Hf/177Hf ratio was measured on 34 ferromanganese nodules, mostly from the Atlantic ocean. The different ocean basins are isotopically distinct with the extreme compositions being less radiogenic in the Atlantic (єHf ∼ +1) than in the Pacific (єHf ∼ +9). A good correlation of єHf and єNd is observed amongst most samples which supports that Hf isotopic compositions in nodules reflect those of ambient seawater. For a given єNd, єHf is more radiogenic in ferromanganese nodules than in rocks from either the mantle or the crust. This correlation makes the coupled Hf-Nd systems a potential paleoceanographic tool. It is argued that a zircon-free clayish component of probable eolian origin may account for the radiogenic Hf in nodules.

Hf-Nd isotope evidence for a transient dynamic regime in the early terrestrial mantle

Modern basalts have seemingly lost all ‘memory’ of the primitive Earths mantle except for an ambiguous isotopic signal observed in some rare gases. Although the Earth is expected to have reached a thermal steady state within several hundred million years (refs 3, 4) of accretion, it is not known how and when the initial chemical fractionations left over from planetary accretion (and perhaps a stage involving a magma ocean) were overshadowed by fractionations imposed by modern-style geodynamics. Because of the lack of samples older than 4 Gyr, this early dynamic regime of the Earth is poorly understood. Here we compare published Hf–Nd isotope data on supracrustals from Isua, Greenland, with similar data on lunar rocks and the SNC (martian) meteorites, and show that, about 3.8 Gyr ago, the geochemical signature of the Archaean mantle was partly inherited from the initial differentiation of the Earth. The observed features seem to indicate that the planet at that time was still losing a substantial amount of primordial heat. The survival of remnants from an early layering in the modern deep mantle may account for some unexplained seismological, thermal and geochemical characteristics of the Earth as observed today.

U–Pb geochronology and Hf–Nd isotope compositions of the oldest Neoproterozoic crust within the Cadomian orogen: new evidence for a unique juvenile terrane

New U–Pb dates, combined with Nd and Hf isotopic data, from rocks within the Port Morvan area of the Baie de St Brieuc region of Brittany identify a unique portion of the Neoproterozoic Cadomia terrane. Two gneisses near Port Morvan yielded U–Pb dates of 754.6±0.8 Ma and 746.0±0.9 Ma, ages that are more than 130 Myr older than the oldest units formed during the main phase of early Cadomian magmatism. Two trondhjemite boulders from the monogenetic facies of the Cesson conglomerate yielded identical ages of 665.2±0.5 Ma and 665.5±0.7 Ma, and a cobble from the polygenetic facies yields a 207Pb–206Pb date of 637±2 Ma. Individual detrital zircons from a sandstone associated with the Cesson conglomerates yield concordant U–Pb dates ranging from 650±3 Ma to 624.1±0.6 Ma. Initial ϵNd values for the rocks in this region range from +5.0 to +6.6, indicative of a substantial input from depleted mantle. Initial ϵHf values determined on zircons from these Neoproterozoic rocks, including the detrital zircons, range from +6.7 to +14.5, consistent with the Nd isotopic results. Maximum initial ϵHf values for two 2 Ga Icartian gneisses, considered basement to Cadomia, average +8.4 and +8.7. In contrast to the results of the Port Morvan rocks, 616–608 Ma syn-tectonic intrusions from Normandy and the British Channel Islands all have negative initial ϵNd values (−10.4 to −8.3) consistent with significant contamination by ancient crust such as the 2 Ga gneisses. The oldest arc-related magmas should have interacted most extensively with Cadomian basement, buffering younger mantle-derived magmas that were generated in subsequent magmatic episodes. The rocks within the Port Morvan region are thus inconsistent as examples of the earliest Cadomian intrusions as they show no evidence of interaction with 2 Ga basement. Instead, the older ages and mantle-like isotopic composition of these rocks suggest they are part of an independent terrane that formed prior to, and independently from, the Cadomian arc. Possible terrane-scale structural boundaries have recently been identified, including the newly recognized Port Morvan thrust fault and the NW-dipping Main Cadomian thrust.

Tectonic and magmatic development of the Salinian Coast Ridge Belt, California

[1] We present new field, structural, petrographic, and geochronologic data on a rare midcrustal (� 25 km) exposure of a Cordilleran arc, the Coast Ridge Belt, located in the Santa Lucia Mountains of central California. The study area is composed primarily of a deformed suite of upper amphibolite to granulite facies rocks (the ‘‘Sur Series’’), which is dominated by metaigneous tonalites, diorites, and gabbros with subordinate metasedimentary quartzite and marble. Inherited zircons in magmatic rocks suggest that the provenance of framework rocks is drawn heavily from miogeoclinal formations and that sedimentation occurred in the late Paleozoic or later. Minor magmatism in the Coast Ridge Belt began in the Early or Middle Cretaceous, but magmatic activity was most intense during a short period time from 93 to 81 Ma, based on U-Pb zircon ages of a felsic gneiss and two less-deformed diorites. The time period 93– 81 Ma also brackets a period of extensive thickening and high-temperature ductile deformation. While a thrusting cause for ductile deformation cannot be ruled out, we favor the hypothesis that the exposed rocks correspond to a zone of return flow of supracrustal rocks locally displaced by granitoid plutons in the shallower crust. Magmatism ended throughout Salinia between81and76Ma,coincidentwiththeattainmentof peak pressure and temperature conditions of 0.75 GPa and 800� C. Exhumation followed immediately, bringing the Coast Ridge Belt to the surface within 8 My at a rate of at least 2–3 mm/yr. Exhumation was coincident with an episode of extensional collapse that has been documented elsewhere in the southern California arc during the early Laramide orogeny and that may be related to underthrusting of the forearc at that time. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 8035 Structural Geology: Pluton emplacement; 3640 Mineralogy and Petrology: Igneous petrology; 8030 Structural Geology: Microstructures; 8124 Tectonophysics: Earth’s interior—composition and state (1212);