Hafnium Isotopic Compositions Research Articles

Ancient and Modern Subduction Zone Contributions to the Mantle Sources of Lavas from the Lassen Region of California Inferred from Lu-Hf Isotopic Systematics

Hafnium isotopic compositions have been determined on a suite of INTRODUCTION calc-alkaline and high-alumina-olivine tholeiitic lavas from the The geochemical and isotopic variability of mantle wedge Lassen region of California and are used, in conjunction with peridotites beneath continental volcanic arcs is poorly previously published mineralogical, geochemical, and isotopic data, constrained. This is a result of the fact that most of what to constrain their petrogenesis. Positive correlation between Hf is known about the geochemistry of the mantle wedge is values and geochemical indices of the modern subduction component inferred from the composition of basaltic arc lavas, which, indicates that the isotopic compositions of the calc-alkaline lavas although derived from the wedge, are affected by variable record addition of radiogenic Hf from the subducted slab. However, contributions from the subducting slab and continental the addition of the modern subduction component increases the Hf crust (e.g. Gill, 1981; Tatsumi et al., 1986; Hawkesworth values of most calc-alkaline lavas by <0·5 units over estimates of et al., 1993, 1994). Despite these limitations there is non-subduction enriched peridotites of the mantle wedge. The Lu–Hf significant evidence to suggest that mantle wedge periisotopic systematics of the Lassen lavas suggest that the calc-alkaline dotites are heterogeneous in composition and mineralogy. magmas have equilibrated with garnet at some point in their history, For example, it has been postulated that the compositions whereas the tholeiitic magmas have not. These observations require of mantle peridotites beneath the Cascade arc vary from the two lava types to be derived from different sources. The isotopic compositions that are similar to sources for mid-ocean variability of the Lassen lavas cannot be produced by mixing mantle ridge basalt (MORB), Hawaiian ocean-island basalt sources inferred to be present in the eastern–central Pacific and (OIB), and high-alumina-olivine tholeiites (HAOT) from western USA with a modern subduction component. Instead, the the Basin and Range province (Hughes, 1990; Leeman isotopic variability is consistent with mixing of a depleted mantle et al., 1990; Bacon et al., 1997; Borg et al., 1997; Clynne source, a more fertile mantle source enriched by an ancient subduction & Borg, 1997). The restite mineralogy of the source component, and a modern subduction component. regions for Cascade magmas is poorly constrained as well. For example, experiments conducted by Baker et al. (1994) found that calc-alkaline basalts from the Mt. Shasta region of California did not have garnet on their liquidus, and Reiners et al. (2000) were able to model

Calibration of the lutetium-hafnium clock.

Well-defined constants of radioactive decay are the cornerstone of geochronology and the use of radiogenic isotopes to constrain the time scales and mechanisms of planetary differentiation. Four new determinations of the lutetium-176 decay constant (lambda176Lu) made by calibration against the uranium-lead decay schemes yield a mean value of 1.865 +/- 0.015 x 10(-11) year(-1), in agreement with the two most recent decay-counting experiments. Lutetium-hafnium ages that are based on the previously used lambda176Lu of 1.93 x 10(-11) to 1.94 x 10(-11) year(-1) are thus approximately 4% too young, and the initial hafnium isotope compositions of some of Earth's oldest minerals and rocks become less radiogenic relative to bulk undifferentiated Earth when calculated using the new decay constant. The existence of strongly unradiogenic hafnium in Early Archean and Hadean zircons implies that enriched crustal reservoirs existed on Earth by 4.3 billion years ago and persisted for 200 million years or more. Hence, current models of early terrestrial differentiation need revision.

Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons

Continental crust forms from, and thus chemically depletes, the Earth's mantle. Evidence that the Earth's mantle was already chemically depleted by melting before the formation of today's oldest surviving crust has been presented in the form of Sm–Nd isotope studies of 3.8–4.0 billion years old rocks from Greenland1,2,3,4,5 and Canada5,6,7. But this interpretation has been questioned because of the possibility that subsequent perturbations may have re-equilibrated the neodymium-isotope compositions of these rocks8. Independent and more robust evidence for the origin of the earliest crust and depletion of the Archaean mantle can potentially be provided by hafnium-isotope compositions of zircon, a mineral whose age can be precisely determined by U–Pb dating, and which can survive metamorphisms4. But the amounts of hafnium in single zircon grains are too small for the isotopic composition to be precisely analysed by conventional methods. Here we report hafnium-isotope data, obtained using the new technique of multiple-collector plasma-source mass spectrometry9, for 37 individual grains of the oldest known terrestrial zircons (from the Narryer Gneiss Complex, Australia, with U–Pb ages of up to 4.14 Gyr (10–13). We find that none of the grains has a depleted mantle signature, but that many were derived from a source with a hafnium-isotope composition similar to that of chondritic meteorites. Furthermore, more than half of the analysed grains seem to have formed by remelting of significantly older crust, indicating that crustal preservation and subsequent reworking might have been important processes from earliest times.

Lu-Hf Isotope Systematics of Garnet Pyroxenites from Beni Bousera, Morocco: Implications for Basalt Origin.

Six garnet pyroxenites from Beni Bousera, Morocco, yield a mean lutetium-hafnium age of 25 +/- 1 million years ago and show a wide range in hafnium isotope compositions (varepsilonHf = -9 to +42 25 million years ago), which exceeds that of known basalts (0 to +25). Therefore, primary melts of garnet pyroxenites cannot be the source of basalts. The upper mantle may be an aggregate of pyroxenites that were left by the melting of oceanic crust at subduction zones and peridotites that were contaminated by the percolation of melts from these pyroxenites. As a consequence, the concept of geochemical heterogeneities as passive tracers is inadequate. Measured lutetium-hafnium partitioning of natural minerals requires a reassessment of some experimental work relevant to mantle melting in the presence of garnet.

Hafnium isotope stratigraphy of ferromanganese crusts

A Cenozoic record of hafnium isotopic compositions of central Pacific deep water has been obtained from two ferromanganese crusts. The crusts are separated by more than 3000 kilometers but display similar secular variations. Significant fluctuations in hafnium isotopic composition occurred in the Eocene and Oligocene, possibly related to direct advection from the Indian and Atlantic oceans. Hafnium isotopic compositions have remained approximately uniform for the past 20 million years, probably reflecting increased isolation of the central Pacific. The mechanisms responsible for the increase in (87)Sr/(86)Sr in seawater through the Cenozoic apparently had no effect on central Pacific deep-water hafnium.

The Source Region and Melting Mineralogy of High-Titanium and Low-Titanium Lunar Basalts Deduced from Lu-Hf Isotope Data

Five high-Ti basalts from the Apollo 11 and 17 landing sites have been analyzed for their hafnium isotope composition. These data serve to better constrain the hafnium isotope variation of the Moon’s mantle. Variations in initial ϵ Hf and ϵ Nd values of low- and high-Ti basalts imply that the source region mineral assemblages of these lunar magma types are distinct. Low-Ti basalts have higher initial ϵ Hf values, at a given ϵ Nd value, than high-Ti basalts. The differences in the hafnium and neodymium isotopic composition of low- and high-Ti basalts reflect the fact that the source of low-Ti basalts had a [Lu/Hf] n ratio approximately four times greater than its [Sm/Nd] n ratio. In contrast, the high-Ti source region had subequal [Lu/Hf] n and [Sm/Nd] n ratios. If it is assumed that mare basalts are partial melts of the Moon’s cumulate mantle, the differences between low- and high-Ti basalts can only be explained by these mare magma types being generated from melting sources with distinctly different mineral assemblages. The large Lu/Hf fractionations, relative to Sm/Nd fractionations, of low-Ti basalts can best be produced by an assemblage of olivine and orthopyroxene with trace amount of clinopyroxene that crystallized early in the history of the Lunar Magma Ocean (LMO). The subequal [Lu/Hf] n and [Sm/Nd] n fractionations of high-Ti basalts can be produced from a variety of ilmenite-bearing mineral assemblages. Low- and high-Ti basalts have similar Lu/Hf ratios, approximately 0.6 times chondrite. The low Lu/Hf ratios measured for these mare magmas contrast sharply with the high Lu/Hf ratios (greater than chondritic) calculated for their sources from initial ϵ Hf values and an assumed chondritic bulk moon initial ϵ Hf value. The difference between the measured Lu/Hf of a lava, vs. the calculated Lu/Hf of its source, implies that during partial melting, Lu was preferentially retained in the residual source, relative to Hf. In order to explain the extreme fractionation of measured Lu/Hf ratios we suggest mare basalts can best be explained using a polybaric melting model. Initial melting of a garnet bearing source followed by continued melting in the spinel stability field can produce the required Lu/Hf fractionations and produce a liquid that last equilibrated with a residuum of olivine and orthopyroxene.

New Hafnium Isotopic Constraints on the Source of MORB

Since Patchett (1983) measured the first hafnium isotopic compositions of MORB samples, it has been known that basalts produced at ridges have Hf isotopic compositions that vary in a manner which is independant of the variations seen in Sr and Nd isotopic compositions. While ocean island basalts display strong correlation between Hf and Nd isotopic compositions, MORB are characterized by a wide scatter in Hf for almost constant Nd isotopic compositions. Since this pioneering work, few other papers have appeared in literature due to the analytical difficulty of obtaining reliable Hf isotopic compositions for Hf-poor basalts. Salters (1996) contributed greatly to the overall limited data set by setting up a new method that allowed the measuremeat of Hf isotopic compositions on smaller amounts of Hf. These new data confirmed the spread previously obtained and highlighted the fact that the MORB source is indeed less homogeous in terms of Hf than Nd isotopes. The development in the early nineties of a new instrument (a mass spectrometer with an ICP source) capable of measuring Hf isotopic compositions on ng quantities of Hf opened up new perspectives and led us to measure MORB samples from the three major ocean basins, the Atlantic, the Pacific, and the Indian Oceans. We primarily selected samples that had already been measured for Nd, Sr, and Pb isotopes and completed the Nd data set when such data were not available. We analysed over 30 samples and data are presented here in conjonction with St, Nd, and Pb isotope data from literature.

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 176 Hf 177 Hf reservoirs in the lower crust. Therefore, melts derived from such garnet-bearing residual assemblages may be high in 176 Hf 177 Hf relative to 143 Nd 144 Nd . 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 Lu Hf and Sm Nd partitioning during melting events in the lower crust indicates that anomalously high 176 Hf 177 Hf 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 176 Hf 177 Hf compositions: all samples (with one exception) have initial Hf and Nd compositions that plot within a ±8 ϵ Hf unit wide band of the reference line ( ϵ Hf = 2 ϵ Nd + 2) for juvenile crust. The absence of high initial 176 Hf 177 Hf 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.

Aluminum depletion in komatiites and garnet fractionation in the early Archean mantle: Hafnium isotopic constraints

Hafnium isotopic compositions were measured in Al-depleted and Al-enriched komatiites from the 3450 Ma old Barberton greenstone belt, southern Africa. All samples have initial ϵ Hf values close to zero. Such values are at variance with the strongly negative or positive values that should be observed if these rocks came from old garnet-depleted or garnet-enriched layers, such as may have formed during the solidification of an ancient terrestrial magma ocean. The garnet fractionation observed in komatiites probably took place during the melting event.

The mean life of continents: Estimates of continent recycling rates from Nd and Hf isotopic data and implications for mantle structure

An analysis of the initial neodymium and hafnium isotopic compositions of modern and ancient mantle‐derived rocks shows that the divergence of the mantle 143Nd/144Nd and 176Hf/177Hf ratios from the chondritic ratios over the past 2 billion years has been much less than would be expected based on the Sm/Nd and Lu/Hf ratios of the mantle source of mid‐ocean ridge basalt. The most likely explanation is that rock material from the continents has been recycled semi‐continuously back into the mantle over this time period. The required rate of return of continents to the mantle, calculated from a simple transport model, is about 0.35±0.15 of the continental mass per billion years (∼2.5 km³/year). This result is consistent with the hypothesis that the mass of the continents has been approximately constant since the late Archean, and implies that recycled continental material must be an important component of the upper mantle.