Albrecht W. Hofmann

Nb and Pb in oceanic basalts : New constraints on mantle evolution

Abstract Nb/U ratios and Ce/Pb ratios are surprisingly uniform at47 ± 10and25 ± 5, respectively, in both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB). We show that these ratios also characterize the mantle sources of both types of oceanic basalts, and that these mantle sources have been fractionated from the primitive-mantle ratios ofNb/U = 30 andCe/Pb = 9. The respective ratios in the continental crust are even lower, namelyNb/U = 10 andCe/Pb = 4. Therefore, OIB cannot be derived from a primitive portion of the mantle, from mixtures of primitive and depleted mantle, or from recycled continental crust. The portion of the primitive mantle from which the continental crust and the residual (MORB plus OIB source) mantle has been differentiated is estimated to be about 50%, but the uncertainties are such that whole-mantle differentiation cannot be ruled out. We propose the following simple model to satisfy the above new constraint on mantle composition: The differentiated part of the mantle, chemically depleted after separation of the major portion of the continental crust, was subsequently internally rehomogenized. This depleted but chemically homogeneous mantle region was then differentiated into MORB and OIB source regions. The primary (continental crust—mantle) differentiation fractionated the Nb/U and Ce/Pb ratios, but the secondary (MORB source-OIB source) differentiation did not. Following the model of Hofmann and White [1,2], we suggest that the mechanism chiefly responsible for the secondary differentiation is the formation and subduction of oceanic crust. It is volumetrically by far the most important ongoing differentiation process on Earth and, over the course of Earth history, has created at least ten times as much oceanic crust as the present-day volume of continental crust. Because the residual mantle was homogenized (though depleted in incompatible elements) after the primary differentiation, the isotopic and chemical heterogeneities exemplified by the isotope ratios of Sr, Nd, Hf, and Pb, and by trace element ratios such as K/Rb, were created during the secondary differentiation. During this process, the bulk partition coefficients of Nb and Ce were very similar to those of U and Pb, respectively. This is in contrast with the primary differentiation, during which U was more incompatible than Nb, and Pb more incompatible than Ce.

The Amount of Recycled Crust in Sources of Mantle-Derived Melts

One proposed strategy for controlling the transmission of insect-borne pathogens uses a drive mechanism to ensure the rapid spread of transgenes conferring disease refractoriness throughout wild populations. Here, we report the creation of maternal-effect selfish genetic elements in Drosophila that drive population replacement and are resistant to recombination-mediated dissociation of drive and disease refractoriness functions. These selfish elements use microRNA-mediated silencing of a maternally expressed gene essential for embryogenesis, which is coupled with early zygotic expression of a rescuing transgene.The phosphoinositide phosphatase PTEN is mutated in many human cancers. Although the role of PTEN has been studied extensively, the relative contributions of its numerous potential downstream effectors to deregulated growth and tumorigenesis remain uncertain. We provide genetic evidence in Drosophila melanogaster for the paramount importance of the protein kinase Akt [also called protein kinase B (PKB)] in mediating the effects of increased phosphatidylinositol 3,4,5-trisphosphate (PIP3) concentrations that are caused by the loss of PTEN function. A mutation in the pleckstrin homology (PH) domain of Akt that reduces its affinity for PIP3 sufficed to rescue the lethality of flies devoid of PTEN activity. Thus, Akt appears to be the only critical target activated by increased PIP3 concentrations in Drosophila.Using genomic and mass spectrometry-based proteomic methods, we evaluated gene expression, identified key activities, and examined partitioning of metabolic functions in a natural acid mine drainage (AMD) microbial biofilm community. We detected 2033 proteins from the five most abundant species in the biofilm, including 48% of the predicted proteins from the dominant biofilm organism, Leptospirillum group II. Proteins involved in protein refolding and response to oxidative stress appeared to be highly expressed, which suggests that damage to biomolecules is a key challenge for survival. We validated and estimated the relative abundance and cellular localization of 357 unique and 215 conserved novel proteins and determined that one abundant novel protein is a cytochrome central to iron oxidation and AMD formation.

An olivine-free mantle source of Hawaiian shield basalts

More than 50 per cent of the Earths upper mantle consists of olivine and it is generally thought that mantle-derived melts are generated in equilibrium with this mineral. Here, however, we show that the unusually high nickel and silicon contents of most parental Hawaiian magmas are inconsistent with a deep olivine-bearing source, because this mineral together with pyroxene buffers both nickel and silicon at lower levels. This can be resolved if the olivine of the mantle peridotite is consumed by reaction with melts derived from recycled oceanic crust, to form a secondary pyroxenitic source. Our modelling shows that more than half of Hawaiian magmas formed during the past 1 Myr came from this source. In addition, we estimate that the proportion of recycled (oceanic) crust varies from 30 per cent near the plume centre to insignificant levels at the plume edge. These results are also consistent with volcano volumes, magma volume flux and seismological observations.

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

HIMU−EM : the French Polynesian connection

Abstract himu, em i and em ii are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteristic himu signature is found in Tubuai, Mangaia and Rurutu, em i is present in the source of Rarotonga and Pitcairn volcanics and em ii dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands. We suggest that the three components are not independent but are physically related in the mantle. The himu component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction. em i and em ii are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust. The trace element pattern and the isotopic composition of himu lavas can be quantitatively modelled using a mixture of ∼ 25% old recycled morb crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of the U/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those of em i and em ii are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlying morb oceanic crust. Pelagic sediments ( μ ∼ 5 and κ ∼ 6 ) account for the Pb isotopic composition of em i whereas terrigenous sediments ( μ ∼ 10 and κ ∼ 4.5 ) evolve towards the em ii end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of the himu component. This, together with the low probability of isolating oceanic crust in the mantle for ⩾ 2 Ga, explains why the extreme himu composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism.

The chemical composition of the Earth

Abstract The bulk composition of the Earth and the composition of the mantle and core are calculated using the ratios of major and trace elements. The ratios of elements which do not enter the core (lithophile) are the same in the bulk Earth as in the mantle. Bulk earth ratios involving an element that does enter the core (siderophile) are therefore determined from meteorite correlation diagrams of siderophile-lithophile ratios vs. lithophile-lithophile ratios and from primitive mantle composition in elements which do not enter the core (e.g., Al). The composition of the core is determined by difference, without resorting to assumptions about core formation processes. It is found that the core contains about 7.3 wt% silicon and 2.3 wt% sulphur. To account for the seismologically determined density deficit of the core, about 4 wt% oxygen must be added. The present results are compatible with the idea that the core material equilibrated at low pressure, in reducing conditions. Furthermore, we propose that the Earth is closer to CM rather than to C1 for non-volatile element ratios.

Precambrian crustal components, plutonic associations, plate environment of the Hercynian Fold Belt of central Europe: Indications from a Nd and Sr isotopic study

Apparent crustal residence ages indicated by Nd model age data for metamorphic rocks, sediments and granitoids of the Hercynian Fold Belt of Europe vary from 1.3 Ga to 3.0 Ga, but are mainly in the range 1.4–1.7 Ga. 2 Ga basement inliers have been documented previously in northern Spain and islands off northwestern France but, in addition to these, old (∼2–3 Ga) model ages are found along the southern margin of the fold belt. These do not identify old inliers but are interpreted to represent Archeanearly Proterozoic crustal components recycled from a southern source. The Nd data, when considered together with the surface geology and recent single-grain zircon ion microprobe data, argue against a binary mixing of Archean components with new Palaeozoic crustal additions to generate the main 1.4–1.7 Ga model age population. Hercynian Europe comprise mainly recycled Proterozoic components although significant new Palaeozoic additions as well as Archean contributions are indicated.Nd and Sr isotopic data together with previous chemical and petrographic observations allow the recognition of a northern belt of continent margin I-type granitoids grading southwards to inner continent S-type plutons in the eastern half of the fold belt. This felsic plutonic association is used to infer a Hercynian plate configuration involving the attachment of the fold belt to a southern parent cratonic block that the model age data suggest may be of early Proterozoic-Archean age.

FOZO, HIMU, and the rest of the mantle zoo

Author Posting.

Coupled major and trace elements as indicators of the extent of melting in mid-ocean-ridge peridotites

Rocks in the Earths uppermost sub-oceanic mantle, known as abyssal peridotites, have lost variable but generally large amounts of basaltic melt, which subsequently forms the oceanic crust. This process preferentially removes from the peridotite some major constituents such as aluminium, as well as trace elements that are incompatible in mantle minerals (that is, prefer to enter the basaltic melt), such as the rare-earth elements. A quantitative understanding of this important differentiation process has been hampered by the lack of correlation generally observed between major- and trace-element depletions in such peridotites. Here we show that the heavy rare-earth elements in abyssal clinopyroxenes that are moderately incompatible are highly correlated with the Cr/(Cr + Al) ratios of coexisting spinels. This correlation deteriorates only for the most highly incompatible elements—probably owing to late metasomatic processes. Using electron- and ion-microprobe data from residual abyssal peridotites collected on the central Indian ridge, along with previously published data, we develop a quantitative melting indicator for mantle residues. This procedure should prove useful for relating partial melting in peridotites to geodynamic variables such as spreading rate and mantle temperature.

An assessment of local and regional isotopic equilibrium in the mantle

Abstract The assumption of local equilibrium during partial melting is fundamental to the interpretation of isotope and trace element data for mantle-derived rocks. If disequilibrium melting is significant, the scale of the chemical and isotopic heterogeneity in the mantle indicated by the data could be as small as the grain size of the mantle rock, and the isotope data themselves are then of doubtful value to the understanding of mantle processes. To assess the scale of isotopic heterogeneity in a partially molten asthenosphere we review the Sr isotopic data of volcanic rocks from oceanic regions and the available experimental data on diffusion kinetics in minerals and melts similar to those existing in the mantle. Although diffusion data are scarce and afflicted with uncertainties, most of the diffusion coefficients for cations in mantle minerals at temperatures of 1000–1200°C appear to be greater than 10−13 cm2 s−1. Sr diffusion in liquid basalt is more rapid, with diffusion coefficients of D = 10−7 to 10−6cm2s−1 near 1300°C. Simple model calculations show that, with these D values, a fluid-free mantle can maintain a state of disequilibrium on a centimeter scale for periods of 108 to 109 years. The state of disequilibrium found in many mantle-derived xenoliths is thus easily explained. A partially molten mantle, on the other hand, will tend to equilibrate locally in less than 105 to 106 years. The analytical data on natural rocks likewise indicate that the inhomogeneities are both old (>FX1.5 b.y.) and regional in character and that the consistent isotopic difference between ocean island and ocean floor volcanics cannot be explained by small-scale heterogeneity of the source rock.