K.P. Jochum

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.

Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the Earth's core

Abstract We have investigated the hypothesis that mantle Pb isotope ratios reflect continued extraction of Pb into the Earths core over geologic time. The Pb, Sr and Nd isotopic compositions, and the abundance of siderophile and chalcophile elements (W, Mo and Pb) and incompatible lithophile elements have been determined for a suite of ocean island and mid-ocean ridge basalt samples. Over the observed range in Pb isotopic compositions for oceanic rocks, we found no systematic variation of siderophile or chalcophile element abundances relative to abundances of similarly incompatible, but lithophile, elements. The high sensitivity of theMo/Pr ratio to segregation of Fe-metal or S-rich metallic liquid (sulfide) and the observed constantMo/Pr ratio rules out the core formation model as an explanation for the Pb paradox. The mantle and crust have the sameMo/Pr and the sameW/Ba ratios, suggesting that these ratios reflect the ratio in the Earths primitive mantle. Our data also indicate that thePb/Ce ratio of the mantle is essentially constant, but the presentPb/Ce ratio in the mantle (≅ 0.036) is too low to represent the primitive value (≅ 0.1) derived from Pb isotope systematics. HigherPb/Ce ratios in the crust balance the lowPb/Ce of the mantle, and crust and mantle appear to sum to a reasonable terrestrialPb/Ce ratio. The constancy of thePb/Ce ratio in a wide variety of oceanic magma types from diverse mantle reservoirs indicates this ratio is not fractionated by magmatic processes. This suggests crust formation must have involved non-magmatic as well as magmatic processes. Hydrothermal activity at mid-ocean ridges may result in significant non-magmatic transport of Pb from mantle to crust and of U from crust to mantle, producing a higherU/Pb ratio in the mantle than in the total crust. We suggest that the lower crust is highly depleted in U and has unradiogenic Pb isotope ratios which balance the radiogenic Pb of upper crust and upper mantle. The differences between thePb/Ce ratio in sediments, this ratio in primitive mantle, and the observed ratio in oceanic basalts preclude both sediment recycling and mixing of primitive and depleted reservoirs from being important sources of chemical heterogeneities in the mantle.

Nb-Th-La in komatiites and basalts" constraints on komatiite petrogenesis and mantle evolution

New spark-source mass spectrometric analyses of Nb, Th, REE and other trace elements in Archaean to Tertiary komatiites and basalts were undertaken to test the model of Hofmann et al. [1] of secular variation of the composition of the upper mantle. Most 3.4 Ga komatiites and basalts have Nb/Th between 7 and 9, values that straddle the primitive mantle ratio of ∼ 8. Several 2.7 Ga komatiites also have Nb/Th= 7–9, but most samples of this age have higher Nb/Th, in the range 10–15. Cretaceous-Tertiary komatiites and basalts from Gorgona Island (Colombia) have Nb/Th between 11 and 24, values that approach those of modern oceanic basalts ( ∼ 15–34). Although these results generally support the model of Hofmann et al., there are several complicating factors: (1) most of the Cretaceous-Tertiary Gorgona komatiites have Nb/Th ratios little higher than those of 2.7 Ga komatiites and primitive mantle, which suggests that low Nb/Th may be a peculiarity of komatiites and not a feature of the Archaean mantle; (2) many Archaean komatiites are depleted in both Nb and Th relative to the REE, a feature that is inconsistent with their derivation from primitive mantle. We speculate that komatiites come from a source that evolved independently from normal upper mantle, and that the depletion of Nb and Th resulted from fractionation of an unknown phase during the deep melting. Certain tholeiitic basalts do not show unusual Nb-Th-La fractionation but nonetheless show a secular increase in Nb/Th. This variation may indicate a change in upper mantle compositions resulting from progressive withdrawal of continental crust during the past 3 Ga.

OCEANIC PLATEAU MODEL FOR CONTINENTAL CRUSTAL GROWTH IN THE ARCHAEAN : A CASE STUDY FROM THE KOSTOMUKSHA GREENSTONE BELT, NW BALTIC SHIELD

Abstract Field studies combined with chemical and isotope data indicate that the Kostomuksha greenstone belt in the NW Baltic Shield consists of two lithotectonic terranes, one mafic igneous and the other sedimentary, separated by a major shear zone. The former contains submarine komatiite–basalt lavas and volcaniclastic lithologies, and the latter is composed of shelf-type rocks and BIF. Komatiitic and basaltic samples yield Sm–Nd and Pb–Pb isochron ages of 2843±39 and 2813±78 Ma, respectively. Their trace-element compositions resemble those of recent Pacific oceanic flood basalts with primitive-mantle normalized Nb/Th of 1.5–2.1 and Nb/La of 1.0–1.5. This is in sharp contrast with island arc and most continental magmas, which are characterized by Nb/(Th,La)N≪1. Calculated initial Nd-isotope compositions (ϵNd(T)=+2.8 to +3.4) plot close to an evolution line previously inferred for major orogens (“MOMO”), which is also consistent with the compositions of recent oceanic plateaux. The high liquidus temperatures of the komatiite magmas (1550°C) and their Al-depleted nature require an unusually hot (1770°C) mantle source for the lavas (>200°C hotter than the ambient mantle at 2.8 Ga), and are consistent with their formation in a deep mantle plume in equilibrium with residual garnet. This plume had the thermal potential to produce oceanic crust with an average thickness of ∼30 km underlain by a permanently buoyant refractory lithospheric mantle keel. Nb/U ratios in the komatiites and basalts calculated on the basis of Th–U–Pb relationships range from 35 to 47 and are thus similar to those observed in modern MORB and OIB. This implies that some magma source regions of the Kostomuksha lavas have undergone a degree of continental material extraction comparable with those found in the modern mantle. The mafic terrane is interpreted as a remnant of the upper crustal part of an Archaean oceanic plateau. When the newly formed plateau reached the active continental margin, its upper part collided with the sedimentary terrane but was too buoyant to subduct. As a result, the volcanic section of the plateau was imbricated and obducted thus becoming a new segment of continental crust. The deeper zones were delaminated and tectonically underplated or subducted.

Th, U and other trace elements in carbonaceous chondrites: Implications for the terrestrial and solar-systemTh/U ratios

We have analyzed Th, U and several other trace elements in eleven carbonaceous chondrites (including the two CI chondrites Orgueil and Ivuna) by mass spectrometric isotope dilution techniques. In contrast to concentrations of Th and other refractory trace elements, U concentrations are highly variable, both among and within different meteorites.Th/U ratios range from about 1 to 4 and correlate inversely with U concentrations. Surprisingly, CI chondrites display by far the largest variation inTh/U. It is shown that the negative correlation betweenTh/U and U cannot be explained by terrestrial contamination or systematic fractionation within the solar nebula; rather it reflects low-temperature mobilization and redistribution of U on the chondrite parent bodies or within the meteorites. The idea of alteration-inducedTh/U variations in chondrites is supported by the covariation of U in Orgueil with other elements which are known to be mobile under aqueous conditions (e.g., Sb, Cs, Ba) and the fact that the analytical data of Th, U lie on binary mixing lines. Our approach to estimating the solar-systemTh/U ratio is based on analyses of carbonaceous chondrites of all types, the combination of data for Th, U with those for other trace elements and on published Pb isotopes in carbonaceous chondrites. The resulting value ofTh/U= 3.9 ± 0.2 (weight ratio) is higher than previous estimates. The corresponding CI chondritic abundances areTh= 28.9 ± 1.5 ppb andU= 7.4 ± 0.5 ppb. This translates into solar-system abundances ofTh= 0.0329 andU= 0.0082 (atomic abundances relative to 106 Si atoms). Assuming that refractory lithophile elements in the primitive mantle have chondritic relative abundances, but are enriched by a factor of 2.5 relative to CI, we obtainTh= 72.2 ppb andU= 18.5 ppb as primitive mantle abundances. If the early Earths mantle possessed aTh/U weight ratio of greater than 4.1 as suggested by the lead isotopes of Archean rocks, this would reflect very early terrestrial ThU fractionation.

Quaternary volcanic activity of the southern Red Sea: new data and assessment of models on magma sources and Afar plume-lithosphere interaction

Chemical compositions and SrNdPb isotopic ratios are reported for Quaternary alkaline volcanic rocks from the southern Red Sea and adjacent regions. Samples from the Ramad seamount (17°N) have highly radiogenic Pb isotope ratios (206Pb/204Pb = 19.59–19.61) and comparatively depleted Sr and Nd signatures. In isotope diagrams the samples from this seamount fall on extensions of linear arrays defined by samples of Red Sea N-type and E-MORB and confirm the existence of an important HIMU-type component in the mantle beneath the southern Red Sea. Alkali basalts from the islands of Hanish and Zubair located in the southernmost part of the Red Sea do not lie on isotope mixing lines defined by Red Sea MORB and Ramad seamount samples. Instead, their data points are shifted towards the data field of Quaternary Afar and Yemen volcanic rocks, thus reflecting the presence of a third isotopic component with hybrid EM1–EM2 characteristics which dominates the continental volcanic rocks on both shoulders of the southernmost Red Sea. The systematic covariation of isotope ratios and geographic position in southern Red Sea volcanic rocks are very similar to those observed along the Gulf of Aden and thus support the torus plume model proposed by Schilling et al. [Schilling, J.-G., Kingsley, R.H., Hanan, B.B., McCully, B.L., 1992. NdSrPb variations along the Gulf of Aden: Evidence for Afar mantle plume-continental lithosphere interaction. J. Geophys. Res. 97, 10927–10966].

The concentrations of the heavy metals in the four new Antarctic meteorites Yamato (a), (b), (c) and (d) and in Orgueil, Murray, Allende, Abee, Allegan, Mocs and Johnstown

In the four meteorites Yamato (a), (b), (c) and (d) recently found in Antarctica, the trace elements W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Th and U were determined by spark source mass spectrometry. The concentrations of these elements were also measured in the meteorites Orgueil (C1), Murray (C2), Allende (C3), Abee (E4), Allegan (H5), Mocs (L6) and Johnstown (Ah). The trace element concentrations confirm the preliminary classification of the Yamato meteorites already given earlier: Yamato (a) = enstatite chondrite, Yamato (b) = Ca-poor achondrite, Yamato (c) = carbonaceous chondrite of type 3, Yamato (d) = ordinary chondrite. The concentrations of Au and Hg are antipathetically correlated. In the achondrite Yamato (b) the concentrations of the siderophile elements W, Re, Os, Ir, and Pt depend strongly on the boiling points of these metals.

The Antarctic meteorite Yamato 74123 — a new ureilite

The rare gases He, Ne, Ar, Kr and Xe were measured in bulk samples of Yamato 74123. The 3He and 21Ne exposure ages are found to be 5.50 Ma and 2.83 Ma, respectively. In addition to the cosmogenic component the samples contain primordial rare gases of the fractionated type in amounts typical of ureilites. In a three-isotope plot neon turns out to be a mixture of planetary neon and cosmogenic neon. The elements Na, Mg, Al, Si, P, S, K, Ca, Cr, Mn, Fe, Co, and Ni have been determined by spark source mass spectrometry in Yamato 74123 and for comparison in the ureilites Haveroand Kenna. The chemical composition as well as the noble gas abundance pattern identify Yamato 74123 as an ureilite.

Massenspektrometrische Bestimmung der Spurenelemente in Steinmeteoriten

In den letzten Jahren wurde ein Verfahren zur Bestimmung aller schweren Spurenelemente von W bis U durch Festkorper-Massenspektrometrie mit Funkenionenquellen erarbeitet. Zur Bestimmung der Empfindlichkeitsfaktoren wurde der Allende-Standardmeteorit herangezogen. Die Konzentrationen der siderophilen schweren Elemente W, Re, Os, Ir, Pt und Au sind in gewohnlichen und kohligen Chondriten untereinander streng korreliert, wahrend Bi zu den siderophilen Elementen antikorreliert ist. Enstatitchondrite verhalten sich etwas anders.