A.W. Hofmann

Boninite-like volcanic rocks in the 3.7-3.8 Ga Isua greenstone belt, West Greenland : Geochemical evidence for intra-oceanic subduction zone processes in the early Earth

Abstract The 3.7–3.8 Ga Isua greenstone belt of southwest Greenland is characterized by variably metamorphosed, metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks. The voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units. The sedimentary rocks consist mainly of banded iron formation, cherts, conglomerates and siliciclastic turbidites. The least altered metavolcanic amphibolites (the Garbenschiefer unit) from the Central Tectonic Domain of the Isua greenstone belt are characterized by high Mg-number (0.60–0.80), MgO (7–18 wt.%), Al2O3 (14–20 wt.%), Ni (60–645 ppm) and Cr (60–1920 ppm) contents, but low TiO2 (0.20–0.40 wt.%), Zr (12–30 ppm), Y (6–14 ppm) and rare earth element (REE) concentrations. These compositional features collectively represent a coherent mafic to ultramafic suite. Chondrite-normalized REE patterns are concave upward. On the primitive mantle-normalized extended trace element diagrams, they are characterized by relative depletion of Nb, but with an enrichment of Zr, relative to neighboring REEs. Alteration, deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition. The average initial eNd value of these metavolcanic rocks is +2. Collectively, these geochemical characteristics are comparable to those of Phanerozoic boninites. Given the observation that in the Tertiary, boninites are exclusively associated with intra-oceanic subduction environments (e.g., Izu–Bonin–Mariana subduction system), this suggests that intra-oceanic subduction zone-like geodynamic processes were operating as early as 3.7–3.8 Ga.

High precision lead isotope systematics of lavas from the Hawaiian Scientific Drilling Project

Abstract We report Pb isotopic compositions for 35 samples of the volcanoes Mauna Loa and Mauna Kea from the Hawaiian Scientific Drilling Project (HSDP-1) core at Hilo. These data were obtained with an external precision of ∼100 ppm (2σext.) on the ratios 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb by using a Pb triple spike to correct for instrumental mass fractionation. The Pb isotopic compositions in the lower section (1200 to 280 m) of the core sample 200 to 400 ka-old Mauna Kea lavas, and display two well-defined linear arrays in 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–206Pb/204Pb isotope spaces. There is a suggestion that Mauna Loa (0 to 280 m depth) also displays such linear array(s). However, analysis of the Mauna Loa samples is complicated by residual contamination and/or sample heterogeneity. While these latter data exhibit a satisfactory array in 208Pb/204Pb vs. 206Pb/204Pb, there still remains scatter in 207Pb/204Pb–206Pb/204Pb space, making it difficult to assess the true Pb isotope systematics of Mauna Loa. The presence of two linear Pb isotopic arrays in Mauna Kea can be interpreted as either reflecting two parallel isochrons or in terms of binary mixing. If interpreted as isochrons, the 207Pb/204Pb–206Pb/204 Pb systematics correspond to an age of ∼1.9 Ga. Comparison of measured Th/U ratios in the lavas and those inferred from Pb isotope systematics strongly suggest that the Pb isotopic arrays reflect binary mixing, and this bears directly on the question of how many distinct components are present in the Hawaiian plume. Most of the new Mauna Kea data lie well outside the mixing-component triangle defined in the literature by the “Kea”, “Loihi”, and “Koolau” components. On the basis of the relationships between Pb isotope ratios in 3D and a principal component analysis of the Mauna Kea Pb isotope dataset, we show here that a three-component mixing model can in principle explain both mixing lines. However, such an explanation requires a highly specific set of mixing conditions in order to produce parallel arrays in Pb isotope space (2D and 3D). Therefore, our preferred interpretation is that the two arrays reflect binary mixing, with four discrete source components involved in the generation of the Kea lavas. Comparison of the Pb isotope characteristics of these lavas with those of East Pacific Rise (EPR) MORB glasses further suggests that EPR-type Pacific lithosphere does not contribute to the source of Kea lavas. The position of samples along the mixing lines does not correlate with stratigraphic height in the core, and therefore the age of the lavas. Rather, it appears as though the relative proportions of the endmembers are controlled by the spatial configuration of these endmembers, and by melting and transport processes in the source itself. The stratigraphic fluctuations of Pb and Sr isotopes contrast with the monotonic decrease of eNd and eHf values as a function of age. This may in part be explained by differences in analytical precision of isotope measurements relative to the total range of values observed. This analytical resolution is far higher for Pb than for the other radiogenic isotopes. Alternatively, the observed fluctuation may be caused by the mobility of lead (as well as Rb and/or Sr) during the ancient differentiation process that created the differences in parent–daughter ratios.

HYDRATION AND DEHYDRATION OF OCEANIC CRUST CONTROLS PB EVOLUTION IN THE MANTLE

Abstract The position of the Pb isotopic compositions of mid-ocean ridge basalts (MORB) to the right of the geochron has long puzzled geochemists. Uranium is more incompatible than Pb during mantle melting, and the mantle source of MORB, being depleted in incompatible elements, should have low Pb isotope ratios and plot to the left of the geochron. This has been called the “lead paradox”. MORB have a second peculiar characteristic: Pb concentrations are drastically depleted relative to other elements of similar compatibility such as Ce or Nd. We suggest that both characteristics can be explained by preferential mobility of Pb during hydrothermal alteration of the oceanic crust associated with sea-floor spreading, and subsequent dehydration during subduction. A portion of this Pb migrates into the mantle wedge and is then added to the continental crust via arc magmas. Parts of the Pb-depleted recycled oceanic crust are stored at some deep level in the mantle and eventually become the source of ocean island basalts, the rest mixes into the mantle to become the MORB source. This model is evaluated quantitatively and special attention is given to the evolution of the Ce/Pb ratio in the depleted mantle from the beginning of Earth history to the present.

Nb in Hawaiian magmas: constraints on source composition and evolution

Abstract Published Th and Nb data for basalts from 11 volcanoes from the Hawaiian Islands and Emperor seamount chain yield a nearly uniform concentration ratio of Nb Th = 15.2 ± 1.7 . The basalts include tholeiites with Nb concentrations ranging down to 4 ppm and alkali basalts with a maximum Nb concentration of 127 ppm. (However, nephelinites and melilitites from the Honolulu Volcanic Series have much lower Nb/Th ratios and are excluded from this analysis.) Using the average Th/U ratio of 3.2 for the five volcanoes of the island of Hawaii, this corresponds to a Nb U = 48.6 ± 5.3 , which is indistinguishable from the mean value of a worldwide sample of OIB and MORB ( Nb U = 47 ± 10 ) given by direct Nb and U analyses. In contrast, the Earths primordial mantle had a Nb/Th ratio of 8.6 and a Nb/U ratio of 30. These data demonstrate that none of the Hawaiian volcanoes, not even the isotopically most nearly primitive Koolau Series from Oahu, has refractory elements derived from an undifferentiated or primordial portion of the mantle. An even less likely source is recycled continental crust, which has a mean Nb/U ratio of 10. A consideration of the chromatographic theory of metasomatism and comparison with metasomatic rocks observed in the continental crust show that large-scale metasomatism in the mantle, which would be needed to account for the very large total volume of Hawaiian-Emperor magmas, is expected to produce a strongly zoned mantle yielding chemically highly heterogeneous magmas unless the metasomatic column has reached a steady state. The expected heterogeneity is difficult to reconcile with observed chemical homogeneity of the main-stage tholeiitic magmas. (The late-stage alkali basalts and the nephelinites must be excluded from this consideration, because their isotopic composition shows that they are derived from the depleted lithosphere or asthenosphere.) The consequence of a steady-state metasomatic column would be that the tholeiitic melt itself represents the metasomatic fluid. The model of recycled oceanic crust and lithosphere is consistent with the chemical and isotopic data. The mantle that originally produced this oceanic crust was already depleted and rehomogenized as a result of prior separation of continental crust. Uniform source ratios of Nb/U and Ba/Rb were established during this rehomogenization process. The general enrichment of the Hawaiian source and the chemical and isotopic heterogeneities found in the present-day lavas were produced during the differentiation into oceanic crust and lithosphere. Secondary Pb/1bPb model ages indicate that this crust was probably produced ∼ 1 Ga ago.

Contrasting bulk and mineral chemistry in depleted mantle peridotites: evidence for reactive porous flow

A series of recent papers have indicated that reconstructed bulk compositions of abyssal peridotites define chemical correlations, namely increasing FeOtot and decreasing SiO2 with increasing MgO, which cannot be produced by simple extraction of partial melts. However, no general consensus exists on the reliability of these trends, because they could be artifacts of the adopted calculations, and on the origin of abyssal peridotites. Specifically, it has been inferred that abyssal peridotite compositions are consistent with combined histories of partial melting and subsequent melt migration which caused either olivine addition, or dissolution and precipitation reaction, by equilibrium porous flow [Niu et al., Earth Planet. Sci. Lett. 152 (1997) 251–265; Asimow, Earth Planet. Sci. Lett. 169 (1999) 303–319]. We report combined bulk-rock and mineral chemical data for ophiolitic peridotites from the Erro-Tobbio (ET) Unit (Voltri Massif, Ligurian Alps), which represent lithosphere remnants of the Jurassic Ligurian Tethys embryonic ocean. These peridotites include sp-lherzolites and sp-harzburgites, and display overall depleted geochemical signature. However, comparison between bulk rock and corresponding mineral compositions reveals that these rocks cannot be residues of simple (equilibrium or fractional) melt extraction. Mineral compositions are similar in all the samples. By contrast, the bulk rock compositions define striking correlations, i.e. increasing FeOtot, Ni, Co, and decreasing Al2O3, SiO2, CaO, Sc, Cr, YbN, with increasing MgO: the MgO–FeOtot and MgO–SiO2 correlations are similar to those recognized in abyssal peridotites. Thus, the ET peridotites provide evidence that the above trends are indeed consistent with similar variations in on-land peridotites; also, these trends cannot simply result from progressive melt depletion, because constituent minerals in the different ET samples have rather uniform composition. Calculated bulk modes indicate that the observed chemical variations are coupled to systematic modal changes, namely decrease in cpx and opx, and increase in olivine, at increasing bulk MgO. The ET peridotites also display decrease of the cpx/opx ratio at increasing bulk MgO, and this argues against a process of simple olivine addition. By contrast, some peculiar bulk-mineral compositional features – e.g. (i) nearly constant olivine Mg* [=Mg/(Mg+Fetot)] values, at increasing modal olivine, (ii) opposite bulk Cr and Ni correlations, (iii) bulk Cr and Yb decrease, and parallel Ni increase, strikingly correlated with progressive modal cpx decrease and concomitant modal olivine increase – are consistent with the expected chemical and modal effects, during a process of interaction between depleted peridotites and melts migrating by equilibrium porous flow (involving pyroxene dissolution and olivine precipitation reactions). Bulk-rock and mineral chemistry data in the ET peridotites thus indicate that the major element correlations inferred by Niu et al. for the abyssal peridotites are reliable, and most likely result from a combined history of partial melting and melt interaction by reactive porous flow.

Geographic control on Pb isotope distribution and sources in Indian Ocean Fe-Mn deposits

Abstract High-precision Pb isotope data have been obtained on Indian Ocean Fe-Mn deposits with a large geographic coverage. These data reveal a strong geographic control on the distribution of Pb isotopes as well as sources of Pb within this basin. The provinciality of Pb isotopes at the scale of the whole Indian Ocean, as well as that of individual basins, broadly matches the pattern of deepwater flow. The existence of several sources of Pb is best illustrated by the presence of three well-defined Pb isotopic arrays, each of which is confined to clear-cut geographic domains. These arrays imply a dominance of binary mixing of Pb sources within each of these domains. The domains consist of the North-Indian (N-Indian; north of 20°S), Southwest-Indian (SW-Indian; 20°S to 50°S, west of 45°E), South-Indian (S-Indian; 20°S to 50°S, east of 45°E) and Antarctic-Indian (A-Indian; south of 50°S), and clearly exhibit a strong control by latitude. The S-Indian domain and Mozambique Channel samples form a cluster, suggesting that there are more than just two sources contributing in these regions. We show that the N-Indian is dominated by sources of Pb derived from the High Himalayas and the Trans-Himalayan Complex, and most likely inherited from interaction at the water interface with Bengal and Indus Fan sediments. The SW-Indian and A-Indian domains share a common unradiogenic component, represented by circumpolar waters derived from the Pacific Ocean and flowing through the Drake Passage into the Atlantic and Indian Oceans. However, the radiogenic sources of Pb in these two domains are clearly distinct. In the SW-Indian, the radiogenic Pb signal reflects the tongue of North Atlantic Deep Water flowing around the tip of South Africa, while in the A-Indian, the source of radiogenic Pb is less certain, but quite probably originates in the Weddell Sea region. These data illustrate the complexity of the spatial Pb isotopic distribution, and Pb sources, within a single ocean basin at the present day, which should be borne in mind when interpreting long-term radiogenic isotope paleorecords from Fe-Mn crusts.

CONSTRAINTS ON EARTH EVOLUTION FROM ANTIMONY IN MANTLE-DERIVED ROCKS

Abstract We have analyzed Sb in a variety of mantle-derived volcanic rocks, peridotites, and in the CI chondrite Orgueil by spark source mass spectrometry. Concentrations vary from 0.02 to 0.8 ppm in oceanic basalts (mid-ocean ridge basalts, MORB; oceanic island basalts, OIB). Antimony is a moderately siderophile element which behaves like the incompatible lithophile element Pr during igneous processes in the mantle. Both MORB and OIB samples have similar Sb/Pr ratios of about 0.02, which are different from those in continental-crustal rocks. Antimony resembles Pb in that it behaves like a highly incompatible element during formation of continental crust, whereas it behaves only moderately incompatible during formation of oceanic basalts (MORB or OIB). Consequently, Sb/Pb ratios of oceanic basalts agree within error limits with those of the continental crust and with the CI chondritic value, indicating that Sb/Pb is not strongly fractionated during crust-mantle-core differentiation. From the Sb/Pb ratios we estimate a Sb concentration of 11 ± 5 ppb for the primitive mantle. An alternative estimate for the primitive-mantle abundance is obtained by assuming the Sb/Pr ratio of the primitive mantle to be intermediate between the MORB-01B value (0.02) and that of the continental crust (0.05). This approach yields Sb = 8 ± 4 ppb for the primitive mantle. Antimony is depleted in the bulk silicate Earth by a factor of 45. The volatility-corrected depletion factor of 7 is similar to other moderately siderophile elements.

Pb isotopic discrimination of crustal domains within the highgrade basement of Sri Lanka

Pb isotope data confirm the recognition of two isotopically distinct crustal domains, originally proposed on the basis of Nd model age data, within the central belt of granulite-grade rocks in Sri Lanka. One domain, the Highland Complex, comprise rocks that have late Archaean-Palaeoproterozoic Nd model ages and unusually high 207Pb204Pb compositions. They plot well above the 207Pb204Pb plumbotectonic growth curves for average upper crust and orogeny of Zartman and Doe (1981), and have isolated from plumbotectonic exchange more than 1 Ga prior to metamorphism ∼ 600 Ma ago. Two Highland samples suggest that, in some cases, this isolation occurred some 2 Ga ago or earlier. This isolation age is considered to date the time the pre-granulite Highland terrain was “cratonised”, i.e. incorporated into a stable shield environment. Pb isotopic compositions of rocks from the second domain, the Wanni Complex, straddle the upper crust and orogeny plumbotectonic curves. In contrast to the Highland granulites, there is no evidence for an extended plumbotectonic isolation period prior to metamorphism. The available UPb zircon, Nd and Pb isotope data indicate that the Wanni Complex represents a mid-Neoproterozoic crustal package (0.7Ga < T< 1.3 Ga) metamorphosed some 600 Ma ago. A time gap exceeding 0.5 Ga separates the age of “cratonisation” ∼1.8−2.0 Ga) of the pre-granulite Highland terrain from the times of formation of the earliest Wanni protoliths (<1.3 Ga). The distinctive isotopic characteristics and the unrelated geological evolution reconstructed from these data indicate that the two now-adjacent Highland and Wanni complexes were geologically unrelated crustal packages prior to high-grade metamorphism and, as such, represent “suspect terranes” brought together during a collision event.