Wafa Abouchami

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.

Crustal growth in West Africa at 2.1 Ga

Birimian (∼2.1 Ga) terranes in the West African craton are a mixture of highly metamorphosed volcanic, sedimentary and plutonic rocks and low grade metavolcanics and metasediments. The volcanic units contain thick, commonly pillowed, tholeiitic basalts overlain by pelagic sediments cherts and black shales; The sedimentary units are characterized by an abundance of clastic turbiditic sediments. Andesites and calc-alkaline felsic volcanics occur at uppermost stratigraphic levels and as dykes. Field relationships between the volcanic and sedimentary units remain a matter of debate. Calc-alkaline and local alkaline granites, which intruded in distinct pulses and occasionally are related to transcurrent tectonics, represent almost half of the Birimian terranes. New isotopic work on the highly metamorphosed units greatly improved the chronology for the Birimian crust. The age of the early Dabakalian event is precisely defined by a U-Pb zircon age at 2186 ± 19 Ma, while Rb-Sr and Sm-Nd methods give ages of 2162 ± 19 Ma and 2141 ± 24 Ma, respectively. A Sm-Nd garnet-whole rock age of 2153 ± 13 Ma suggests that metamorphism culminated at about the same time. In contrast, the most precise zircon U-Pb and Sm-Nd data for the more widespread Birimian terranes (sensu stricto), from this study and from the literature, cluster between 2.12 and 2.07 Ga. The major evolution of the Birimian crust apparently lasted less than 50 Ma. Isotopic evidence indicates that Birimian granitoids contain a negligible component of Archean crust: eNd(2.1-Ga) values are positive and similar to those of Birimian basalts, crustal residence times are shorter than 200 Ma, U-Pb ages for detrital zircons from clastic sediments range from 2098 ±11 Ma to 2125 ± 17 Ma, while granite chemistry and Nd isotopic characteristics are unrelated. Only very locally in Guinea is there isotopic evidence of interaction between Birimian felsic magmas and the Archean rocks from the Man craton. In accord with Abouchami et al.s (1990) suggestion that Birimian basalts represent oceanic plateaus, the present data argue that the protolith of much of the West African continent was created around 2.1 Ga in an environment remote from Archean crust. Intrusion of calc-alkaline magmas into the tholeiitic units suggests that island arcs formed on top of the assumed oceanic plateaus which then collided with the Man Archean craton. Taking the Birimian formations from the Guyana shield into account, the minimum crustal growth rate at 2.1 Ga is about 1.6 km3/a, some ∼60% higher than the present growth rate. Birimian crust growth at 2.1 Ga is reminiscent of Archean processes but contrasts with 1.7 – 1.9 Ga crust formation in the North Atlantic continent which generally involved significantly more interaction with older continental crust. A comparison of the Birimian crustal growth rate with the average crustal growth rate over the Earth history implies that a large part of the Birimian crust has been recycled into the mantle or incorporated into younger orogenic segments. This apparent deficit in the crustal budget is even more dramatic for the Archean crust.

Lead isotopes reveal bilateral asymmetry and vertical continuity in the Hawaiian mantle plume

The two parallel chains of Hawaiian volcanoes (‘Loa’ and ‘Kea’) are known to have statistically different but overlapping radiogenic isotope characteristics. This has been explained by a model of a concentrically zoned mantle plume, where the Kea chain preferentially samples a more peripheral portion of the plume. Using high-precision lead isotope data for both centrally and peripherally located volcanoes, we show here that the two trends have very little compositional overlap and instead reveal bilateral, non-concentric plume zones, probably derived from the plume source in the mantle. On a smaller scale, along the Kea chain, there are isotopic differences between the youngest lavas from the Mauna Kea and Kilauea volcanoes, but the 550-thousand-year-old Mauna Kea lavas are isotopically identical to Kilauea lavas, consistent with Mauna Keas position relative to the plume, which was then similar to that of present-day Kilauea. We therefore conclude that narrow (less than 50 kilometres wide) compositional streaks, as well as the larger-scale bilateral zonation, are vertically continuous over tens to hundreds of kilometres within the plume.

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.

Pb and Nd isotopes in NE Atlantic Fe-Mn crusts : proxies for trace metal paleosources and paleocean circulation

Abstract We report high precision Pb isotopic data (2σ ≤ 100 ppm) together with Nd isotopes on depth profiles from two Fe–Mn crusts from the eastern Atlantic basin. The profiles provide a 13 Ma record of changes in eastern North Atlantic Deep Water (ENADW), and over the past 8 Ma for Mediterranean Outflow Water (MOW). Pb isotope ratios in the two records display gradual and systematic changes, tracking each other through time. The highly precise Pb isotopic data also resolve, for the first time, changes in all three Pb isotope ratios in a single Fe–Mn crust, and exhibit well-defined binary mixing lines in Pb isotope space. The Pb isotopic record of the Tropic Seamount crust shows that eastern Atlantic Pb has been dominated by binary mixing throughout the last 13 Ma. The Pb binary mixing lines further demonstrate that a change in Pb provenance to the eastern Atlantic occurred at 8 and 4 Ma. This is shown by the distinctive Pb isotopic compositions of the mixing components in the time intervals 4–0 Ma and 13–8 Ma. Consideration of Pb and Nd isotope systematics show that from 4 Ma to the present, the two endmember components correspond to NADW and Southern Component Water (SCW). The quasi-cyclic character of 206Pb/204Pb variations, as well as age progressive changes in the Pb mixing proportions of NADW and SCW endmembers, appear to reflect changes in deep water circulation. In particular, strengthening of the Pb and Nd isotopic signal associated with NADW since 3–4 Ma in both the eastern and western Atlantic basins implies that NADW advection from the western into the eastern Atlantic has been in operation over the past 4 Ma. During the period from 13 to 8 Ma, two entirely different sources of Pb and Nd existed in the eastern Atlantic. The radiogenic Pb and unradiogenic Nd component may have originated in the Norwegian–Greenland seas and the low 206Pb/204Pb—high eNd endmember component in the Southern ocean or Tethys. This suggestion is supported by evidence from paleoceanographic studies indicating the importance of Greenland–Scotland ridge activity in Miocene deep water production, and also the presence of a distinctive high δ13C water mass in the Southern Ocean, derived either locally or flowing in via Tethys. The 8 Ma record of the Lion Seamount crust (65GTV) demonstrates that Pb and Nd sources in MOW switched from predominantly internal, Mediterranean (European) sources prior to 4 Ma, to mainly external (Saharan) sources after 4 Ma. The gradual increase of Pb isotope ratios seen following the end of the Messinian reflects enhanced input of Saharan dust into the water column in the eastern Atlantic. The strengthening of the “Saharan” isotopic signal from about 4 Ma also matches the documented increase in aridity and dustiness in the Saharan and sub-Saharan regions and coincides with the re-establishment of water exchange between the Atlantic and Mediterranean after the Messinian. Although Pb and, to a lesser extent, Nd isotope ratios are distinct during the Messinian (6.5–5 Ma), there is no clear evidence for either a shutdown of MOW or a stronger North Atlantic signal during this period. The fact that a similar isotopic signal is observed, with an even higher amplitude in the ENADW record, shows that this signal is a feature of the whole eastern Atlantic. From 3 to 4 Ma ago, a source of radiogenic Pb and unradiogenic Nd appears to have dominated not only the eastern Atlantic but the world oceans, since it is seen ubiquitously in other Fe–Mn crusts at this time. This isotopic signal must be conveyed around the globe via the ocean circulation or the atmosphere. While the closure of the Panama gateway may have played an important role in these global changes in source(s) and/or fluxes of Pb and Nd to the oceans 3–4 Ma ago other events, in conjunction, such as the emergence of the Greenland–Scotland ridge, were probably also as influential.

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.

Secular changes of lead and neodymium in central Pacific seawater recorded by a FeMn crust

Oceanic and atmospheric circulation patterns varied considerably during the Tertiary and Quaternary and influenced the geochemical cycles of elements in seawater. We report the first resolution lead and neodymium isotopic record of such changes at a high time resolution in two depths profiles from a hydrogenous FeMn crust. The crust, Va13-2, is located in the central Pacific (146°W, 9°25′N, 4830 m) and has previously been dated by 230Th and 10Be. The first profile was drilled with a sample time resolution of ∼3 kyr and allows evaluation of short-term changes to lead and neodymium sources to central Pacific seawater over the last 400 kyr (marine δ18O stages 2 to 11). Longer-term changes were monitored at lower time resolution in a second profile to an age of 10 Ma. Short-term variations in lead and neodymium isotope ratios are resolved in the high resolution profile (0 to 400 kyr). Superimposed on the short-term variations is a secular decrease in 206Pb204Pb ratios beginning at ∼130 kyr in marine δ18O stage 5, implying a change in the lead sources to the central Pacific. Lead and neodymium isotopic compositions indicate an increased influence from Central American eolian sources to Pacific seawater at this time. Lead isotopes are found to be statistically more variable during interglacial than glacial periods. These observations are supported by the greater eolian dust fluxes found in sediment cores from the equatorial Pacific during interglacial stages. The most important paleoceanographic event of the last 10 Ma to affect Pacific seawater was the closure of the Panama gateway. Changes in lead and neodymium isotopes in Val3-2 during the last 10 Ma occurred along with gradual closure of the Panama straits. However, these changes did not occur in tandem: while neodymium isotope ratios increase between 10 and 8 Ma, lead isotope ratios remain constant. In contrast, the period 7 to 1 Ma is marked by a secular increase in lead isotope ratios but nearly constant neodymium. These changes are consistent with a source of radiogenic lead and neodymium conveyed by the Circumpolar Current into the Pacific, rather than by the Panama gateway, and involve 20 to 40% Southern Component Water (SCW) input of lead and neodymium. Modelling of lead and neodymium isotopic mixing between the different water masses involved in generating Pacific deep waters lead us to the following conclusions: (1) Small variations in the strength and composition of North Atlantic Deep Water (NADW) have a relatively minor effect on the amounts of lead and neodymium from SCW contributing to the Pacific and (2) an enhanced SCW flow with an open isthmus of Panama, as suggested by General Circulation Models (GCM), requires a corresponding reduction in NADW Pb and Nd contributions to SCW. The general agreement between the isotopic compositions of surface layers of Mn nodules, integrated over such long time intervals, and those of present-day bottom waters at their respective locations show that the present-day ocean circulation pattern has dominated through the Pleistocene. Our study of Mn crust Va13-2 shows that shorter-term changes in lead and neodymium isotope ratios can be resolved, provided that such crusts are sampled at an appropriate time resolution.

A lead isotopic study of circum-antarctic manganese nodules

Abstract Lead isotope ratios in manganese nodules from the Circum-Antarctic ocean display systematic geographical variations. Eastward, in the direction of current movement, from south of Australia, they progressively decrease in the Pacific sector and into the western Atlantic sector until ∼30°W. From this longitude, lead isotope ratios increase in the eastern Atlantic sector and reach their highest values in the Indian sector near 110°E. South of western Australia they abruptly decrease. These two lead isotope domains correspond closely to the Circum-Antarctic neodymium isotope domains previously recognized by Albarede and Goldstein (1992). However, the intraocean isotopic variations are much clearer for Pb than Nd, reflecting the shorter oceanic residence time of Pb (∼102 y) compared with Nd (∼103 y), and confirming that Pb is a much more sensitive marine tracer. Throughout the Circum-Antarctic ocean, the lead isotopic changes follow the pattern of present-day oceanic circulation. The Pb shows an overall continental signature, which is strongest in the Indian sector. The most probable candidate as the source of the continental signal is North Atlantic Deep Water (NADW ) which enters the Circum-Antarctic in the south Atlantic. As NADW mixes with Circumpolar water, lead isotope ratios increase eastward in the Indian sector, reflecting progressive loss of the Pacific-derived Pb signal. The abrupt decrease of lead isotope ratios between the Indian and Pacific sectors appears to be a consequence of addition of Pacific seawater. Through the Pacific, lead isotope ratios gradually decrease eastward, approaching values of East Pacific Rise metalliferous sediments, ocean ridge basalts, and south Pacific Mn nodules. This trend results from an increasing influence of Pacific ocean water containing a component derived from Pacific volcanism. The Pacific signature persists into the Argentine basin in the southwest Atlantic Ocean, reflecting significant input of water from the Drake Passage northward into the Atlantic. The Pacific Pb component is seen in the Atlantic and Indian oceans near southern Africa as well as in the Atlantic and Indian sectors of the Circum-Antarctic. Lead and neodymium isotopes correlate in the Atlantic-Indian sectors but not in the Pacific sector, reflecting differences in residence times and modes of water mass mixing. The renewal of Circum-Antarctic water occurs on a timescale which is short compared to the residence time of Nd but long compared to Pb. As a result, dissolved Pb is subject to considerable changes over the timescale of Circumpolar current movement, while Nd is less easily affected. NADW is added locally in the south Atlantic and moves eastward as a water mass. As NADW mixes with Circumpolar water (CPW) through the Indian sector, changes affecting Nd/Pb in NADW and CPW occur in tandem, and neodymium and lead isotopes are correlated. Through the Pacific sector, gradual addition of Pacific seawater occurs over a large region and Nd/Pb relationships are not coupled. Because simple isotopic mixing systematics are not conserved there, neodymium and lead isotopes vary independently. The consistency of lead isotope variations with present-day ocean bottom water circulation is remarkable, considering that the Mn nodules as sampled reflect the integrated flux of Pb over ∼ 105-106 y, the oceanic mixing time is ∼103 y, the residence time for Pb is ∼102 y, and the circulation time of the CircumAntarctic ocean is ∼30 y. Processes occurring over such different timescales can show such a relationship only if the system is stable over the longest time interval. Thus, on average, element sources and ocean circulation through the Pleistocene have been similar to the present-day, despite significant perturbations during glacial periods.

Geochemical characteristics of West Molokai shield‐ and postshield‐stage lavas: Constraints on Hawaiian plume models

There are systematic geochemical differences between the < 2 Myr Hawaiian shields forming the subparallel spatial trends, known as Loa and Kea. These spatial and temporal geochemical changes provide insight into the spatial distribution of geochemical heterogeneities within the source of Hawaiian lavas, and the processes that create the Hawaiian plume. Lavas forming the similar to 1.9 Ma West Molokai volcano are important for evaluating alternative models proposed for the spatial distribution of geochemical heterogeneities because ( 1) the geochemical distinction between Loa and Kea trends may end at the Molokai Fracture Zone and ( 2) West Molokai is a Loa- trend volcano that has exposures of shield and postshield lavas. This geochemical study ( major and trace element abundances and isotopic ratios of Sr, Nd, Hf, and Pb) shows that the West Molokai shield includes lavas with Loa- and Kea- like geochemical characteristics; a mixed Loa- Kea source is required. In contrast, West Molokai postshield lavas are exclusively Kea- like. This change in source geochemistry can be explained by the observed change in strike of the Pacific plate near Molokai Island so that as West Molokai volcano moved away from a mixed Loa- Kea source it sampled only the Kea side of a bilaterally zoned plume.

A crustal section of an intra-oceanic island arc : The Late Jurassic-Early Cretaceous Guanajuato magmatic sequence, central Mexico

Abstract The Late Jurassic-Early Cretaceous Alisitos-Teloloapan intra-oceanic arc, which extends along the Pacific coast of Mexico, is overlain by the extensive thick calc-alkaline Tertiary volcanics of the Sierra Madre Occidental. In central-southern Mexico this arc can be divided in to two major units. Its base, which is mainly exposed in the Guanajuato-Leon area, consists of a volcano-plutonic assemblage refered to as the Guanajuato magmatic sequence. The top consists of pillowed calc-alkaline basalts and andesites overlain and/or interbedded with a thick volcaniclastic sedimentary pile in which lenses of reworked Albian-Aptian reefal limestones are interlayered. The Guanajuato magmatic sequence represents the crustal section of an intra-oceanic nascent island arc and from top to base is made up of: (1) volcaniclastic sedimentary cover, (2) a predominantly pillowed basaltic sequence, (3) a doleritic and basaltic dyke complex, (4) silicic plutonic rocks and salite diorite, (5) Mg-hornblende diorite, (6) layered cumulate gabbros and, (7) ultramafic-mafic cumulates. Plutonic rocks of intermediate to silicic composition are volumetrically abundant. All the rocks have suffered low-grade metamorphism with magmatic textures preserved. Metamorphic textures do not occur. Mg-rich olivine is restricted to the ultramafic cumulates, whereas Ca-rich clinopyroxene is present in all the rock types, although with the exception of the most differentiated silica rich plutonic rocks. The textural relationships and mineral chemistry of the clinopyroxenes and hornblende in the ultramafic cumulates and in the clinopyroxene gabbros point to an origin by igneous accumulation from a tholeiitic parent magma at shallow levels above the mantle-crust boundary. Al-rich basalts and dolerites belong to a low-K tholeiitic suite. The Nb contents, which are abnormaly high for arc tholeiites, are in the range of N-MORB. Such enrichments have been related to an immature stage of the arc development, the mantle wedge being still enriched. This is in agreement with the evidence that the Guanajuato tholeiitic magma represents the earliest products of the Alisitos-Teloloapan arc magmatic activity. The Guanajuato basic rocks and hornblende diorite show similar high eNd ratios which fall in the range of the values of oceanic arcs. The most siliceous plutonic rocks differ from the basic suite by virtue of their slightly lower eNd and higher eSr ratios, which may be explained by an assimilation-fractional crystallization (AFC) process. The siliceous melts differentiated by crystal fractionation from the tholeiitic mantle magma and assimilated material such as metamorphosed oceanic crust and sediments, which form the wall rocks of the magma chamber. The high eSr ratios of the basalts and their feeder dykes are probably related to the polyphase low-grade metamorphism (i.e. hydrothermal oceanic alteration and regional metamorphism) that has affected the uppermost levels of the arc.