Anton P. le Roex

Isotope characteristics of the Okenyenya igneous complex, northwestern Namibia: constraints on the composition of the early Tristan plume and the origin of the EM 1 mantle component

Sr, Nd and Pb isotope data are presented for a variety of intrusive rocks from the Mesozoic age Okenyenya igneous complex, which is temporally and spatially associated with the Etendeka Group volcanic rocks in northwestern Namibia. On the basis of bulk rock geochemistry the Okenyenya intrusions can be subdivided into tholeiitic and alkaline suites. The tholeiitic suite has a wide range in isotope composition; for example, initial ϵSr (ϵSr(i)) from 1.2 to 150 with decrease in initial ϵNd (ϵNd(i)) from 4.8 to −3.9. In contrast, the undersaturated rock types show a more restricted range and, in terms of ϵSr(i) (− 11.0–15.1) and ϵNd(i) (0.3–5.0), plot within the mantle array and close to Bulk Earth values. The range in isotope composition shown by the Okenyenya intrusions is similar to that shown by the Etendeka Group volcanic rocks. The tholeiitic suite is comparable in isotope composition to the Etendeka low TiZr (LTZ) basalts and defines a trend towards continental crust, whereas the alkaline suite is similar to the Etendeka Tafelkop basalts. The Etendeka high TiZr (HTZ) basalts do not have an isotopic equivalent amongst the Okenyenya intrusions, but are indistinguishable from basalts in DSDP Hole 525A on the Walvis Ridge; both are strongly displaced towards enriched mantle (EM 1) sources. The large variation in ϵSr(i) shown by the tholeiitic suite and Etendeka LTZ basalts appears to reflect extensive crustal contamination of the magmas, whereas the HTZ basalts, which trend towards EM 1, owe their isotope composition to melting of ancient continental lithospheric mantle. The alkaline gabbros and the Tafelkop basalts have compositions similar to the present-day composition of the Tristan plume and are interpreted as direct melts of the upwelling Tristan mantle plume at the time of continental break-up. An analogous relationship is observed between the Marion plume, Madagascan Upper Cretaceous basalts, and MORB erupted at the intersection between the Southwest Indian Ridge (SWIR) and the Madagascar Ridge. The strong EM 1 signature of basalts found in DSDP Hole 525A on the Walvis Ridge, and at 39–41°E on the SWIR is attributed to melting at shallow depth of ancient continental lithospheric mantle thermally eroded and rafted into the surrounding asthenosphere at the time of continental breakup.

Petrology and geochemistry of basalts from the American-Antarctic Ridge, Southern Ocean: implications for the westward influence of the Bouvet mantle plume

Ridge segments and fracture zones from the American-Antarctic Ridge have been systematically dredge sampled from ∼4° W to ∼18° W. Petrographic studies of the dredged basalts show that the dominant basalt variety is olivine-plagioclase basalt, although olivine-plagioclase-clinopyroxene basalt is relatively common at some localities. Selected samples have been analysed for major and trace elements, rare earth elements and Sr and Nd isotopes. These data show that the majority of samples are slightly evolved (Mg#=69-35) N-type MORB, although a small group of samples from a number of localities have ‘enriched’ geochemical characteristics (T- and P-type MORB).These different types of MORB are readily distinguished in terms of their incompatible trace element and isotopic characteristics: N-type MORB have high Zr/Nb (17–78), Y/Nb (4.6–23) and 143Nd/144Nd (0.51303–0.51308) ratios, low Zr/Y (2.2–4.2) and 87Sr/86Sr (0.70263–0.70295) ratios and have (La/Sm)N<1.0; T-type MORB have lower than chondritic Zr/Nb ratios (8.8–15.5), relatively low Y/Nb (1.9–4.3) and 143Nd/144Nd (0.51296–0.51288) ratios and relatively high Zr/Y (3.1–4.7), 87Sr/86Sr (0.70307–0.70334) and (La/Sm)N (1.1–1.5) ratios; the single sample of P-type MORB has low Zr/Nb (6.3), Y/Nb (0.9) and 143Nd/144Nd (0.51287) ratios and high Zr/Y (7.1), 87Sr/86Sr (0.70351) and (La/Sm)N (2.4) ratios. The geochemical characteristics of this sample are essentially identical to those of the Bouvet Island lavas.Geochemically ‘enriched’ MORB are less abundant on the American-Antarctic Ridge than on the Southwest Indian Ridge but their geochemical characteristics are identical. The compositions of T- and P-type MORB are consistent with a regional mixing model involving normal depleted mantle and Bouvet plume type magma. On a local scale the composition of T-type MORB is consistent with derivation from depleted mantle which contains ∼4% veins of P-type melt.We propose a model for the evolution of the American-Antarctic Ridge lavas in which N-type MORB is derived from mantle with negligible to low vein/mantle ratios, T-type MORB is derived from domains with moderate and variable vein/mantle ratios and P-type MORB from regions with very high vein/mantle ratios where vein material comprises the major portion of the melt. The sparse occurrence of ‘enriched’ lavas and by implication ‘enriched’ mantle beneath the American-Antarctic Ridge, some distance (500–1,200 km) from the Bouvet plume location, is interpreted to be the result of lateral dispersion of enriched mantle domains by asthenospheric flow away from the Bouvet mantle plume towards the American-Antarctic Ridge.

South Atlantic hot spot-plume systems: 1. Distribution of volcanism in time and space

Abstract New Ar—Ar dating of rocks dredged from seamounts and ridges distributed along the St. Helena and Gough volcanic chains suggests that these features were formed by, respectively, the activity of the St. Helena and Walvis hot spot-plume systems. The St. Helena and Walvis (referred to elsewhere as Tristan) hot spots probably consist of broad zones of diffuse volcanism (i.e. oceanic islands, seamounts, and small ridges), at least 500 km in diameter. It remains unclear as to whether one or several narrow plume(s) is (are) upwelling to form these broad zones of hot spot volcanism, which results from decompression melting across parts of the broad, impacted ‘mushroom head’ of the plume(s). The very slow velocity of the African plate, in association with the westward flow of St. Helena and Walvis plume material to the South Atlantic spreading-axis, are likely to be important factors in the development of these broad fields of mid-plate volcanism. On a more localized scale, lithospheric structure (e.g. fracture zones) probably controls the locations of sites of hot spot volcanism. The distributions of Ar—Ar ages along the St. Helena and Gough chains, in conjunction with the proposition of their having been formed by broad hot spots, have been incorporated into a reconstruction of African plate motion over hot spot-plume systems since the opening of the South Atlantic. Estimates of the velocity of the African plate suggest that the African plate might have slowed significantly between ∼ 31 and 0 Ma. The South Atlantic spreading axis migrated westward away from the (fixed?) St. Helena and Walvis hot spot-plume systems, this migration beginning between ∼ 80 and 70 Ma. This led to a transition from an on-spreading-axis to a mid-plate constructional setting along the St. Helena Chain and the Walvis Ridge.

Geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the ‘famous’ region

Bulk rock major and trace element variations in selected basalts from the Famous area, in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types. The distribution of NiO in olivine and Cr-spinel phenocrysts distinguishes the picritic basalts, plagioclase phyric basalts and plagioclase-pyroxene basalts from the olivine basalts. For a given Mg/Mg+Fe2+ atomic ratio of the mineral, the NiO content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts. The Zr/Nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and, in addition, distinguishes the picritic basalts from the other basalt types. These differences indicate that the different magma groups could not have been processed through the same magma chamber, and preclude any direct inter-relationship via open or closed system fractional crystallization.The Fe-Mg partitioning between olivine and host rock suggests that the picritic basalts represent olivine (±Cr-spinel) enriched magmas, derived from a less MgO rich parental magma. The partitioning of Fe and Mg between olivine, Cr-spinel and coexisting liquid is used to predict a primary magma composition parental to the picritic basalts. This magma is characterized by relatively high MgO (12.3%) and CaO (12.6%) and low FeO* (7.96%) and TiO2 (0.63%).Least squares calculations indicate that the plagioclase phyric basalts are related to the plagioclase-pyroxene basalts by plagioclase and minor clinopyroxene and olivine accumulation. The compositional variations within the olivine basalts can be accounted for by fractionation of plagioclase, clinopyroxene and olivine in an open system, steady state, magma chamber in the average proportions 45∶32∶23. It is suggested that the most primitive olivine basalts can be derived from a pristine mantle composition by approximately 17% equilibrium partial melting. Although distinguished by its higher Zr/Nb ratio and lower NiO content of phenocryst phases, the magma parental to the picritic basalts can be derived from a similar source composition by approximately 27% equilibrium partial melting. It is suggested that the parental magma to the plagioclase-pyroxene and plagioclase phyric basalts might have been derived from greater depth resulting in the fractionation of the Zr/Nb ratio by equilibration with residual garnet.

Petrogenesis of anomalous K-enriched MORB from the Southwest Indian Ridge: 11°53′E to 14°38′E

Glassy pillow basalts with unusual geochemical characteristics for mid-ocean ridge basalt (MORB) have been dredge sampled from the Southwest Indian Ridge between 12 and 15°E during Leg ANT IV/4 of the F.S. POLARSTERN. Lavas from 4 of 6 dredges are moderately nepheline normative, highly K-enriched (0.5–1.77 wt% K2O) alkali basalts and hawaiites. Mg-numbers indicate that many of the lavas are fairly primitive (Mg No.=63–67), yet show extreme enrichment in incompatible elements; e.g. Nb (24–60 ppm), Ba (170–470 ppm) and Sr (258–460 ppm). Incompatible-element ratios such as Zr/Nb (3–5) and Y/Nb (0.46–1.1) are extremely low even for E-type (enriched) MORB, whereas (La/Yb)n ratios are particularly high (3.4–7.8). 87Sr/86Sr (0.70290–0.70368), 143Nd/144Nd (0.51302–0.51284) and 206Pb/204Pb (18.708–19.564) isotopic ratios further indicate the geochemically ‘enriched’ nature of these lavas, which range from the compositional field for depleted N-type (normal) MORB towards the composition of Bouvet Island lavas. Mutually correlated incompatible-element and Sr-, Nd- and Pb-isotopic ratios allow a fairly well constrained model to be developed for the petrogenesis of these unusually alkalic mid-ocean ridge lavas. The alkalic nature and degree of enrichment in incompatible elements is ascribed to particularly low degrees of partial melting (3–5 wt%), at greater than usual depth, of a source region that has experienced prior geochemical enrichment (by veining) related to the upwelling Bouvet mantle plume. To account for the observed compositional variations, a model is proposed whereby mixing between partial melts derived from these geochemically enriched silicate veins, and an incipient to low percentage (±2%) melt from the surrounding geochemically depleted suboceanic asthenosphere occurs as a consequence of increasing degree of melting with adiabatic upwelling. Eruption of these alkalic lavas in this spreading ridge environment is attributed to a temporary hiatus in tholeiitic volcanism and associated spreading along this section of the Southwest Indian Ridge, related to readjustment of spreading direction to a more stable plate geometry.

Local and regional heterogeneity in MORB from the Mid-Atlantic Ridge between 54.5°S and 51°S: Evidence for geochemical enrichment☆

Abstract Basaltic lavas from the southern Mid-Atlantic Ridge between the Bouvet triple junction at 54.5°S 1°W and 51°S, show a range in texture from aphyric to highly olivine or plagioclase porphyritic. Olivine ± Cr-spinel ± plagioclase are the dominant phenocryst phases, and clinopyroxene occurs as a phenocryst or microphenocryst phase in many lavas. The range in differentiation can be attributed to crystal fractionation accumulation ; basalts from fracture zones show a greater range than basalts from ridge segments. The fracture zone basalts also show a minor transform fault effect in that their compositions are consistent with lower degrees of partial melting than the ridge axis basalts. This difference is, however, not manifested in incompatible trace element ratios. The incompatible trace element ratios range from normal depleted (N-type) MORB through transitional varieties to moderately enriched (E-type) MORB, yet have higher Sr and lower Nd isotopic ratios than is generally accepted for depleted MORB. Characteristic trace element and isotopic ratios of these lavas are: Zr Nb = 8–38 , Y Nb = 2.1–15.7 , Zr Y = 2.0–4.0 , 143 Nd 144 Nd = 0.51303–0.51282 , 87 Sr 86 Sr = 0.70290–0.70364 . The geochemical signature of enriched MORB from this region is distinct from that associated with the Bouvet mantle plume and enriched MORB from the Southwest Indian and the American-Antarctic Ridges. This is best illustrated in terms of the covariation of Zr Nb ratio and 143 Nd 144 Nd and 87 Sr 86 Sr ratios. The source of enrichment in these lavas may be the recently proposed Shona hotspot. The enriched signature results from source region mixing achieved by veining of normal depleted mantle by low volume partial melts associated with the rising Shona plume. Subsequent partial melting, during upwelling below the ridge axis, has led to the geochemical enrichment observed in MORB from this region.

Isotope Geochemistry of the Oceanic Mantle Near the Bouvet Triple Junction

Abstract We have measured the isotopes of helium and lead in three suites of dredged basalts from the Southwest Indian Ridge, the America-Antarctic Ridge, and the Mid-Atlantic Ridge near the Bouvet triple junction. The magmatic 3He/4He ratios range from 6.5 to 14.2 times atmospheric (Ra), which encompasses the entire range of existing MORB data, and differs significantly from the commonly accepted value for depleted MORB of roughly eight times atmospheric (Ra). Several samples from near the Bouvet triple junction have extremely low helium concentrations, low 3He/4He ratios, and display internal isotopic disequilibrium between glass and vesicles. These basalt glasses are, therefore, unique among the sample suite in having a significant contribution from post-eruptive accumulation of radiogenic 4He. Although these data do not reflect the mantle source, they can be used to calculate eruption ages for the basalts, which fall between 7 and 100 kyr, and demonstrates the potential utility of He-U ages for MORB. Lead isotopic compositions from the dredged basalt glasses range from depleted MORB to those of Bouvet Island itself (206Pb/204Pb of 18.2–19.6). The results are consistent with a mantle plume origin of Bouvet Island in that high 3He/4He and 206Pb/204Pb ratios are observed both on Bouvet island and the ridge segment immediately adjacent to it. The American-Antarctic Ridge has lower He, Sr and Pb isotope ratios than those of the Southwest Indian Ridge, which suggests a lesser influence of the plume. The dredge samples from near the triple junction have relatively low 3He/4He ratios (7.4 ± 0.2 Ra), suggesting that the plume is presently centered beneath Bouvet Island rather than beneath the triple junction. The data from the ridge segment adjacent to Bouvet Island, on the Southwest Indian Ridge, define a trend toward high helium, strontium, and lead isotope ratios, which could be explained by mixing processes. A single dredge from one ridge segment at 7°E, between the Islas Orcadas and Shaka Fracture Zones, defines most of the helium isotopic variability found throughout the region and defines an isotopic composition unique in the region. The highest 3He/4He ratios were obtained from this ridge segment (∼14 Ra) and are associated with relatively unradiogenic strontium and lead isotopic compositions. The remarkable isotopic variability over short distances along the Southwest Indian Ridge and the America-Antarctic Ridge may be related to their slow spreading rates and the lack of steady state magma chambers which could homogenize geochemically distinct magma batches. Alternatively, there is a different scale of heterogeneity in the mantle beneath this region.

Analysis of rare-earth elements in geological samples by gradient ion chromatography: An alternative to ICP and INAA

Abstract An application of high-performance ion chromatography (HPIC) for the analysis of rare-earth elements (REE) in geological samples is described. The technique (developed by Heberling and coworkers on synthetic solutions) is shown to be capable of determining 12 of the REE in a wide variety of rock types with an accuracy and precision comparable to that of inductively coupled plasma (ICP) atomic emission spectroscopy. By this HPIC method it is not possible to obtain data for Ho, due to interference by Y, or Lu which is not sufficiently well resolved from Yb. The HPIC instrument used in this study is a Dionex® 4000 i Gradient ion chromatograph in which eluant composition and flow rate are microprocessor controlled. Separation of the REE was effected using a Dionex® CS 5 solumn with conditions similar to those outlined by Heberling and coworkers. The eluted REE were reacted with a colour complexing agent and detected photometrically using a UV/visible light detector at a wavelength of 520 nm. Analysis by HPIC requires that the sample be taken into solution and the matrix removed. Sample dissolution and removal of matrix elements, that would otherwise interfere with the chromatogram, were achieved using standard cationexchange procedures routinely applied in the preparation of samples for ICP analysis. Providing that a transition metal-free solution remains after matrix removal, the full range of REE can be determined in ∼20 min. However, if ppb levels of the transition metals are present in the analyte solution, these must be removed during the early part of the HPIC analysis so increasing the run time to ∼42 min. REE data obtained by this HPIC technique for a number of international rock standards compare favourably with the published data on these well-characterised samples.

Petrography and geochemistry of basaltic rocks from the Conrad fracture zone on the America-Antarctica Ridge

Intrusive and extrusive basaltic rocks have been dredged from the Conrad fracture zone (transecting the slow-spreading America-Antarctica Ridge). The majority of rocks recovered are holocrystalline with the dominant mineral assemblage being plagioclase plus clinopyroxene with or without minor Fe-Ti oxides (olivine occurs in only three samples) and many of the samples show evidence of extensive alteration. Secondary minerals include chlorite, actinolite, K- and Na-feldspar, analcite and epidote. In terms of bulk chemistry the rocks are characterized by their generally evolved and highly variable compositions (e.g.Mg*=0.65−0.35;TiO2=0.7−3.6%;Zr=31−374ppm;Nb=<3−21ppm;Y=17−96ppm;Ni=100−9ppm), but with respect to the immobile incompatible element ratios (e.g. Zr/Nb, Y/Nb, La/Sm) are similar to “normal” or depleted mid-oceanic ridge basalts. Quantitative major and trace element modelling indicate that most of the variation observed can be attributed to low-pressure fractional crystallization of plagioclase plus clinopyroxene in approximately equal proportions with or without minor Fe-Ti oxides. The range in composition can be accounted for by up to 76% fractional crystallization. Although ferrobasalts have not frequently been associated with slow spreading ridges, the extreme differentiation observed in the Conrad fracture zone basalts implies some additional constraint other than spreading rate on the formation of ferrobasalt and reaffirms the importance of extensive crustal differentiation during the production of this basalt type.

Continental material in the shallow oceanic mantle¿How does it get there?

Unusual compositions of some oceanic basalts have been attributed to their sources containing continental lithosphere detached during the breakup of Gondwana. However, the processes of how such continental lithospheric material is detached and transported into the ocean basin have not been constrained. Here we identify Walvis Ridge, where it has been argued that Deep Sea Drilling Project (DSDP) Site 525A contains continental material, as a unique location to constrain these processes. Absolute plate motion (relative to the Tristan mantle plume) and relative plate motion (between Africa and South America) of the African plate are oblique to one another, such that tectonic detachment versus hotspot-related thermal erosion should sample spatially separated continental units of different age. We present isotopic compositions of xenoliths representing the neo-Proterozoic lithosphere at the inferred site for tectonic detachment during continental breakup and show that this process does not explain the Walvis Ridge DSDP Site 525A mantle source. Rather, thermal erosion of ancient cratonic mantle by the Tristan mantle plume is indicated. A convective return flow is required to transport the eroded subcontinental lithospheric mantle to the site of plume activity some ∼50 m.y. later and provides constraints on the direction and velocity of mantle flow in the upper mantle.