A.J. Erlank

Sr, Nd and Pb isotope and minor element geochemistry of lamproites and kimberlites

All samples studied here exhibit loweNd (lamproites; Western Australia = −10 to −19.2 and Smoky Butte = −21.6 to −25.9: Group II kimberlites: Finsch Mine = −5.8 to −7.4) and(87Sr/86Sr)i > Bulk Earth. They are all LREE enriched (up to 1000 × chondrite) and have high trace element contents (i.e. Rb, Rb, Ba, Th, K, Ta, Sr, P, Hf, Zr and Ti). Smoky Butte and Finsch rocks plot below the Pb-ore growth curve(206Pb204Pb) = 16.02−16.64 and 17.74−17.99, and207Pb204Pb= 15.19−15.28 and 15.48−15.57, respectively) consistent with two-stage evolutions whereas the Western Australia rocks exhibit high207Pb204Pb (15.71−15.75) with unradiogenic206Pb204Pb (17.23−17.57) indicative of a more complex three-stage evolution. These lamproites and Group II kimberlites were at least predominantly derived from source regions which had had low, but variableUP/b andRb/Sr for between 1 and 2.5 Ga. Such source regions are inferred to exist within the subcontinental mantle lithosphere. Different styles of trace element enrichment are recognised which with time develop different isotope-isotope trends. HighRb/Sr,Rb/Ba andK/Ti ratios observed in Group II kimberlites and metasomatised peridotites are probably related to the migration of H2O-rich fluids within the upper mantle, while the lamproites have features consistent with the introduction of small volume silicate melts to their source regions, i.e. highT/iK,Ta/Yb and lowRb/Ba. At Smoky ButteRb/Sr was also low, but the Western Australia rocks have highRb/Sr because they are relatively less enriched in Sr. Hence highRb/Sr ratios (and with time high87Sr/86Sr) are developed in mantle source with both high and lowRb/Ba. Inferred μ for the last stage of evolution of their source regions is < 7.5 for all the rocks studied and there is a striking positive correlation betweeneNd and206Rb/204Pb, which indicates that lowSm/Nd andU/Pb ratios coexist (over long periods of time) within the subcontinental mantle lithosphere.

Sea floor basalt alteration: Some chemical and Sr isotopic effects

Major element, Sr isotope and trace element data for 16 elements are reported for various weathered zones in three submarine basalt pillows. During the initial stages of alteration, it appears that SiO2, Al2O3, CaO, S, and Ga are lost from the basalt, whereas Fe2O3, total Fe, MnO, K2O, H2O, Cl, B, Rb, and Cs increase. Sr87/Sr86 ratios also increase during weathering. MgO, Na2O, P2O5, Ba, Ni and Cu show significant (10–50%) but less consistent changes. TiO2, Zr, Sr and V show only minor changes (<7%). Zn, Cr, Co, Y and Nb show no changes outside experimental error.

Correlated Nd, Sr and Pb isotope variation in Walvis Ridge basalts and implications for the evolution of their mantle source

Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge [13, 14]. The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and consistent with formation at the paleo mid-ocean ridge [14]. The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other. The low 143Nd/144Nd (0.51238), 206Pb/204Pb (17.54), 208Pb/204Pb (15.47), 208Pb/204Pb (38.14) and high87Sr/86Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan de Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher143Nd/144Nd (0.51270), 206Pb/204Pb (18.32), 207Pb/204Pb (15.52), 208Pb/204Pb (38.77) and lower 87Sr/86Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing od depleted and enriched end member melts or partial melting of an inhomogenous, variably enriched mantle source. However, observe ZrBaNbY interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.

Kimberlite-borne garnet peridotite xenoliths from old enriched subcontinental lithosphere

Abstract High-temperature deformed garnet lherzolite xenoliths from mantle beneath the northern Lesotho margin of the Archaean craton in southern Africa are inferred to have been in internal Nd and Sr isotopic equilibrium at the time of sampling by kimberlite 90 Ma ago. Their trace element and isotopic signatures resemble those of depleted (high Sm/Nd, low Rb/Sr) MORB-type mantle from the convective asthenosphere. The common lower-temperature coarse and deformed garnet lherzolite xenoliths from beneath the Kimberley area in the craton interior display internal isotopic equilibrium or, alternatively, inverse isotopic disequilibrium attributable to recent mantle metasomatic component addition or redistribution. These lherzolites bear trace element and isotopic signatures of variously enriched (low Sm/Nd, high Rb/Sr) mantle stabilized in subcontinental lithosphere for up to 3 Ga. As a group the lithospheric peridotites provide an extreme range of isotopic compositions for potential contamination of continental volcanics.

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.

The significance of incompatible elements in mid-Atlantic ridge basalts from 45°N with particular reference to Zr/Nb

K, Rb, Ba, Sr, Y, Zr and Nb have been determined in samples of MORB (mid-ocean ridge basalt) from the mid-Atlantic ridge at 45 °N by X-ray fluorescence spectrometry. This suite is characterised by higher concentrations of K, Rb, Ba and Nb (and lower K/Rb, K/Ba and Zr/Nb) than low-K tholeiites (typical MORB) previously described in the literature. Available data from other sources also shows that the 45 °N suite has higher Cs and U contents, rare earth-patterns enriched in the light REE, and higher 87Sr/86Sr ratios relative to typical MORB. The inter-element and isotopic ratios of typical MORB have been previously interpreted to imply that these samples have been derived from source areas which have undergone earlier differentiation and/or melting episodes. The 45 °N MORB samples are considered to be derived from more primitive or relatively undepleted mantle. It is noteworthy that these samples have inter-element ratios very similar to those obtained for oceanic island basalts associated with the mid-ocean ridges.The significance of the relative enrichment or depletion of Nb in MORB has not been previously noted and the use of the Zr/Nb ratio to illustrate these effects is emphasised. The abundances of Zr and Nb are apparently unaffected by sea-water alteration and thus the Zr/Nb ratio is potentially a more useful measure of depletion than ratios involving K and Rb, which are very sensitive to sea-water alteration. Brief mention is made of the variation of Zr/Nb in other MORB samples to demonstrate the existence of varying degrees of depletion in the respective source areas of these samples.

Mantle metasomatism: Isotope and trace-element trends in xenoliths from Kimberley, South Africa

Modal metasomatism in peridotite xenoliths from Kimberley, South Africa, is observed in the development of phlogopite, K-richterite, diopside and LIMA. The average bulk-rock minor- and trace-element contents of different peridotite types range up to 100× primitive mantle, and the bulk-rock compositions reflect the modal proportions of metasomatic minerals in individual samples. Present-day whole-rock 87Sr86Sr=0.7045–0.7118 and 206Pb204Pb=17.48–21.24, and they have been little affected by interaction with the host kimberlite which has 87Sr86Sr=0.7042 and 206Pb204Pb=18.6–19.2. The preferred interpretation is that these isotope ratios reflect Rb/Sr and U/Pb fractionation during the modal metasomatism ∼150 Ma ago, i.e. after Karoo magmatism at ∼190 Ma and prior to emplacement of the kimberlite at 90 Ma. Present-day ϵNd= −9.3 to +2.4, reflecting both the presence of low-SmNd Proterozoic mantle prior to metasomatism at 150 Ma, and the inferred shift to relatively high ϵNd during that metasomatism. Rb/SrU/Pb fractionation during metasomatism has resulted in a distinctive negative correlation between present-day 87Sr86Sr and 206Pb204Pb. This contrasts with positive SrPb isotope trends in MORB glasses and selected OIB, but it is similar to that observed in certain CFB provinces. The trend to relatively high Rb/Sr at low U/Pb appears to be linked to the development of a potassic phase, typically phlogopite, and as such it arguably offers the first evidence for a style of minor- and trace-element enrichment which is restricted to shallow levels in the Earths mantle.

Metasomatism of the mantle lithosphere recorded by rare earth elements in garnets

Abstract Garnets derived from peridotite and kimberlite concentrates in southern African kimberlites show normal and sinusoidal REE patterns. Both types of REE patterns are interpreted to have resulted from the metasomatism of previously existing mantle garnets by LREE-enriched melts or fluids. In this model, metasomatism proceeded in accordance with garnet-liquid K d s which increase by orders of magnitude from the LREE and MREE to the HREE, thus fractionating the REE into three groups. The development of sinusoidal REE patterns resulted from the HREE lagging behind the LREE during metasomatic readjustment in composition. Sinusoidal REE patterns represent a state of disequilibrium during the re-equilibration process and are retained by garnets of relatively refractory compositions. Garnets from off-craton and on-craton regions show both types of REE patterns, suggesting that the presence of refractory garnets typical of cratonic regions is not restricted to the craton. Calculated liquid compositions for the metasomatic melts resemble kimberlite and lamproite, but were probably unrelated to the host kimberlites. Melts from off-craton regions are less LREE rich than melts from cratonic regions. These results suggest a different lithosphere in the off-craton region, possibly reflecting extensional tectonics and a higher degree of mantle upwelling.

On the origin of eucrites and diogenites

Abstract Eleven eucrites have been analysed for major, minor and some trace (K, Sr, Zr, Y, Ba and Ni) constituents. These data are interpreted in terms of an igneous fractionation model according to which the observed enrichment trends of various elements in eucrite liquids are considered to be indicative of the simultaneous fractionation of plagioclase and pyroxene. Serra de Mageand Moore County are representatives of the cumulates thus formed. The achondrite Binda, a monomict breccia of howarditic composition, is interpreted as a possible precursor to the eucrite liquids. The derivation of a parent eucrite liquid from material of this composition could have occurred by fractionation of orthopyroxene. Diogenites are considered to represent the orthopyroxenites thus formed. The original liquid from which Binda and the eucrites were derived must, in terms of this model, have been more mafic than Binda.

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.