Sulphide inclusions in diamonds from the Koffiefontein kimberlite, S Africa: constraints on diamond ages and mantle Re–Os systematics

Re–Os isotope compositions of syngenetic sulphide inclusions in both eclogite suite (E-type) and peridotite suite (P-type) parageneses in diamonds from the Koffiefontein mine, South Africa have been analysed using a technique capable of analysing single inclusion grains, or, in some cases multiple inclusions from the same diamonds. Sulphide inclusion Ni contents broadly correlate with Os abundances in that low-Ni (6.8–8.7% Ni), E-type sulphides have 4.7 to 189 ppb Os whereas the two high-Ni (25%), P-type sulphides have 5986 and 6097 ppb Os. Two P-type sulphides from the same diamond define the first mineral isochron obtained for a single diamond which has an age of 69±30 Ma with chondritic initial ¹⁸⁷Os/¹⁸⁸Os. This indicates that the sulphides, and hence the host diamond, crystallised close to the time of kimberlite emplacement (90 Ma), in the Mesozoic. This is supported by Pb isotopic measurements of a fragment from one of the sulphides, together with the absence of significant Type IaB nitrogen aggregation in the host diamond lattice. E-type sulphide inclusions have radiogenic Os isotopic compositions, ¹⁸⁷Os/¹⁸⁸Os 0.346 to 2.28, and Re–Os model ages from 1.1 to 2.9 Ga. They define an array on a Re–Os isochron diagram that may be interpreted as defining a single period of E-type sulphide growth at 1.05±0.12 Ga, with an elevated initial ¹⁸⁷Os/¹⁸⁸Os. Alternatively, two episodes of sulphide crystallisation, from a chondritic reservoir, may be invoked in the Archaean and in the Proterozoic. The results for both P- and E-type diamonds point to a spectrum of diamond crystallisation ages. High contents of both Re and Os, and the similarity of Re/Os ratios of sulphide inclusions in diamonds to whole rock eclogite and peridotite xenoliths indicate that small amounts of sulphides can dominate the mantle budget of both these elements during melting. Recent addition to the lithospheric mantle of high-Os material similar to that from which the P-type sulphides crystallised may explain the variable, sometimes young Os model ages seen in whole rock xenolith Re–Os data.     

Coupled Os–Os enrichments in the Earth’s mantle – core–mantle interaction or recycling of ferromanganese crusts and nodules?

¹⁸⁶Os enrichments in volcanic rocks and peridotite-derived iridosmine grains have been attributed to contributions from Earth’s outer core to the mantle, and apparently constrain the scale of mantle convection and an early timing for inner–outer core segregation more than 3.5 Gyr ago. Here, we highlight that marine ferromanganese crusts and nodules are characterised by high Pt/Os ratios and Pt–Os contents that develop much larger ¹⁸⁶Os excesses over geological time (≥0.2%/Gyr) than those hypothesised for Earth’s outer core (<0.005–0.01%/Gyr). ¹⁸⁷Os/¹⁸⁸Os ratios in ferromanganese crusts are radiogenic due to sequestering of continental Os from seawater. Similarly, ancient ferromanganese materials may have had ¹⁸⁶Os excesses (>0.1%) as a result of high Pt/Os ratios in continental crust, even prior to in-growth of ¹⁸⁶Os after formation due to their high Pt/Os ratios. Past recycling of small amounts of these materials into the Earth’s mantle will produce coupled ¹⁸⁷Os–¹⁸⁶Os excesses and little change in Re and platinum-group-element concentrations, as observed in Hawaiian picrites, and in contrast to the predicted result of outer core addition to the mantle. ¹⁸⁷Os and ¹⁸⁶Os enrichments in the Hawaiian mantle source are potentially consistent with it comprising recycled oceanic lithosphere, pelagic sediments and ferromanganese materials, and questions the notion that Os isotopes can be used to uniquely identify core–mantle interactions and the depth at which mantle sources for volcanism originate.     

Temporal–compositional trends over short and long time-scales in basalts of the Big Pine Volcanic Field, California

Primitive basaltic single eruptions in the Big Pine Volcanic Field (BPVF) of Owens Valley, California show systematic temporal–compositional variation that cannot be described by simple models of fractional crystallization, partial melting of a single source, or crustal contamination. We targeted five monogenetic eruption sequences in the BPVF for detailed chemical and isotopic measurements and ⁴⁰Ar/³⁹Ar dating, focusing primarily on the Papoose Canyon sequence. The vent of the primitive (Mg# = 69) Papoose Canyon sequence (760.8 ± 22.8 ka) produced magmas with systematically decreasing (up to a factor of two) incompatible element concentrations, at roughly constant MgO (9.8 ± 0.3 (1σ) wt.%) and Na₂O. SiO₂ and compatible elements (Cr and Ni) show systematic increases, while ⁸⁷Sr/⁸⁶Sr systematically decreases (0.7063–0.7055) and ε_Nd increases (− 3.4 to − 1.1). ¹⁸⁷Os/¹⁸⁸Os is highly radiogenic (0.20–0.31), but variations among four samples do not correlate with other chemical or isotopic indices, are not systematic with respect to eruption order, and thus the Os system appears to be decoupled from the dominant trends. The single eruption trends likely result from coupled melting and mixing of two isotopically distinct sources, either through melt-rock interaction or melting of a lithologically heterogeneous source. The other four sequences, Jalopy Cone (469.4 ± 9.2 ka), Quarry Cone (90.5 ±17.6 ka), Volcanic Bomb Cone (61.6 ± 23.4 ka), and Goodale Bee Cone (31.8 ± 12.1 ka) show similar systematic temporal decreases in incompatible elements. Monogenetic volcanic fields are often used to decipher tectonic changes on the order of 10⁵–10⁶ yr through long-term changes in lava chemistry. However, the systematic variation found in Papoose Canyon (10⁰–10² yr) nearly spans that of the entire volcanic field, and straddles cutoffs for models of changing tectonic regime over much longer time-scales. Moreover, ten new ⁴⁰Ar/³⁹Ar ages combined with chemistry from all BPVF single eruption sequences show the long-term trend of BPVF evolution comprises the overlapping, temporal–compositional trends of the monogenetic vents. This suggests that the single eruption sequences contain the bulk of the systematic chemical variation, whereas their aggregate compositions define the long-term trend of volcanic field evolution.     

The marineOs/Os record of the past 80 million years

We report new¹⁸⁷Os/¹⁸⁶Os data and Re and Os concentrations in metalliferous sediments from the Pacific to construct a composite Os isotope seawater evolution curve over the past 80 m.y. Analyses of four samples of upper Cretaceous age yield¹⁸⁷Os/¹⁸⁶Os values of between 3 and 6.5 and¹⁸⁷Re/¹⁸⁶Os values below 55. Mass balance calculations indicate that the pronounced minimum of about 2 in the Os isotope ratio of seawater at the K-T boundary probably reflects the enormous input of cosmogenic material into the oceans by the K-T impactor(s). Following a rapid recovery to¹⁸⁷Os/¹⁸⁶Os of 3.5 at 63 Ma, data for the early and middle part of the Cenozoic show an increase in¹⁸⁷Os/¹⁸⁶Os to about 6 at 15 Ma. Variations in the isotopic composition of leachable Os from slowly accumulating metalliferous sediments show large fluctuations over short time spans. In contrast, analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data. These results together with sediment leaching experiments indicate that dissolution of non-hydrogenous Os can occur during the hydrogen peroxide leach and demonstrate that Os data from pelagic clay leachates do not always reflect the Os isotopic composition of seawater.

New data for the late Cenozoic further substantiate the rapid increase in the¹⁸⁷Os/¹⁸⁶Os of seawater during the past 15 Ma. We interpret the correlation between the marine Sr and Os isotope records during this time period as evidence that weathering within the drainage basin of the Ganges-Brahmaputra river system is responsible for driving seawater Sr and Os toward more radiogenic isotopic compositions. The positive correlation between⁸⁷Sr/⁸⁶Sr and U concentration, the covariation of U and Re concentrations, and the high dissolved Re, U and Sr concentrations found in the Ganges-Brahmaputra river waters supports this interpretation. Accelerating uplift of many orogens worldwide over the past 15 Ma, especially during the last 5 Ma, could have contributed to the rapid increase in¹⁸⁷Os/¹⁸⁶Os from 6 to 8.5 over the past 15 Ma. Prior to 15 Ma the marine Sr and Os record are not tightly coupled. The heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic Os and Sr to the oceans and account for the periods of decoupling of the marine Sr and Os isotope records.     

Os isotope systematics in ocean island basalts

New ReOs isotopic results for Os-poor basalts from St. Helena, the Comores, Samoa, Pitcairn and Kerguelen dramatically expand the known range of initial ¹⁸⁶Os/¹⁸⁷Os ratios in OIBs to values as high as 1.7. In contrast to the Os isotopic uniformity of Os-rich basalts from the HIMU islands of Tubuai and Mangaia found by Hauri and Hart [1], our values for St. Helena span most of the known range of Os isotopic variability in oceanic basalts (initial ¹⁸⁷Os/¹⁸⁶Os ranges from 1.2 to 1.7). Generation of such radiogenic Os in the mantle requires melting of source materials that contain large proportions of recycled oceanic crust. The very low Os concentrations of most of the basalts analyzed here, however, leave them susceptible to modification via interaction with materials containing radiogenic Os in the near-surface environment. Thus the high ¹⁸⁶Os/¹⁸⁷Os ratios may result from assimilation of radiogenic Os-rich marine sediments, such as Mn oxides, within the volcanic piles traversed by these magmas en route to the surface. Furthermore, the Os isotopic signatures of Os-rich, olivine-laden OIBs may reflect the accumulation of lithospheric olivine, rather than simply their mantle source characteristics. The extent to which these processes alter the view of the mantle obtained via study of ReOs systematics in oceanic basalts is uncertain. These effects must be quantified before ReOs systematics in OIBs can be used with confidence to investigate the nature of mantle heterogeneity and its causes.     

Helium isotope geochemistry of mid-ocean ridge basalts from the South Atlantic

We report new helium isotope results for 49 basalt glass samples from the Mid-Atlantic Ridge between 1°N and 47°S.³He/⁴He in South Atlantic mid-ocean ridge basalts (MORB) varies between 6.5 and 9.0 R_A (R_A is the atmospheric ratio of1.39 × 10⁻⁶), encompassing the range of previously reported values for MORB erupted away from high³He/⁴He hotspots such as Iceland. He, Sr and Pb isotopes show systematic relationships along the ridge axis. The ridge axis is segmented with respect to geochemical variations, and local spike-like anomalies in³He/⁴He, Pb and Sr isotopes, and trace element ratios such as(La/Sm)_N are prevalent at the latitudes of the islands of St. Helena, Tristan da Cunha and Gough to the east of the ridge. The isotope systematics are consistent with injection beneath the ridge of mantle “blobs” enriched in radiogenic He, Pb and Sr, derived from off-axis hotspot sources. The variability in³He/⁴He along the ridge can be used to refine the hotspot source-migrating-ridge sink model.

MORB from the 2–7°S segment are systematically the least radiogenic samples found along the mid-ocean ridge system to date. Here the depleted mantle source is characterized by⁸⁷Sr/⁸⁶Sr of 0.7022, Pb isotopes close to the geochron and with²⁰⁶Pb/²⁰⁴Pb of 17.7, and³He/⁴He of 8.6–8.9 R_A. The “background contamination” of the subridge mantle, by radiogenic helium derived from off-ridge hotspots, displays a maximum between 20 and 24°S. The HePb and HeSr isotope relations along the ridge indicate that the³He/⁴He ratios are lower for the hotspot sources of St. Helena, Tristan da Cunha and Gough than for the MORB source, consistent with direct measurements of³He/⁴He ratios in the island lavas. Details of the HeSrPb isotope systematics between 12 and 22°S are consistent with early, widespread dispersion of the St. Helena plume into the asthenosphere, probably during flattening of the plume head beneath the thick lithosphere prior to continental breakup. The geographical variation in theHe/Pbratio deduced from the isotope systematics suggests only minor degassing of the plume during this stage. Subsequently, it appears that the plume component reaching the mid-Atlantic ridge was partially outgassed of He during off-ridge hotspot volcanism and related melting activity.

Overall, the similar behavior of He and Pb isotopes along the ridge indicates that the respective mantle sources have evolved under conditions which produced related He and Pb isotope variations.     

Isotope geochemistry of Pantelleria volcanic fluids, Sicily Channel rift: a mantle volatile end-member for volcanism in southern Europe

Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manifestations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Channel rift zone, south of the African–European collision plate boundary. We find that Pantelleria fluids contain a CO₂–He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a main thermal (100°C) aquifer of mixed marine–meteoric water. The measured ³He/⁴He ratios and δ¹³C of both the free gases (4.5–7.3 R_a and −5.8 to −4.2‰, respectively) and dissolved helium and carbon in waters (1.0–6.3 R_a and −7.1 to −0.9‰), together with their covariation with the He/CO₂ ratio, constrain a ³He/⁴He ratio of 7.3±0.1 R_a and a δ¹³C of ca. −4‰ for the magmatic end-member. These latter are best preserved in fluids emanating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir. Outside the caldera, the magmatic component is more affected by air dilution and, at a few sites, by mixing with either organic carbon and/or radiogenic ⁴He leached from the U–Th-rich trachytic host rocks of the aquifer. Pantelleria magmatic end-member is richer in ³He and has a lower (closer to MORB) δ¹³C than all fluids yet analyzed in volcanic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9±0.2 R_a, δ¹³C=−3±1‰). This observation is consistent with a south to north increasing imprint of subducted crustal material in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of the African–Eurasian plate collision boundary. Our results for Pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or ‘less contaminated’, ³He-richer mantle source beneath the Sicily Channel rift zone. The lower than MORB ³He/⁴He ratio but higher than MORB CO₂/³He ratio of Pantelleria volatile end-member are compatible with petro-geochemical evidence that this mantle source includes an upwelling HIMU–EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle.     

Glacial–interglacial changes in Subantarctic sea surface temperature and δO-water using foraminiferal Mg

Laboratory culturing experiments with living Globigerina bulloides indicate that Mg/Ca is primarily a function of seawater temperature and suggest that Mg/Ca of fossil specimens is an effective paleotemperature proxy. Using culturing results and a core-top Neogloboquadrina pachyderma calibration, we have estimated glacial–interglacial changes in sea surface temperature (SST) using planktonic Mg/Ca records from core RC11-120 in the Subantarctic Indian Ocean (43°S, 80°E) and core E11-2 in the Subantarctic Pacific Ocean (56°S, 115°W). Our results suggest that glacial SST was about 4°C cooler in the Subantarctic Indian Ocean and 2.5°C cooler in the Subantarctic Pacific. Comparison of SST and planktonic δ¹⁸O records indicates that changes in SST lead changes in δ¹⁸O by on average 1–3 kyr. The glacial–interglacial temperature change indicated by the Subantarctic Mg/Ca records suggests that temperature accounts for 40–60% of the foraminiferal δ¹⁸O change. We have used the Mg/Ca-based SST estimates and δ¹⁸O determinations to generate site-specific seawater δ¹⁸O records, which suggest that seawater δ¹⁸O was on average 1‰ more positive during glacial episodes compared with interglacial episodes.     

Refining the noble gas record of the Réunion mantle plume source: Implications on mantle geochemistry

We report isotope analyses of helium, neon, argon, and xenon using different extraction techniques such as stepwise dynamic and static crushing, and high-resolution stepwise heating of three mantle xenoliths from Réunion Island. He and Ne isotopic compositions were similar to previously reported Réunion data, yielding a more radiogenic composition when compared to the Hawaiian or Icelandic mantle plume sources. We furthermore observed correlated ¹²⁹Xe/¹³⁰Xe and ¹³⁶Xe/¹³⁰Xe ratios following the mantle trend with maximum values of 6.93 ± 0.14 and 2.36 ± 0.06, respectively. High-resolution argon analyses resulted in maximum ⁴⁰Ar/³⁶Ar ratios of 9000–11,000, in agreement with maximum values obtained in previous studies. We observed a well-defined hyperbolic mixing curve between an atmospheric and a mantle component in a diagram of ⁴⁰Ar/³⁶Ar vs. ²⁰Ne/²²Ne. Using a mantle ²⁰Ne/²²Ne of 12.5 (Ne–B) a consistent ⁴⁰Ar/³⁶Ar value of 11,053 ± 220 in sample ILR 84-4 was obtained, whereas extrapolations to a higher mantle ²⁰Ne/²²Ne ratio of 13.8 (solar wind) would lead to a much higher ⁴⁰Ar/³⁶Ar ratio of 75,000, far above observed maximum values. This favours a mantle ²⁰Ne/²²Ne of about 12.5 considered to be equivalent to Ne–B. Extrapolated and estimated ⁴⁰Ar/³⁶Ar ratios of the Réunion, Iceland, Loihi, and MORB mantle sources, respectively, tend to be linearly correlated with air corrected ²¹Ne/²²Ne and show the same systematic sequence of increasing relative contributions in radiogenic isotopes (Iceland–Loihi–Réunion–MORB) as observed for ⁴He/³He. In general, He–Ne–Ar isotope systematics of the oceanic mantle can be explained by following processes: (i) different degree of mixing between pure radiogenic and pure primordial isotopes generating the MORB and primitive plume (Loihi-type) endmembers; (ii) relatively recent fractionation of He relative to Ne and Ar, in one or both endmembers; (iii) after the primary fractionation event, different degrees of mixing between melts or fluids of MORB and primitive plume affinity generate the variety of observed OIB data, also on a local scale; (iv) very late-stage secondary fractionation during magma ascent and magma degassing leads to further strong variation in He/Ne and He/Ar ratios.     

Geochemical morphology of the North Mid-Atlantic Ridge, 10°–24°N: Trace element-isotope complementarity

The new data presented here from a 10–24°N segment of the North Mid-Atlantic Ridge show that this segment is the most depleted of the 10–70°N ridge section. They also show the existence of: (1) a geochemical gradient from the 14°N anomaly to 17°10′N; (2) a very depleted mantle source (the lowest Sr isotopic ratios found so far in the North Atlantic); and (3) a geochemical limit located at about 17°10′N without any obvious relation with any structural feature. The 15°20′N fracture zone does not show any relationship with respect to this gradient. The basalts located north of 17°10′N have very homogeneous features, which allow their characteristics to be averaged (i.e., ⁸⁷Sr/⁸⁶Sr= 0.70238 ± 0.00004, (Nb/Zr)_N = 0.28 ± 0.1) and they are defined as normal mid-ocean ridge basalts. The basaltic glasses located south of 17°10′N present a wide spectrum of isotopic compositions and extended rare earth element patterns (from depleted to enriched). Despite this, they have a constant K/Nb of 233 ± 9 (1s_M, n = 18) whereas this ratio is 344 ± 29 north of 17°10′N. These observations illustrate the strong coherence of behaviour between K and Nb (Ta) during the petrogenic processes involved in the generation of these mid-ocean ridge basalts and also their fractionation during previous mantle processes. Possible interpretations of mixing processes are discussed and sources at the ridge segment scale are favoured. However, when looking in detail, local heterogeneities are still common and can even be traced back off-axis to 115 my.

Placed in the context of the North Atlantic Ridge from 10° to 70°N, the Sr isotopic ratios reveal the Azores superstructure (23–50°N), whereas the trace element ratios (La/Sm-Nb/Zr) trace the second-order structures (33–40°N, 42–48°N) superimposed on the superstructure. This study illustrates the complementarity of information given by certain well chosen trace element ratios on the one hand and by isotopic ratios on the other. Since there is evidence of decoupling between isotopic ratios and/or trace element ratios, it introduces the notion of complementary “chemical memory” as recorded by a given type of trace element ratio or a given type of isotopic ratio     

Martian mantle mineralogy investigated by the Lu–Hf and Sm–Nd systematics of shergottites

Chemical heterogeneities in the Martian mantle are believed to result from the crystallization of a magma ocean in the first 100 million years of its history. Shergottite meteorites from Mars are thought to retain a compositional record of such early differentiation and the resulting mineralogy at different depths. The coupled ¹⁷⁶Lu–¹⁷⁶Hf and ¹⁴⁷Sm–¹⁴³Nd isotope systematics in 9 shergottites are used here to investigate these issues. Three compositional groups in the shergottites display distinct isotope systematics. One group, commonly termed as depleted, is characterized by positive ¹⁷⁶Hf_i from + 46.2 to + 50.4 and ¹⁴³Nd_i from + 36.2 to + 39.1. Another, termed as enriched, has negative ¹⁷⁶Hf_i = − 16.5 to − 13.2 and ¹⁴³Nd_i = − 7.0 to − 6.5. The third group is intermediate between the depleted and enriched groups with positive ¹⁷⁶Hf_i = + 30.0 to + 33.4 and ¹⁴³Nd_i = + 16.9. Together, they describe mixing curves between ¹⁷⁶Hf/¹⁷⁷Hf, ¹⁴³Nd/¹⁴⁴Nd, Lu/Hf, and Sm/Nd, implying that they sample two distinct sources in the Martian mantle. All shergottites are characterized by (Sm/Nd)_source < (Sm/Nd)_sample, but (Lu/Hf)_source > (Lu/Hf)_sample. This decoupling can be explained by two successive partial melting episodes in the depleted shergottite source and localized in the Martian upper mantle. The genesis of shergottites can be modeled using non-modal equilibrium partial melting in a source initially composed of 60% olivine, 21% clinopyroxene, 9% orthopyroxene, and 10% garnet, with degrees of partial melting of 8.8% and 3.9%, respectively, for the two successive events. The enriched end-member of the shergottite mixing curve is best modeled by late-stage quenched residual melt resulting from the crystallization of a magma ocean. The depleted shergottite source may be modeled as a mixture of cumulates and residual melt, as convection in the Martian magma ocean is expected to reduce the incompatible trace element heterogeneity in the final solidified layers. Consequently, equilibrium crystallization is preferred to model the crystallization of the Martian magma ocean. The models that best explain the shergottite data are those where the magma ocean is at a depth of at least 1350 km in Mars.     

A critical evaluation of young (near-zero) K–Ar ages

An evaluation of the precision and resolution of the unspiked K–Ar dating method is presented with particular regard to the statistical significance of ages that are measured near or at the detection limit of the technique. Near-zero (historical) ages can be measured by the unspiked K–Ar technique with a precision that is essentially controlled by the precision with which the ⁴⁰Ar/³⁶Ar of the sample can be resolved from the present-day atmospheric value of 295.5. The best analytical precision on the isotopic ratio is ±0.05% (1σ) by this technique, which currently limits the lower detection limit of unspiked K–Ar ages to samples featuring at least 0.14% of radiogenic ⁴⁰Ar. The corresponding youngest resolvable K–Ar age depends on the K content and atmospheric contamination of the sample. Total-fusion analysis of high-K refractory minerals like sanidine is not practicable via K–Ar, and the lowest resolvable age for medium-K samples more amenable to complete fusion is around 1.5 ka (on a single-run basis). It is argued that near-zero age measured with a probability density straddling or narrowing the time-origin cannot be handled without accounting for the non-negativity constraint imposed by the physical requirement of a positive age. The pertinent equations are derived both for the single-run case and for the case of independent replicates made on a single sample. We show that pooled K–Ar replicates can theoretically reduce the nominal uncertainty of individual unspiked ages (typically ±1.5 ka, 2σ) to a value that is close to the smallest ⁴⁰Ar/³⁹Ar isochron age uncertainty achievable on sanidine in the 0–2 ka range (±0.2 ka, 2σ). However, this performance is obtained at the cost of prohibitively large-sample statistics (n≥15) for medium-K feldspars datable via K–Ar. Coupled with the inability of the K–Ar approach to obviate the problems of excess/fractionated ⁴⁰Ar and/or xenocrystic contamination, this makes the ⁴⁰Ar/³⁹Ar technique the method of choice for dating historical events by the K–Ar scheme.     

Osmium-strontium-neodymium-lead isotopic covariations in mid-ocean ridge basalt glasses and the heterogeneity of the upper mantle

Osmium, strontium, neodymium, and lead isotopic data have been obtained for 30 hand picked samples of basaltic glass from the Pacific, Atlantic and Indian mid-oceanic ridges. Large variations in Os isotopic ratios exist in the glasses, from abyssal peridotite-like values to radiogenic compositions similar to oceanic island basalts (¹⁸⁷Os/¹⁸⁶Os and ¹⁸⁷Os/¹⁸⁸Os ratios range from 1.06 to 1.36 and from 0.128 to 0.163, respectively). Os isotopic and elemental data suggest the existence of mixing correlations. This relationship might be ascribed to secondary contamination processes; however, such a hypothesis cannot account for the negative correlation observed between Os and Nd isotopes and the existence of complementary covariations between Os and SrPb isotopes. In this case, OsSrNdPb isotopic variations are unrelated to late post-eruption or shallow level contamination. These relationships provide strong evidence that the Os isotopic composition of the samples are derived from the mantle and thus implies a global chemical heterogeneity of the oceanic upper mantle. The results are consistent with the presence of recycled oceanic crust in the mantle sources of mid-ocean ridge basalts, and indicate that the unique composition of the upper mantle below the Indian ocean results from its contamination by a mantle component characterized by radiogenic Os and particularly unradiogenic Nd and Pb isotopic compositions.     

Alkaline syenites in eastern Cathaysia (South China): link to Permian–Triassic transtension

Two alkaline syenite plutons, the Tieshan and Yangfang plutons, have recently been recognized within NE-trending fault zones in eastern Cathaysia, South China. The rocks are very enriched in K₂O (6.28–9.39 wt.%), rare earth elements (REE; particularly light REE) and large ion lithophile elements, but are relatively low in high field strength elements. Isotopically, they are characterized by high initial ⁸⁷Sr/⁸⁶Sr (0.7093 to 0.7123) and low _Nd(t) values (−5.64 to −10.63). The geochemical data suggest that the alkaline syenites most likely formed via fractional crystallization of enriched mantle-derived magmas. Sensitive High-Resolution Ion Microprobe zircon U–Pb dating indicates that these two intrusions have Late Permian (254±4 Ma) and Early Triassic (242±4 Ma) crystallization ages, respectively. Our data suggest that a tectonic regime dominated by transtension probably existed from at least the latest Permian into the Triassic and was responsible for the formation of the Tieshan and Yangfang alkaline syenites. When combined with previous paleomagnetic, structural, and sedimentology data, we suggest that the transtension along the NE-trending strike-slip fault zones was related to oblique subduction of the Pacific plate underneath South China.     

U,U andTh in seawater

We have developed techniques to determine²³⁸U,²³⁴U and²³²Th concentrations in seawater by isotope dilution mass spectrometry. U measurements are made using a²³³U²³⁶U double spike to correct for instrumental fractionation. Measurements on uranium standards demonstrate that²³⁴U/²³⁸U ratios can be measured accurately and reproducibly.²³⁴U/²³⁸U can be measured routinely to ± 5‰ (2σ) for a sample of 5 × 10⁹ atoms of²³⁴U (3 × 10⁻⁸ g of total U, 10 ml of seawater). Data acquisition time is 1 hour. The small sample size, high precision and short data acquisition time are superior to-counting techniques.²³⁸U is measured to ± 2‰ (2σ) for a sample of 8 × 10¹² atoms of²³⁸U ( 3 × 10⁻⁹ g of U, 1 ml of seawater).²³²Th is measured to ± 20‰ with 3 × 10¹¹²³²Th atoms (10⁻¹⁰ g²³²Th, 1 1 of seawater). This small sample size will greatly facilitate investigation of the²³²Th concentration in the oceans. Using these techniques, we have measured²³⁸U,²³⁴U and²³²Th in vertical profiles of unfiltered, acidified seawater from the Atlantic and²³⁸U and²³⁴U in vertical profiles from the Pacific. Determinations of²³⁴U/²³⁸U at depths ranging from 0 to 4900 m in the Atlantic (7°44′N, 40°43′W) and the Pacific (14°41′N, 160°01′W) Oceans are the same within experimental error (± 5‰,2σ). The average of these²³⁴U/²³⁸U measurements is 144 ± 2‰ (2σ) higher than the equilibrium ratio of 5.472 × 10⁻⁵. U concentrations, normalized to 35‰ salinity, range from 3.162 to 3.281 ng/g, a range of 3.8%. The average concentration of the Pacific samples (31°4′N, 159°1′W) is 1% higher than that of the Atlantic (7°44′N, 40°43′W and 31°49′N, 64°6′W).²³²Th concentrations from an Atlantic profile range from 0.092 to 0.145 pg/g. The observed constancy of the²³⁴U/²³⁸U ratio is consistent with the predicted range of²³⁴U/²³⁸U using a simple two-☐ model and the residence time of deep water in the ocean determined from¹⁴C. The variation in salinity-normalized U concentrations suggests that U may be much more reactive in the marine environment than previously thought.     

Isotope and trace element geochemistry of young Pacific seamounts: implications for the scale of upper mantle heterogeneity

Basalts from young seamounts situated within 6.8 m.y. of the East Pacific Rise, between 9° and 14°N latitude, display significant variations in ¹⁴³Nd/¹⁴⁴Nd (0.51295–0.51321), ⁸⁷Sr/⁸⁶Sr (0.7025–0.7031), and(La/Sm)_N (0.415–3.270). Nd and Sr isotope ratios are anti-correlated and form a trend roughly parallel to the “mantle array” on a¹⁴³Nd/¹⁴⁴Nd vs.⁸⁷Sr/⁸⁶Sr variation diagram. Nd and Sr isotope ratios display negative and positive correlations, respectively, with(La/Sm)_N. The geochemical variations observed at the seamounts are nearly as great or greater than those observed over several hundred kilometers of the Reykjanes Ridge, or at the islands of Iceland or Hawaii.

Samples from one particular seamount, Seamount 6, display nearly the entire observed range of chemical variations, offering an ideal opportunity to constrain the nature of heterogeneities in the source mantle. Systematics indicative of magma mixing are recognized when major elements, trace elements, trace element ratios, and isotope ratios are compared with each other in all possible permutations. The source materials required to produce the end-member magmas are: (1) a typical MORB-source-depleted peridotite; and (2) a relatively enriched material which may represent ancient mantle segregations of basaltic melt, incompletely mixed remnants of subducted ocean crust, or metasomatized peridotite such as that found at St. Paul's Rocks or Zabargad Island. Due to the proximity of the seamounts to the East Pacific Rise (EPR), the source materials are thought to comprise an intimate mixture in the mantle immediately underlying the seamounts and the adjacent EPR. Lavas erupted at the ridge axis display a small range of isotopic and incompatible trace element compositions because the large degrees of melting and presence of magma chambers tend to average the chemical characteristics of large volumes of mantle.

If the postulated mantle materials, with large magnitude, small-scale heterogeneities, are ubiquitous in the upper mantle, chemical variations in basalts ranging from MOR tholeiites to island alkali basalts may reflect sampling differences rather than changes in bulk mantle chemistry.     

A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems

¹⁴³Nd/¹⁴⁴Nd ratios, and Sm and Nd abundances, are reported for particulates from major and minor rivers of the Earth, continental sediments, and aeolian dusts collected over the Atlantic, Pacific, and Indian Oceans. Overall, Sm/Nd ratios and Nd isotopic compositions in contemporary continental erosion products vary within the small ranges of ¹⁴⁷Sm/¹⁴⁴Nd= 0.115 ± 0.01 and¹⁴³Nd/¹⁴⁴Nd= 0.51204 ± 0.0002 (ε_Nd = −11.4 ± 4). The average period of residence in the continental crust is estimated to be1.70 ± 0.35Ga.

These results combined with data from the literature have implications for the age, history, and composition of the sedimentary mass and the continental crust: (1) The average “crustal residence age” of the whole sedimentary mass is about 1.9 Ga. (2) The range of Nd isotope compositions in the continent derived particulate input to the oceans is the same as Atlantic sediments and seawater, but lower than those of the Pacific, demonstrating the importance of Pacific volcanism to Pacific Nd chemistry. (3) The average ratio of Sm/Nd is about 0.19 in the upper continental crust, and has remained so since the early Archean. This precludes the likelihood of major mafic to felsic or felsic to mafic trends in the overall composition of the upper continental crust through Earth history. (4) Sediments appear to be formed primarily by erosion of continental crust having similar Sm/Nd ratios, rather than by mixing of mafic and felsic compositions. (5) The average ratio of ¹⁴³Nd/¹⁴⁴Nd≈ 0.5117 (ε_Nd ≈ −17) in the upper continental crust, assuming its mean age is about 2 Ga. (6) The uniformity of the SmNd isotopic systematics in river and aeolian particulates primarily reflects efficient recycling of old sediment by sedimentary processes on a short time scale compared to the amount of time the material has resided in the crust.     

Equilibrium Fe isotope fractionation between inorganic aqueous Fe(III) and the siderophore complex, Fe(III)-desferrioxamine B

In oxic oceans, most of the dissolved iron (Fe) exists as complexes with siderophore-like, strongly coordinating organic ligands. Thus, the isotope composition of the little amount of free inorganic Fe that is available for precipitation and preservation in the geological record may largely be controlled by isotope fractionation between the free and complexed iron. We have determined the equilibrium Fe isotope fractionation induced by organic ligand activity in experiments with solutions having co-existing inorganic Fe(III) species and siderophore complexes, Fe-desferrioxamine B (at pH 2). The two differently complexed Fe(III) pools were separated by addition of Na₂CO₃, which led to immediate precipitation of the inorganic Fe without causing significant dissociation of Fe-desferrioxamine complexes. Experiments using enriched ⁵⁷Fe tracer showed that isotopic equilibration between the ⁵⁷Fe-labelled inorganic species and the isotopically “normal” siderophore-bound Fe was rapid during the first few seconds and then became slower. Consequently, the data fitted poorly to first and second order reaction equations. However, with a two-stage reaction, the data fit perfectly with a first order equation for the slower stage, indicating that approximately 40% re-equilibration may take place during the separation of the two pools. To further test if the induced precipitation leads to experimental artefacts, the fractionation during precipitation of inorganic Fe was determined. Assuming a Rayleigh-type fractionation during precipitation, this experiment yielded an isotope fractionation factor of ⁵⁶Fe_{solution-solid} = 1.00027. Calculations based on these results indicate that isotopic re-equilibration is unlikely to significantly affect our determined equilibrium Fe isotope fractionation between inorganically and organically complexed Fe. To determine the equilibrium Fe isotope fractionation between inorganically and organically bound Fe(III), experiments with variable proportions of inorganic Fe were carried out at 25 °C. Irrespective of the proportion of inorganic Fe, equilibrium fractionation factors were within experimental uncertainty, yielding an average fractionation factor, Δ⁵⁶Fe_DFOB-inorg of 0.60 ± 0.15‰. The results indicate that equilibrium Fe isotope fractionation induced by strongly coordinating organic ligands may fractionate Fe isotopes in nature. The fractionation is likely to be important in oxic, Fe(III)-bearing environments, such as soils and rivers, and may, for example, largely control the Fe isotope composition of marine Fe–Mn crusts.     

Helium isotope geochemistry of some volcanic rocks from Saint Helena

³He/⁴He ratios have been measured for olivine and clinopyroxene phenocrysts in 7–15 m.y. old basaltic lavas from the island of St. Helena. Magmatic helium was effectively resolved from post-eruptive radiogenic helium by employing various extraction techniques, includingin vacuo crushing, and stepwise heating or fusion of the powders following crushing. The inherited³He/⁴He ratio at St. Helena is 4.3–5.9 R_A. Helium isotope disequilibrium is present within the phenocrysts, with lower³He/⁴He upon heating and fusion of the powders following crushing, due to radiogenic ingrowth or to -particle implantation from the surrounding(U + Th)-rich lavas.

A single crushing analysis for clinopyroxene in a basalt from Tubuaii gave³He/⁴He= 7.1 R_A.³He/⁴He ratios at St. Helena and Tubuaii (HIMU hotspots characterized by radiogenic Pb isotope signatures) are similar to³He/⁴He ratios previously measured at Tristan da Cunha and Gough Island (EM hotspots characterized by low²⁰⁶Pb/²⁰⁴Pb). Overall, the HeSrPb isotope systematics at these islands are consistent with a mantle origin as contiguous, heterogeneous materials, such as recycled crust and/or lithosphere.³He/⁴He ratios at HIMU hotspots are similar to mantle xenoliths which display nearly the entire range of Pb isotope compositions found at ocean islands, and are only slightly less than values found in mid-ocean ridge basalts (7–9 R_A). This suggests that the recycled materials were injected into the mantle within the last 10⁹ yrs.     

A metamorphic history from micron-scale Pb/Pb chronometry of Archean monazite

Ion microprobe measurements of Pb isotope ratios in monazites have been obtained, in situ, from thin sections using the Cambridge ISOLAB 120. Molecular interferences are sufficiently resolved at an RP of 6500 to allow ²⁰⁷Pb/²⁰⁶Pb dating of monazite with precisions as low as 4–5 Ma (2σ). The results presented here provide important information on the chronological history of the Late Archean metamorphism of the Wind River Range, Wyoming (USA).

Matrix monazites and monazite inclusions in garnets from a metapelite from the northern Wind River Range have been analysed by SIMS. In a previous study peak metamorphic conditions (T = 800°C; P = 8 ± 1 kb^*) were estimated using inclusion assemblages in garnets from this same sample. Isolated monazite inclusions in garnet yield ²⁰⁷Pb/²⁰⁶Pb age estimates of 2781 ± 6 to 2809 ± 10 Ma. Those along fractures yield lower ages (2603–2687 Ma) which are similar to TIMS and SIMS ages of matrix monazites. A single large (500 μm) monazite grain locally preserves growth zoning, but has a recrystallised core and a resorbed (recrystallised?) rim. Age estimates for these three regions are 2788 ± 9 Ma, 2663 ± 4 and 2523 ± 6 Ma, respectively. Thus the inclusion assemblages of Sharp and Essene^* may record peak metamorphic conditions at ca. 2.8 Ga, and indicate a phase of metamorphism that predates by over 100 Ma the emplacement of the Bridger Batholith, the major lithologic component of the northern Wind River Range.

The analysed monazite grains appear to preserve ca. 300 Ma history, even within a single grain. Monazite inclusions in garnet that are fully armoured may provide estimates for the time of garnet growth, even in high grade terranes where most chronometers are reset. The age pattern preserved by the large monazite grain cannot be simply related to diffusion controlled closure. Instead, a chronology is preserved which can be related to the petrographic setting of indicidual grains through in situ analysis.