Robert W. Nesbitt

Chemical characteristics of fluid phase released from a subducted lithosphere and origin of arc magmas: Evidence from high-pressure experiments and natural rocks

Abstract Dehydration experiments on synthetic serpentine spiked with 11 trace elements (Cs, Rb, K, Ba, Sr, La, Sm, Tb, Y, Yb and Nb) have been carried out at 12 kbar and 850°C to examine the nature of element migration processes in the upper mantle beneath volcanic arcs. The results indicate that an element with a larger ionic radius is more readily transported by the aqueous fluid phase through the dehydration process. The geochemistry of arc magnesian andesite and basalt, which are part of the compositional spread included in the range of arc primary magmas, has shown that source mantle materials of subduction zone magmas are enriched in incompatible elements with larger ionic radii compared to the source mantle of oceanic magmas; the enrichment should be caused by the transfer of elements by the fluid phase from the downgoing lithosphere. The downgoing slab is anhydrous beneath volcanic arcs, and the relatively shallow breakdown of hydrous phases means that the mantle wedge peridotite is infiltrated by the slab-derived fluid phase beneath the fore-arc regions. Drag of the metasomatized peridotite wedge downwards with the subducted lithosphere transports a larger-ion-element-enriched source to the regions of arc-magma generation.

U-Pb isotope geochronology of zircon: evaluation of the laser probe-inductively coupled plasma mass spectrometry technique

Abstract Using a laser ablation microprobe-inductively coupled plasma mass spectrometer (LP-ICPMS) we have determined 238U, 207Pb, 206Pb, and 204Pb abundances of several zircon populations whose ages have previously been measured by other techniques (principally ion microprobe). Ages of the samples range from 360–2800 Ma. A frequency quadrupled Nd-YAG UV laser (266 nm) which produces pit sizes of 10–15 μm was used to ablate the zircon samples. Prolonged measurements of the 206Pb/238U isotope ratio using ablation techniques show a serious fractionation effect because of nonconstant focusing of the laser. This ablation fractionation dramatically increases the Pb/U isotopic ratio because of the greater volatility of Pb. If the beam is constantly refocussed during ablation (active focusing), the effect of fractionation is minimised and the duration of the signal intensity increased. 207Pb/206Pb isotopic ratio measurements on zircons with ages greater than 2400 Ma previously measured by SHRIMP give an excellent agreement. Typical precision of the measurement for 207Pb/206Pb isotopic and Pb/U elemental ratios determined on single zircon grains is about 0.6–5% and 3–20%, respectively. In the case of Phanerozoic zircons (c.

Isotopic characteristics of subduction fluids in an intra-oceanic setting, Izu–Bonin Arc, Japan

New radiogenic isotope and trace element data are presented for the volcanic sequences along 600 km of the active Izu–Bonin arc, the Oligocene Izu arc, and their associated rift basins. As with many intra-oceanic island arcs, the Pliocene–Recent Izu–Bonin frontal-arc lavas are highly depleted in Zr, Nb and the rare-earth elements relative to typical mid-ocean ridge basalt (MORB), indicating that the mantle wedge source has undergone a previous episode of melting. Ratios between these elements (such as Nb/Zr and La/Sm), as well as 143Nd/144Nd, do not vary significantly along the length of the frontal-arc. These parameters suggest that each of the arc volcanoes is derived from similar melt fractions of the mantle wedge. However, Ba/Zr, Ba/Rb and 87Sr/86Sr increase along the frontal-arc to the north. This leads us to propose that a variable enrichment in Ba and radiogenic Sr is superimposed on the mantle wedge. Sr–Nd and Pb–Nd isotope variation indicate that both Sr and Pb become more radiogenic after fluid addition. However, Pb isotope ratios do not correlate with increases in Pb concentration or ratios such as Ba/Zr and Nb/Pb. In other words, the Pb isotopic composition of the arc lavas appears to be independent of the amount of Pb introduced by subduction fluids into the mantle source. This buffering of Pb isotopes along the frontal-arc means that the isotopic composition of the lavas is indistinguishable from that of the fluid. Isotopic mixing models presented for the arc are only illustrative of the many plausible combinations of components and quantities. Despite this, we are able to determine that the mantle wedge has isotopic characteristics similar to Indian Ocean MORB, and that the subduction-fluid solute is primarily derived from subducted oceanic basalt with a <2% contribution from subducted sediment. Lavas in the Oligocene Izu arc and fore-arc basin were derived from a mantle wedge of similar composition to the active arc. Despite levels of Pb enrichment comparable to those of the modern arc, the Pb isotopes of the Oligocene volcanics indicate a lower sediment input into the melting region.

An improved method for extracting marine sediment fractions and its application to Sr and Nd isotopic analysis

The radiogenic isotopic composition of both detrital and Fe–Mn fractions in marine sediments can be used in paleoceanography to infer changes of bottom-water circulation. We have examined various chemical techniques for the analysis of Sr and Nd isotope ratios in these fractions and present a robust method that can be used to separate both the Fe–Mn oxides and the detrital fraction from a marine sediment sample for isotopic analysis. Our sequential leaching procedure involves the use of 10% acetic acid, followed by 1 M hydroxylamine hydrochloride in 25% acetic acid to remove the carbonate component and the Fe–Mn oxide fraction, respectively. The applicability of our chemical procedure is illustrated with examples from a marine sediment core raised from the northern Cape Basin, southeast Atlantic Ocean.

Fluid-mantle interaction in an intra-oceanic arc: constraints from high-precision Pb isotopes

Abstract We present new isotopic and trace element data, including high-precision double-spike Pb isotope measurements, for back-arc lavas from the Izu–Bonin arc. Systematic along-arc isotopic variation of lavas has been identified in the Izu–Bonin arc that is coherent between the volcanic front and back-arc. The Sr isotopes are more radiogenic in the north, while Pb isotopes are less radiogenic in the north compared to the central part of the arc. This is particularly apparent in the back-arc seamounts. Decoupled variation of Pb and Sr isotopes cannot be explained by changes in the amount of a single subduction component. Almost parallel but distinct trends on Pb–Pb isotope plots imply differing mantle sources in the northern and central parts of the arc. The decoupling of Sr and Pb isotopic variation for both volcanic front and back-arc can be explained by the presence of two mantle components: a MORB source observed in the back-arc basins of the Philippine Sea Plate and a Pacific MORB-like source. An internally consistent model which explains along- and across-arc isotopic trends can be obtained by assuming mixing between the two mantle sources, but a lesser contribution of Pacific MORB-type source in the northern part of the arc. The source mantle of the rifting-related volcanism is distinct from the back-arc seamount chain volcanism and has a much stronger signature of Pacific MORB. A strong correlation between Sr isotopes and fluid-mobile element enrichment in the volcanic front lavas imply a significant contribution of slab-derived fluid to the source of volcanic front. In contrast, back-arc seamounts and rifting-related volcanism show a more limited influence of a fluid phase contribution to their source. Instead, high Δ7/4 and low 143Nd/144Nd associated with high Th/Ce imply that subducted bulk sediment is an important component in the back-arc. By assuming the regional mantle end-members, the relative contribution of a subduction-related component can be estimated. The magmatism in the back-arc seamounts is estimated to have a 0.2–0.3% bulk sediment addition to the source for both the northern and central parts of the arc. Minimal addition of fluid from altered oceanic crust is also predicted. For the rifting-related magmatism, a lower sediment contribution is predicted. Volcanic front magmatism is compatible with having no direct bulk sediment input, but is likely to involve the contribution of 2–2.5% of a fluid derived from altered oceanic crust and sediment in a mixing ratio of about 99:1.

Geochemistry of the quaternary volcanic rocks of the northeast Japan arc

Major and trace-element data are presented for a series of lavas from 17 volcanic centres in the NE Japan arc. These represent a transect of the Quaternary arc from its volcanic front (type A volcanoes) in the east, across its central zone (type B) to its western margin (type C). Rocks range from basalt to dacite in composition and the variation is attributed to fractionation of plagioclase, mafic silicates and titanomagnetite. For comparative purposes trace-element data for each volcano are presented on a 55 wt.% SiO2 normalised basis. The volcanoes display the characteristic features of arc volcanism viz enrichment in large ion lithophile elements (Rb, Sr, Ba, K) and depletion in high field strength elements (Zr, Nb, Hf, Ta). Using these data and derived primary magma compositions (Tatsumi et al., 1983), mixing calculations give the percentages of fractionating mineral phases necessary to derive the 55 wt.% SiO2 liquids. These data together with published distribution coefficients allow the calculation of trace-element abundances in the primary magmas. Reciprocal trace-element plots indicate that irrespective of position with respect to the volcanic front, the elements, La, Ba, K, Sr, Nb, Y and Zr of the primary magmas all lie on a common line strongly suggesting that the melts are the products of varying degrees of melting of a common homogeneous source. However, those melts close to the volcanic front (type A) show anomalous Rb, Th and Pb values suggesting that their mantle source was enriched in these elements. Further calculations suggest that the HFS depletion is not due to retention by residual mineral phases but is a feature of the source. Thus the data suggest: (a) that the source of the arc volcanism was homogeneous; and (b) that the characteristic chemical features were an inherent feature of the source. Since Zr/Nb ratios in the source melts are close to those found in N-type MORB it is suggested that a major component of arc-volcanic source rocks was a depleted mantle of the type which has given melts of N-type MORB composition. LIL elements added to such a depleted source are derived by the loss of silica-rich aqueous fluids from the descending (subducting) slab. Such losses occur within the first 100 km and are fixed in the overlying mantle wedge. This contaminated material is transported down by drag-induced convection, producing a homogeneous, but metasomatised mantle within the melting zone. Diapiric uprise produced by small-scale melting, produces further melting, with the major control of melt chemistry being the degree of partial melting. Superimposed on this chemistry is a localised addition of Pb, Rb and Th in the source of those volcanoes nearest the volcanic front.

Zircon geochronology of Archaean felsic sequences in the Zimbabwe craton: a revision of greenstone stratigraphy and a model for crustal growth

Abstract U-Pb ion-microprobe (SHRIMP) work on zircon populations from 13 Zimbabwean Archaean felsic rocks are presented and interpreted. Samples were extracted from felsic volcanic sequences from most of the major greenstone belts and represent the first zircon geochronological data from within the greenstone belts themselves. The data demonstrate a Late Archaean volcanicity spanning 250 Ma which began at least 2900 Ma ago and ended at 2650 Ma. The intrusion of extensive granitoid sills of the Chilimanzi suite at c. 2.6 Ga marks the widespread stabilization of the craton. Based on the new zircon data and a re-evaluation of published mapping, a new stratigraphic subdivision is presented for the Late (<2.9 Ga) Archaean of Zimbabwe. A feature of the stratigraphic model is the cyclicity of magmatism which begins with ultramafic-mafic rocks, progresses through felsic volcanism and ends with a granitoid event. These cycles are repeated at least three or four times in the 250 Ma time span. An important characteristic of the felsic volcanic rocks is that the bulk of the material examined contains inherited, xenocrystic zircons whose ages range from 1000 Ma to 20 Ma older than the host rocks. The oldest xenocrystic zircons (c. 3.6 Ga) are restricted to volcanic rocks which erupted through the Tokwe segment; itself the only known > 3.3 Ga fragment of Archaean crust in Zimbabwe. These data suggest that the Early Archaean crust is restricted to the Tokwe segment in the south of the country. Since even the oldest of the felsic volcanics (2.90 Ga, Lower Belingwean) have zircons which are 50 Ma older, it is suggested that remnants of earlier cyclic greenstone-granitoid events must underlie the present craton and that all of the currently exposed greenstone belts of Zimbabwe were developed on sialic crust.

Degassing and contamination of noble gases in Mid‐Atlantic Ridge basalts

New He, Ne, Ar and CO2 stepped-crushing data from the Mid-Atlantic Ridge show that contamination of basalts by atmospheric noble gases involves three or more components: unfractionated air, fractionated air with high 36Ar/22Ne (45) and fractionated air with low 36Ar/22Ne (5). In addition, the magmatic noble gases trapped in these basaltic glasses are variably fractionated such that 4He/40Ar* (where the asterisk indicates corrected for atmospheric contamination based on all 36Ar being atmospheric in origin) is in the range 3–12. Single samples have a range in 4He/40Ar* with the highest ratios in the final crush steps, consistent with the most fractionated (highest 4He/40Ar*) volatiles trapped in the smallest vesicles. It is not possible to distinguish between batch and Rayleigh degassing mechanisms. The complexities of the contamination and magmatic fractionation processes means that it is not possible to estimate 40Ar/36Ar of the mantle source to these basalts other than it must be higher than the highest ratio measured (26,200 ± 5200). Noble gas/CO2 ratios are also variable. While some CO2 adsorption during crushing exaggerates the variations in He/CO2 and Ar/CO2, we show that it is not possible to account for the entire variation as an analytical artefact: some of the variation is present in the vesicles. Variations in He/CO2 cannot be attributed to solubility controlled degassing because of the broadly similar solubilities of He and CO2 in tholeiitic magmas. The large range in He/CO2 in these glasses (factor of 10) is not accompanied by indications of major changes in melting regime or source region chemistry, therefore is thought to reflect late-stage (magmatic) fractionation of CO2 from the noble gases. It is not possible to identify an explicit mechanism, although both CO2 reduction (e.g., to hydrocarbons or graphite) and kinetic CO2-noble gas fractionation could account for the variations.

U-Pb zircon evidence for an extensive early Archean craton in Zimbabwe: A reassessment of the timing of craton formation, stabilization, and growth

U-Pb single-zircon analyses provide direct evidence for an enlarged early Archean craton forming the core to the present Zimbabwe craton. Virtually identical dates from the south-central Tokwe segment (3455 ± 2 Ma) and Midlands (3456 ± 6 Ma) parts of the craton strongly suggest their synchronous formation, during an event that formed a single early cratonic nucleus which we propose to call the “Sebakwe protocraton.” This is considered to underlie most of the current Zimbabwe craton. Parts of the craton are at least 3565 ± 21 Ma, a rock age reported here that represents the oldest rock dated from Zimbabwe. A ca. 3350 Ma relatively undeformed and unmetamorphosed intrusive granitic phase constrains the timing of the high-grade metamorphism and the stabilization of the protocraton. Comparison with published Re-Os data for the Zimbabwe craton strongly indicates a depleted subcontinental lithospheric mantle underlying the entire Sebakwe protocraton. Subsequent intrusive and volcanic activity from 3.0 to 2.6 Ga represents a second major period of magma genesis and crustal formation within which the predominant rocks of the exposed Zimbabwe craton were generated.

U–Pb dating of stockwork zircons from the eastern Iberian Pyrite Belt

Zircons found in the stockwork zones of the massive sulphide Los Frailes deposit, Iberian Pyrite Belt, are interpreted to have grown during the hydro-thermal alteration of host felsic volcanic rocks. Ion microprobe (SHRIMP) dating gives a206Pb/238 U age of 345.7 ± 4.6 Ma (2σ) and together with published spore data from the deposit suggest an age of uppermost Devonian to lowermost Carboniferous. Stockwork zircons offer the possibility of precisely defining the emplacement ages of these giant sulphide accumulations over the whole Pyrite Belt and from this will emerge estimates of the thermal budget necessary to generate the deposits.