Tod E. Waight

A FIFTY YEAR PERSPECTIVE OF MAGMATIC EVOLUTION ON RUAPEHU VOLCANO, NEW ZEALAND : VERIFICATION OF OPEN SYSTEM BEHAVIOUR IN AN ARC VOLCANO

Geochemical and petrological data for samples from well documented eruptions that occurred at Ruapehu volcano over the period 1945–1996 can be used to illustrate the complexity of short term geochemical variation in an arc-type volcano. Collectively, data from Ruapehu Volcano show trends with time of increasing SiO2 abundance and rising 87Sr/86Sr ratios, consistent with broad control by assimilation and crystal fractionation processes (AFC). However, the magmas emplaced during the past fifty years show geochemical variability that spans most of the range shown by lavas erupted over the entire history of the volcano. Magma compositions fluctuate through wide ranges over relatively short time intervals reflecting the effects of processes associated with magma recharge events within the volcano. These complex trends are also manifested when the geochemistry and petrography of sequences of prehistoric lavas are examined in detail and they arise from short-term effects that are imposed during recharge on the overall AFC trend. We show that the temporal geochemical and petrographic variations among erupted magmas are modulated by processes of mixing and mingling between fresh magma from below and stagnant melt and entrained crystals from earlier events remaining in the volcanic edifice, probably in dikes and sills. These processes are probably replicated over longer time periods (hundreds to thousands of years) as melts arrested at different levels in the near surface conduit system are progressively displaced by new magma batches. Arc type volcanoes such as Ruapehu are characterised by pulsatory growth in which bursts of high magma production are superimposed on a background of subdued but more or less continuous activity. This style of activity is difficult to predict through the usual (seismicity, ground deformation, lake water geochemistry) volcano monitoring techniques, and petrology and geochemistry may provide the basis for an alternative strategy.

Rb isotope dilution analyses by MC-ICPMS using Zr to correct for mass fractionation: towards improved Rb–Sr geochronology?

A new technique is presented where mass fractionation during Rb isotope dilution analyses by multi-collector inductively coupled plasma mass spectrometry is corrected for by measuring the amount of fractionation on admixed Zr. Replicate analyses of natural Rb interspersed with analyses of 87Rb tracer enriched samples yield a mean 87Rb/85Rb=0.38540±19 (0.05%, 2 s.d.), assuming a natural 90Zr/91Zr of 4.588. Each Rb analysis takes 1 min, consumes 20 ng of Rb and has an internal precision of ∼0.02% (2 s.e.). Washouts between samples take 5 min. Persistent but small stable Rb backgrounds are overcome by an on-peak-zeroes (OPZ) measurement prior to data acquisition. Close examination of measured 87Rb/85Rb and 90Zr/91Zr ratios indicate small changes in relative fractionation of Rb and Zr during plasma ionisation occur when different sample introduction techniques are used (e.g., ‘wet’ vs. ‘dry’ nebulisation), although the differences are insignificant compared to the level of precision required for isotope dilution measurements. Replicate analyses of whole rock samples suggest a reproducibility for Rb concentration measurements of ≤0.5% and 87Rb/86Sr measurements of 0.2% when interfering Sr is reduced to satisfactory levels. However, it is difficult to ascertain to what extent this reproducibility reflects the limit of the technique or powder heterogeneity. Much of the error involved in the Rb isotope dilution and Sr isotope ratio measurements by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) is derived from uncertainties as to which 87Sr/86Sr (and 87Rb/85Rb) ratios to use when correcting for isobaric interferences due to the presence of spike Sr and Rb at mass 87. If isobaric interferences are minimised by efficient separation of Rb from Sr during cation exchange chemistry, the use of natural ratios for isobaric interference corrections yields the most reproducible data, indicating that the interferences are derived from environmental blank. Larger isobaric interferences at mass 87 are indicative of inefficient chemical separations, and the measured ratio from the complementary analysis provide more reproducible data. Burning off of Rb during conventional thermal ionisation mass spectrometry (TIMS) Sr isotope analysis nullifies this isobaric interference, and therefore, TIMS remains the method of choice for reliable and precise 87Sr/86Sr determinations on spiked samples. Application of our technique to minerals separated from Tertiary to Palaeozoic plutons yields age data consistent with previous determinations. Where different two-point isochron ages can be calculated for individual plutons, the ages reproduce to ≤±0.3%. The method represents an initial improvement in Rb isotope dilution measurements over TIMS by allowing a quantifiable correction to be made for mass fractionation, confirmed by duplicate analyses of standards and samples by both TIMS and MC-ICPMS. Mass fractionation corrected Rb isotope dilution analyses should result in: (1) improved Rb–Sr geochronology in examples where the Rb–Sr ratio provides the largest source of error; (2) application of this improved method to Rb–Sr geochronology on smaller samples such as single mica-flakes and micro-drill samples and; (3) by comparison with other geochronological techniques, more detailed cooling and crystallisation histories of igneous and metamorphic rocks. Taking advantage of these improvements requires a reevaluation of the Rb decay constant, which this technique should also permit.

Geochemical investigations of microgranitoid enclaves in the S-type Cowra Granodiorite, Lachlan Fold Belt, SE Australia ☆

The Cowra Granodiorite is a relatively mafic, enclave-rich, S-type pluton in the Lachlan Fold Belt, which has been cited as a type example of a restitic origin for all varieties of enclaves in Lachlan Fold Belt S-type granites. Microgranitoid enclaves from the pluton are subordinate to metasedimentary varieties and can be subdivided into two groups according to their mafic mineral assemblage: pyroxene microtonalites and biotite microgranites. No geochemical or isotopic distinction can be made between the two varieties. Petrographic evidence (acicular apatites, xenocrysts from the host granite) suggests an origin as mingled, more mafic, magmas, which have been variably contaminated by the more felsic host magma. This is supported by the fact that the microgranitoid enclaves have isotopic compositions (87Sr/86Sr(i)=0.7095 to 0.7144, eNd(i)=−9.2 to −6.9) that are generally more primitive than, or similar to, those of the host granite (87Sr/86Sr(i)=0.7142, eNd(i)=−8.8). The spread in isotopic compositions, like their trace element compositions, is considered to be the consequence of variable degrees of diffusive exchange between the felsic and more mafic magmas during slow cooling. Several studied metasedimentary enclaves are not in isotopic equilibrium with their host granite and therefore cannot represent pristine samples of the bulk source region of the granite. Instead, they represent portions of a lithologically and compositionally diverse source terrane or accidental xenoliths entrapped during emplacement.

SHRIMP U-Pb geochronology of Cretaceous magmatism in northwest Nelson-Westland, South Island, New Zealand

Abstract Ion microprobe U‐Pb zircon ages have been obtained from four samples of Cretaceous granitoid and two samples of volcanogenic sediment from the northwest Nelson‐Westland region of the South Island of New Zealand. Crow Granite, which intrudes lower Paleozoic metasedi‐mentary rocks in the Buller Terrane on the eastern side of the Karamea Batholith, has given a crystallisation age of 137 ± 3 Ma (2a). This age is typical of the Jurassic‐Early Cretaceous plutonic rocks that dominate the Median Tectonic Zone, and raises the possibility that the Western Province and the Median Tectonic Zone were linked some 20 m.y. earlier than previously proposed. The “Gouland granod‐iorite”, which forms a large pluton at the northeastern margin of the Karamea Batholith, has a crystallisation age of 119 ± 2 Ma (2a). This age is similar to the Separation Point Batholith (118 Ma), and the distinctive chemistry of the batholith (high Na, Al, Sr, and low Y) is also displayed by the Gouland granodiorite. The “Big Deep granit...

Mid-Cretaceous granitic magmatism during the transition from subduction to extension in southern New Zealand: a chemical and tectonic synthesis

Abstract Regional geochronological studies indicate that mid-Cretaceous plutonism (the Hohonu Suite at ∼110 Ma) in the Hohonu Batholith, Western Province of New Zealand, occurred during a period of rapid tectonic change in the SW Pacific portion of Gondwana. The 30–40 m.y. preceding Hohonu Suite magmatism were dominated by the subduction-related plutonism of the Median Tectonic Zone volcanic arc. Between 125–118 Ma there was a major collisional event, inferred to be the result of collision between the Median Tectonic Zone and the Western Province. This collision resulted in melting of the Median Tectonic Zone arc underplate and generation of a distinctive suite of alkali-calcic granitoids, termed the Separation Point Suite. At ∼110 Ma there was another pulse of magmatism, restricted to the Buller terrane of the Western Province, and including the Hohonu Suite granitoids. This was followed almost immediately by extension, culminating in the opening of the Tasman Sea some 30 m.y. later. The Hohonu Suite granitoids overlap temporally with the last vestiges of collisional Separation Point magmas and the onset of crustal extension in the Western Province, and thus represent magmatism in a post-collisional setting. Hohonu Suite magmas are typically calc-alkaline, but retain a chemical signature which suggests that the earlier Separation Point Suite magmas and/or sources were involved in Hohonu Suite petrogenesis. A model is proposed in which rapid isothermal uplift, resulting from the post-collisional collapse of continental crust previously thickened during the Median Tectonic Zone collision, caused melting of lower continental crust to generate the Hohonu Suite granitoids. In this example, granitoid composition is a consequence of the composition of the source rocks and the conditions present during melting, and no geochemical signature indicative of the tectonic setting during magmatism is present.

Rapid and highly reproducible analysis of rare earth elements by multiple collector inductively coupled plasma mass spectrometry

Abstract A method has been developed for the rapid chemical separation and highly reproducible analysis of the rare earth elements (REE) by isotope dilution analysis by means of a multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS). This technique is superior in terms of the analytical reproducibility or rapidity of analysis compared with quadrupole ICP-MS or with thermal ionization mass spectrometric isotope dilution techniques. Samples are digested by standard hydrofluoric–nitric acid–based techniques and spiked with two mixed spikes. The bulk REE are separated from the sample on a cation exchange column, collecting the middle-heavy and light REE as two groups, which provides a middle-heavy REE cut with sufficient separation of the light from the heavier REE to render oxide interferences trivial, and a Ba-free light REE cut. The heavy (Er-Lu), middle (Eu-Gd), and light REE (La-Eu) concentrations are determined by three short (1 to 2 min) analyses with a CETAC Aridus desolvating nebulizer introduction system. Replicate digestions of international rock standards demonstrate that concentrations can be reproduced to

Lead isotopic disequilibrium between sulfide and plagioclase in the bushveld complex and the chemical evolution of large layered intrusions

Abstract The Pb isotopic compositions of coexisting plagioclase and sulfide from the Bushveld Complex were determined by laser ablation multi-collector ICPMS (LA MC-ICPMS). The samples are of the upper Critical Zone in the northeast corner of the Complex and were collected from drill core and underground mine exposures. All the rocks are fresh and exhibit no evidence for alteration, weathering, or disruption of the Pb isotope systematics subsequent to the initial cooling of the intrusion. Furthermore, individual plagioclase and sulfide crystals do not contain enough U to warrant correction for radiogenic in-growth. For these reasons, the measured Pb isotope ratios approximate the initial ones. For plagioclase, 207Pb/206Pb ranges from 0.98 to 1.02 and 208Pb/206Pb from 2.26 to 2.35. Low 207Pb/206Pb and 208Pb/206Pb ratios characterize grain boundaries and partially annealed microcracks, some of which contain minute fragments of sulfide and other phases, and this accounts for most, if not all, the heterogeneity exhibited by individual samples. Real compositional differences exist, however, in plagioclase from different lithologic layers. For example, plagioclase 207Pb/206Pb values vary from 1.004 in norite beneath the Merensky pyroxenite to 1.009 in the mineralized pyroxenite, and 0.997 in overlying norite. In most samples in which sulfide and plagioclase coexist, the sulfide 207Pb/206Pb ratio is lower and 208Pb/206Pb ratio higher than the corresponding ones in plagioclase. For example, in a mineralized Merensky reef sample, average sulfide 207Pb/206Pb and 208Pb/206Pb ratios are 0.993 and 2.313, respectively, while those in plagioclase are 1.000 and 2.292. In one sample, the sulfide is extremely heterogeneous, with 207Pb/206Pb and 208Pb/206Pb ratios as low as 0.84 and 2.12. In this particular sample, the compositions must represent an isolated occurrence of addition of a young Pb component. The array of sulfide and plagioclase compositions requires multiple sources of Pb at the time of crystallization or soon thereafter. The disequilibrium between plagioclase and sulfide implies that some of the Pb originated from the isotopically distinct country rocks and was introduced at temperatures at which the composition of sulfide but not plagioclase could be modified. Thus, Bushveld sulfide, and to some extent plagioclase, do not reliably record the initial Pb isotopic composition(s) of the parent magma(s).

Rapid sample digestion by fusion and chemical separation of Hf for isotopic analysis by MC-ICPMS

A new method for rapid sample digestion and efficient chemical separation of Hf and REE from rock samples for precise isotopic analysis is presented. Samples are digested by fusion in the presence of a lithium borate flux at 1100 degrees C and dissolved whilst molten in dilute nitric or hydrochloric acid. Prior to chemical separation using ion exchange techniques, Li and B from the flux material and Si from the sample are separated from the remaining major elements, REE and high field strength elements (HFSE) in the sample by Fe-hydroxide co-precipitation. The chemical separation of Hf is a two-stage procedure designed to first remove the remaining matrix elements (e.g. Fe, Ba) in the sample using standard cation exchange techniques, followed by separation of Hf from the REE and HFSE on TEVA extraction chromatographic resin. Hf yields are >90% and total procedural blanks are ca. 50 pg. Hf isotope ratios of a synthetic standard solution and replicate digestions of international rock standards BHVO-1 and BCR-1 measured on multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS) reproduce similarly to </=50 ppm (2 S.D.). The following elemental ratios are routinely obtained for elements, which interfere isobarically or may affect the ionisation and/or fractionation behaviour of Hf during analysis: (176)Yb/(176)Hf<0.0001; (176)Lu/(176)Hf<0.00001; Ti/Hf<0.05. This technique also provides a means of separating Nd from the REE fraction for isotopic analysis and, potentially, may be adapted for measurement of Lu/Hf ratios by isotope dilution techniques.

Hydrothermal-metasomatic and tectono-metamorphic processes in the Isua supracrustal belt (West Greenland): A multi-isotopic investigation of their effects on the Earth's oldest oceanic crustal sequence

Abstract Despite superimposed metamorphic overprinting and metasomatic alterations, primary volcanic features remain preserved in low-strain domains of mafic volcanic sequences in the western Isua supracrustal belt (ISB, West Greenland). These basaltic successions represent the hitherto oldest known fragments of oceanic crust on Earth. Early Archean metasomatic fluids, rich in light rare earth elements (LREE), Th, U, Pb, Ba, and alkalies, invaded the supracrustal package and distinctively altered the basaltic sequences. Field relationships, source characteristics traced by Pb isotopes, and geochronological results provide indications that these fluids were genetically related to the emplacement of tonalite sheets into the ISB between 3.81 and 3.74 Ga ago. Subsequent early Archean metamorphism homogenized the mixed primary and metasomatic mineral parageneses of these metavolcanic rocks. Allanite occurs as the most characteristic and critical secondary metasomatic-metamorphic phase and is developed in macroscopically discernible zones of increased metsomatic alteration, even in domains of low strain. Because of its high concentration of LREE, Th, and U, this secondary mineral accounts for much of the disturbances recorded by the Sm-Nd and Th-U-Pb isotope systematics of the pillowed metabasalts. The supracrustal sequences were tectono-metamorphically affected to varying degrees during a late Archean, ∼2.6- to 2.8-Ga-old event, also recognized in the adjacent gneiss terranes of the Isuakasia area. The degree to which bulk rocks were isotopically reequilibrated is directly dependent on the different relative contributions of allanite-hosted parent-daughter elements to the overall whole-rock mass budget of the respective isotope systems. Although low-strained (initially only weakly metasomatized) pillow basalts remained more or less closed with respect to the U-Pb and Rb-Sr systems since ∼3.74 Ga, the Sm-Nd system appears to have been partially opened on a whole-rock scale during the late Archean event. This diversified behavior of the whole-rock isotope systems with respect to late Archean overprinting is explained by the combination of mass budget contributions of the respective elements added during metasomatism and the partial opening of metasomatic macroenvironments during late Archean recrystallization processes with associated renewed fluid flow. In reactivated zones of high strain, where primary metasomatic alteration is most prominently developed, late Archean partial resetting also of the U-Pb isotope system on a whole-rock scale occurred. This is consistent with an apparent late Archean age of kyanite, which initially crystallized during the early Archean metamorphism. Its age is controlled by the U-Pb systematics of allanite inclusions, which have exchanged their isotopic properties during the tectono-metamorphic event that overprinted the oceanic crustal sequence at Isua more than 1000 myr later. These results underline the need for care in the interpretation of whole-rock geochemical data from polymetamorphic rocks in general, and from the Isua oceanic crustal sequences in particular, to constrain isotopic models of early Earth’s evolution. Likewise, this study cautions against the indiscriminate use of geochemical data of metavolcanic rocks from Isua to infer models for geotectonic settings relevant for their formation.

Field characteristics, petrography, and geochronology of the Hohonu Batholith and the adjacent Granite Hill Complex, North Westland, New Zealand

Abstract Detailed geological mapping, petrography, geochemistry, and geochronological studies in the Hohonu Batholith, North Westland, have identified 10 granitoid plutons emplaced during three intrusive episodes. The earliest episode is represented by a single dated Paleozoic pluton, Summit Granite (new) (381.2 ± 7.3 Ma), which is correlated with a discrete pulse of Mid—Late Devonian plutonism recognised in the Karamea Batholith. The undated Mount Graham Granite (new) is also likely to be Paleozoic, based on chemical and petrographic characteristics. The bulk of the batholith (seven plutons) was emplaced in the mid Cretaceous (114–109 Ma) and comprises two related, yet distinct, geochemical suites, which correlate with the previously defined Rahu Suite. The plutons identified are (from north to south): Pah Point Granite; Jays Creek Granodiorite (new); Uncle Bay Tonalite; Te Kinga Monzogranite; Deutgam Granodiorite; Turiwhate Granodiorite (new); and Arahura Granite (new). Mid‐Cretaceous plutonism in the W...