Hans Friedrichsen

HELIUM AND ARGON ISOTOPE SYSTEMATICS OF THE CENTRAL LAU BASIN AND VALU FA RIDGE : EVIDENCE OF CRUST/MANTLE INTERACTIONS IN A BACK-ARC BASIN

Abstract We report helium and argon isotope analyses of fresh Lau Basin volcanic glasses from six sites within the central basin (~18°S), six localities along the Valu Fa Ridge (21°–22.4°S), and a single site from the northeastern basin (~ 15°S). Central basin basalts have 3 He 4 He ratios (R) between 8.2–8.5 R A (R A = air 3 He 4 He ), 40 Ar 36 Ar ratios significantly greater than atmosphere (up to 4900), and 4He contents from 3.5–9.4 × 10−6cm3 STP/g, similar to N-MORBs worldwide. These results are consistent with trace element and other (radiogenic) isotope data on these samples which indicate derivation from a depleted mantle source region. In contrast, evolved lavas from the Valu Fa Ridge all have R R A and show evidence of mixing with a component rich in radiogenic helium. There is a clear relationship between the 3 He 4 He ratios of these samples and their chemistry: basaltic andesites have 6.02 R c R a , andesites have lower ratios (2.37−2.65 RA), and a dacite has the lowest value of the entire sample suite (1.19 RA). All lavas have 40 Ar 36 Ar ratios similar to the atmospheric value and low helium concentrations, from 3–11 × 10−8cm3 STP/g, or between 30 and 300 times less than the central basin basalts. Although the helium isotope results of the Valu Fa lavas mirror the shift to more radiogenic values of other isotope systems (e.g., 206 Pb 204 Pb , 87 Sr 86 Sr ) which indicate addition of subducted sediment to these magma sources, we find no evidence that the radiogenic helium has a mantle or slab derivation or is in any way coupled to these other tracers. Instead, the most plausible mechanism to explain its incorporation into the Valu Fa lavas is by assimilation of old Lau crust in the near-surface environment by previously degassed magma. We argue that this mechanism has general applicability and can explain a number of hitherto apparently paradoxical geochemical features of some back-arc and ocean ridge lavas such as their high volatile and LIL element contents with low rare gas concentrations, and their mantle 3 He 4 He ratios with (hydrated oceanic) crustal D/H values. The realisation that the helium and argon systematics of the Valu Fa lavas are controlled by crust/mantle interactions has important implications for distinguishing between a number of models proposed for the formation of such evolved lavas, and we show that fractional crystallisation processes can most readily account for the low concentrations of, and systematic trends in, the mantle-derived helium and argon component of these lavas. In addition, because pre-existing crust in the Lau Basin must be old and/or altered enough to supply the radiogenic helium and atmospheric-like 40 Ar 36 Ar component to the Valu Fa lavas, the occurrence of crust/mantle interactions implies that old (forearc) crust may have been trapped within the Lau Basin: such a scenario has a clear bearing on ideas of the tectonic development of the basin. Finally, because of the potential of crust/mantle interactions to modify 3 He 4 He and 40 Ar 36 Ar ratios of mantle-derived melts, we assess the implications for using He and Ar tracers to characterise mantle sources in arcs, back-arcs, and spreading ridges, and consider the consequences for the combined use of rare gases with other (radiogenic) isotopic tracers of magma provenance at such settings. The basaltic andesite from the northeastern basin may also be influenced by the same kind of crustal interaction as the Valu Fa lavas as it falls within the 3 He 4 He range (6.9RA) of the other basaltic andesites. Interestingly, other helium isotope studies indicate that this part of the basin is characterised by a wide range in 3 He 4 He ratios, from MORB values up to 22RA. The low 3 He 4 He ratio of the basaltic andesite, therefore, serves to illustrate the possible effects of magma chamber processes on the rare gas and other volatile characteristics of hotspot lavas: an observation which is important not only for this part of the Lau Basin but for other localities worldwide.

Isotope studies of hydrogen and oxygen in ground ice experiences with the equilibration technique

Abstract Equilibration technique suitable for a large amount ofsamples is described for hydrogen and oxygen isotope analyses of ground ice, especially ice wedges, including the sampling strategy and the analytical procedure as well as the calibration of the Finnigan MAT Delta-S mass spectrometer in June, 1999. Since for future analyses of ice wedges, a higher sampling resolution with limited sample volume is required, the limit of the equliibration technique for small water samples size of between 0.05 and 5 ml was checked. For water samples smaller than 1ml, corresponding to a molar ratio [H2O]/[H2] of smaller than 0.994, a balance correction has to be applied. The experimental errors due to partial evaporation during evacuation, the balance calcultion of the isotope equilibration process, the linearity as well as memory effects of the mass spectrometer for smaples with large differences in δ18O and δD are tackled in this paper. In the polar regions of Northern Siberia without Late Pleistocene and Holocene glaciation, ground ice is used as an archive for paleoclimate studies. First results of stable isotope measurements on ice wedges clearly show a shift towards heavier isotopes and thus warmer winter temperatures as well as a change in the source of the precipitation between Late Pleistocene and Holocene. These results indicate the high potential of ground ice for paleoclimate studies.

O-Sr isotopic variations in Miocene granitoids from the Aegean: evidence for an origin by combined assimilation and fractional crystallization

Sr, O, and D/H isotopic compositions have been analyzed in Miocene metaluminous to slightly peraluminous (I-type) granitoids of the central Aegean. Individual plutonic complexes display significant variations in their δ18O and initial87Sr/86Sr compositions.δD and δ18O compositions of minerals and whole-rocks are mostly in the magmatic range. Some samples from Naxos and Mykonos/Delos show low δD and δ18O values characteristic of meteoric-water-hydrothermal interaction, but as a whole the changes in δ18O and Sr isotopic compositions as a result of hydrothermal alteration were slight, even in instances where marked alteration is petrographically observable. Consequently, the bulk-rock variations of δ18O from 8.1‰ to 12.0‰ and of87Sr/86Sr from 0.70438 to 0.71450 may be regarded as primary and indicative of the conditions of their evolution. Heterogeneous isotopic compositions observed in the individual plutons of Serifos, Ikaria, Samos and Kos may be caused by the multiple intrusion of chemically and isotopically distinct magma pulses, with high viscosities and relatively rapid consolidation in most cases preventing complete homogenization. The granitoids of Serifos, Ikaria and Kos display weak correlations between the initial87Sr/86Sr and δ18O and 1/Sr. The granitoid province shows a positive correlation between87Sr/86Sr and δ18O and a non-linear relationship between87Sr/86Sr and 1/Sr, whereby 1/Sr increases more rapidly than the isotopic ratio as the degree of fractionation of the rocks increases. It is argued that assimilation of older continental material by mantle-derived arc magmas with combined fractionation (AFC) is the most plausible model to explain the chemical and isotopic characteristics of the granitoids and the geological situation in which rock-types trend from granodiorites in the (south)west, near the inferred Oligocene-Miocene suture, to granites in the center and monzonites in the (north)east of the province.

Lower crustal melting and the role of open-system processes in the genesis of syn-orogenic quartz diorite-granite-leucogranite associations: constraints from Sr-Nd-O isotopes from the Bandombaai Complex, Namibia

Abstract The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous hornblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U–Pb zircon age of 540±3 Ma. Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (eNd (540 Ma)=−6.3 to −19.8; δ18O=9.0–11.6‰) but rather low and uniform initial Sr isotope compositions (87Sr/86Srinitial=0.70794–0.70982). Two leucogranites and one aplite have higher initial 87Sr/86Sr ratios (0.70828–0.71559), but similar initial eNd (−11.9 to −15.8) and oxygen isotope values (10.5–12.9‰). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K2O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr ( 1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material.

3He evidence for a wide zone of active mantle melting beneath the Central Andes

We report results of a regional survey of helium isotopes measured in water and gas samples in volcanic sulfataras and geothermal springs from the Central Andes of northern Chile and Bolivia between the latitudes 15°S and 23°S. The highest 3He4He ratios (reported as RRA ratios: R = sample 3He4He, RA = air 3He4He) are associated with the active volcanic arc of the Western Cordillera (0.92 < RRA < 5.52) and approach ratios found at other convergent margins in the circum-Pacific region. A significant 3He component is also present in fluid and gas samples from the high Altiplano plateau (0.48 < RRA < 3.56) and the Eastern Cordillera (0.03 < RRA < 1.2), up to 300 km east of the active arc and more than 300 km above the subducting slab. This wide zone of 3He anomalies is delineated both to the east and the west by regions with low 3He4He ratios ( ⩽ 0.2RA), typical of radiogenic helium production in the crust. Studies of the regional groundwater regime suggest that the wide zone of elevated 3He4He values away from the active volcanic arc is unlikely to be caused by lateral and shallow transport of magmatic helium and there is no evidence for significant crustal sources of 3He. The high 3He4He ratios are interpreted as reflecting degassing of volatiles from mantle-derived magmas emplaced over an area 400 km wide beneath and into crust up to 75 km thick. The subducting slab is at depths of 100–350 km in this region. In the west, underneath the active volcanic arc, mantle melting is probably largely controlled by mantle hydration and dehydration and the helium isotope data can be used to delineate the extent of the asthenospheric mantle wedge at depth. In contrast, mantle melting behind the arc, beneath the Altiplano and Eastern Cordillera, may be a result of convective removal of the base of the lithosphere. The sharp cut-off in the mantle helium signal in the east is interpreted as marking the western edge of thick and relatively cold lithosphere, devoid of mantle melts, which could transport mantle volatiles towards the surface. This may coincide with the limit of underthrusting of the Brazilian shield beneath the eastern margin of the Central Andes.

40Ar/39Ar and RbSr analyses from ductile shear zones from the Atacama Fault Zone, northern Chile: the age of deformation

Abstract The influence of deformation on the K-Ar and the Rb-Sr isotope system is investigated. It is assumed that, due to the diffusion processes involved, deformation has a similar effect on isotopic equilibrium as has temperature. In order to examine the influence of deformation on the K-Ar and the Rb-Sr isotope systems two shear zones from the Atacama Fault Zone (AFZ), situated in the north Chilean Coastal Cordillera, have been investigated. The AFZ, which was active as a sinistral strike-slip fault during the Mesozoic, has two sets of shear zones, one formed under amphibolite (SZ1), one under greenschist facies conditions (SZ2), Rb-Sr and 40Ar/39Ar age determinations were conducted on samples from cross sections of each set. In SZ1 the hornblendes and bioties from a weakly deformed sample reveal cooling ages of 153-152 and 150 ± 1 Ma, respectively. Biotite from the center of the shear zone of SZ1 gave an isochron of 143.9 ± 0.3 Ma (MSWD = 0.04) which is interpreted as the age of deformation which produced resetting of the mineral system. In SZ2 hornblendes yielded 40Ar/39Ar plateau (cooling) ages of ∼ 138 Ma. Biotites from undeformed samples gave Rb-Sr and 40Ar/39Ar total degassing ages of 130 ± 1 Ma, whereas biotite from the mylonitic rocks yielded 126-125 Ma which dates the time of deformation. Sr isotope homogenization occurred in the mylonitic rocks, and is most likely a result of deformation. The formation of SZ1 can be correlated to the Araucanian (= Nevadan) phase. The deformation in SZ2 is related to the onset of uplift and cooling of the Coastal Cordilleran magmatic arc.

Oxygen and hydrogen isotopic composition of Alpine and pre-Alpine minerals of the Swiss Central Alps

The rocks of the Swiss Central Alps consist of pre-Mesozoic and Mesozoic rocks which were metamorphosed during the Alpine orogeny. Eightysix samples from this area have been analyzed for their isotopic composition (310 mineral phases for their δ18O values and 99 mineral phases for their δD values).The mineral phases of pre-Mesozoic and Mesozoic rocks differ significantly in their stable isotope composition. The minerals in pre-Mesozoic rocks display a rather uniform oxygen and hydrogen isotope composition indicative of large-scale homogenization with magmatic fluids. The mineral phases of Mesozoic rocks, on the other hand, show a large variation in their isotopic composition, their δ18O values are heavier, and their δD values are isotopically lighter than the pre-Mesozoic phases. These data indicate the lack of a large-scale water supply to the gneissic cores of the Penninic nappes during Alpine metamorphism.Equilibrium conditions, as indicated by concordant oxygen isotope temperatures, are attained in several samples; disequilibrium, however, is more frequently observed, mainly in the central part of the Lepontin area. The pre-Mesozoic rocks recrystallized during Alpine metamorphism. This process was accompanied by partial reequilibration of the oxygen isotopes, and took place in a closed system. In the pre-Mesozoic rocks, the oxygen isotope fractionations, therefore, reflect the temperatures at the time of this recrystallization which, in many cases, is not the maximum temperature of Alpine metamorphism. There is strong evidence that oxygen isotope ratios are frozen during the progressive phase of a metamorphic event.Oxygen isotope fractionations indicate temperatures of Alpine metamorphism ranging from 500 ° C near Andermatt to 700 ° C near the Bergell granite.

A high‐performance, safer and semi‐automated approach for the δ18O analysis of diatom silica and new methods for removing exchangeable oxygen

The determination of the oxygen isotope composition of diatom silica in sediment cores is important for paleoclimate reconstruction, especially in non-carbonate sediments, where no other bioindicators such as ostracods and foraminifera are available. Since most currently available analytical techniques are time-consuming and labour-intensive, we have developed a new, safer, faster and semi-automated online approach for measuring oxygen isotopes in biogenic silica. Improvements include software that controls the measurement procedures and a video camera that remotely records the reaction of the samples under BrF(5) with a CO(2) laser. Maximum safety is guaranteed as the laser-fluorination unit is arranged under a fume hood in a separate room from the operator. A new routine has been developed for removing the exchangeable hydrous components within biogenic silica using ramp degassing. The sample plate is heated up to 1100 degrees C and cooled down to 400 degrees C in approximately 7 h under a flow of He gas (the inert Gas Flow Dehydration method--iGFD) before isotope analysis. Two quartz and two biogenic silica samples (approximately 1.5 mg) of known isotope composition were tested. The isotopic compositions were reproducible within an acceptable error; quartz samples gave a mean standard deviation of <0.15 per thousand (1sigma) and for biogenic silica <0.25 per thousand (1sigma) for samples down to approximately 0.3 mg. The semi-automated fluorination line is the fastest method available at present and enables a throughput of 74 samples/week.

A high performance, safe and semi-automated approach on the δ18O analysis of diatom silica and new methods for removing exchangeable oxygen

The determination of the oxygen isotope composition of diatom silica in sediment cores is important for paleoclimate reconstruction, especially in non-carbonate sediments, where no other bioindicators such as ostracods and foraminifera are available. Since most currently available analytical techniques are time-consuming and labour-intensive, we have developed a new, safer, faster and semi-automated online approach for measuring oxygen isotopes in biogenic silica. Improvements include software that controls the measurement procedures and a video camera that remotely records the reaction of the samples under BrF(5) with a CO(2) laser. Maximum safety is guaranteed as the laser-fluorination unit is arranged under a fume hood in a separate room from the operator. A new routine has been developed for removing the exchangeable hydrous components within biogenic silica using ramp degassing. The sample plate is heated up to 1100 degrees C and cooled down to 400 degrees C in approximately 7 h under a flow of He gas (the inert Gas Flow Dehydration method--iGFD) before isotope analysis. Two quartz and two biogenic silica samples (approximately 1.5 mg) of known isotope composition were tested. The isotopic compositions were reproducible within an acceptable error; quartz samples gave a mean standard deviation of <0.15 per thousand (1sigma) and for biogenic silica <0.25 per thousand (1sigma) for samples down to approximately 0.3 mg. The semi-automated fluorination line is the fastest method available at present and enables a throughput of 74 samples/week.

The seasonal sea-ice zone in the glacial Southern Ocean as a carbon sink.

Reduced surface–deep ocean exchange and enhanced nutrient consumption by phytoplankton in the Southern Ocean have been linked to lower glacial atmospheric CO2. However, identification of the biological and physical conditions involved and the related processes remains incomplete. Here we specify Southern Ocean surface–subsurface contrasts using a new tool, the combined oxygen and silicon isotope measurement of diatom and radiolarian opal, in combination with numerical simulations. Our data do not indicate a permanent glacial halocline related to melt water from icebergs. Corroborated by numerical simulations, we find that glacial surface stratification was variable and linked to seasonal sea-ice changes. During glacial spring–summer, the mixed layer was relatively shallow, while deeper mixing occurred during fall–winter, allowing for surface-ocean refueling with nutrients from the deep reservoir, which was potentially richer in nutrients than today. This generated specific carbon and opal export regimes turning the glacial seasonal sea-ice zone into a carbon sink.