Konrad Hammerschmidt

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.

Helium isotope characteristics of Andean geothermal fluids and lavas

The first comprehensive helium isotope survey of the Andes is reported here. We have sampled geothermal fluids and phyric lava flows from the Southern (svz) and Central (cvz) Volcanic Zones, the volcanically active Pun˜a region and the Precordillera, Salta Basin, Longitudinal Valley and the aseismic region between the two volcanic zones. Although the active areas are characterised by significant differences in crustal age and thickness, the svz, cvz and Pun˜a are characterised by a wide and overlapping range in 3He/4He ratios (for fluids and phenocrysts) from predominantly radiogenic values to close to the MORB ratio. The measured ranges in 3He/4He ratios (R) (reported normalised to the air 3He/4He—RA) are: svz (0.18 < R/RA< 6.9); cvz (0.82 < R/RA< 6.0); and Pun˜a (1.8 < R/RA< 5.4). Modification of magmatic 3He/4He ratios by water/rock interactions (fluids) or post-eruptive grow-in of radiogenic 4He or preferential diffusive loss of 3He (phenocrysts) is considered unlikely; this means that the wide range reflects the helium isotope characteristics of magma bodies in the Andean crust. The mechanism controlling the 3He/4He ratios appears to be a mixing between mantle (MORB-like) helium and a radiogenic helium component derived from radioactive decay within the magma (magma aging) and/or interaction with 4He-rich country rock: a process expected to be influenced by pre-eruptive degassing of the mantle component. Assimilation of lower crust is also capable of modifying 3He/4He ratios, albeit to a much lesser extent. However, it is possible that the highest measured values in each zone were established by the addition of lower crustal radiogenic helium to MORB helium. In this case, the higher ‘base level’ ratios of the svz would reflect the younger crustal structure of this region. In contrast to helium, there is no overlap in the Sr or Pb isotope characteristics of lavas from the active zones: in all areas, therefore, 3He/4He ratios appear to vary independently of Sr and Pb isotope variations. This decoupling between the lithophile tracers and helium reflects the different processes controlling their isotopic characteristics: crust-mantle interactions, alone, for Sr and Pb but for helium the effects of pre-eruptive degassing and possibly magma aging are possibly superimposed. The presence of mantle helium in the Pun˜a region, and to a lesser extent in the Salta Basin, gives an across-arc perspective to the helium isotope distribution and shows mantle melting to occur significantly to the east of the active arc: this is most probably a consequence of lithospheric delamination. The Precordillera hot spring water has the only pure radiogenic helium signal of the entire sample suite and thus marks the western limit of asthenospheric mantle under the Andes.

Mapping magma sources in the east Sunda-Banda arcs, Indonesia: Constraints from helium isotopes

Abstract We report new helium isotope analyses of olivine and clinopyroxene separates from recent lavas for eleven volcanoes from Flores in the east Sunda arc through the inactive segment between the arcs to Banda Island at the extreme of the contiguous Banda arc. In the east Sunda arc, 3 He 4 He ratios (R) vary between 4.5 RA ( R A = air 3 He 4 He ) for the leucitic Batu Tara volcano to a remarkable low of pure radiogenic helium (0.0075 RA) for Werung at the southern tip of Lomblen Island. Lavas from the inactive zone, which represents the locus of collision of the Australian continent with the arc, have a narrower range in R R A —from 3.9 for Kisu in the straits of Pantar to 1.0 for Romang Island. Our one locality (Banda Island) in the Banda arc gives the highest R R A ratio (3.1) observed along this arc to date. The results are consistent with the involvement of crustal material in magma genesis throughout the east Sunda/Banda arcs, as far west as Iya in central Flores. We combine these helium isotope results with published and on-going strontium isotope studies, and show that the source of the helium in the crustal component is unlikely to be terrigenous sediments derived from the Australian continent; rather, degassing of Australian continental crust appears to be the dominant process controlling the helium budget. The He-Sr isotope systematics also provide a framework to account for the areal pattern of 3 He 4 He in this part of the arcs: the radiogenic crustal component is diluted with mantle helium both in a down-dip direction and with increasing lateral distance from the collision zone. These factors result in an excellent first-order relationship between the 3 He 4 He ratio, degree of He Sr enrichment (relative to the postulated mantle endmember), and alkalinity of the erupted lavas. Such a relationship has a direct bearing on models of the tectonic evolution of the collision zone, and on the observation that helium isotopes are decoupled from strontium and other geochemical signatures along the Banda arc.

Eocene high pressure metamorphism in the Penninic units of the Tauern Window (Eastern Alps): evidence from40Ar−39Ar dating and petrological investigations

The40Ar-39Ar degassing spectra of white micas and amphiboles from three tectonic units of the central Tauern Window (Pennine basement and cover in the Eastern Alps) have been measured. White micas are classified as (1) pre-Alpine low-Si relic micas with an age value of 292 Ma, variously disturbed by the Alpine metamorphism; (2) Alpine phengitic micas of variable composition with an age between 32 and 36 Ma; (3) Alpine low-Si micas with a maximum age of 27 Ma. We attribute the higher Alpine ages to a blueschist facies event, whereas the lower age reflects the late cooling of the nappe pile. Blueschist facies phengites from the basement (Lower Schieferhülle) and the tectonic cover (Upper Schieferhülle) crystallized at a temperature below the closure temperature (Tc) for argon diffusion in white mica and record ages of 32 to 36 Ma. At the same time a thin, eclogite facies unit (Eclogite Zone) was thrust between the Lower and the Upper Schieferhülle and cooled from eclogite facies conditions at about 600°C at 20 kbar to blueschist facies conditions at 450°C or even 300°C at >10 kbar. Eclogite facies phengites closed for argon diffusion and record cooling ages, coinciding with the crystallization ages in the hanging and the footwall unit. Amphibole age spectra (actinolite, glaucophane, barroisite) are not interpretable in terms of geologically meaningful ages because of excess argon.

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.

Helium isotope evidence for off-axis degassing of the Icelandic hotspot

Abstract We report a helium and carbon survey of 16 hydrothermal localities from Vestfirdir—the northwest peninsula of Iceland. The region is situated ∼250 km away from the Icelandic rift-axis, and is characterized by low-temperature ( 3 He / 4 He ratios ( R / R A ) values between those typical of MORB (8±1) and 30—the highest value matches the most extreme magmatic ratios reported worldwide to date. Four other localities have R / R A ∼MORB ratios with the remaining three showing heavy dilution with crustal radiogenic helium ( R / R A δ 13 C ( CO 2 ) (−6 to −16‰ PDB), and CO 2 / 3 He ratios vary over 4 orders of magnitude (4×10 6 to 2.9×10 10 ). There appears no systematic relationship between the volatile systematics and geographic distribution or age of reservoir rocks. Stable isotope ( δ 18 O and δ D) variations are consistent with a localized meteoric origin for the hydrothermal waters—the carrier phase of the volatiles. We conclude that release of magmatic volatiles in Vestfirdir occurs predominantly via incipient mantle melting with a negligible input of re-distributed volatiles from the rift zones. It is also highly unlikely that leaching of old reservoir rocks contribute to the mantle volatile flux: however, release of radiogenic helium from the uppermost crust does occur and this is accompanied by addition of isotopically light carbon most likely of organic origin. The light carbon input is most discernible when the mantle-derived carbon signal is reduced—most probably by reaction to form calcite. Although Vestfirdir makes a discernible (off-axis) input to the degassing history of the Icelandic hotspot, its contribution is extremely minor (∼0.04%) when compared to the degassing flux associated with rifting and volcanism along the Icelandic spreading zones.

Plume related alkaline magmatism in central Africa—the Meidob Hills (W Sudan)

Abstract We investigated in a combined petrological, geochemical and isotope study the Miocene to Holocene volcanic activity in the Meidob Hills Volcanic Field, the north-eastern part of the Darfur Dome Volcanic Province in western Sudan. The Darfur Dome is related to a domal uplift, associated with a negative Bouguer anomaly, and crustal shearing along the Central African Fault Zone. Magmas of the Meidob Hills evolved from basanites to phonolites by fractionation of olivine, clinopyroxene, magnetite, and in a late stage by plagioclase–anorthoclase and apatite. Significant contamination by crustal rocks is observed in basaltic to benmoreitic–trachytic (the latter mostly pyroclastic) rocks. Isotope data demonstrate the presence of a HIMU-like component, which is most pronounced in 7 Ma old basanites ( 87 Sr / 86 Sr =0.70306, 206 Pb / 204 Pb =20.075, 207 Pb / 204 Pb =15.690, 208 Pb / 204 Pb =39.785 ). The basanites are interpreted as a mixture between hypothetical sources of depleted mantle and HIMU with small amounts of some other sources. 3 He / 4 He -isotope ratios in olivine from mafic rocks are much lower than the primitive He mantle source (PHEM) and range between 6.6 and 9.2 Ra, corrected for atmospheric contamination, measured values between 5.4 and 7.5 Ra. Trace element ratios of Ba, Nb, La and Zr show a wide range and indicate source heterogeneities, possibly a subcontinental lithospheric source. The Darfur Dome provides an example for an isolated intracontinental mantle plume with a low He—high HIMU-type composition. Its igneous activity is not related to graben structures, and we see no connection to the Cameroon Line, the Tibesti and the volcanism of the Red Sea and the Afar plume.

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.

Retrograde evolution of eclogites: evidences from microstructures and 40Ar/39Ar white mica dates, Münchberg Massif, northern Bavaria

Phengites from eclogites and pegmatites (3T, 2M1, coarse-grained and recrystallized) of the Münchberg Massif (Weissenstein and Oberkotzau) have been dated by the 40Ar/39Ar method. 3T-micas from the eclogites yielded plateau and isochron ages of 365±7 Ma. 2M1-micas show disturbed degassing spectra. Micas from pegmatites show a slight excess Ar component, with an isochron age of 353 to 351±3 Ma. An age component of approximately 300 Ma was also detected. In combination with age values from the literature, the cooling history of the Münchberg Massif from eclogite-facies conditions (390 Ma) to cooling below 350°C (350 Ma) is documented. The age component of 300 Ma is attributed to a low-grade stage of mineral growth accompanied by a transitional ductile-brittle deformation. The petrological effects include formation of pumpellyite-prehnite-facies minerals, frequently precipitated in microcraks and cleavage planes of earlier formed minerals. This stage has to be seen in conjunction with the intrusions of the Fichtelgebirge granite.

No slab‐derived CO2 in Mariana Trough back‐arc basalts: Implications for carbon subduction and for temporary storage of CO2 beneath slow spreading ridges

The Southern Mariana Trough is particularly well suited to study mass balance in subduction zones because the flux of material recycled from the subducted slab has been shown to diminish to negligible levels in the southernmost part of the area. We present new He and Ar concentration and isotopic data for 16 back-arc basaltic glasses and combine these with previously published CO2 and H2O concentration and δ13C data to explore the recycling of carbon and light noble gases in the Mariana back arc. Degassing has affected all samples and is particularly extensive in more water-rich samples, i.e., those containing the largest recycled component. The degassing history features three stages: (1) deep degassing which commenced when the melt reached saturation of CO2 and noble gases in the mantle, (2) preeruptive degassing during storage in the crust-mantle transition zone which involved addition of extraneous CO2 to the vapor phase, and (3) eruption. CO2 released during stage 1 was, at least partially, incorporated into wall rock and subsequently remobilized during stage 2 degassing of later magma batches. Reconstructed parental values for 3He/4He, δ13C, CO2/3He, and CO2/40Ar* are indistinguishable from those of mid-ocean ridge basalt. This implies that there is negligible recycling of subducted carbon, helium, or argon into the source of Mariana Trough basalt.