Jörg Erzinger

Noble gas evidence for a lower mantle component in MORBs from the southern East Pacific Rise: Decoupling of helium and neon isotope systematics

Abstract Abundances and isotopic compositions of all noble gases have been determined in basalt glasses from different latitudes on the 13–23°S section of the East Pacific Rise. In this region earlier work has identified isotopic signatures of Sr, Nd, and Pb as well as He which indicate a plume-like component admixed to the depleted MORB mantle between about 15.8° and 20.7°S (Bach et al., 1994; Mahoney et al., 1994; Kurz et al., 1997), although incompatible trace element ratios yield no such evidence. Our noble gas data confirm plume-like isotope ratios for He, Ne, and probably Ar, whereas Kr and Xe compositions are atmospheric. Interestingly, He and Ne data do not correlate very well, as the plume-like Ne component can be traced farther south than plume-like He. A similar decoupling of isotopic patterns has been observed for He vs. the radiogenic isotopes (Sr, Nd, Pb) by Mahoney et al. (1994) and is confirmed here. Mixing models suggest that between 13.0° and 15.8°S, a small amount of undegassed material from the lower mantle is admixed to the depleted MORB mantle, whereas from 15.8° to 20.7°S a considerable, but constant proportion of plume material, which prior to mixing has been degassed to progressively larger extents proceeding to the south, was present in the magma source. South of 20.7°S the lower mantle component is virtually absent. Several processes are discussed to explain these observations, such as passive mantle heterogeneities (Mahoney et al., 1994), plume-ridge interaction (e.g., Poreda et al., 1993), or a leaking lower mantle. Based on our neon isotope data and references from the literature, we discuss the evidence for a lower 20 Ne 22 Ne ratio in the MORB reservoir than in the plume source, as might be expected if the nucleogenic 21 Ne 22 Ne production ratio in the mantle is smaller than predicted.

The geochemical consequences of late-stage low-grade alteration of lower ocean crust at the SW Indian Ridge: Results from ODP Hole 735B (Leg 176)

Chemical exchange between oceanic lithosphere and seawater is important in setting the chemical composition of the oceans. In the past, budgets for chemical flux in the flanks of mid-ocean ridges have only considered exchange between basalt and seawater. Recent studies have shown that lower crustal and upper mantle lithologies make up a significant fraction of sea floor produced at the global mid-ocean ridge system. Moreover, the rugged topography of slow spread crust exposing lower crust and upper mantle facilitates prolonged fluid circulation, whereas volcanic ridge flanks are more rapidly isolated from the ocean by a sediment seal. Hence, elemental fluxes during lower crust-seawater reactions must be assessed to determine their role in global geochemical budgets. ODP Hole 735B penetrates more than 1500 m into lower ocean crust that was generated at the very slow spreading Southwest Indian Ridge and later formed the 5-km-high Atlantis Bank on the inside corner high of the Atlantis II Fracture Zone. The gabbroic rocks recovered from Hole 735B preserve a complex record of plastic and brittle deformation and hydrothermal alteration. High-temperature alteration is rare below 600 m below seafloor (mbsf), but the lowermost section of the hole (500 -1500 mbsf) has been affected by a complex and multistage low-temperature (,250°C) alteration history probably related to the tectonic uplift of the basement. This low-T alteration is localized and typically confined to fractured regions where intense alteration of the host rocks can be observed adjacent to veins/veinlets filled with smectite, smectite- chlorite mixed layer minerals, or chlorite 6 calcite 6 zeolite 6 sulfide6 Fe-oxyhydroxide. We have determined the bulk chemistry and O and Sr isotope compositions of fresh/altered rock pairs to estimate the chemical fluxes associated with low-temperature interaction between the uplifted and fractured gabbroic crust and circulating seawater. The locally abundant low-temperature alteration in crust at Site 735 has significantly changed the overall chemical composition of the basement. The direction of these changes is similar to that defined for volcanic ridge flanks, with low-temperature alteration of gabbroic crust acting as a sink for the alkalis, H2O, C, U, P, 18 O, and 87 Sr. The magnitudes of element fluxes are similar to volcanic ridge flanks for some components (C, P, Na) but are one or two orders of magnitude lower for others. The flux calculations suggest that low-temperature fluid circulation in gabbro massifs can result in S uptake (3% of riverine sulfate input) in contrast to the S losses deduced for volcanic ridge flanks. Copyright

Continuous H2O, CO2, 222Rn and temperature measurements on Merapi Volcano, Indonesia

Abstract A set of analytical instruments, consisting of a gas chromatograph for water and carbon dioxide, an alpha scintillation counter to measure radon and a temperature sensor, was especially designed and constructed to monitor volcanic gases and was successfully operated over weeks in the high temperature fumaroles at the summit of Merapi Volcano, Indonesia. The analytical unit runs automatically and is remote controlled through a radio-link. Over a period of several weeks, concentrations of H 2 O and CO 2 were measured every 35 min and Rn activity was monitored at about 90-min intervals. The fumarole temperatures were continuously recorded every minute from July 2000 to the end of January 2001. The composition and temperature of the gas emitted at Merapi Volcano show significant variations. Due to a regular stronger degassing of the magma, concentrations of CO 2 increase periodically while H 2 O concentrations decrease. On the other hand, Rn activity rises with increasing atmospheric pressure and higher water concentrations. After intensive rainfall, infiltration of cold meteoric water into the volcanic edifice possibly causes an increase in the concentration of water vapour and a decrease in the temperature of the gases in the fumarole. Several seismic events recorded at Merapi correlate with gas temperature.

A helium isotope cross-section study through the San Andreas Fault at seismogenic depths

We have analyzed noble gas isotopes in 19 mud gas samples from 116–3943 m borehole depth of the San Andreas Fault Observatory at Depth (SAFOD) Main Hole in the context of origin and spatial variability of fluids occurring at seismogenic depths. The measured 3He/4He ratios range between 0.40 and 1.02 Ra (Ra is the atmospheric 3He/4He ratio of 1.39 × 10−6), with 4He/20Ne values between 0.33 and 4.92, revealing a mixture of three components to the total helium: (1) atmospheric helium, (2) helium with a crustal signature, and (3) mantle-derived helium. The air-corrected 3He/4He ratios fall between 0.2 Ra and 0.9 Ra. Samples from the 2117–3196 m depth show a relatively constant helium isotope composition (0.35–0.46 Ra), indicating that ∼5% of the helium in this section the Pacific Plate is derived from the mantle. The contribution of mantle-derived helium increases slightly in the transition from the Pacific Plate to the North American Plate and reaches maximal values of ∼12% on the North American Plate (below ∼3500 m borehole depth). On the basis of our observations, we suggest that the San Andreas Fault plays a role for fluid flux from greater depths, but higher amounts of mantle-derived fluids rise up through other, more permeable faults, situated on the North American Plate of the San Andreas Fault Zone (SAFZ). Lateral fluid dispersion at shallow depths through permeable country rock of the North American Plate may explain the observed increase in 3He/4He ratios with increasing distance to the SAF.

REE mobilization during small-scale high-pressure fluid–rock interaction and zoisite/fluid partitioning of La to Eu

A zoisite–quartz segregation and its adjacent host metabasite from the Tauern Window (Austria) have been investigated in detail to study the geochemical behaviour of REE during high-pressure fluid–rock interaction and the role of zoisite. The segregation formed at T = 500 to 550°C/P ≫ 0.6 GPa because of decompressional dehydration of the host metabasite. During the formation of the segregation the REE La to Tb and Dy to Lu were geochemically decoupled. In the metabasite La to Tb are incorporated in allanite and were leached from the host rock by the fluid phase. This decoupling and the leaching of La to Tb significantly alter the REE patterns of the host rock in the immediate vicinity of the segregation from LREE enriched to LREE depleted. Based on a mass-balance calculation for La, Ce, Nd, Sm, and Eu the composition of the fluid from which the segregation was precipitated is calculated. The fluid is LREE enriched with (La/Sm)N = 3.7. Individual REE contents range from Eu = 2.4 ± 1.1 ppm up to Ce = 90 ± 20 ppm, ΣREE = 190 ± 50 ppm. Calculated partition coefficients DREEzoisite/fluid are generally <1 and increase from DLazoisite/fluid = 0.08 ± 0.02 to DEuzoisite/fluid = 0.8 ± 0.4, indicating a stronger preference of zoisite for MREE than for LREE. To describe the fractionation of La relative to Sm between zoisite and fluid, the La-Sm exchange coefficient between zoisite and fluid is calculated. The derived exchange coefficient KD(La-Sm)zoisite/fluid = 0.2 ± 0.1 suggests that a fluid in equilibrium with zoisite will have a three- to ten-times higher La/Sm ratio than coexisting zoisite.

Chemical fluxes in the Tonga Subduction Zone: Evidence from the Southern Lau Basin

Chemical and isotopic data for volcanic glasses from the Valu Fa Ridge (VFR) in the southern Lau Basin closely resemble the composition of volcanics from the nearby Tonga arc. The most primitive basalts from the VFR were used to reconstruct the primary magma composition which is believed to reflect the composition of the bulk crust. The magma production rate at the Valu Fa Ridge was estimated (0.7−1.5×106 kg/a/m), and chemical fluxes of various elements have been calculated based on these values and the primary magma composition. The chemical and isotopic composition of VFR lavas is totally consistent with the release of fluid-soluble elements from the subducting altered oceanic crust and sediments. Mass balance calculations indicate the important role of the altered basaltic basement as a source of H2O, K, Rb, Cs, Sr, U, and Pb in supra-subduction zone rocks.

A HELIUM, ARGON, AND NITROGEN RECORD OF THE UPPER CONTINENTAL CRUST (KTB DRILL HOLES, OBERPFALZ, GERMANY) : IMPLICATIONS FOR CRUSTAL DEGASSING

Abstract An extensive set of bulk rock samples and mineral separates from the German Deep Drilling Project (KTB) has been analyzed for nitrogen, helium, and argon concentrations. The samples comprise metabasites, paragneisses, and hornblende–gneisses from a 7-km section in the crystalline basement metamorphosed during the Variscan orogeny. The data indicate a dependence of the N and Ar concentrations on the mica content of the rocks. The He–N 2 –Ar composition of fluid inclusions in quartz from felsic veins suggests that the fluids are old basement brines with large proportions of radiogenic He and Ar. There is no systematic down-hole variation in N and Ar concentrations, but He concentrations appear to increase with depth. Calculated degassing rates suggest near complete degassing of He from the gneisses, but some He retention in the metabasites. Nonetheless, overall degassing fluxes of He (1.2–4.0×10 10 atoms m −2 s −1 ) are not in conflict with estimates of the He flux in continental crust based on the steady-state degassing model. In contrast, our estimates indicate that Ar fluxes are significantly lower than predicted by a steady-state degassing model which is thus probably inappropriate to describe Ar fluxes correctly. The down-hole trend of increasing He concentration is interpreted as a degassing profile. A model of simultaneous radiogenic ingrowth and diffusive loss of He has been applied to determine the bulk transport coefficient of He in crystalline basement, which was found to be between 10 −10 and 10 −7 m 2 s −1 . This is much higher than reasonable diffusion rates, suggesting that episodes of enhanced degassing, probably related to hydrothermal or tectonic activities, played an important role in the degassing history of the crust. The relationship between the transport of heat and He in the crust in the KTB area is examined. Under the assumption that the ratio of heat flow and He flux from the mantle is the same as in ocean basins, the He isotopic composition of the basement fluids in the KTB boreholes is consistent with the inferred reduced heat flow at the KTB site. On a global scale, however, heat flow and flux of mantle He are probably not correlated.

Real-time drilling mud gas monitoring for qualitative evaluation of hydrocarbon gas composition during deep sea drilling in the Nankai Trough Kumano Basin

BackgroundIntegrated Ocean Drilling Program Expedition 338 was the second scientific expedition with D/V Chikyu during which riser drilling was conducted as part of the Nankai Trough Seismogenic Zone Experiment. Riser drilling enabled sampling and real-time monitoring of drilling mud gas with an onboard scientific drilling mud gas monitoring system (“SciGas”). A second, independent system was provided by Geoservices, a commercial mud logging service. Both systems allowed the determination of (non-) hydrocarbon gas, while the SciGas system also monitored the methane carbon isotope ratio (δ13CCH4). The hydrocarbon gas composition was predominated by methane (> 1%), while ethane and propane were up to two orders of magnitude lower. δ13CCH4 values suggested an onset of thermogenic gas not earlier than 1600 meter below seafloor. This study aims on evaluating the onboard data and subsequent geological interpretations by conducting shorebased analyses of drilling mud gas samples.ResultsDuring shipboard monitoring of drilling mud gas the SciGas and Geoservices systems recorded up to 8.64% and 16.4% methane, respectively. Ethane and propane concentrations reached up to 0.03 and 0.013%, respectively, in the SciGas system, but 0.09% and 0.23% in the Geoservices data. Shorebased analyses of discrete samples by gas chromatography showed a gas composition with ~0.01 to 1.04% methane, 2 – 18 ppmv ethane, and 2 – 4 ppmv propane. Quadruple mass spectrometry yielded similar results for methane (0.04 to 4.98%). With δD values between -171‰ and -164‰, the stable hydrogen isotopic composition of methane showed little downhole variability.ConclusionsAlthough the two independent mud gas monitoring systems and shorebased analysis of discrete gas sample yielded different absolute concentrations they all agree well with respect to downhole variations of hydrocarbon gases. The data point to predominantly biogenic methane sources but suggest some contribution from thermogenic sources at depth, probably due to mixing. In situ thermogenic gas production at depths shallower 2000 mbsf is unlikely based on in situ temperature estimations between 81°C and 85°C and a cumulative time-temperature index of 0.23. In conclusion, the onboard SciGas data acquisition helps to provide a preliminary, qualitative evaluation of the gas composition, the in situ temperature and the possibility of gas migration.

INVESTIGATIONS ON THE INFLUENCE OF GUEST MOLECULE CHARACTERISTICS AND THE PRESENCE OF MULTICOMPONENT GAS MIXTURES ON GAS HYDRATE PROPERTIES

In this study, we investigated the molecular characteristics of hydrates which were synthesized from gas mixtures containing the two isomers of butane, or the pentane isomers neopentane and isopentane, in excess methane. Thereto various techniques, including Raman spectroscopy, powder and single crystal X-ray diffraction and C NMR spectroscopy were employed. It turned out that shape and conformation of the guest molecule and hydrate structure both influence each other. In case of the mixed butane hydrate it could be confirmed that n-butane is enclathrated in its gauche conformation. This was verified by Raman spectroscopy, single crystal X-ray diffraction and calculated data. While isopentane is known as a structure H former, our results from powder X-ray diffraction, C NMR and ab initio calculations show that it can be also incorporated into structure II when the hydrate is formed from a neopentane/isopentane/methane gas mixture.