Pyrohydrolysis-IRMS determination of silicate chlorine stable isotope compositions. Application to oceanic crust and meteorite samples

This contribution describes the optimization of chlorine extraction from silicate samples by pyrohydrolysis prior to the precise determination of Cl stable-isotope compositions (637 Cl) by gas source, dual inlet Isotope Ratio Mass Spectrometry (IRMS) on CH(3)Clg. The complete method was checked on three international reference materials for Cl-content and two laboratory glass standards. Whole procedure blanks are lower than 0. 5 mu mol, corresponding to less than 10 wt.% of most of the sample chloride analysed. In the absence of international chlorine isotope rock, we report here Cl extracted compared to accepted Cl contents and reproducibilities on Cl and delta Cl-37 measurements for the standard rocks. After extraction, the Cl contents of the three international references compared within error with the accepted values (mean yield = 94 +/-10%) with reproducibilities better than 12% (10). The laboratory glass standards - andesite SO100DS92 and phonolite S9(2) - were used specifically to test the effect of chloride amount on the measurements. They gave Cl extraction yields of 100 +/-6% (1 sigma-; n = 15) and 105 +/- 8% (1 sigma-; n = 7), respectively, with delta Cl-37 values of -0.51 0.14%o and -0.39 0.17%o (1g). In summary, for silicate samples with Cl contents between 39 and 9042 ppm, the Pyrohydrolysis/HPLC method leads to overall CI extraction yields of 100 8%, reproducibilities on Cl contents of 7% and on delta Cl-37 measurements of 0.12%o (all 1 sigma). The method was further applied to ten silicate rocks of various mineralogy and chemistry (meteorite, fresh MORB glasses, altered basalts and setpentinized peridotites) chosen for their large range of Cl contents (70-2156 ppm) and their geological significance. delta Cl-37 values range between -2.33 and -0.50%o. These strictly negative values contrast with the large range and mainly positive values previously reported for comparable silicate samples and shown here to be affected by analytical problems. Thus we propose a preliminary, revised terrestrial CI cycle, mainly dominated by negative and zero delta Cl-37 values. (C) 2007 Elsevier B.V. All rights reserved.

Oceanic crust and island arc formation in Central Asia during late Neoproterozoic times - evidence from petrological and geochemical studies

Die vorliegende Arbeit behandelt die Entwicklung des 570 Ma alten, neoproterozoischen Agardagh - Tes-Chem Ophioliths (ATCO) in Zentralasien. Dieser Ophiolith liegt südwestlich des Baikalsees (50.5° N, 95° E) und wurde im frühen Stadium der Akkretion des Zentralasiatischen Mobilgürtels auf den nordwestlichen Rand des Tuvinisch-Mongolischen Mikrokontinentes aufgeschoben. Bei dem Zentralasiatische Mobilgürtel handelt es sich um einen riesigen Akkretions-Subduktionskomplex, der heute das größte zusammenhängende Orogen der Erde darstellt. Im Rahmen dieser Arbeit wurden eine Reihe plutonischer und vulkanischer Gesteine, sowie verschiedene Mantelgesteine des ATCO mittels mikroanalytischer und geochemischer Verfahren untersucht (Elektronenstrahlmikrosonde, Ionenstrahlmikrosonde, Spurenelement- und Isotopengeochemie). Die Auswertung dieser Daten ermöglichte die Entwicklung eines geodynamisch-petrologischen Modells zur Entstehung des ATCO. Die vulkanischen Gesteine lassen sich aufgrund ihrer Spurenelement- und Isotopenzusammensetzung in inselbogenbezogene und back-arc Becken bezogene Gesteine (IA-Gesteine und BAB-Gesteine) unterscheiden. Darüber hinaus gibt es eine weitere, nicht eindeutig zuzuordnende Gruppe, die hauptsächlich mafische Gänge umfasst. Der grösste Teil der untersuchen Vulkanite gehört zur Gruppe der IA-Gesteine. Es handelt sich um Al-reiche Basalte und basaltische Andesite, welche aus einem evolvierten Stammmagma mit Mg# 0.60, Cr ~ 180 µg/g und Ni ~ 95 µg/g hauptsächlich durch Klinopyroxenfraktionierung entstanden sind. Das Stammmagma selbst entstand durch Fraktionierung von ca. 12 % Olivin und geringen Anteilen von Cr-Spinell aus einer primären, aus dem Mantel abgeleiteten Schmelze. Die IA-Gesteine haben hohe Konzentrationen an inkompatiblen Spurenelementen (leichte-(L)- Seltenerdelement-(SEE)-Konzentrationen etwa 100-fach chondritisch, chondrit-normierte (La/Yb)c von 14.6 - 5.1), negative Nb-Anomalien (Nb/La = 0.37 - 0.62) und niedrige Zr/Nb Verhältnisse (7 - 14) relativ zu den BAB-Gesteinen. Initiale eNd Werte liegen bei etwa +5.5, initiale Bleiisotopenverhältnisse sind: 206Pb/204Pb = 17.39 - 18.45, 207Pb/204Pb = 15.49 - 15.61, 208Pb/204Pb = 37.06 - 38.05. Die Anreicherung lithophiler inkompatibler Spurenelemente (LILE) in dieser Gruppe ist signifikant (Ba/La = 11 - 130) und zeigt den Einfluss subduzierter Komponenten an. Die BAB-Gesteine repräsentieren Schmelzen, die sehr wahrscheinlich aus der gleichen Mantelquelle wie die IA-Gesteine stammen, aber durch höhere Aufschmelzgrade (8 - 15 %) und ohne den Einfluss subduzierter Komponenten entstanden sind. Sie haben niedrigere Konzentrationen an inkompatiblen Spurenelementen, flache SEE-Muster ((La/Yb)c = 0.6 - 2.4) und höhere initiale eNd Werte zwischen +7.8 und +8.5. Nb Anomalien existieren nicht und Zr/Nb Verhältnisse sind hoch (21 - 48). Um die geochemische Entwicklung der vulkanischen Gesteine des ATCO zu erklären, sind mindestens drei Komponenten erforderlich: (1) eine angereicherte, ozeaninselbasalt-ähnliche Komponente mit hoher Nb Konzentration über ~ 30 µg/g, einem niedrigen Zr/Nb Verhältnis (ca. 6.5), einem niedrigen initialen eNd Wert (um 0), aber mit radiogenen 206Pb/204Pb-, 207Pb/204Pb- und 208Pb/204Pb-Verhältnissen; (2) eine N-MORB ähnliche back-arc Becken Komponente mit flachem SEE-Muster und einem hohen initialen eNd Wert von mindestens +8.5, und (3) eine Inselbogen-Komponente aus einer verarmten Mantelquelle, welche durch die abtauchende Platte geochemisch modifiziert wurde. Die geochemische Entstehung der ATCO Vulkanite lässt sich dann am besten durch eine Kombination aus Quellenkontamination, fraktionierte Kristallisation und Magmenmischung erklären. Geodynamisch gesehen entstand der ATCO sehr wahrscheinlich in einem intraozeanischen Inselbogen - back-arc System. Bei den untersuchten Plutoniten handelt es sich um ultramafische Kumulate (Wehrlite und Pyroxenite) sowie um gabbroische Plutonite (Olivin-Gabbros bis Diorite). Die geochemischen Charakteristika der mafischen Plutonite sind deutlich unterschiedlich zu denen der vulkanischen Gesteine, weshalb sie sehr wahrscheinlich ein späteres Entwicklungsstadium des ATCO repräsentieren. Die Spurenelement-Konzentrationen in den Klinopyroxenen der ultramafischen Kumulate sind extrem niedrig, mit etwa 0.1- bis 1-fach chondritischen SEE-Konzentrationen und mit deutlich LSEE-verarmten Mustern ((La/Yb)c = 0.27 - 0.52). Berechnete Gleichgewichtsschmelzen der ultramafischen Kumulate zeigen grosse Ähnlichkeit zu primären boninitischen Schmelzen. Die primären Magmen waren daher boninitischer Zusammensetzung und entstanden in dem durch vorausgegangene Schmelzprozesse stark verarmten Mantelkeil über einer Subduktionszone. Niedrige Spurenelement-Konzentrationen zeigen einen geringen Einfluss der abtauchenden Platte an. Die Spurenelement-Konzentrationen der Gabbros sind ebenfalls niedrig, mit etwa 0.5 - 10-fach chondritischen SEE-Konzentrationen und mit variablen SEE-Mustern ((La/Yb)c = 0.25 - 2.6). Analog zu den Vulkaniten der IA-Gruppe haben alle Gabbros eine negative Nb-Anomalie mit Nb/La = 0.01 - 0.31. Die initialen eNd Werte der Gabbros variieren zwischen +4.8 und +7.1, mit einem Mittelwert von +5.9, und sind damit identisch mit denen der IA-Vulkanite. Bei den untersuchten Mantelgesteinen handelt es sich um teilweise serpentinisierte Dunite und Harzburgite, die alle durch hohe Mg/Si- und niedrige Al/Si-Verhältnisse gekennzeichnet sind. Dies zeigt einen refraktären Charakter an und steht in guter Übereinstimmung mit den hohen Cr-Zahlen (Cr#) der Spinelle (bis zu Cr# = 0.83), auf deren Basis der Aufschmelzgrad der residuellen Mantelgesteine berechnet wurde. Dieser beträgt etwa 25 %. Die geochemische Zusammensetzung und die petrologischen Daten der Ultramafite und Gabbros lassen sich am besten erklären, wenn man für die Entstehung dieser Gesteine einen zweistufigen Prozess annimmt. In einer ersten Stufe entstanden die ultramafischen Kumulate unter hohem Druck in einer Magmenkammer an der Krustenbasis, hauptsächlich durch Klinopyroxen-Fraktionierung. Bei dieser Magmenkammer handelte es sich um ein offenes System, dem von unten laufend neue Schmelze zugeführt wurde, und aus dem im oberen Bereich evolviertere Schmelzen geringerer Dichte entwichen. Diese evolvierten Schmelzen stiegen in flachere krustale Bereiche auf und bildeten dort meist isolierte Intrusionskörper. Diese Intrusionskörper erstarrten ohne Magmen-Nachschub, weshalb petrographisch sehr unterschiedliche Gesteine entstehen konnten. Eine geochemische Modifikation der abkühlenden Schmelzen erfolgte allerdings durch die Assimilation von Nebengestein. Da innerhalb der Gabbros keine signifikante Variation der initalen eNd Werte existiert, handelte es sich bei dem assimilierten Material hauptsächlich um vulkanische Gesteine des ATCO und nicht um ältere, möglicherweise kontinentale Kruste.

The distribution of sulfur in oceanic crust with respect to oxidative diagenesis (Table 1)

A detailed geochemical-petrological examination of layer 2 basalts recovered during Leg 37 of the DSDP has revealed that the original distribution, form and abundance of igneous sulfide have been profoundly altered during low-grade oxidative diagenesis. The net result appears to have been a rather pervasive remobilization of igneous sulfide to form secondary pyrite accompanied by a bulk loss of sulfur equivalent to about 50-60% of the original igneous value, assuming initial saturation. It is suggested that during infiltration of seawater into the massive crystalline rock, igneous sulfide has experienced pervasive oxidation, under conditions of limited oxidation potential, to form a series of unstable, soluble sulfur species, primarily in the form of SO3[2-] and S2O3[2-]. Spontaneous decomposition of these intermediate compounds through disproportionation has resulted in partial reconstitution of the sulfur as secondary pyrite and the generation of SO4[2-] ion, which, due to its kinetic stability, has been lost from the basalt system and ultimately transferred to the ocean. This model not only satisfies the geochemical and petrological observations but also provides a suitable explanation for the highly variable delta34S values which characterize secondary sulfides in deep ocean floor basalts.

Geochemistry of the oldest Atlantic oceanic crust

Controversy has surrounded the issue of whether mantle plume activity was responsible for Pangaean continental rifting and massive flood volcanism (resulting in the Central Atlantic Magmatic Province or CAMP, emplaced around 200 Ma) preceding the opening of the central Atlantic Ocean in the Early Mesozoic. Our new Sr-Nd-Pb isotopic and trace element data for the oldest basalts sampled from central Atlantic oceanic crust by deep-sea drilling show that oceanic crust generated from about 160 to 120 Ma displays clear isotopic and chemical signals of plume contamination (e.g., 87Sr/86Sr(i) = 0.7032-0.7036, epsilonNd(t) =+6.2 to +8.2, incompatible element patterns with positive Nb anomalies), but these signals are muted or absent in crust generated between 120 and 80 Ma, which resembles young Atlantic normal mid-ocean ridge basalt. The plume-affected pre-120 Ma Atlantic crustal basalts are isotopically similar to lavas from the Ontong Java Plateau, and may represent one isotopic end-member for CAMP basalts. The strongest plume signature is displayed near the center of CAMP magmatism but the hotspots presently located nearest this location in the mantle reference frame do not appear to be older than latest Cretaceous and are isotopically distinct from the oldest Atlantic crust. The evidence for widespread plume contamination of the nascent Atlantic upper mantle, combined with a lack of evidence for a long-lived volcanic chain associated with this plume, leads us to propose that the enriched signature of early Atlantic crust and possibly the eruption of the CAMP were caused by a relatively short-lived, but large volume plume feature that was not rooted at a mantle boundary layer. Such a phenomenon has been predicted by recent numerical models of mantle circulation.

Helium and neon isotopes in oceanic crust of ODP Hole 118-735B

In an attempt to determine the helium and neon isotopic composition of the lower oceanic crust, we report new noble gas measurements on 11 million year old gabbros from Ocean Drilling Program site 735B in the Indian Ocean. The nine whole rock samples analyzed came from 20 to 500 m depth below the seafloor. Helium contents vary from 3.3*10**-10 to 2.5*10**-7 ccSTP/g by crushing and from 5.4*10**-8 to 2.4*10**-7 ccSTP/g by melting. 3He/4He ratios vary between 2.2 and 8.6 Ra by crushing and between 2.9 and 8.2 by melting. The highest R/Ra ratios are similar to the mean mid-ocean ridge basalt (MORB) ratio of 8+/-1. The lower values are attributed to radiogenic helium from in situ alüha-particle production during uranium and thorium decay. Neon isotopic ratios are similar to atmospheric ratios, reflecting a significant seawater circulation in the upper 500 m of exposed crust at this site. MORB-like neon, with elevated 20Ne/22Ne and 21Ne/22Ne ratios, was found in some high temperature steps of heating experiments, but with very small anomalies compared to air. These first results from the lower oceanic crust indicate that subducted lower oceanic crust has an atmospheric 20Ne/22Ne ratio. Most of this neon must be removed during the subduction process, if the ocean crust is to be recirculated in the upper mantle, otherwise this atmospheric neon will overwhelm the upper mantle neon budget. Similarly, the high (U+Th)/3He ratio of these crustal gabbros will generate very radiogenic 4He/3He ratios on a 100 Ma time scale, so lower oceanic crust cannot be recycled into either MORB or oceanic island basalt without some form of processing.

Geochemistry of the oceanic crust from ocean drilling samples

This book presents new data on chemical and mineral compositions and on density of altered and fresh igneous rocks from key DSDP and ODP holes drilled on the following main tectonomagmatic structures of the ocean floor: 1. Mid-ocean ridges and abyssal plains and basins (DSDP Legs 37, 61, 63, 64, 65, 69, 70, 83, and 91 and ODP Legs 106, 111, 123, 129, 137, 139, 140, 148, and 169); 2. Seamounts and guyots (DSDP Legs 19, 55, and 62 and ODP Legs 143 and 144); 3. Intraplate rises (DSDP Legs 26, 33, 51, 52, 53, 72, and 74 and ODP Legs 104, 115, 120, 121, and 183); and 4. Marginal seas (DSDP Legs 19, 59, and 60 and ODP Legs 124, 125, 126, 127, 128, and 135). Study results of altered gabbro from the Southwest Indian Ridge (ODP Leg 118) and serpentinized ultramafic rocks from the Galicia margin (ODP Leg 103) are also presented. Samples were collected by the authors from the DSDP/ODP repositories, as well as during some Glomar Challenger and JOIDES Resolution legs. The book also includes descriptions of thin sections, geochemical diagrams, data on secondary mineral assemblages, and recalculated results of chemical analyses with corrections for rock density. Atomic content of each element can be quantified in grams per standard volume (g/1000 cm**3). The suite of results can be used to estimate mass balance, but parts of the data need additional work, which depends on locating fresh analogs of altered rocks studied here. Results of quantitative estimation of element mobility in recovered sections of the upper oceanic crust as a whole are shown for certain cases: Hole 504B (Costa Rica Rift) and Holes 856H, 857C, and 857D (Middle Valley, Juan de Fuca Ridge).

Vein mineral ages of oceanic crust from the Atlantic Ocean

Vein smectites with large Rb/Sr enrichments from extensively altered basaltic oceanic crust in Deep Sea Drilling Project hole 417A in the western Atlantic define a highly constrained Rb/Sr isochron age of 108 +/- 3 m.y. This age is identical to a less well constrained age of 108 +/- 17 m.y. for vein smectites with lower Rb/Sr enrichments from adjacent hole 418A and to the 108 m.y. age of crust formation derived by paleontological and magnetic anomaly correlation. Reasonable agreement exists between the 87Sr/86Sr ratio of vein calcites from both sites and the seawater 87Sr/86Sr ratio at the time. Pervasive low-temperature alteration in the contrasting environments of sites 417 and 418 appears to be coeval and essentially coincident with basement formation. Alteration may be used to advantage in determining ages of old oceanic crust.

Geochemistry of oceanic crust of the Cocos Plate

New geochemical data from the Cocos Plate constrain the composition of the input into the Central American subduction zone and demonstrate the extent of influence of the Galápagos Hotspot on the Cocos Plate. Samples include sediments and basalts from Ocean Drilling Program (ODP) Site 1256 outboard of Nicaragua, gabbroic sills from ODP Sites 1039 and 1040, tholeiitic glasses from the Fisher Ridge off northwest Costa Rica, and basalts from the Galápagos Hotspot Track outboard of Central Costa Rica. Site 1256 basalts range from normal to enriched MORB in incompatible elements and have Pb and Nd isotopic compositions within the East Pacific Rise MORB field. The sediments have similar 206Pb/204Pb and only slightly more radiogenic 207Pb/204Pb and 208Pb/204Pb isotope ratios than the basalts. Altered samples from the subducting Galápagos Hotspot Track have similar Nd and Pb isotopic compositions to fresh Galápagos samples but have significantly higher Sr isotopic composition, indicating that the subduction input will have a distinct geochemical signature from Galápagos-type mantle material that may be present in the wedge beneath Costa Rica. Gabbroic sills from Sites 1039 and 1040 in East Pacific Rise (EPR) crust show evidence for influence of the Galápagos Hotspot ?100 km beyond the morphological hotspot track.

B, d11B, K and d18O data for altered oceanic crust at DSDP Sites 417/418

Samples of drilled oceanic crust, from DSDP Holes 417A, 417D and 418A and ODP Hole 735B, and oceanic crust from the Oman and Cyprus ophiolites, were analyzed for B contents and d11B. Composite samples from DSDP Holes 417A, 417D and 418A were used to represent the upper 550 m of altered oceanic crustal Layer 2A. Whole-rock samples from the Troodos ophiolite, Cyprus, and the Oman ophiolite were selected to represent crustal Layer 2B dikes. Composite samples from ODP Hole 735B were used to represent crustal Layer 3. The B content of the DSDP composites ranges from 7.2 ppm to 104 ppm and correlates with both d1818O and K, showing that it is a good indicator of the extent of low temperature alteration. The d11B of the DSDP composites varies between -2.5? and 5.4?. The B content of the samples from the Troodos ophiolite ranges from 2.4 ppm to 8.1 ppm; d11B varies from -0.9? to 7.8?. The B content of the Oman ophiolite samples ranges from 5.0 ppm to 11.1 ppm; d11B varies from -1.6? to 16.9?. The B content of the samples from ODP Hole 735B ranges from 1.1 ppm to 7.1 ppm; d11B varies from -4.3? to 24.9?. The general pattern displayed by these samples is one of greatest (and most variable) B enrichment at the top of the crust and least enrichment at the bottom of the section. All of these samples are enriched compared to unaltered MORB, which is believed to have a B content of approximately 0.5 ppm. The d11B values of deeper samples, from Layers 2B and 3, are more variable and generally higher than those from Layer 2A. Boron contents and d11B are not correlated. The data from the DSDP Site 417/418 composites indicate that the d11B of fluid circulating in the upper crust changes only slightly during alteration, increasing by an average of 5.1? with an accompanying decrease in B concentration of 7%. Low temperature alteration appears to be a water-dominated process resulting in minor modification of circulating seawater. A minimum water-rock ratio of 400 is calculated for these samples, implying a minimum low-temperature seawater flux through the upper oceanic crust of 3.4?10**14 l/y. The average B content of altered oceanic crust, as represented by these samples, is 5.2+/-1.7 ppm and the average d11B is 3.4+/-1.1?. This average isotopic composition is measurably different from the apparent average of oceanic sediments, supporting the idea that d11B could be useful for identifying the source(s) of B in island arcs.

Rubidium and Strontium chemistry of oceanic crust

Times of vein mineral deposition in the ocean crust have been determined both by Rb-Sr isochron ages of vein smectites and by comparison of 87Sr/86Sr ratios of vein calcites with the known variations of seawater 87Sr/86Sr ratio with time. Results from drilling sites 105, 332B and 418A, Atlantic Ocean, which have basement formation ages of 155 m.y., 3.5 m.y., and 110 m.y., respectively, show that vein deposition is essenrially complete within 5-10 m.y. after formation of the basaltic crust. This provids direct evidence that hydrothermal circulation of sea-water through the oceanic crust is an important process for only 5-10 m.y. after crust formation.

Platinum group elements and geochemistry of altered oceanic crust of ODP Hole 140-504B

Primary chemical heterogeneity in the sheeted dike complex in Deep Sea Drilling Project Hole 504B makes these rocks unsuitable for conventional mass balance calculations in determining element mobility associated with hydrothermal alteration. Due to the original heterogeneity and variable degrees of fractionation in the dikes, an appropriate reference sample on which calculations can be based is difficult to find. Therefore, the use of incompatible element ratios is developed to evaluate geochemical changes during alteration(s). For example, on a Zr/Yb-La/Yb plot, scatter along a straight line suggests tapping of a variably depleted mantle source and deviation from the line suggests element mobility (gain or loss). Using this method, our data indicates that the hydrothermal evolution of the sheeted dike complex was accompanied by significant loss of Cu, Zn, and Ti and some loss of La. The sheeted dike complex has low platinum group element (PGE) concentrations and steep PGE patterns, typical of mid-ocean ridge basalts (MORBs) on the global scale. We propose that the unusual PGE patterns of MORBs cannot be entirely generated by a partial melting and sulfide segregation model; instead, these patterns in part must have been inherited from their mantle source. The Au data show no evidence for mobilization during hydrothermal alteration of the dikes.

Mineral and oxygen isotopic compositions in a profile through the altered lower oceanic crust, ODP Hole 176-735B

ODP Hole 735B located on the Southwest Indian Ridge at 57°E is an in situ sampled long, continuous section of lower oceanic crust. Oxygen isotope compositions of constituent minerals of Leg 176 gabbros have been measured by UV-laser oxygen isotope microprobe. Together with existing data from Leg 118, a complete oxygen isotope profile through the lower oceanic crust has been obtained. Most clinopyroxenes and olivines have normal mantle values of ~5.5 per mil and ~5.2 per mil, respectively, while plagioclases show slight d18O enrichment relative to its mantle value of 6.1per mil. Down-hole variations of Hole 735B gabbro indicate a downward decreasing d18O profile, with a kink at a depth of about 800 m below sea floor. Above this depth, gabbros are depleted in 18O relative to unaltered basalts, while below ~800 m they show nearly unmodified d18O values. Abundant seawater penetration appears to be limited to the upper part of the lower crust at ODP site 735 (~800 m into the gabbroic layer and ~2-2.5 km into the oceanic crust from the top of pillow basalts). Mass balance calculations show that the lower crust formed under this ultra-slow-spreading ridge has an average d18O value of 5.5 per mil. The whole crust at Site 735 has an overall 18O enrichment with d18O values of 6.0 per mil to 7.8 per mil, depending on the possible variation of the d18O values of the upper pillow basalts and sheeted dykes. The apparent difference in oxygen isotope compositions of ocean crusts formed with different spreading rates has important implications on the buffering of ocean water over geological time, as well as on the oxygen recycling between crust and mantle through subduction. The difference of seawater penetration between fast- and slow-spreading ridges could be related to their particular magmatic-tectonic history during the formation and aging of the crust. However, more analyses on continuous sections through oceanic and ophiolitic crust in different tectonic settings are required to derive any predictive models.