Oblique nonvolcanic seafloor spreading in Lena Trough, Arctic Ocean

Passive rifting and the early non-volcanic formation of ocean basins are fundamental aspects of the plate tectonic cycle. Cenozoic plate margins where this has occurred are rare. Here we present new observations from Lena Trough in the Arctic Ocean that bear on the early phase of oceanic spreading in such rifts. Lena Trough is an oblique seafloor rift system bounding the North American and Eurasian plates, and connecting neighboring Gakkel Ridge with the rest of the global mid-ocean ridge system. Mapping and sampling show widespread mantle outcrop along two parallel basement ridges bounded by steeply dipping normal faults. Volcanism is limited to the intersection with Gakkel Ridge and to minor eruption of strongly potassic alkali basalts in a single location. Non-eruptive magmatism is shown by an increase in plagioclase-and vein-bearing lithologies over residual peridotite in the center of Lena Trough. Normal mid-ocean ridge stairstep geometry and obvious low-angle detachments as seen at other ridges are absent. Lena Trough thus is an example of a young nonvolcanic continental rift that is just now beginning the transition to oblique nonvolcanic seafloor spreading. This style of oblique rifting, without the formation of striated large-scale low-angle detachments appears to be a major mode of crust formation on ultraslow spreading ridges. The sharp transition from the continental margins on either side to nonvolcanic rifting, with mantle slab exhumation in the rift may provide a model for the early evolution of oblique continental rifts, such as the Cote d\u27Ivoire/NE Brazil conjugate margins. Copyright 2011 by the American Geophysical Union

Deciphering petrological signatures of reactive melt stagnation and cooling in the oceanic mantle underneath ultraslow-spreading ridges

The global mid-ocean ridge system creates oceanic crust and lithosphere that covers more than two-thirds of the Earth. Basalts are volumetrically the most important rock type sampled at mid-ocean ridges. For this reason, our present understanding of upper mantle dynamics and the chemical evolution of the earth is strongly influenced by the study of mid-ocean ridge basalts (MORB). However, MORB are aggregates of polybarically generated small melt increments that can undergo a variety of physical and chemical processes during their ascent and consequently affect their derivative geochemical composition. Therefore, MORB do not represent “direct” windows to the underlying upper mantle. Abyssal peridotites, upper mantle rocks recovered from the ocean floor, are the residual complement to MORB melting and provide essential information on melt extraction from the upper mantle. In this study, abyssal peridotites are examined to address these overarching questions posed by previous studies of MORB: How are basaltic melts formed in the mantle, how are they extracted from the mantle and what physical and chemical processes control mantle melting? The number of studies on abyssal peridotites is small compared to those on basalts, in part because seafloor exposures of abyssal peridotites are relatively rare. For this reason, abyssal peridotite characteristics need to be considered in the context of subaerially exposed peridotites associated with ophiolites, orogenic peridotite bodies and basalt-hosted xenoliths. However, orogenic peridotite bodies are mainly associated with passive continental margins, most ophiolites are formed in supra-subduction zone settings, and peridotite xenoliths are often contaminated by their host magma. Therefore, studies of abyssal peridotites are essential to understanding the primary characteristics of the oceanic upper mantle free from the influence of continental rifting, subduction and tectonic emplacement processes. Nevertheless, numerous processes such as melt stagnation and cooling-induced, inter-mineral exchange can affect residual abyssal peridotite compositions after the cessation of melting. The aim of this study is to address these post-melting modifications of abyssal peridotites from a petrological-geochemical perspective. The samples in this study were dredged along the axis of the ultraslow-spreading Gakkel Ridge in the Arctic Ocean within the “Sparsely Magmatic Zone”, a 100 km ridge section where only mantle rocks are exposed. During two expeditions (ARK XVII-2 in 2001 and ARK XX-2 in 2004), exceptionally fresh peridotites were recovered. The boulders and cobbles collected cover a range of mantle rock compositions, with most characterized as plagioclase-free spinel peridotites or plagioclase- spinel peridotites. This thesis investigates melt stagnation and cooling processes in the upper mantle and is divided into two parts. The first part focuses on processes in the stability field of spinel peridotites (>10 kb) such as melt refertilization and cooling related trace element exchange, while the second part investigates processes in the stability field of plagioclase peridotites (< 10 kb) such as reactive melt migration and melt stagnation. The dissertation chapters are organized to follow the theoretical ascent of a mantle parcel upwelling beneath the location where the samples were collected.Im Bereich der mittelozeanischen Rücken wird die ozeanische Kruste gebildet, die mehr als zwei Drittel der Erde bedeckt. Basalte stellen dabei die volumetrisch bedeutendste Gesteinsgruppe dar, die infolgedessen die Hauptgrundlage unseres Wisssens über geodynamische und geochemische Prozesse im Erdmantel bilden. Basalte von mittelozeanischen Rücken (MORB) unterliegen während ihrer Bildung und beim Transport zur Oberfläche einer Vielzahl von Prozessen, die ihre geochemische Zusammensetzung modifizieren können. Daher erlauben MORB keine direkten Einblicke in Mantelprozesse. Tiefsee-Peridotite, also Mantelgesteine, die direkt am Ozeanboden aufgeschlossen sind, repräsentieren das Komplementärgestein zu den MORB-Basalten und sind daher mindestens ebenso wichtig für die Erforschung von Schmelzprozessen und Schmelzmigration im Mantel. Infolgedessen werden zentrale Fragen von Schmelzbildung und -transport im ozeanischen Mantel anhand von Tiefsee-Peridotiten untersucht. Die Zahl der wissenschaftlichen Untersuchungen an Tiefsee-Peridotiten ist noch vergleichsweise gering, was zum Teil an der relativen Seltenheit liegt, dass Tiefsee-Peridotite direkt am Ozeanboden aufgeschlossen sind. Unser Wissen über Mantelgesteine beruht auf Studien an Ophioliten, alpinotypen Peridotiten, und an Xenolithen, die daher auch immer im Kontext betrachten werden müssen. Allerdings sind erstere häufig durch Obduktionsprozesse und letztere durch die sie transportierenden Schmelzen modifiziert. Weiterhin ist ein Großteil der Ophiolite in Back-Arc Becken entstanden, während alpinotype Peridotite mit der Entstehung an kontinentalen Rändern in Zusammenhang gebracht werden. Daher erlauben sie keine direkten Einblicke in Mantelprozesse an mittelozeanischen Rücken. Dies macht die Erforschung von Tiefsee- Peridotiten umso wichtiger. Allerdings werden auch Tiefsee-Peridotite nach Beendigung des Aufschmelzungsprozesses noch weiteren Prozessen, wie z.B. Abkühlung und Schmelzrefertilisierung unterworfen. Ziel dieser Doktorarbeit ist es, diese Prozesse aus petrologisch-geochemischer Perspektive zu untersuchen. Neuere Erkenntnisse haben gezeigt, das die sogenannten ultra-langsam spreizenden mittelozeanischen Rücken eine fundamental andere Dynamik aufweisen, als ihre schneller spreizenden Äquivalente. Geringe Magmenproduktion, typisch für langsam spreizende Rücken, und damit einhergehende niedrigere Verhältnisse von Schmelzen zu Umgebungsgestein erlauben die detaillierte Studie von Modifikationsprozessen der Zusammensetzung von residuellen Peridotiten. Die Proben für diese Arbeit stammen aus der Amagmatischen Zone (Sparsely Magmatic Zone) am Gakkel-Rücken, der das langsam spreizende Endglied im System der mittelozeanischen Rücken darstellt. Der zentrale Bereich des Gakkel-Rückens wird von einem 100 km langen Abschnitt gebildet, an dem nur Mantelgesteine direkt am Ozeanboden aufgeschlossen sind. Am westlichen Ende dieses Bereiches wurden am gleichen Ort in zwei aufeinanderfolgenden Expeditionen (ARK XVII-2 im Jahre 2001 und ARK XX-2 im Jahre 2004) ausserordentlich frische Mantelperidotite gedredgt, die die Grundlage dieser Studie darstellen. Diese Arbeit ist thematisch in zwei Teile gegliedert. Der erste Teil untersucht Prozesse im Stabilitätsbereich der Spinelperidotite (>10 kbar), während der zweite Teil sich mit Prozessen bei niedrigeren Drücken (<10 kbar) im Stabilitätsbereich der Plagioklasperidotite befasst. Die Reihenfolge der Kapitel folgt einem theoretischen Aufstiegspfad von Mantelgesteinen unter einem mittelozeanischen Rücken

Trace element zoning in pyroxenes from ODP Hole 735B gabbros: Diffusive exchange or synkinematic crystal fractionation?

Major and trace element profiles of clinopyroxene grains in oceanic gabbros from ODP Hole 735B have been investigated by a combined in situ analytical study with ion probe, and electron microprobe. In contrast to the homogeneous major element compositions, trace elements (REE, Y, Cr, Sr, and Zr) show continuous core to rim zoning profiles. The observed trace element systematics in clinopyroxene cannot be explained by a simple diffusive exchange between melts and gabbros along grain boundaries. A simultaneous modification of the melt composition is required to generate the zoning, although Rayleigh fractional crystallization modelling could mimic the general shape of the profiles. Simultaneous metasomatism between the cumulate crystal and the porous melt during crystal accumulation is the most likely process to explain the zoning. Deformation during solidification of the crystal mush could have caused squeezing out of the incompatible element enriched residual melts (interstitial liquid). Migration of the melt along grain boundaries might carry these melt out of the system. This process named as synkinematic differentiation or differentiation by deformation (Natland and Dick in J Volcanol Geotherm Res 110(3-4):191-233, 2001) may act as an important magma evolution mechanism in the oceanic crust, at least at slow-spreading ridges. © Springer-Verlag 2006

Coupled hydrogen and fluorine incorporation in garnet: New constraints from FTIR, ERDA, SIMS, and EPMA

International audienceIt is well known that some garnet compositions can incorporate hydrogen and/or fluorine at levels up to several wt%. However, accurate measurement of these elements can be difficult at trace to minor concentration levels, so they are frequently ignored in routine chemical analysis. Furthermore, the mechanisms of H incorporation are still under debate, and only one mechanism for F substitution is commonly considered. We employed infrared spectroscopy (FTIR), elastic recoil detection analysis (ERDA), secondary ion mass spectrometry (SIMS), and electron probe microanalysis (EPMA) to measure H and F concentrations and constrain incorporation mechanisms in ten grossular garnets. We also present SIMS data for 11 spessartine and two andradite garnets. Three grossular garnets were measured with ERDA to obtain an infrared integral molar absorption coefficient (εi) for H2O of 13 470 L/(mol·cm2). Grossular H2O and F concentrations range from 0.017 to 0.133 wt% and 0.012 to 0.248 wt%, respectively. Correlations between 16OH and 19F and interpretation of FTIR spectra prompt us to consider various coupled substitutions of H and F for Si, which can explain some high-frequency IR absorption bands that have been attributed previously to “hydrogrossular clusters” (variably sized clusters in which 4H substitute for Si) or to inclusions of hydrous minerals. A strong correlation between 16OH and 19F in spessartine and similar high-frequency IR bands implies a similar role for H-F substitution. Coupled H-F substitution is also probably relevant to some andradite-rich garnets, rare pyrope from the Dora Maira massif, and some synthetic garnets. Improvements in analytical methods for trace to minor H and F open up more possibilities for using these elements to calculate the activities of H2O and F-species in fluids that were in equilibrium with garnet-bearing phase assemblages, as well as constraining the recycling of these elements into the mantle via study of xenoliths

How can we characterise graphite via electron microscopy?

Graphite is one of the most electrically-conductive and mechanically weak minerals commonly encountered in crustal rocks, so its presence affects geophysical properties and rheology. It also behaves as a buffer for oxidation reactions, can record peak temperature through crystallinity if undeformed, has been encountered as a minor ‘pinning’ phase affecting grain size, and can impact the ability of dissolved species to permeate along grain boundaries. However, because our typical rock sample preparation methods involve use of resins containing carbon, and because we typically coat samples with carbon to ensure conductivity in electron beam instruments, there have been comparatively few attempts to map and describe its distribution in metamorphic rocks. We have attempted to characterize graphite using a wide variety of sample preparation and analytical methods, and have found: (i) Specimens for scanning electron microscopy (SEM), electron microprobe (EMP), and transmission electron microscopy (TEM) analyses should be prepared as polished billets rather than thin sections without use of carbon-containing compounds. Analyses should target freshly polished surfaces where there would be no residual/smeared epoxy/plastic. (ii) Mechanical polishing, even with colloidal silica, does not yield a sample surface suitably crystalline to be analysed using EBSD but broad ion beam (BIB)-polishing does. (iii) Element maps made using a TEM as well as an EMP with wavelength and energy dispersive spectroscopy (WDS and EDS) detectors optimized for light elements, and soft X-ray detectors (SXES) can reliably demonstrate the presence of C even if it is disseminated on grain boundaries in layers only tens of nm thick. If SXES is to be employed, we found it useful to first map a wider variety of elements as well as using a large area WDS pseudocrystal optimized for light elements (e.g. the JEOL LDE6L) to narrow down the target regions and then acquiring SXES data from the same areas. This is because SXES map acquisition is currently several times slower than WDS in stage mapping mode and still slightly slower using beam scanning. High vacuum Ir-coating is preferred; a thin 1 nm coat does the job well and allows BSE imaging with only slight extraneous peaks in the spectra. Field emission source X-ray mapping at lower kV (e.g. 10 kV and lower) and lower current (e.g. 10 nA) reduces the C Ka X-ray range and keeps the electron beam compact. (iv) The crystalline and molecular-scale structure of carbon can be characterized using electron loss spectroscopy (EELS) on a TEM. (v) Raman spectroscopy provides a good alternative to electron beam methods to map C at grain scale. These methods have allowed us to demonstrate that carbon fills quartz triple junctions and lies along grain and phase boundaries in <50nm thick layers in quartz-feldspar mixtures typically encountered in Alpine Fault Zone mylonites