Constraints on mantle melting from major and trace element systematics in residual abyssal peridotites

ZusammenfassungDie Bildung von mittelozeanischen Rückenbasalten (MORB) ist einer der wichtigsten Stoffflüsse der Erde. Jährlich wird entlang der 75.000 km langen mittelozeanischen Rücken mehr als 20 km3 neue magmatische Kruste gebildet, das sind etwa 90 Prozent der globalen Magmenproduktion. Obwohl ozeanische Rücken und MORB zu den am meisten untersuchten geologischen Themenbereichen gehören, existieren weiterhin einige Streit-fragen. Zu den wichtigsten zählt die Rolle von geodynamischen Rahmenbedingungen, wie etwa Divergenzrate oder die Nähe zu Hotspots oder Transformstörungen, sowie der absolute Aufschmelzgrad, oder die Tiefe, in der die Aufschmelzung unter den Rücken beginnt. Diese Dissertation widmet sich diesen Themen auf der Basis von Haupt- und Spurenelementzusammensetzungen in Mineralen ozeanischer Mantelgesteine.Geochemische Charakteristika von MORB deuten darauf hin, dass der ozeanische Mantel im Stabilitätsfeld von Granatperidotit zu schmelzen beginnt. Neuere Experimente zeigen jedoch, dass die schweren Seltenerdelemente (SEE) kompatibel im Klinopyroxen (Cpx) sind. Aufgrund dieser granatähnlichen Eigenschaft von Cpx wird Granat nicht mehr zur Erklärung der MORB Daten benötigt, wodurch sich der Beginn der Aufschmelzung zu geringeren Drucken verschiebt. Aus diesem Grund ist es wichtig zu überprüfen, ob diese Hypothese mit Daten von abyssalen Peridotiten in Einklang zu bringen ist. Diese am Ozeanboden aufgeschlossenen Mantelfragmente stellen die Residuen des Aufschmelz-prozesses dar, und ihr Mineralchemismus enthält Information über die Bildungs-bedingungen der Magmen. Haupt- und Spurenelementzusammensetzungen von Peridotit-proben des Zentralindischen Rückens (CIR) wurden mit Mikrosonde und Ionensonde bestimmt, und mit veröffentlichten Daten verglichen. Cpx der CIR Peridotite weisen niedrige Verhältnisse von mittleren zu schweren SEE und hohe absolute Konzentrationen der schweren SEE auf. Aufschmelzmodelle eines Spinellperidotits unter Anwendung von üblichen, inkompatiblen Verteilungskoeffizienten (Kd's) können die gemessenen Fraktionierungen von mittleren zu schweren SEE nicht reproduzieren. Die Anwendung der neuen Kd's, die kompatibles Verhalten der schweren SEE im Cpx vorhersagen, ergibt zwar bessere Resultate, kann jedoch nicht die am stärksten fraktionierten Proben erklären. Darüber hinaus werden sehr hohe Aufschmelzgrade benötigt, was nicht mit Hauptelementdaten in Einklang zu bringen ist. Niedrige (~3-5%) Aufschmelzgrade im Stabilitätsfeld von Granatperidotit, gefolgt von weiterer Aufschmelzung von Spinellperidotit kann jedoch die Beobachtungen weitgehend erklären. Aus diesem Grund muss Granat weiterhin als wichtige Phase bei der Genese von MORB betrachtet werden (Kapitel 1).Eine weitere Hürde zum quantitativen Verständnis von Aufschmelzprozessen unter mittelozeanischen Rücken ist die fehlende Korrelation zwischen Haupt- und Spuren-elementen in residuellen abyssalen Peridotiten. Das Cr/(Cr+Al) Verhältnis (Cr#) in Spinell wird im Allgemeinen als guter qualitativer Indikator für den Aufschmelzgrad betrachtet. Die Mineralchemie der CIR Peridotite und publizierte Daten von anderen abyssalen Peridotiten zeigen, dass die schweren SEE sehr gut (r2 ~ 0.9) mit Cr# der koexistierenden Spinelle korreliert. Die Auswertung dieser Korrelation ergibt einen quantitativen Aufschmelz-indikator für Residuen, welcher auf dem Spinellchemismus basiert. Damit kann der Schmelzgrad als Funktion von Cr# in Spinell ausgedrückt werden: F = 0.10×ln(Cr#) + 0.24 (Hellebrand et al., Nature, in review; Kapitel 2). Die Anwendung dieses Indikators auf Mantelproben, für die keine Ionensondendaten verfügbar sind, ermöglicht es, geochemische und geophysikalischen Daten zu verbinden. Aus geodynamischer Perspektive ist der Gakkel Rücken im Arktischen Ozean von großer Bedeutung für das Verständnis von Aufschmelzprozessen, da er weltweit die niedrigste Divergenzrate aufweist und große Transformstörungen fehlen. Publizierte Basaltdaten deuten auf einen extrem niedrigen Aufschmelzgrad hin, was mit globalen Korrelationen im Einklang steht. Stark alterierte Mantelperidotite einer Lokalität entlang des kaum beprobten Gakkel Rückens wurden deshalb auf Primärminerale untersucht. Nur in einer Probe sind oxidierte Spinellpseudomorphosen mit Spuren primärer Spinelle erhalten geblieben. Ihre Cr# ist signifikant höher als die einiger Peridotite von schneller divergierenden Rücken und ihr Schmelzgrad ist damit höher als aufgrund der Basaltzusammensetzungen vermutet. Der unter Anwendung des oben erwähnten Indikators ermittelte Schmelzgrad ermöglicht die Berechnung der Krustenmächtigkeit am Gakkel Rücken. Diese ist wesentlich größer als die aus Schweredaten ermittelte Mächtigkeit, oder die aus der globalen Korrelation zwischen Divergenzrate und mittels Seismik erhaltene Krustendicke. Dieses unerwartete Ergebnis kann möglicherweise auf kompositionelle Heterogenitäten bei niedrigen Schmelzgraden, oder auf eine insgesamt größere Verarmung des Mantels unter dem Gakkel Rücken zurückgeführt werden (Hellebrand et al., Chem.Geol., in review; Kapitel 3).Zusätzliche Informationen zur Modellierung und Analytik sind im Anhang A-C aufgeführtAbstractThe generation of mid-ocean ridge basalts (MORB) is one of the most important mass transfer processes on Earth. Each year, more than twenty km3 of magmatic crust is produced along 75,000 km of ocean ridge, which is over ninety percent of the global magma production. Although ocean ridges and MORB are among the most studied geological features, several interlocking issues concerning melt generation at mid-ocean ridges are still not well understood. The most important of these are the role of geodynamic boundary conditions, such as spreading rate or vicinity to plumes and large transform offsets, as well as the absolute extent of melting or the depth at which melting initiates. This thesis examines these issues on the basis of mineral major and trace element compositions of oceanic mantle rocks.Geochemical characteristics of MORB suggest that melting begins in the stability field of garnet peridotite, at depths exceeding 80 km. Recent experiments, however, have shown that heavy rare earth elements (REE) are compatible in clinopyroxene (cpx) at the lherzolite solidus. This garnet-like partitioning behavior of cpx implies that garnet is no longer needed and that melting begins at significantly lower pressures. Therefore it is important to know, whether this hypothesis is consistent with compositional variations of abyssal peridotites. These fragments of the oceanic mantle are the residues of MORB melting and are now exposed on the ocean floor. Their mineral chemistry preserved the conditions at which melts were created in the mantle. Major and trace element data obtained by electron and ion microprobe on residual abyssal peridotite samples from the Central Indian Ridge (CIR) are compared to published data. Cpx in the CIR peridotites have very fractionated REE patterns, i.e. low Sm/Yb ratios, but high absolute Yb concentrations. Fractional melting models (which simulate the most extreme fractionation theoretically possible) of a spinel peridotite using commonly accepted partition coefficients cannot reproduce the observed characteristics of the CIR peridotites. Application of the new partition coefficients, in which the heavy REE are compatible in cpx, may explain the measured data better than the models that use standard partition coefficients. Still, the observed fractionations require extremely high degrees of melting, inconsistent with the only moderate degrees of melting estimated from major elements. Small (~3-5%) degrees of melting of a garnet peridotite, followed by subsequent melting under spinel-facies conditions can account for the observations. Therefore, garnet remains an important phase during partial melting beneath mid-ocean ridges (Chapter 1).A further stumbling block for the quantitative understanding of melting beneath mid-ocean ridges has been the lack of correlation between major and trace elements in residual abyssal peridotites. The Cr/(Cr+Al) ratio (Cr#) of spinel is accepted as a good qualitative indicator for the extent of partial melting. In fact, heavy REE (Dy, Er, and Yb) in cpx are highly correlated (r2 ~ 0.9) with Cr# of coexisting spinels using the CIR mineral data and published data from other abyssal peridotites. Application of this correlation yields a quantitative melting indicator for melting residues that is based on spinel major element compositions. Now, the absolute degree of melting can be expressed as a function of spinel Cr#: F = 0.10×ln(Cr#) + 0.24 (Hellebrand et al., Nature, in review; Chapter 2). Application of this tool to melting residues for which ion probe data are not available enables us to link geochemical and geophysical data on melting beneath mid-ocean ridges. From a geodynamic perspective, Gakkel Ridge in the Arctic Ocean is important for the understanding of partial melting process, because it is the slowest spreading mid-ocean ridge on Earth and it is devoid of large-offset transform faults. Published basalt data imply an extremely low degree of melting, in accordance with global correlations between basalt composition and estimated degree of melting. Pervasively altered mantle peridotites from one location along the virtually unexplored Gakkel Ridge were studied for relict primary minerals. Only one sample contained oxidized spinel pseudomorphs in which traces primary spinel are still preserved. Their composition reflect a higher degree of melting than indicated by the basalt compositions and their Cr# are significantly higher than in peridotites from faster spreading ridges. Crustal thickness at Gakkel Ridge, calculated from this spinel composition, is thicker than the observed crustal thickness estimated from gravity data, or predicted from global correlations between spreading rate and seismically determined crustal thickness. This unexpected result may be due to enhanced compositional heterogeneity at low degrees of melting, or may reflect a higher overall degree of depletion for the Gakkel Ridge than was previously thought (Hellebrand et al., Chem.Geol., in review; Chapter 3). Additional material is provided in Appendices A-C, which contain details relevant to the analysis and modeling presented in Chapters 1-3

Diffusion-limited REE uptake by eclogite garnets and its consequences for Lu-Hf and Sm-Nd geochronology

Garnets from the Zermatt-Saas Fee eclogites contain narrow central peaks for Lu+Yb+Tm±Er and at least one additional small peak towards the rim. The REE Sm+Eu+Gd+Tb±Dy are depleted in the cores but show one prominent peak close to the rim. These patterns cannot be modeled using Rayleigh fractionation accompanied by mineral breakdown reactions. Instead, the patterns are well explained using a transient matrix diffusion model where REE uptake is limited by diffusion in the matrix surrounding the porphyroblast. Observed profiles are well matched if a roughly linear radius growth rate is used. The secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism. The model predicts anomalously low 176Lu/177Hf and 147Sm/144Nd ratios in garnets where growth rates are fast compared to diffusion of the REE, and these results have important implications for Lu-Hf and Sm-Nd geochronology using garne

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

Earth Total Water Content

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Magmatic Products by Ocean Floor Spreading in MAR : Preliminary Analyses of Peridotites from IODP Exp.304/305 at Atlantis Massif, MAR 30°N

金沢大学大学院自然科学研究科4the International Symposium of the Kanazawa University 21st-Century COE Program,　Promotion Envirnmental Research in Pan-Japan Sea Area -Young Researchers\u27 Network- , DATE:March 8-10,2006, PLACE: Kanazawa Excel Hotel Tokyu, Japan, Sponsors: Japan Sea Research Institute / UNU-IAS(United Nation University Institute of Advanced Studies), Ishikawa Prefectural Government, City of Kanazaw

Protracted timescales of lower crustal growth at the fast-spreading East Pacific Rise

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 5 (2012): 275-278, doi:10.1038/ngeo1378.Formation of the oceanic crust at mid-ocean ridges is a fundamental component of plate tectonics. A majority of the crust at many ridges is composed of plutonic rocks that form by crystallization of mantle-derived magmas within the crust. Recent application of U/Pb dating to samples from in-situ oceanic crust has begun to provide exciting new insight into the timing, duration and distribution of magmatism during formation of the plutonic crust1-4. Previous studies have focused on samples from slow-spreading ridges, however, the time scales and processes of crustal growth are expected to vary with plate spreading rate. Here we present the first high-precision dates from plutonic crust formed at the fast-spreading East Pacific Rise (EPR). Individual zircon minerals yielded dates from 1.420–1.271 million years ago, with uncertainties of ± 0.006–0.081 million years. Within individual samples, zircons record a range of dates of up to ~0.124 million years, consistent with protracted crystallization or assimilation of older zircons from adjacent rocks. The variability in dates is comparable to data from the Vema lithospheric section on the Mid-Atlantic Ridge (MAR)3, suggesting that time scales of magmatic processes in the lower crust may be similar at slow- and fast-spreading ridges.This research was partially funded by NSF grant OCE-0727914 (SAB), a Cardiff University International Collaboration Award (CJL) and NERC grant NE/C509023/1 (CJM).2012-07-2

MPI-DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented. Copyright 2006 by the American Geophysical Union

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30°N

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): B07103, doi:10.1029/2010JB007931.Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100–220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45° rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge

Large scale multifactorial likelihood quantitative analysis of BRCA1 and BRCA2 variants: An ENIGMA resource to support clinical variant classification

The multifactorial likelihood analysis method has demonstrated utility for quantitative assessment of variant pathogenicity for multiple cancer syndrome genes. Independent data types currently incorporated in the model for assessing BRCA1 and BRCA2 variants include clinically calibrated prior probability of pathogenicity based on variant location and bioinformatic prediction of variant effect, co-segregation, family cancer history profile, co-occurrence with a pathogenic variant in the same gene, breast tumor pathology, and case-control information. Research and clinical data for multifactorial likelihood analysis were collated for 1,395 BRCA1/2 predominantly intronic and missense variants, enabling classification based on posterior probability of pathogenicity for 734 variants: 447 variants were classified as (likely) benign, and 94 as (likely) pathogenic; and 248 classifications were new or considerably altered relative to ClinVar submissions. Classifications were compared with information not yet included in the likelihood model, and evidence strengths aligned to those recommended for ACMG/AMP classification codes. Altered mRNA splicing or function relative to known nonpathogenic variant controls were moderately to strongly predictive of variant pathogenicity. Variant absence in population datasets provided supporting evidence for variant pathogenicity. These findings have direct relevance for BRCA1 and BRCA2 variant evaluation, and justify the need for gene-specific calibration of evidence types used for variant classification

Large scale multifactorial likelihood quantitative analysis of BRCA1 and BRCA2 variants: An ENIGMA resource to support clinical variant classification

Abstract The multifactorial likelihood analysis method has demonstrated utility for quantitative assessment of variant pathogenicity for multiple cancer syndrome genes. Independent data types currently incorporated in the model for assessing BRCA1 and BRCA2 variants include clinically calibrated prior probability of pathogenicity based on variant location and bioinformatic prediction of variant effect, co-segregation, family cancer history profile, co-occurrence with a pathogenic variant in the same gene, breast tumor pathology, and case-control information. Research and clinical data for multifactorial likelihood analysis were collated for 1395 BRCA1/2 predominantly intronic and missense variants, enabling classification based on posterior probability of pathogenicity for 734 variants: 447 variants were classified as (likely) benign, and 94 as (likely) pathogenic; 248 classifications were new or considerably altered relative to ClinVar submissions. Classifications were compared to information not yet included in the likelihood model, and evidence strengths aligned to those recommended for ACMG/AMP classification codes. Altered mRNA splicing or function relative to known non-pathogenic variant controls were moderately to strongly predictive of variant pathogenicity. Variant absence in population datasets provided supporting evidence for variant pathogenicity. These findings have direct relevance for BRCA1 and BRCA2 variant evaluation, and justify the need for gene-specific calibration of evidence types used for variant classification. This article is protected by copyright. All rights reserved.Peer reviewe