Chemical and isotopic systematics of oceanic basalts. Implications for Mantle Composition and Processes

Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.

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Composition of the Continental Crust

3.01 – Composition of the Continental Crust

A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post-Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present-day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond-hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction-related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post-Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.

The geochemical evolution of the continental crust

Effects of Continental Weathering and Sedimentary Sorting on the Hf, Nd, and Pb Isotopic Composition of Sediments and Implications for Dissolved Input to the Ocean

The Southern Hemisphere westerly winds (SWW) play a critical role in global climate, yet their behavior on decadal to centennial timescales, and the mechanisms driving these changes during the preindustrial era, remain poorly understood. We present a decadally resolved record of dust compositions using strontium and neodymium isotope ratios in mineral dust from the Siple Dome ice core, Antarctica, to explore the potential that abrupt changes in SWW behavior occurred over the past millennium. The record spans portions of the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) intervals as defined in the Northern Hemisphere. We find evidence of an abrupt strengthening of atmospheric circulation during the MCA at ∼1125 CE (825 BP) that persisted for about 60 yr, indicating increased influence of Patagonia‐sourced dust. This occurs during an extended positive phase of Southern Annular Mode (SAM+)‐like conditions, characterized by high SWW velocities and a southerly shift of the main wind belt toward ∼60°S, suggesting that rapid changes in SWW strength could occur under the present SAM+ pattern. A second 20 yr long shift in dust compositions during the LIA at ∼1748 CE (200 BP) is coincident with higher dust delivery to Siple Dome, and may indicate increased dust emissions related to glacier activity in Patagonia. The new Siple Dome ice core data set demonstrates that Sr‐Nd isotopes can be used to trace shifts in atmospheric circulation on decadal timescales.

Abrupt Changes in Atmospheric Circulation During the Medieval Climate Anomaly and Little Ice Age Recorded by Sr‐Nd Isotopes in the Siple Dome Ice Core, Antarctica

Li isotopes in HP-LT rocks; insights into the role of sediment-derived fluids

The ultimate goal of research data management is to achieve the long-term utility and impact of data acquired by research projects. Proper data management ensures that all researchers can validate and replicate findings, and reuse data in the quest for new discoveries. Research data need to be open, consistently and comprehensively documented for meaningful evaluation and reuse following domain-specific guidelines, and available for reuse via public data repositories that make them Findable, persistently Accessible, Interoperable, and Reusable (FAIR).In the early 2000&#8217;s, the development of geochemical databases such as GEOROC and PetDB underscored that the reporting and documenting practices of geochemical data in the scientific literature were inconsistent and incomplete. The original data could often not be recovered from the publications, and essential information about samples, analytical procedures, data reduction, and data uncertainties was missing, thus limiting meaningful reuse of the data and reproducibility of the scientific findings. In order to avoid that such poor scientific practice might potentially damage the health of the entire discipline, we launched the Editors Roundtable in 2007, an initiative to bring together editors, publishers, and database providers to implement consistent publication practices for geochemical data. Recognizing that mainstream scientific journals were the most effective agents to rectify problems in data reporting and implement best practices, members of the Editors Roundtable created and signed a policy statement that laid out &#8216;Requirements for the Publication of Geochemical Data&#8217; (Goldstein et al. 2014, http://dx.doi.org/10.1594/IEDA/100426). This presentation will examine the impact of this initial policy statement, assess the current status of best practices for geochemical data management, and explore what actions are still needed.&#160;While the Editors Roundtable policy statement led to improved data reporting practices in some journals, and provided the basis for data submission policies and guidelines of the EarthChem Library (ECL), data reporting practices overall remained inconsistent and inadequate. Only with the formation of the Coalition for Publishing Data in the Earth and Space Sciences (COPDESS, www.copdess.org), which extended the Editors Roundtable to include publishers and data facilities across the entire Earth and Space Sciences, along with the subsequent AGU project &#8216;Enabling FAIR Data&#8217;, has the implementation of new requirements by publishers, funders, and data repositories progressed and led to significant compliance with the FAIR Data Principles. Submission of geochemical data to open and FAIR repositories has increased substantially. Nevertheless, standard guidelines for documenting geochemical data and standard protocols for exchanging geochemical data among distributed data systems still need to be defined, and structures to govern such standards need to be identified by the global geochemistry community. Professional societies such as the Geochemical Society, the European Association of Geochemistry, and the International Association of GeoChemistry can and should take a leading role in this process.

Managing Open and FAIR Data in Geochemistry: Where are we a decade after the Editors Roundtable?

Petrology of Gakkel Ridge Basalts: Preliminary Results

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