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Posts Tagged ‘December 2017’

Peace and War

At the end of July, I had a very special holiday, with some of my family, in Orkney, the cluster of small, wind-swept islands (Fig. 1) that stand bravely between the Atlantic and the North Sea off the extreme north-east corner of Scotland. I had always wanted to visit Orkney because my great-grandmother, Isabella Allan, was born on the tiny island of Stronsay in 1843. By any standards, these islands have an extraordinary human history, stretching from Neolithic times, some 5,000 years ago to the two great wars of the 20th century.

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Proper Terminology in Analytical Geochemistry

Success in science has basically two components: (1) what one might call the “research component,” involving the testing of a hypothesis or the acquisition of a unique data set that provides insight into the workings of nature; (2) the “communications component”, which involves the dissemination and archiving of the knowledge that has been won. For me, the importance of this second, communications, aspect within the scientific process seems all too often underappreciated. Problems that many of us have certainly encountered in the geochemical literature range from incomplete descriptions of an analytical method, to the use of ambiguous jargon, to authors simply using the wrong terminology, etc.

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Layered Mafic–Ultramafic Intrusions of Fennoscandia: Europe’s Treasure Chest of Magmatic Metal Deposits

Northeastern Fennoscandia hosts a rich diversity of mafic–ultramafic intrusions of variable shape and size, emplaced in different tectonic regimes over a period spanning ~600 million years (between 1.88 Ga and 2.5 Ga). Several of the bodies contain world-class ore deposits, notably the Kemi chromium deposit and the Pechenga nickel deposits. Other deposits include nickel and copper at Kevitsa, Kotalahti and Sakatti; vanadium at Koillismaa; and platinum-group elements at Portimo and Penikat. These deposits constitute important resources that could shield Europe from potential future supply shortages of these key industrial metals.

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Quantitative Textural Insights into the Formation of Gabbro in Mafic Intrusions

Rock textures provide a key to deciphering the physical processes by which gabbro forms in mafic intrusions. Developments in both direct optical and crystallographic methods, as well as indirect magnetic fabric measurements, promise significant advances in understanding gabbroic textures. Here, we illustrate how bulk magnetic fabric data, particularly from intrusions with sparse silicate-hosted magnetite, may be used to extend direct crystallographic observations from thin sections. We also present a scheme for characterizing crystallographic foliation and lineation and use this to suggest that the strength of gabbro plagioclase foliations and lineations varies significantly with geodynamic environment.

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Plagioclase Zonation: An Archive of Trapped Liquid and Crustal Contamination

Many cumulates in layered intrusions contain plagioclase crystals that are compositionally zoned in terms of their major elements, and, less commonly, in their 87Sr/86Sr isotopic ratios. Major-element zoning in plagioclase is best explained by trapped liquid in the pore spaces between cumulus crystals, which is a result of the complex interplay between the rate of crystal growth and the cooling rate. Isotopic zoning in feldspars likely reflects crystal growth in a magma that is becoming, or has become, isotopically contaminated through wall rock partial melting and assimilation processes. Mineral-scale isotopic zoning, such as detected in plagioclase, can be used to infer the cooling rates of layered intrusions.

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Metasomatic Chromitite Seams in the Bushveld and Rum Layered Intrusions

Millimeter–centimeter thick layers of chromite-rich rock (chromitites) are rare, but ubiquitous, features of the Bushveld (South Africa) and Rum (Scotland) layered intrusions. Despite their meager dimensions, the chromitites provide insight into processes that modify igneous layering and, in the Bushveld, the formation of the platinum-group element–rich Merensky Reef. The Merensky Reef chromitites represent reaction zones formed in a compositional gradient between hydrous silicate melt and a crystalline cumulate assemblage, analogous to reaction zones in metamorphic systems. At Rum, the chromitites formed at the melting front between newly injected magma and the magma chamber floor, an analogous process but one driven by thermal, rather than chemical, energy.

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From the Executive Committee

On 13 August 2017, the Elements Executive Committee met at the Goldschmidt conference in Paris. Representatives from 11 of the 17 societies that participate in Elements were able to attend the meeting and discuss the current state and the future of the magazine. There was a full agenda and plenty of discussion, which occupied the entire afternoon but fortunately ended in time for the Goldschmidt ice breaker. The Executive Committee is the main channel through which the member societies are represented in the management of Elements, and the committee serves to provide not only financial oversite but also guidance over new appointments and new initiatives.

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The Skaergaard Intrusion of East Greenland: Paradigms, Problems and New Perspectives

The Skaergaard Intrusion of East Greenland is the quintessential example of low-pressure closed-system fractionation of basaltic magma. Field evidence of extensive layering and associated quasi-sedimentary structures, and the resultant ‘cumulate’ paradigm of crystal settling in magma chambers, has led to many long-standing controversies. Of particular significance is the lack of consensus about the microstructural record and the mechanisms by which interstitial liquid is expelled from solidifying crystal mushy zones. Skaergaard remains a cradle for new insights into igneous processes, with recent work highlighting the importance of separation of immiscible liquids on magma evolution.

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A Personal Perspective on Layered Intrusions

Last year (2016), when I spoke at the Geological Society of America’s Penrose Conference on “Layered Mafic Intrusions and Associated Economic Deposits” held in Red Lodge, Montana, I noted that I gave my first professional talk some 35 years ago. As a young researcher back then, I had thought that all the interesting questions in layered intrusions were soon to be solved, leaving little room for me to make a name for myself. I was wrong!

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Layered Intrusions: From Petrological Paradigms to Precious Metal Repositories

Layered mafic–ultramafic intrusions have occupied a position of central importance in the field of igneous petrology for almost a century. In addition to underpinning petrological paradigms such as cumulus theory, some layered intrusions are exceptionally enriched in base and precious metals, including the platinum-group elements. Technological advances are driving the current and future state-of-the-art in the study of layered intrusions and, looking forward, it is clear that these bodies will continue to inspire and challenge our understanding of magmatic systems and magma solidification for many years to come.

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