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"igneous rocks"
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The key role of mica during igneous concentration of tantalum
Igneous rocks with high Ta concentrations share a number of similarities such as high Ta/Nb, low Ti, LREE and Zr concentrations and granitic compositions. These features can be traced through fractionated granitic series. Formation of Ta-rich melts begins with anatexis in the presence of residual biotite, followed by magmatic crystallization of biotite and muscovite. Crystallization of biotite and muscovite increases Ta/Nb and reduces the Ti content of the melt. Titanium-bearing oxides such as rutile and titanite are enriched in Ta and have the potential to deplete Ta at early stages of fractionation. However, mica crystallization suppresses their saturation and allows Ta to increase in the melt. Saturation with respect to Ta and Nb minerals occurs at the latest stages of magmatic crystallization, and columbite can originate from recrystallization of mica. We propose a model for prediction of intrusion fertility for Ta.
Journal Article
Igneous rocks
Explores Earth science's natural processes, how geologists study igneous rocks, and how igneous rocks relate to the our daily life.
Book
Iron isotopic fractionation during eclogite anatexis and adakitic melt evolution: insights into garnet effect on Fe isotopic variations in high-silica igneous rocks
How and what extent residual garnet in the source and garnet fractionation contribute to the heavy Fe isotopic compositions in high-silica igneous rocks remain unexplored. We here measured the Fe isotopic compositions of garnet-bearing leucosomes, garnet-free leucosomes, garnetites, garnet-amphibolites, eclogites and their constituent minerals in the North Qaidam ultrahigh pressure metamorphic belt. Garnet-amphibolites were derived from the retrogression of eclogites. Garnet-bearing leucosomes were formed by partial melting of eclogites with residual garnet in the source, while garnet-free leucosomes and garnetites originated from garnet-bearing leucosomes by garnet fractionation and accumulation, respectively. Iron isotopic measurements show that eclogites and garnet-amphibolites have similar Fe isotopic compositions with δ
56
Fe of ~ 0.05‰, indicating that fluids involved in retrograde metamorphism are internally buffered and thus did not modify the Fe elemental and isotopic budget of metamorphic rocks. Garnet-bearing leucosomes have heavier Fe isotopic compositions (δ
56
Fe ~ 0.09‰) than eclogites and garnet-amphibolites, consistent with residual isotopically light Fe-rich garnet in the source and the preferential extraction of isotopically heavy Fe-rich omphacite into melts during eclogite partial melting. The δ
56
Fe values of garnet-bearing and garnet-free leucosomes increase with the increase of SiO
2
, Fe
3+
/ΣFe and (Na + K)/Al, and with the decrease of FeO
T
, suggesting that isotopically light Fe-rich garnet fractionation, together with compositional changes of magmas, controls the observed Fe isotopic variations. Rayleigh-fractionation modeling estimates Fe isotopic fractionation factors between garnet and felsic melts to be
α
garnet-felsic melt
= 0.999958–0.999988. Such small Fe isotopic fractionation factors suggest that except for garnet fractionation, fractionation of other isotopically light Fe-rich minerals (e.g., ulvospinel–titanomagnetite, ilmenite, biotite) is also required to explain the extremely heavy Fe isotopic enrichments observed in high-silica igneous rocks worldwide.
Journal Article
What are igneous rocks?
\"Bursting volcanoes, cooling magma, crystals, and granite: this title covers everything igneous rock related. Written for a lower-elementary-level audience, the science behind the rock cycle and the formation of igneous rocks is presented in clear and easy-to-understand language.\"--Provided by publisher.
Book
Global atmospheric oxygen variations recorded by Th/U systematics of igneous rocks
Atmospheric oxygen has evolved from negligible levels in the Archean to the current level of about 21% through 2 major step rises: The Great Oxidation Event (GOE) in the early Proterozoic and the Neoproterozoic Oxygenation Event (NOE) during the late Proterozoic. However, most previous methods for constraining the time of atmospheric oxygenation have relied on evidence from sedimentary rocks. Here, we investigate the temporal variations of the Th/U of arc igneous rocks since 3.0 billion y ago (Ga) and show that 2 major Th/U decreases are recorded at ca. 2.35 Ga and ca. 0.75 Ga, coincident with the beginning of the GOE and NOE. The decoupling of U from Th is predominantly caused by the significant rise of atmospheric oxygen. Under an increasingly oxidized atmosphere condition, more uranium in the surface environment became oxidized from the water-insoluble U4+ to the water-soluble U6+ valance and incorporated in the sea water and altered oceanic crust. Eventually, the subduction of this altered oceanic crust produced the low-Th/U signature of arc igneous rocks. Therefore, the sharp decrease of Th/U in global arc igneous rocks may provide strong evidence for the rise of atmospheric oxygen. We suggest that the secular Th/U evolution of arc igneous rocks could be an effective geochemical indicator recording the global-scale atmospheric oxygen variation.
Journal Article
A felsic meta-igneous source for Li-F-rich peraluminous granites: insights from the Variscan Velay dome (French Massif Central) and implications for rare-metal magmatism
The Velay anatectic dome in the Variscan French Massif Central exposes a low-pressure–high-temperature metamorphic sequence, which represents an ideal natural laboratory for documenting the behavior of rare-metals and fluxing elements during crustal melting. We investigated the silicate and bulk-rock geochemistry of sub- to suprasolidus metapelites and orthogneisses, as well as related granites, and performed forward thermodynamically constrained geochemical modeling to quantify the respective effects of melting pressure, temperature, H2O activity, and protolith composition on the Li and F contents of granitic melts. We find that biotite compositions are good proxies of melt compositional evolutions during prograde melting. The crystallization of peritectic cordierite at low pressure (< 5 kbar) and “water-fluxed” melting both inhibit the Li enrichment of anatectic melts. Metapelite-derived melts consistently show modest Li–F contents, and a decoupling is observed as melts with the highest Li concentrations (~ 200–400 ppm) are produced below 750 °C, whereas F-richest melts (~ 0.2–0.4 wt%) are produced above 750 °C near the biotite-out isograd. Peraluminous orthogneiss anatexis can generate a melt that is concomitantly enriched in both F (~ 0.3–1 wt%) and Li (~ 600–1350 ppm) at relatively low temperature (< 750 °C), which can evolve toward rare-metal granite compositions (~ 10,000 ppm Li; ~ 2 wt% F) after 80–90 wt% of fractional crystallization. Melting of felsic meta-igneous rocks followed by magmatic differentiation is thus a viable mechanism to form Li-F-rich rare-metal granites and pegmatites, providing a direct link between protracted crust recycling and rare-metal magmatism in late-orogenic settings.
Journal Article
Zircon stability grids in crustal partial melts: implications for zircon inheritance
Zircon inheritance is a common phenomenon in igneous rocks, although more frequent in granitoids. Zircons inherited from granite magmas mostly come from the source, not from wall rocks or xenoliths. Consequently, they can provide invaluable information about the source materials, melting temperature, and melt segregation conditions. Miller et al. (Geology 31:529–532, 2003) divided granite rocks according to their zircon saturation temperature (TZr) into “hot” (TZr > 800 °C, with little or no inherited zircon) and “cold” (TZr < 800 °C; with abundant inherited zircon). Nevertheless, we have found that coeval and neighboring two-mica granites with TZr < 750 °C, presumably derived from similar sources, may have a radically different inheritance, from about 95 to near 0%. This paper aims to understand the reasons for these differences, in particular, and the survival of source zircons in granitoids, in general. To this end, we modeled the relationships between source composition, temperature, pressure, water content, zircon solubility, and melt fraction, on one hand; and melt production and zircon solution rates, on the other hand. Our results foresee that zircon survival during crustal melting is more probable if the source is a fertile peraluminous metasedimentary rock than if it is a metaluminous source with similar SiO2. Elevated zircon inheritance is characteristic of mid-crustal S-type, water-rich granite magmas generated within 4.5 and 6 kbar. Moderate or no inheritance is characteristic of water-poor granite magmas, because their sources require higher temperatures to produce the same melt fraction. Fast melt extraction does not cause perceptible effects on our models, because melt generation is slower than zircon dissolution, except in the case of crustal underplating by hot mafic magmas. We propose to refine the “hot” and “cold” classification by splitting the “cold” granites (TZr < 800 °C) into two categories, “dry” with little inheritance and “wet” with a very high zircon inheritance. Wet granites require a source water-fluxed from outside. They are characteristic of mid-crustal anatectic complexes with highly fertile gneisses alternating with unfertile mica-rich metapelites. We suggest that the extra water should come in most cases from dehydration reactions in the unfertile metasedimentary rocks beneath the crustal section undergoing anatexis.
Journal Article