scholarly journals Mantle heating at ca. 2 Ga by continental insulation: Evidence from granites and eclogites

Geology ◽  
2021 ◽  
Author(s):  
R. Tamblyn ◽  
D. Hasterok ◽  
M. Hand ◽  
M. Gard
Keyword(s):  
Earth Element ◽  
Thermal Conditions ◽  
Metamorphic Rocks ◽  
Critical Threshold ◽  
Heavy Rare Earth ◽  
Heavy Rare Earth Element ◽  
Thermal Budget ◽  
Thermally Driven ◽  
Cooling Trend ◽  
Cooling Model

Igneous and metamorphic rocks contain the mineralogical and geochemical record of thermally driven processes on Earth. The generally accepted thermal budget of the mantle indicates a steady cooling trend since the Archean. The geological record, however, indicates this simple cooling model may not hold true. Subduction-related eclogites substantially emerge in the rock record from 2.1 Ga to 1.8 Ga, indicating that average mantle thermal conditions cooled below a critical threshold for widespread eclogite preservation. Following this period, eclogite disappeared again until ca. 1.1 Ga. Coincident with the transient emergence of eclogite, global granite chemistry recorded a decrease in Sr and Eu and increases in yttrium and heavy rare earth element (HREE) concentrations. These changes are most simply explained by warming of the thermal regime associated with granite genesis. We suggest that warming was caused by increased continental insulation of the mantle at this time. Ultimately, secular cooling of the mantle overcame insulation, allowing the second emergence and preservation of eclogite from ca. 1.1 Ga until present.

Rare-earth crystal chemistry of thalénite-(Y) from different environments

2018 ◽  
Vol 82 (2) ◽  
pp. 313-327
Author(s):  
Markus B. Raschke ◽  
Evan J. D. Anderson ◽  
Jason Van Fosson ◽  
Julien M. Allaz ◽  
Joseph R. Smyth ◽  
...  
Keyword(s):  
Rare Earth ◽  
Rare Earth Element ◽  
Earth Element ◽  
Spatial Scales ◽  
Structure Refinement ◽  
X Ray Diffraction ◽  
Heavy Rare Earth ◽  
Heavy Rare Earth Element ◽  
X Ray ◽  
Golden Horn

ABSTRACTThalénite-(Y), ideally Y3Si3O10F, is a heavy-rare-earth-rich silicate phase occurring in granite pegmatites that may help to illustrate rare-earth element (REE) chemistry and behaviour in natural systems. The crystal structure and mineral chemistry of thalénite-(Y) were analysed by electron microprobe analysis, X-ray diffraction and micro-Raman spectroscopy from a new locality in the peralkaline granite of the Golden Horn batholith, Okanogan County, Washington State, USA, in comparison with new analyses from the White Cloud pegmatite in the Pikes Peak batholith, Colorado, USA. The Golden Horn thalénite-(Y) occurs as late-stage sub-millimetre euhedral bladed transparent crystals in small miarolitic cavities in an arfvedsonite-bearing biotite granite. It exhibits growth zoning with distinct heavy-rare-earth element (HREE) vs. light-rare-earth element (LREE) enriched zones. The White Cloud thalénite-(Y) occurs in two distinct anhedral and botryoidal crystal habits of mostly homogenous composition. In addition, minor secondary thalénite-(Y) is recognized by its distinct Yb-rich composition (up to 0.8 atoms per formula unit (apfu) Yb). Single-crystal X-ray diffraction analysis and structure refinement reveals Y-site ordering with preferential HREE occupation of Y2 vs. Y1 and Y3 REE sites. Chondrite normalization shows continuous enrichment of HREE in White Cloud thalénite-(Y), in contrast to Golden Horn thalénite-(Y) with a slight depletion of the heaviest REE (Tm, Yb and Lu). The results suggest a hydrothermal origin of the Golden Horn miarolitic thalénite-(Y), compared to a combination of both primary magmatic followed by hydrothermal processes responsible for the multiple generations over a range of spatial scales in White Cloud thalénite-(Y).

scholarly journals Petrogenesis of Heavy Rare Earth Element Enriched Rhyolite: Source and Magmatic Evolution of the Round Top Laccolith, Trans-Pecos, Texas

Minerals ◽  
2018 ◽  
Vol 8 (10) ◽  
pp. 423 ◽  
Author(s):  
Brent Elliott
Keyword(s):  
Rare Earth ◽  
Rare Earth Element ◽  
Earth Element ◽  
Geochemical Evolution ◽  
Magmatic Differentiation ◽  
Magmatic System ◽  
Heavy Rare Earth ◽  
Heavy Rare Earth Element ◽  
Element Concentrations ◽  
Stage Crystallization

The Round Top rhyolite located in Trans-Pecos Texas is enriched in Be, F, Li, Nb, Rb, Sn, Th, U, Y, Zr, and rare earth elements (REEs). REE-bearing minerals are mainly ubiquitous nano-scale accessory phases throughout the groundmass, incorporated in synchysite-group minerals, xenotime-(Y), Y- and Ce-rich fluorite, and zircon. The rhyolite is peraluminous, high-silica, alkaline (not peralkaline), with elevated heavy rare earth element concentrations and anonymously negative Eu values. Pervasive spongy groundmass and recrystallization textures are consistent with the elevated and remobilized Zr, Th, and Y + HREE (heavy rare earth element) concentrations and a high field strength element (HFSE) soluble, sub-alkalic, F-rich, magmatic system. REE-bearing minerals are present as late-magmatic, interstitial phases and attributed with closed-system, post-magmatic, hydrothermal alteration. Petrogenetic modeling provides scenarios that explain the geochemical evolution and REE complexing behavior in evolved rhyolite magmas, and determines possible source compositions and evolution. Trace element models suggest a system typical of having extensive magmatic differentiation. The resulting rhyolite magma is indicative of a silica-rich magmatic system enriched in H2O, Li, and/or F that could be considered transitional between pure silicate melt and hydrothermal fluid, where fluorine-ligand complexing was prevalent through late magmatic cooling and crystallization processes. Thorough differentiation and high fluorine activity contributed to the late stage crystallization of REE-bearing minerals in the Round Top rhyolite.

The role of clay minerals in formation of the regolith-hosted heavy rare earth element deposits

2020 ◽  
Vol 105 (1) ◽  
pp. 92-108 ◽  
Author(s):  
Martin Yan Hei Li ◽  
Mei-Fu Zhou
Keyword(s):  
Rare Earth ◽  
Adsorption Capacity ◽  
Clay Minerals ◽  
Rare Earth Element ◽  
Earth Element ◽  
Modern Society ◽  
Heavy Rare Earth ◽  
Heavy Rare Earth Element ◽  
Adsorption Sites ◽  
Shallow Soils

Abstract Rare earth elements (REEs) have become increasingly important to our modern society due to their strategical significance and numerous high technological applications. Regolith-hosted heavy rare earth element (HREE) deposits in South China are currently the main source of the HREEs, but the ore-forming processes are poorly understood. In these deposits, the REEs are postulated to accumulate in regolith through adsorption on clay minerals. In the Zudong deposit, the world's largest regolith-hosted HREE deposit, clay minerals are dominated by short, stubby, nanometer-scale halloysite tubes (either 10 or 7 Å) and microcrystalline kaolinite in the saprolite and lower pedolith and micrometer-sized vermicular kaolinite in the humic layer and upper pedolith. A critical transformation of the clay minerals in the upper pedolith is coalescence and unrolling of halloysite to form vermicular kaolinite. Microcrystalline kaolinite also transformed to large, well-crystalline vermicular kaolinite. This transformation could result in significant changes in different physicochemical properties of the clay assemblages. Halloysite-abundant clay assemblages in the deep regolith have specific surface area and porosity significantly higher than the kaolinite-dominant clay assemblages in the shallow soils. The crystallinity of clay minerals also increased, exemplified by decrease in Fe contents of the kaolinite group minerals (from ~1.2 wt% in the lower saprolite to ~0.35 wt% in the upper pedolith), thereby indicative of less availability of various types of adsorption sites. Hence, halloysite-abundant clay minerals of high adsorption capacity in deep regolith could efficiently retain the REEs released from weathering of the parent granite. Reduction in adsorption capacity during the clay transformation in shallow depth partially leads to REE desorption, and the released REEs would be subsequently transported to and adsorbed at deeper part of the soil profile. Hence, the clay-adsorbed REE concentration in the lower pedolith and saprolite (~2500 ppm on average) is much higher than the uppermost soils (~400 ppm on average). Therefore, weathering environments that favor the release of the REEs in the shallow soils but preservation of halloysite in the deep regolith can continuously adsorb REEs in the clay minerals to form economically valuable deposits.

Rare metal indicator minerals in bedrock and till at the Strange Lake peralkaline complex, Quebec and Labrador, Canada

2019 ◽  
Vol 56 (8) ◽  
pp. 857-869
Author(s):  
M.B. McClenaghan ◽  
R.C. Paulen ◽  
I.M. Kjarsgaard
Keyword(s):  
Rare Earth ◽  
Specific Gravity ◽  
Rare Earth Element ◽  
Earth Element ◽  
Rare Metal ◽  
Heavy Mineral ◽  
Surficial Sediments ◽  
Heavy Rare Earth ◽  
Heavy Rare Earth Element ◽  
Indicator Minerals

A study of rare metal indicator minerals and glacial dispersal was carried out at the Strange Lake Zr – Y – heavy rare earth element deposit in northern Quebec and Labrador, Canada. The heavy mineral (>3.2 specific gravity) and mid-density (3.0–3.2 specific gravity) nonferromagnetic fractions of mineralized bedrock from the deposit and till up to 50 km down ice of the deposit were examined to determine the potential of using rare earth element and high fileld strength element indicator minerals for exploration. The deposit contains oxide, silicate, phosphate, and carbonate indicator minerals, some of which (cerianite, uraninite, fluorapatite, rhabdophane, thorianite, danburite, and aeschynite) have not been reported in previous bedrock studies of Strange Lake. Indicator minerals that could be useful in the exploration for similar deposits include Zr silicates (zircon, secondary gittinsite (CaZrSi2O7), and other hydrated Zr±Y±Ca silicates), pyrochlore ((Na,Ca)2Nb2O6(OH,F)), and thorite (Th(SiO4))/thorianite (ThO2) as well as rare earth element minerals monazite ((La,Ce,Y,Th)PO4), chevkinite ((Ce,La,Ca,Th)4(Fe,Mg)2(Ti,Fe)3Si4O22), parisite (Ca(Ce,La)2(CO3)3F2), bastnaesite (Ce(CO3)F), kainosite (Ca2(Y,Ce)2Si4O12(CO3)·H2O), and allanite ((Ce,Ca,Y)2(Al,Fe)3(SiO4)3(OH)). Rare metal indicator minerals can be added to the expanding list of indicator minerals that can be recovered from surficial sediments and used to explore for a broad range of deposit types and commodities that already include diamonds and precious, base, and strategic metals.

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