Phosphogenesis and REE+Y diagenesis of recent and Paleozoic phosphorites
Date
2021-09-04
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Abstract
Fosfor on elusloodusele kandva tähtsusega keemiline element ning ühtlasi primaarproduktsiooni kontrolliv toitaine. Fosfori settese sidumise mehhanism ehk fosfogenees on siiani paljuski ebaselge. Peamine fosforit sisaldav mineraal setetes on apatiit, mille kristallstruktuuri on võimalik asenduda suurel hulgal erinevatel keemilistel elementidel. Üks olulisemaid asenduvaid elementide rühmasi on haruldased muldmetallid ja üttrium ehk REE+Y. Käesolevas doktoritöös uuriti <5 miljoni aasta vanuseid fosforiite Namiibia šelfil ning ~ 490 miljoni aasta vanuseid Eesti biogeenseid fosforiite. Töö peamisteks eesmärkideks oli selgitada fosfogeneesiks vajalikud keskkonnatingimused ning haruldaste muldmetallide sidustamise ja ümberjaotumine mehhanismid. Töö fookuses olid REE+Y süstemaatika fosforiitides nii lühiajalisel kui pikaajalisel skaalal, stabiilsete isotoopide ja jälgelementide käitumine apatiidi väljasettimise ajal ning väljasettinud apatiidi mikromorfoloogia. Käesoleva töö tulemused näitavad, et fosfogeneesi levikut kontrollivad hapnikurikaste ja –vaesete tingimuste varieerumine settes ning apatiidi settimiseks vaja minevate orgaaniliste substraatide olemasolu. REE+Y sidustamine apatiiti toimub geoloogilisel ajaskaalal kiiresti ning ka väga noortel fosforiitidel on jälgelementide koostis oluliselt erinev algupärasest. Peamiseks REE+Y allikaks apatiidis on hapnikuvaene poorivesi. Apatiidi settimise ajal poorivee valdav keemiline koostis kontrollib REE+Y sidustamist ja seeläbi nende elementide sisaldust fosforiidis. Sellest tulenevalt võivad üksteisest paarikümne kilomeetri kaugusel paiknevate fosforiidi leiukohtade REE+Y kontsentratsioonid varieeruda enam kui kümme korda.
Phosphorus is an essential element for all life on Earth as well as the key limiting nutrient on geological timescales. However, the formation of solid phosphate phases – phosphogenesis – is poorly constrained. The main phosphate containing mineral in marine sediments is apatite. Due to its complex crystal structure, apatite is susceptible to a wide variety of substitutions. One of the most important chemical substitutions in apatite are a group of 15 elements called Rare Earth Elements and Yttrium (REE+Y). In this thesis, Recent Namibian sedimentary phosphorites and ~490 mln year old Estonian phosphorites were studied. The main goal of this doctoral thesis was to decipher the environmental conditions during phosphogenesis and the diagenetic evolution of sedimentary phosphorites. The specific aims of this thesis were to study the stable isotopic and trace element systematics in Recent sedimentary apatites and the post-depositional short- and long-term changes in apatite REE+Y composition. The thesis concludes that dominantly suboxic conditions punctuated by short-term sulfidic conditions, coupled with organic matter derived nucleation surfaces control both the depth and spatial distribution of phosphogenesis. Uptake of REE+Y by apatite is a fast-paced process on geological timescales and primary REE+Y signatures are already altered in Recent phosphorites. The dominant source for REE+Y in apatite is suboxic-sulfidic pore-water. The chemical environment of pore-water immediately after deposition of apatite controls the degree of enrichment of REE+Y in phosphorites, and can result in up to 10-fold REE+Y abundance variability between phosphorite localities a few tens of kilometres apart.
Phosphorus is an essential element for all life on Earth as well as the key limiting nutrient on geological timescales. However, the formation of solid phosphate phases – phosphogenesis – is poorly constrained. The main phosphate containing mineral in marine sediments is apatite. Due to its complex crystal structure, apatite is susceptible to a wide variety of substitutions. One of the most important chemical substitutions in apatite are a group of 15 elements called Rare Earth Elements and Yttrium (REE+Y). In this thesis, Recent Namibian sedimentary phosphorites and ~490 mln year old Estonian phosphorites were studied. The main goal of this doctoral thesis was to decipher the environmental conditions during phosphogenesis and the diagenetic evolution of sedimentary phosphorites. The specific aims of this thesis were to study the stable isotopic and trace element systematics in Recent sedimentary apatites and the post-depositional short- and long-term changes in apatite REE+Y composition. The thesis concludes that dominantly suboxic conditions punctuated by short-term sulfidic conditions, coupled with organic matter derived nucleation surfaces control both the depth and spatial distribution of phosphogenesis. Uptake of REE+Y by apatite is a fast-paced process on geological timescales and primary REE+Y signatures are already altered in Recent phosphorites. The dominant source for REE+Y in apatite is suboxic-sulfidic pore-water. The chemical environment of pore-water immediately after deposition of apatite controls the degree of enrichment of REE+Y in phosphorites, and can result in up to 10-fold REE+Y abundance variability between phosphorite localities a few tens of kilometres apart.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone
Keywords
rare earth metals, diagenesis, apatite, phosphorite, sedimentary rocks, Paleozoic