『Abstract
The rare earth (REE) contents of soils and pan sediments of the
Darling Hills granitic terrain of southwestern South Africa were
determined to study the behavior of REE during weathering in a
semi-arid, Mediterranean climate. The P-rich, S-type bedrock granite
has a middle REE (MREE) enrichment relative to post-Archean Australian
sedimentary rocks (PAAS) that primarily reflects the REE content
of feldspar and accessory apatite. Initial chemical weathering
of the granite results in the loss of REE, particularly Ce, Eu
and the heavy REE (HREE). The enrichment of REE in soils relative
to bedrock granite corresponds to clay mineral and amorphous Fe
oxide content in the case of the light REE (LREE) and precipitation
of pedogenic calcite in the case of the HREE. Quartz sand dilutes
the REE content, and concentration of K-feldspar produces positive
Eu anomalies. Cerium is rapidly precipitated during weathering
and retained in clayey, upper soil horizons as insoluble CeO2. Most pan sediments have a bowl-shaped REE pattern
that is derived from the accumulation of eroded catchment soils
modified by precipitation of evaporative carbonates. REE enrichment,
particularly LREE, and the large negative Ce anomaly of oxidized
basal pan sediment relative to the bedrock granite suggest that
REE entering the pan by groundwater flow are sorbed onto amorphous
Fe oxides. In contrast, goethite-rich nodules from the pan have
a low REE content and show HREE enrichment relative to the granite.
Precipitation of pedogenic calcite and evaporative pan carbonates
greatly increases the retention of the HREE in these semi-arid
soils. Diagenetic pan dolomite has a REE pattern similar to pedogenic
and evaporative calcite, but with a significantly lower REE content.
The REE patterns indicate that, although most abundant in the
bedrock granite, the MREE are least retained by soils during weathering.
The LREE are generally retained by clayey and amorphous Fe-oxide-rich
soils and sediments, whereas HREE are retained by precipitation
of soil and pan carbonates.
Keywords: Rare earth elements (REE); Granite; Soil; Carbonate;
Weathering; Saline pans; Dolomite; Apatite; Fe oxides』
『南アフリカの南西部にあるDarling Hills花崗岩地域の土壌と盤層堆積物の希土類元素(REE)含有量が半乾燥の地中海気候における風化中のREEの挙動を研究するために決定された。Pに富むSタイプの基盤花崗岩は、長石と副成分燐灰石のREE含有量を主に反映する後始生代オーストラリア堆積岩(PAAS)に比べて中間のREE(MREE)に富んでいる。花崗岩の初期化学風化で、REE、とくにCe、Eu、および重いREE(HREE)が失われている。基盤花崗岩に比べて土壌中のREEが富むのは、軽いREE(LREE)の場合は粘土鉱物と非晶質Fe酸化物含有量に、そしてHREEの場合は土壌生成方解石の沈殿に対応している。石英砂はREE含有量を希釈し、カリ長石の濃集は正のEu異常を生じる。セリウムは風化の間に急速に沈殿し、不溶性CeO2として粘土質の上部土壌層に留まる。大部分の盤層堆積物はわん形のREEパターンをもち、これは蒸発による炭酸塩の沈殿によって変化を受けた浸食による流域土壌の集積に由来する。基盤花崗岩に比べて、酸化した基底の盤層堆積物がREEとくにLREEに富み大きな負のCe異常なのは、地下水流によって盤層へ浸入するREEは非晶質Fe酸化物に吸着されることを示している。対照的に、盤層からの針鉄鉱に富む団塊は低いREE含有量をもち、花崗岩に比べてHREEに富むことが示される。土壌生成方解石と蒸発による盤層炭酸塩の沈殿は、これらの半乾燥土壌におけるHREEの保存を非常に増加させる。続成作用による盤層苦灰石は土壌生成および蒸発による方解石と似たREEパターンをもつが、かなり低いREE含有量をもつ。REEパターンは、基盤花崗岩では最も多いけれどもMREEは風化の間に土壌に留まりにくいことを示している。LREEは一般に粘土質で非晶質Fe酸化物に富む土壌および堆積物に保持されるが、一方HREEは土壌と盤層の炭酸塩の沈殿により保持される。』
1. Introduction
2. Geological setting
3. Materials and methods
4. Results
4.1. Granite
4.2. Granitic soils
4.3. Salt pan sediments
5. Discussion
5.1. REE mobility during granite weathering
5.2. REE mobility in granitic soils
5.3. REE behavior in salt pan sediments
6. Conclusions
Acknowledgements
References