Petrography and the REE-composition of apatite in the Paleoproterozoic Pilgujärvi Sedimentary Formation, Pechenga Greenstone Belt, Russia

Abstract

The first globally significant phosphorous-rich deposits appear in the Paleoproterozoic at around 2 Ga, however, the specific triggers leading to apatite precipitation are debated. We examine phosphorous-rich rocks (up to 8 wt% P 2 O 5 ) in 1.98-1.92 Ga old Pilgujärvi Sedimentary Formation, Pechenga Greenstone Belt, Russia. Phosphates in these rocks occur as locally derived and resedimented sand-to-gravel/pebble sized grains consisting of apatitecemented muddy sediments. Phosphatic grains can be subdivided into four petrographic types (A-D), each has a distinct REE signature reflecting different early-to-late diagenetic conditions and/or metamorphic overprint. Pyrite containing petrographic type D, which typically has a flat REE pattern, negative Ce anomaly and positive Eu anomaly, is the best preserved of the four types and best records conditions present during apatite precipitation. Type D phosphatic grains precipitated under (sub)oxic basinal conditions with a significant hydrothermal influence. These characteristics are similar to Zaonega Formation phosphates of NW Russia's Onega Basin, and consistent with phosphogenesis triggered by the development of anoxic(sulfidic)-(sub)oxic redoxclines at shallow sediment depth during the Paleoproterozoic.

On the track of a Scottish impact structure: a detrital zircon and apatite provenance study of the Stac Fada Member and wider Stoer Group, NW Scotland

“…Secondly, apatite is more likely than zircon to represent first-cycle detritus as it is prone to dissolution at source by acidic meteoric and pedogenic waters (Morton & Hallsworth, 1999). Despite being prone to dissolution at source (Joosu et al 2016) apatite is found in non-trivial abundance in nearly all Quaternary sediments (Nechaev & Isphording, 1993) and, because it is stable during diagenesis (due to the liberation of organic P and P adsorbed onto the surface of clay minerals; e.g. Bouch et al 2002), the detrital apatite signal is likely to be preserved once buried.…”

Archaean phosphates: a case study of transformation processes in apatite from the Barberton greenstone belt

“…Measurements of the REEs and Y in 2.5-3.7 Ga-old BIFs and metacherts (e.g., Bolhar et al 2004Bolhar et al , 2005Nishizawa et al 2005;Hofmann et al 2013) as well as in apatite (Lepland et al 2002;Nishizawa et al 2004;Joosu et al 2015Joosu et al , 2016Wudarska et al 2018) show that they contain "sea water fingerprints" with a Y anomaly and flat REE distribution patterns, identical to the pattern found in modern sea water (Elderfield and Greaves 1982). The Y anomaly seems to be present in geological records throughout the entire Precambrian, both after and prior to the great oxidation event (GOE) and up to the Eoarchean (Joosu et al 2015(Joosu et al , 2016Wudarska et al 2018). However, unlike modern seawater, the sedimentary rocks prior to the GOE are characterized by a lack of a negative Ce anomaly, which implies anoxic conditions (e.g., Lepland et al 2002;Joosu et al 2015Joosu et al , 2016Wudarska et al 2018), whereas a positive Eu anomaly indicates significant hydrothermal input into the Archaean oceans (Veizer et al 1989).…”

“…The Y anomaly seems to be present in geological records throughout the entire Precambrian, both after and prior to the great oxidation event (GOE) and up to the Eoarchean (Joosu et al 2015(Joosu et al , 2016Wudarska et al 2018). However, unlike modern seawater, the sedimentary rocks prior to the GOE are characterized by a lack of a negative Ce anomaly, which implies anoxic conditions (e.g., Lepland et al 2002;Joosu et al 2015Joosu et al , 2016Wudarska et al 2018), whereas a positive Eu anomaly indicates significant hydrothermal input into the Archaean oceans (Veizer et al 1989). Due to the mafic melt differentiation rules, apatite crystallizes as one of the latest phases, and therefore they exhibit a negative Eu anomaly (Puchelt and Emmermann 1976).…”

“…Many papers have studied phosphates of Precambrian age (e.g., Mojzsis et al 1996;Lepland et al 2002Lepland et al , 2013Nishizawa et al 2004Nishizawa et al , 2005Blake et al 2010;Joosu et al 2015Joosu et al , 2016Alibert 2016;Wudarska et al 2018) with the scope to deduce their primary formation conditions. In the oldest known supracrustal rocks from the Isua Supracrustal Belt (≈ 3.7 Ga) (Moorbath et al 1973;Nutman et al 1997), metamorphism (e.g., Rosing et al 1996;Rollinson 2003) caused obliteration of primordial isotopic signatures (Wudarska et al 2018), whereas REE signatures have been partially preserved (Lepland et al 2002;Wudarska et al 2018).…”

“…The structure and its transformation were investigated by transmission electron microscopy (TEM), and an auxiliary tool was cathodoluminescence (CL). Paleoproterozoic apatite from the Zaonega formation is used as a reference material, giving insight into the relationship between the primary/secondary signatures resulting from the formation/transformation of the crystals (Joosu et al 2015(Joosu et al , 2016.…”