Phosphate behavior during submarine hydrothermal alteration of ca. 3.455 Ga basaltic seafloor rocks from Pilbara, Western Australia

Abstract

Phosphorus is an essential element for various biomolecules and biochemical processes. A phosphate-poor ocean model is popular for the Archean oceans, because of the potential absence of continental land masses and phosphorus scavenging by BIF. On the other hand, recent studies have suggested the possibility of a high phosphorus flux from Archean submarine hydrothermal activity, yielding phosphate-rich oceans. Here, we performed mineralogical and geochemical analyses for the 3.455 Ga Apex Basalt in ABDP#1 core from East Pilbara Terrane of the Pilbara Craton, Australia, to examine whether Archean submarine hydrothermal activity could play the role of a phosphate source to contemporary oceans.The 60 m section of the Apex Basalt was divided into three zones in this study. Samples in Zone A (top 10 m section) comprised actinolite, epidote, albite, and chlorite. Less altered characteristics were indicated by low chemical index of alteration (CIA) values (0.42 to 0.44). Samples of Zone B (middle 25 m section) were extensively chloritized, with abundant siderite and quartz. This indicates that CO2-rich hydrothermal fluids were responsible for the alteration of Zone B. Samples of Zone C (lower 25 m section) comprised chlorite, sericite, and sulfides. Sulfidic hydrothermal fluids were responsible for the alteration of Zone C. CIA values of both Zones B and C ranged from 0.77 to 0.99, suggesting intensive hydrothermal alteration. Concentrations of P2O5in Zone A were 0.10 ± 0.01 wt%, and these values were most likely close to the original igneous composition. Euhedral apatite crystals in Zone A are interpreted as the remnants of igneous phosphates in the Apex Basalt. It was found that most parts of Zones B and C were depleted in P2O5(0.01 ± 0.01 wt%). Such depletion was caused by 3.455 Ga submarine hydrothermal activity. A few layers of Zones B and C had moderate concentrations of P2O5(0.06 ± 0.01 wt%). In these layers, various phosphate minerals were found, including secondary apatite, xenotime (YPO4), berlinite (AlPO4), and florencite (Ce, La, Nd)Al3(PO4)2(OH)6). Berlinite and florencite were mostly formed during hydrothermal alteration of the host rocks, and apatite and xenotime were also formed in the same manner, although difficulties exist to distinguish hydrothermal apatite from potential metamorphic apatite. Carbonatization associated with siderite formation in Zone B was notable in the ABDP #1 core samples. Phosphorus was significantly depleted in the carbonatization zones. Stability fields of siderite indicate that temperatures of fluids were relatively low (< 200 °C), and displayed weakly acidic to alkaline pH. In addition, it is found that sulfidic and acidic hydrothermal alteration also leached phosphorus from the host rocks. Calculations of apatite solubility suggested that hydrothermal fluids percolating Zone B could contain at least similar level as modern marine phosphorus concentration (2.3 µM) and could reach up to 2 mM. The estimated phosphate flux from Archean hydrothermal activity could have exceeded the modern riverine flux, which is the primary phosphorus source to the modern ocean. Therefore, the Archean phosphorus flux would likely have been significant enough to support the early biosphere.