Rielli Andrea, Tomkins Andrew G, Nebel Oliver, Brugger Joël, Etschmann Barbara, Paterson David
Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126, Pisa, Italy.
School of Earth, Atmosphere and the Environment, Monash University, Melbourne, Victoria, 3800, Australia.
Sci Rep. 2018 Nov 6;8(1):16411. doi: 10.1038/s41598-018-34669-0.
Changes in the oxygen fugacity (fO) of the Earth's mantle have been proposed to control the spatial and temporal distribution of arc-related ore deposits, and possibly reflect the evolution of the atmosphere over billions of years. Thermodynamic calculations and natural evidence indicate that fluids released from subducting slabs can oxidise the mantle, but whether their oxidation potential varied in space and time remains controversial. Here, we use garnet peridotites from western Norway to show that there is a linear decrease in maximum fO with increasing depth in the mantle wedge. We ascribe this relation to changes in the speciation of sulfur released in slab fluids, with sulfate, controlling maximum oxidation, preferentially released at shallow depths. Even though the amount of sulfate in the Precambrian oceans, and thus in subducted lithologies, is thought to have been dramatically lower than during the Phanerozoic, garnet peridotites metasomatised during these two periods have a comparable fO range. This opens to the possibility that an oxidised mantle with fO similar to modern-day values has existed since the Proterozoic and possibly earlier. Consequently, early magmas derived from partial melting of metasomatised mantle may have had suitable fO to generate porphyry Cu-Au and iron-oxide Cu-Au deposits.
有人提出，地球地幔的氧逸度（fO）变化控制着与弧相关的矿床的时空分布，并可能反映了数十亿年来大气的演化。热力学计算和自然证据表明，俯冲板块释放的流体可以使地幔氧化，但它们的氧化电位在空间和时间上是否变化仍存在争议。在这里，我们使用挪威西部的石榴石橄榄岩表明，地幔楔中最大fO随深度增加呈线性下降。我们将这种关系归因于板块流体中释放的硫的形态变化，其中控制最大氧化作用的硫酸盐优先在浅深度释放。尽管前寒武纪海洋中以及因此在俯冲岩性中的硫酸盐含量被认为比显生宙期间要低得多，但在这两个时期经历交代作用的石榴石橄榄岩具有相当的fO范围。这就有可能自元古代甚至更早以来就存在fO与现代值相似的氧化地幔。因此，由交代地幔部分熔融产生的早期岩浆可能具有合适的fO来形成斑岩型铜金矿床和铁氧化物铜金矿床。
