Abstract Basalts from the East Pacific Rise (EPR), Siqueiros transform zone and Mid-Atlantic Ridge (MAR) area have been analyzed for the platinum-group elements (PGE) and a wide range of incompatible elements. The low PGE content of the most primitive mid-ocean ridge basalts (MORB) suggests that they leave the mantle sulfide-saturated but become sulfide under-saturated, as a consequence of decompression, during their ascent from the mantle to the MOR magma chamber. Because the pressure drop is small, the ascending magma enters the magma chamber slightly sulfide under-saturated and requires only a small amount of fractional crystallization to return it to sulfide-saturation. Sulfide-saturation is marked by the Pd content of the melt falling by over an order of magnitude at ca. 9.5 wt.% MgO. However, once the magma has become sulfide-saturated, it shows no evidence of a further decline in Pd with decreasing MgO as the system evolves. This must result from regular replenishment of the underlying axial magma chamber by fresh batches of primitive magma. Most replenishments occur when the MgO content of the resident melt in the magma chamber is between ∼9 to 6.5 wt.% MgO. We combine the tight constraints imposed by highly compatible Pd abundances, with those imposed by strongly incompatible elements, to produce the first model that successfully accounts for the variations of both classes of elements in open system mid-ocean ridges magma chambers for normal-type MORB (N-MORB). We show that the lack of decline in the Pd content of sulfide-saturated mid-ocean ridge basalts, with decreasing MgO, requires frequent small replenishments (between 1% and 4%) of similar magnitude, rather than large initial inputs that systematically decline from near 100% to near zero during the life of an individual ridge system. Assuming the spreading rate of ∼110 mm/yr for the EPR at 9°N, and 14 yr between the 1991–1992 and 2005–2006 eruptions, the calculated volume of erupted and intruded magma is 2.3 × 103 m3/m of ridge length. If the expelled melt represents 0.5% of the accessible magma in the chamber, as suggested by the Pd modeling, the volume of the magma accessible during tapping is 4.6 × 105 m3/m of ridge length.

中文翻译：

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使用贵金属探针量化洋中脊岩浆过程

摘要 分析了东太平洋隆起 (EPR)、Siqueiros 转换带和大西洋中脊 (MAR) 地区的玄武岩中的铂族元素 (PGE) 和多种不相容元素。最原始的洋中脊玄武岩 (MORB) 的低 PGE 含量表明，它们在从地幔上升到 MOR 岩浆房期间，由于减压，使地幔硫化物处于饱和状态，但硫化物处于欠饱和状态。由于压降小，上升的岩浆进入岩浆房时硫化物略为欠饱和，只需要少量的分步结晶即可使其恢复硫化物饱和。硫化物饱和的特征是熔体的 Pd 含量在大约 10 倍时下降了一个数量级。9.5 重量％的氧化镁。然而，一旦岩浆变得硫化物饱和，随着系统的发展，没有证据表明 Pd 随着 MgO 的减少而进一步下降。这一定是由于新鲜批次的原始岩浆定期补充下方的轴向岩浆房所致。大多数补充发生在岩浆房中滞留熔体的 MgO 含量在 ~9 至 6.5 wt.% MgO 之间时。我们将高度相容的 Pd 丰度强加的严格约束与强不相容元素强加的约束相结合，以产生第一个模型，该模型成功地解释了开放系统洋中脊岩浆房中正常型 MORB 的两类元素的变化(N-MORB)。我们表明，随着 MgO 的减少，硫化物饱和的洋中脊玄武岩的 Pd 含量没有下降，需要频繁的类似幅度的小补充（1% 到 4%），而不是在单个山脊系统的生命周期内系统地从接近 100% 下降到接近零的大量初始输入。假设 EPR 在 9°N 的扩散率为 ~110 mm/yr，以及 1991-1992 和 2005-2006 喷发之间的 14 年，计算的喷发和侵入岩浆体积为 2.3 × 103 m3/m 脊长. 如果排出的熔体占腔室中可进入的岩浆的 0.5%，如 Pd 模型所建议的那样，则在出钢过程中可进入的岩浆体积为 4.6 × 105 m3/m 脊长。