Depths and temperatures of mid-ocean-ridge magma chambers and the composition of their source magmas
Depths and temperatures of mid-ocean-ridge magma chambers and the composition of their source magmas
复制标题
大洋中脊岩浆房的深度和温度及其源岩浆的成分
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发表时间：
1984
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通讯作者：
M. Fisk
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文献类型：
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作者：
M. Fisk
对加拉帕戈斯隆起和大西洋中脊（60-63°N）海底玄武岩玻璃和共存橄榄石的电子探针分析表明，橄榄石斑晶实际上是不分带的，橄榄石成分与假设橄榄石在主玻璃的液相温度下结晶所预测的橄榄石成分相同。橄榄石的化学分带的缺失表明，在橄榄石斑晶存在的岩浆中，它们的寄主岩浆的冷却温度必须小于15℃，岩浆的MgO/FeO必须几乎恒定（小于3wt %的橄榄石被去除）。橄榄石斑晶与宿主岩浆的再平衡相对较快，喷发前24小时内发生的任何温度或成分变化都会记录在橄榄石成分中。与岩浆上升速度相比，1毫米橄榄石斑晶的沉降速度太慢，无法使早期形成的橄榄石沉降。在那里，许多在海中脊峰喷发的岩浆必须以几乎等温和等化学的方式从最终的储藏库上升，或者它们在超流体温度下上升，并在喷发前冷却到低于流体温度。在这两种情况下，岩浆都能在喷发前立即识别出温度（±15°C）和岩浆库的成分。这些岩浆储层的深度可以通过海洋玄武岩压力-温度相图的实验和经验测定来估计。对于加拉帕戈斯隆起和中大西洋脊玄武岩，这些储层位于脊壳下3 - 25公里处，在某些情况下，单个储层可能位于±3公里处。
Summary Electron-microprobe analyses of submarine basalt glass and coexisting olivine from the Galapagos Rise and the Mid-Atlantic Ridge (60–63°N) reveal that olivine phenocrysts are virtually unzoned and olivine compositions are identical to those predicted by assuming that the olivine crystallized at the liquidus temperature of the host glass. The absence of chemical zoning of olivine indicates that their host magmas must have cooled by less than 15°C and the MgO/FeO of the magma must have been nearly constant (less than 3 wt% olivine removed) during the time that olivine phenocrysts existed in the magma. Olivine phenocrysts re-equilibrate with their host magma relatively quickly and any changes in temperature or composition that occurred in the 24 h previous to eruption would be recorded in the olivine composition. The settling rates of 1-mm olivine phenocrysts are too slow compared to magma ascent rates to allow early-formed olivine to settle out. There, many magmas that erupt at mid-ocean-ridge crests must ascend from their final reservoir nearly isothermally and isochemically, or alternatively they ascend at superliquidus temperatures and cool below the liquidus just prior to eruption. In either case the magmas identify the temperature (±15°C) and the composition of the magma reservoir immediately prior to eruption. The depth of these magma reservoirs may be estimated through experimental and empirical determination of the pressure-temperature phase diagrams of oceanic basalts. For Galapagos Rise and Mid-Atlantic Ridge basalts these reservoirs are at 3–25 km beneath the ridge crust and in some cases individual reservoirs may be located within ±3 km.