Strontium- and lead-isotope composition of some basalts at DSDP Hole 69-504B

Seventeen whole-rock samples, generally taken at 25- to 50-meter intervals from 5 to 560 meters sub-basement in Deep Sea Drilling Project Hole 504B, were analyzed for 87Sr/86Sr ratios, and rubidium and strontium concentrations. Ten of these samples also were analyzed for Pb-isotope composition. Strontium-isotope ratios for eight samples from the upper 260 meters of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the interval 330 to 560 meters, five samples have a restricted range of 0.70259 to 0.70279, with a mean of 0.70266, almost identical to the average value of fresh mid-ocean-ridge basalts. In the interval 260 to 330 meters, approximately intermediate strontium- isotope ratios are found.
The higher 87Sr/86Sr ratios in the upper part of the hole can be interpreted in terms of strontium-isotope alteration during basalt-sea-water interaction. Relative to average fresh mid-ocean ridge basalts, the upper 260 meters of basalts are enriched by an average of about 9% in sea-water strontium 87Sr/86Sr = 0.7091). This Sr presumably is located in the smectites, which, as the main secondary minerals throughout the hole, replace olivine and matrix glass and locally fill vesicles (analyzed samples contained no veins).
The strontium-isotope data strongly suggest that the integrated flux of sea water through the upper part of the Hole 504B crust has been greater than through the lower part. This is also suggested by (1) the common occurrence of Feoxide- hydroxide minerals as alteration products above 270 meters, but their near absence below 320 meters, (2) the presence of vein calcite above 320 meters, but its near absence below this level, and (3) the occurrence of vein pyrite only below a depth of 270 meters. Sea-water circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below 230 meters sub-basement. Although sufficient sea water was present within the lower part of the hole to produce smectitic alteration products, the overall water /rock ratio was low enough to prevent significant modification of strontium-isotope ratios.
Lead-isotope ratios of Hole 504B basalts form approximately linear arrays in plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb. The arrays are similar to those reported for basalts from other mid-ocean ridges. There is no trend in Hole 504B lead-isotope ratios with vertical position in the basement. The arrays indicate that the lead-isotope composition of the upper mantle from which the Hole 504B basaltic melts were derived was inhomogeneous.

本次研究对深海钻探计划（Deep Sea Drilling Project, DSDP）504B孔基底以下5米至560米段的全岩样品进行了分析，样品采集间距普遍为25至50米，共计17件。分析内容包括87锶/86锶（87Sr/86Sr）比值、铷与锶浓度；其中10件样品额外开展了铅同位素组成测试。
该孔上部260米段的8件样品的87Sr/86Sr比值介于0.70287至0.70377之间，平均值为0.70320；330米至560米段的5件样品的比值范围狭窄，为0.70259至0.70279，平均值0.70266，与新鲜洋中脊玄武岩（mid-ocean-ridge basalts）的平均比值几乎一致；而260米至330米段则呈现出介于两者之间的锶同位素比值。
该孔上部地层较高的87Sr/86Sr比值，可通过玄武岩与海水相互作用过程中的锶同位素蚀变来解释。相较于新鲜洋中脊玄武岩的平均比值，上部260米段的玄武岩中海水来源锶（87Sr/86Sr=0.7091）的平均富集程度约为9%。这类锶赋存于蒙脱石（smectites）中，蒙脱石作为全孔主要的次生矿物，交代橄榄石（olivine）与基质玻璃（matrix glass），并局部充填气孔（vesicles）（本次分析的样品未含脉体）。
锶同位素数据强烈表明，504B孔上部地壳的海水综合通量高于下部地层。这一结论也得到以下证据的支持：(1) 270米以上地层普遍发育铁氧化物-氢氧化物（Feoxide-hydroxide）矿物蚀变产物，而320米以下几乎未见此类矿物；(2) 320米以上存在方解石脉（vein calcite），但该深度以下几乎无方解石脉产出；(3) 仅在270米以深地层发现黄铁矿脉（vein pyrite）。基底以下230米以深区域发育大量相对不透水的块状熔岩流（massive lava flows），可能部分限制了下部玄武岩地层的海水循环。尽管孔内下部地层仍有足够海水发生蒙脱石蚀变作用，但整体水岩比（water/rock ratio）极低，不足以使锶同位素比值发生显著改变。
504B孔玄武岩的铅同位素比值在以208Pb/204Pb、207Pb/204Pb与206Pb/204Pb为轴的图解中近似呈线性阵列，该阵列与其他洋中脊玄武岩的报道结果相似。504B孔的铅同位素比值并未随基底内的垂直深度呈现出变化趋势。该线性阵列表明，形成504B孔玄武质熔体的源区上地幔（upper mantle）具有同位素不均一性。