Geochemistry of Atlantis Massif crust

The Atlantis Massif (Mid-Atlantic Ridge, 30°N) is an oceanic core complex marked by distinct variations in crustal architecture, deformation and metamorphism over distances of at least 5 km. We report Sr and Nd isotope data and Rare Earth Element (REE) concentrations of gabbroic and ultramafic rocks drilled at the central dome (IODP Hole 1309D) and recovered by submersible from the southern ridge of the massif that underlie the peridotite-hosted Lost City Hydrothermal Field. Systematic variations between the two areas document variations in seawater penetration and degree of fluid-rock interaction during uplift and emplacement of the massif and hydrothermal activity associated with the formation of Lost City. Homogeneous Sr and Nd isotope compositions of the gabbroic rocks from the two areas (87Sr/86Sr: 0.70261-0.70429 and epsilon-Nd: +9.1 to +12.1) indicate an origin from a depleted mantle. At the central dome, serpentinized peridotites are rare and show elevated seawater-like Sr isotope compositions related to serpentinization at shallow crustal levels, whereas unaltered mantle isotopic compositions preserved in the gabbroic rocks attest to limited seawater interaction at depth. This portion of the massif remained relatively unaffected by Lost City hydrothermal activity. In contrast, pervasive alteration and seawater-like Sr and Nd isotope compositions of serpentinites at the southern wall (87Sr/86Sr: 0.70885-0.70918; epsilon-Nd: -4.7 to +11.3) indicate very high fluid-rock ratios (~20 and up to 10**6) and enhanced fluid fluxes during hydrothermal circulation. Our studies show that Nd isotopes are most sensitive to high fluid fluxes and are thus an important geochemical tracer for quantification of water-rock ratios in hydrothermal systems. Our results suggest that high fluxes and long-lived serpentinization processes may be critical to the formation of Lost City-type systems and that normal faulting and mass wasting in the south facilitate seawater penetration necessary to sustain hydrothermal activity.

亚特兰蒂斯地块（位于大西洋中脊30°N处）是一处洋核杂岩，其地壳结构、变形作用与变质作用在至少5公里的范围内存在显著差异。本研究报道了采自该地块中央穹窿（国际大洋发现计划[IODP]1309D钻孔）以及通过载人潜水器从地块南脊回收的辉长岩和超镁铁质岩的锶（Sr）、钕（Nd）同位素数据与稀土元素（Rare Earth Element）浓度；上述岩石赋存于以橄榄岩为宿主的失落城热液场之下。两个采样区域间的系统差异，揭示了该地块抬升与就位过程中海水渗透程度、流体-岩石相互作用程度，以及与失落城形成相关的热液活动的变化特征。两处辉长岩的锶、钕同位素组成均一（87Sr/86Sr比值为0.70261~0.70429，εNd值为+9.1至+12.1），表明其起源于亏损地幔。在中央穹窿区域，蛇纹石化橄榄岩较为罕见，其呈现出与地壳浅部蛇纹石化作用相关的类海水锶同位素组成；而辉长岩中保留的未蚀变幔源同位素特征，则证明深部环境中海水与岩石的相互作用十分有限，该区域基本未受失落城热液活动的影响。与之相反，南壁的蛇纹岩普遍遭受蚀变，且其锶、钕同位素组成与海水相似（87Sr/86Sr比值为0.70885~0.70918，εNd值为-4.7至+11.3），这表明热液循环过程中流体-岩石比极高（约为20，最高可达10^6）且流体通量显著增强。本研究表明，钕同位素对高流体通量最为敏感，因此可作为量化热液系统中水-岩比的重要地球化学示踪剂。研究结果显示，高流体通量与长期持续的蛇纹石化作用，可能是形成失落城型热液系统的关键条件；而南部区域的正断层作用与块体运动，则促进了维持热液活动所需的海水渗透。