Hydroschorlomite in altered basalts from Hole 1256D, ODP Leg 206: The transition from low‐temperature to hydrothermal alteration
Hydroschorlomite in altered basalts from Hole 1256D, ODP Leg 206: The transition from low‐temperature to hydrothermal alteration
复制标题
ODP 第 206 段第 1256D 洞蚀变玄武岩中的水软绵岩：从低温到热液蚀变的转变
DOI：
--
复制
发表时间：
2006
期刊：
影响因子：
--
通讯作者：
M. Bohn
中科院分区：
文献类型：
--
作者：
C. Laverne;O. Grauby;J. Alt;M. Bohn
在大洋钻探计划206航段(赤道东太平洋)1256D孔钻探的玄武岩最深处(661-749 Mbsf)，发现了一种富钛、钙、铁的放射状石榴石，首次在洋壳中发现。利用光学显微镜、电子探针、扫描和透射电子显微镜以及显微拉曼光谱等手段对水生石榴石进行了详细的岩石学和矿物学研究，并对其与其他矿物的关系进行了表征。在1256D井中，水硬铝石呈细小的(5-50μm)无面体或自面体晶体，与青瓷岩脉附近黑色晕中的青铜矿共生，或与皂石和氢氧化铁矿脉相邻的棕色蚀变晕中的棕色皂质层状硅酸盐共生。这两种晕都是在低温(低于约100°C)下形成的。结构观察表明，水钠闪石的形成与层状硅酸盐是同时代的。水硬铝石富CaO(22.5~26.5 wt%)、TiO2(22.0~28.6 wt%)、FeOt(6.2~12.9 wt%)，富含F(高达0.85 wt%)和Zr2O(高达0.34 wt%)。显微拉曼光谱中3557 cm−1处的OH振动证实了氧化物总百分含量较低(95.2-97.3wt%)的羟基的存在。化学填图表明，水硬铝石没有分带，总是与青山质或皂质型层状硅酸盐共生。一些水硬铝石晶体部分包括微小的(<；10μm)骨架钛磁铁矿。1256D孔玄武岩中水硬铝石的产出与斜长石的蚀变和少量绿泥石-蒙皂石混层产出的温度和整体蚀变强度的普遍下降相吻合。形成水长石所需的钛是由原始微小(<；10μm)钛磁铁矿的分解提供的，而钙是由钠长石取代斜长石提供的。因此，随着钛铁矿的形成，在低温蚀变向热液变质过渡的条件下，水硬铝石是钛磁铁矿蚀变的指示剂，并可能影响岩石的磁性。
Hydroschorlomite, a Ti‐, Ca‐, Fe‐rich andraditic garnet present in the deepest cores of basalts (661–749 mbsf) drilled in Hole 1256D during Ocean Drilling Program (ODP) Leg 206 (equatorial east Pacific), is reported here for the first time in oceanic crust. Detailed petrological and mineralogical studies by optical microscope, electron microprobe, scanning and transmission electron microscope, and micro‐Raman spectroscopy are used to characterize this hydrogarnet and its relationships with other minerals. Hydroschorlomite occurs in Hole 1256D as small (5–50 μm) anhedral or euhedral crystals associated either with celadonite in black halos adjacent to celadonite veins or with brown saponitic phyllosilicate in brown alteration halos adjacent to veins of saponite and iron oxyhydroxides. Both types of halos are formed at low temperature (less than about 100°C). Textural observations suggest that hydroschorlomite formation is contemporaneous with the phyllosilicates. Hydroschorlomite is rich in CaO (22.5–26.5 wt%), TiO2 (22.0–28.6 wt%), and FeOt (6.2–12.9 wt%) and contains significant F (up to 0.85 wt%) and Zr2O3 (up to 0.34 wt%). The presence of OH suggested by the low total percentages of oxides (95.2–97.3 wt%) is confirmed by the OH vibration at 3557 cm−1 in the micro‐Raman spectrum. Chemical mapping indicates that hydroschorlomite is not zoned and is always associated with either celadonitic or saponitic phyllosilicates. Some hydroschorlomite crystals partly include tiny (<10 μm) skeletal titanomagnetite. The occurrence of hydroschorlomite in Hole 1256D basalts coincides with a general downward increase in temperatures and overall intensity of alteration manifest by the alteration of plagioclase and the occurrence of small amounts of mixed‐layer chlorite‐smectite. The titanium necessary to form hydroschorlomite is provided by the breakdown of primary tiny (<10 μm) titanomagnetite, while calcium is provided by the replacement of plagioclase by albite. Hydroschorlomite is thus an indicator of alteration of titanomagnetite under conditions transitional from low‐temperature alteration to hydrothermal metamorphism with formation of titanite and may affect magnetic properties of the rocks.