Hydrothermal alteration of plagioclase microphenocrysts and glass in basalts from the East Pacific Rise near 13°N: An SEM-EDS study

The interactions of seafloor hydrothermal fluid with igneous rocks can result in leaching elements from the rocks,creating potential ore-forming fluids and influencing the chemical compositions of near-bottom seawater.The hydrothermal alteration of plagioclase microphenocrysts and basaltic glass in the pillow basalts from one dredge station(103°57.62′′W,12°50.55′N,water depth 2480 m)on the East Pacific Rise(EPR)near 13°N were analyzed using a scanning electron microscope(SEM)and energy dispersive X-ray spectrometry(EDS).The results show that the edges of the plagioclase microphenocrysts and the basaltic glass fragments are altered but the pyroxene and olivine microphenocrysts in the interior of the pillow basalts appear to be unaffected by the hydrothermal fluids.In addition,our results show that the chemical alteration at the rims of the plagioclase microphenocrysts and the edges of basaltic glass fragments can be divided into separate types of alteration.The chemical difference in hydrothermal alteration of the plagioclase microphenocrysts and the basaltic glass indicate that different degrees of hydrothermal fluid-solid phase interaction have taken place at the surface of the pillow basalts.If the degree of hydrothermal fluid-solid phase interaction is relatively minor,Si,Al,Ca and Na diffuse from the inside of the solid phase out and as a result these elements have a tendency to accumulate in the edge of the plagioclase microphenocrysts or basaltic glass.If the degree of hydrothermal fluid-solid phase interaction is relatively strong,Si,Al,Ca and Na also diffuse from the inside of solid phase out but these elements will have a relatively low concentration in the edge of the plagioclase microphenocrysts or basaltic glass.Based on the chemical variation observed in the edges of plagioclase microphenocrysts and basaltic glass,we estimate that the content of Si,Al and Fe in the edges of plagioclase microphenocrysts can have a variation of 10.69%,17.59%and 109%,respectively.Similarly,the Si,Al and Fe concentrations in the edges of basaltic glass can have a variation of 9.79%,16.30%and 37.83%,respectively,during the interaction of hydrothermal fluids and seafloor pillow basalt.

Hydrothermal alteration of plagioclase microphenocrysts and glass in basalts from the East Pacific Rise near 13°N: An SEM-EDS study

The interactions of seafloor hydrothermal fluid with igneous rocks can result in leaching elements from the rocks, creating potential ore-forming fluids and influencing the chemical compositions of near-bottom seawater. The hydrothermal alteration of plagioclase microphenocrysts and basaltic glass in the pillow basalts from one dredge station (103°57.62″W, 12°50.55′N, water depth 2480 m) on the East Pacific Rise (EPR) near 13°N were analyzed using a scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS). The results show that the edges of the plagioclase microphenocrysts and the basaltic glass fragments are altered but the pyroxene and olivine microphenocrysts in the interior of the pillow basalts appear to be unaffected by the hydrothermal fluids. In addition, our results show that the chemical alteration at the rims of the plagioclase microphenocrysts and the edges of basaltic glass fragments can be divided into separate types of alteration. The chemical difference in hydrothermal alteration of the plagioclase microphenocrysts and the basaltic glass indicate that different degrees of hydrothermal fluid-solid phase interaction have taken place at the surface of the pillow basalts. If the degree of hydrothermal fluid-solid phase interaction is relatively minor, Si, Al, Ca and Na diffuse from the inside of the solid phase out and as a result these elements have a tendency to accumulate in the edge of the plagioclase microphenocrysts or basaltic glass. If the degree of hydrothermal fluid-solid phase interaction is relatively strong, Si, Al, Ca and Na also diffuse from the inside of solid phase out but these elements will have a relatively low concentration in the edge of the plagioclase microphenocrysts or basaltic glass. Based on the chemical variation observed in the edges of plagioclase microphenocrysts and basaltic glass, we estimate that the content of Si, Al and Fe in the edges of plagioclase microphenocrysts can have a variation of 10.69%, 17.59% and 109%, respectively. Similarly, the Si, Al and Fe concentrations in the edges of basaltic glass can have a variation of 9.79%, 16.30% and 37.83%, respectively, during the interaction of hydrothermal fluids and seafloor pillow basalt.