Diffuse spreading, a newly recognized mode of crustal accretion in the southern Mariana Trough backarc basin

South of the latitude of Guam, the Mariana Trough exhibits both trench-parallel and trench-normal extension. In this study, we examined the locus of trench-normal extension separating the Philippine Sea plate from the broadly deforming Mariana platelet. Along this boundary, we identified three distinct modes of extension and described their distinguishing characteristics using deep- and shallow-towed side-scan sonar and ship multibeam data along with regional geophysical, geochemical, and seismicity data. In the west, the Southwest Mariana Rift is an active tectonic rift exhibiting abundant strong earthquakes up to mb 6.7 and limited evidence of volcanism. In the east, the Malaguana-Gadao Ridge is a seafloor spreading center producing few and weak earthquakes less than mb 5. Between these zones, there is an ~20–40-km-wide and ~120-km-long area of high acoustic backscatter characterized by closely spaced volcano- tectonic ridges and small volcanic cones with distributed intermediate-strength seismicity up to mb 5.7. Fresh-looking volcanic rocks with high water contents and strong arc chemical affinities have been recovered from the high-backscatter zone. We interpret this morphologically and geophysically distinct zone as undergoing diffuse spreading, a distributed form of magmatic crustal accretion where new crust forms within a broad zone tens of kilometers across rather than along a narrow spreading axis. Diffuse spreading appears to be a rheological threshold effect enabled by slow opening rates and a high slab-fluid flux that facilitate the formation of a broad zone of weak hydrous lithosphere, within which new crust is accreted. Our findings describe a poorly understood process in plate tectonics, and observations of similar terrains in other backarc basins suggest that this process is not unique to the Mariana Trough.

ESTABLISHMENT OF ERYTHROCYTE SAMPLES FOR IN-AIR MICRO-PIXE ANALYSIS AND ITS APPLICATION TO ERYTHROCYTES IN HEMODIALYSIS PATIENTS

The present study aims to analyze the elemental distribution in erythrocytes from five hemodialysis patients treated with erythropoietin in comparison to four healthy controls. Using in-air micro-PIXE we determined that iron dots were distributed in the peripheral region of control erythrocytes, which were made up of two to four compartments. Iron dots tend to aggregate partially in hemodialysis patients. Calcium was rendered to form small dense nodules in erythrocytes of controls, and small dense nodules became marked in some hemodialysis patients. Nodular formation of phosphorous was weakened in both control and hemodialysis patients. Potassium aggregated focally and formed small dense granules in erythrocytes of healthy controls. On the other hand, hemodialysis patients showed enrichment of potassium, with diffuse spreading partially to all over erythrocytes. These findings indicate that in-air micro-PIXE is a useful tool for analyzing the elemental distributions in erythrocytes of hemodialysis patients.