Origin of atoll dolomites distinguished by geochemistry and crystal chemistry: Kita-daito-jima, northern Philippine Sea

Abstract

Kita-daito-jima is a carbonate island located at the northwestern region of the Philippine Sea (25°55.6′–57.6′N, 131°16.9′–19.8′E). Dolomites extend from the island surface to a depth of 100 m below the ground surface (mbgs). Strontium isotope stratigraphy indicates that subsurface Units C1 (0–49.7 mbgs) and C2 (49.7–103.4 mbgs) were dolomitized at 5.5 Ma and 2.0 Ma, respectively, and that island-surface dolomites are products of dolomitization at 1.6–2.0 Ma. X-ray diffraction analysis indicates that the island-surface and borehole dolomites comprise variable mixtures of four and three dolomite crystal phases, respectively. Each of these phases is distinguished by a different Ca and Mg content. Three textural types can be recognized in the Kita-daito-jima dolomites, fabric-preserving crystalline nonmimetic (CNM), crystalline mimetic (CM), and fabric-preserving microsucrosic (MS). CNM dolomites contain more calcian phases, whereas MS dolomites commonly are richer in more stoichiometric phases. Backscattered electron images indicate that calcian dolomite phases were precipitated earlier than the more stoichiometric dolomite phases and that there is no significant hiatus between the phases, although they are diachronous. Both the island surface and borehole cores dolomites show linear relationships between whole-rock δ 18O composition and Mg contents and between whole-rock trace element concentrations (Sr and Na) and Mg contents. These covariances result from phase mixing, not stoichiometric effects. Deconvolution of whole-rock isotopic and elemental compositions based on the relative abundance of phases reveals that each phase has a distinct chemical and isotopic composition. Oxygen isotopic compositions of the island surface and borehole dolomites suggest that all dolomite phases formed in seawater and that dolomitization primarily occurred during glacio-eustatic sea-level lowstands and cooler ocean temperatures.