English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

The <63-μm fractions of serpentinite muds from two seamounts on the Mariana and Izu-Bonin forearcs were analyzed for mineral composition by X-ray diffraction and for chemical composition by X-ray fluorescence. The silt fraction of the muds consists predominantly of chrysotile, brucite, and ample amorphous constituents. Chlorite and smectite are less abundant components. Of special interest is the occurrence of iowaite, a brucite-like, Cl-bearing mineral with a layered structure. Iowaite was not found in the samples from the summit site of one of the seamounts drilled; however, it is scattered throughout the strata, composing the flanks of both seamounts investigated. No systematic change of the iowaite abundance with depth was observed. The distribution of iowaite is confined to the surface of the flanks of the seamount. Based on the distribution on the mineral and its chemical composition, we suggest that the iowaite formed by oxidation of some of the ferrous iron in brucite contained in the serpentine mud as it contacted abyssal seawater during protrusion onto the seafloor. The resulting positive charge imparted to the brucite was compensated by the uptake of seawater chloride. Consequently, the formation of iowaite is restricted to the seafloor where oxygen and chloride are available for these reactions. The availability of oxygen is considered the limiting factor. We conclude that iowaite formation cannot be a major cause for the low chlorinity of pore fluids inside the seamounts. INTRODUCTION One of the drilling objectives of Leg 125 was to obtain additional information on the composition of the pore fluids circulating through serpentinite seamounts on the Mariana and Izu-Bonin forearcs (Fryer et al., 1990). The pore fluids recovered from cores of Sites 778-780 and 784 (Fig 1.) are characterized by a strong decrease in chlorinity at shallow depths below seafloor (Fryer, Pearce, Stokking, et al., 1990; Mottl, this volume). Aboard the ship the interpretation of this dramatic decrease in chlorinity was that the fluids most likely were derived from dehydration of hydrous minerals at depth (Fryer, Pearce, Stokking, et al., 1990). The dehydration reactions generate relatively fresh water, which flows upward and decreases the chlorinity of the pore waters. The chloride-bearing mineral iowaite was found by X-ray diffraction (XRD) analysis of the mineral content of serpentinite mud samples. The detection of iowaite prompted our interest in addressing the question of whether the formation of iowaite might be a possible alternative cause for the low chlorinity of the interstitial waters recovered from the serpentinite seamounts. METHODS Sample Preparation The silt fraction (<63 μm) was separated from the bulk sample by sieve and sedimentation techniques, eliminating the sandand gravelsized fraction of unaltered parent rock and coarser serpentinite. Thus, only the "matrix" fraction of the serpentinite muds was analyzed. Because the serpentinite muds strongly tended to flocculate, sedimentation methods to separate a finer fraction often failed. The silt fractions of such samples were obtained by using a <63-μm sieve. Careful desalting by dialysis for 24 hr did not stop flocculation completely; however, unlimited dialysis could not be run without removing water-soluble components from the samples; in particular, 'Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992. Proc. ODP, Sci. Results, 125: College Station, TX (Ocean Drilling Program). Institut für Sedimentforschung, Universita^ Heidelberg, Federal Republic of Germany. chloride. Thus, allowance was made for minor salt contents rather than risk the alteration of the chemical composition of the sample. Most of the minerals were identified by XRD of oriented specimens. Nonoriented specimens were prepared by filling a rectangular sample holder of 1-cm volume with the powdered samples. X-ray Diffraction Analysis X-ray diffraction was performed by a Siemens 500 automatic diffractometer using Ni-filtered CuKα-radiation. Scanning speed with routine runs was 17min. Air-dried, oriented samples were scanned from 1° to 40° 2a Samples suspected of containing expandable clays were run again after glycolation and heat treatment. Nonoriented specimens were scanned from 2° to 70° 2θ. Slower scanning speeds over particular ranges were used when the identification of a particular mineral phase demanded higher resolution and precision. Semiquantitative estimates of the abundances of the mineral species were based on the peak areas. The calculated abundances were revised so that the sum of all identified components totaled 100%. When applying this standard procedure to a quantitative estimate of minerals, we must make allowances for large margins of error. Because of the lack of suitable reference samples of known abundances and, above all, because of large differences in the degree of crystallographic order of the different minerals, an exact estimate with a precision of 1% cannot be obtained. This is true in particular for the minerals iowaite and chrysotile. Abundances of iowaite will probably be overestimated. On the other hand, abundances of serpentine minerals might be underestimated. Nevertheless, the calculated abundances are reported even in numbers below 10. This is done to show the differences in composition between the samples rather than to put major significance on the absolute values. If the sample preparation and machine settings are kept constant, differences in abundances among the analyzed samples can be evaluated with sufficient reliability. Mineral Identification Clay minerals were recognized in general by their basal reflections in oriented samples. The various nonclay minerals were detected by their prominent reflections unless the reflections interfered with those of other minerals present. Brucite was recognized by its peaks at

Iowaite in Serpentinite Muds at Sites 778, 779, 780, and 784: A Possible Cause for the Low Chlorinity of Pore Waters