A Technique for Obtaining Pore-Water Chemical Composition from Indurated and Hydrothermally Altered Sediment and Basalt: The Ground Rock Interstitial Normative Determination (Grind)

Abstract

During Ocean Drilling Program Leg 139 at Middle Valley, Juan de Fuca Ridge, we recovered indurated sediment and hydrothermally altered diabases and gabbros from an active hydrothermal reservoir at Sites 857 and 858. Because few of these samples yielded pore water upon squeezing even at the maximum pressure afforded by the hydraulic presses aboard JOIDES Resolution, we developed the GRIND technique (ground rock interstitial normative determination). The GRIND technique consists of fragmenting a freshly collected sample, grinding the sample with distilled water in a ball mill, squeezing the ground mixture, and analyzing the effluent.Concentrations of Mg, Ca, Na, K, Rb, and, from Site 857 only, Sr, measured in GRIND samples are comparable to measurements of squeezed samples, in contrast to alkalinity, sulfate, dissolved silica, Ba, Mn, Li, and B, which differ substantially between the two methods. GRIND samples with chlorinity greater than about 100 mmol/kg have only minimal artifacts caused by ion exchange. Ion exchange in more diluted pore-water samples causes an increase in Na and a decrease in Mg, Ca, and Sr. The GRIND method could be an invaluable tool for estimating the chemical composition of pore waters in basement.

Methods

“…A 5-10 cm long sample was taken from every IW WRC sample and put inside a glove bag, in which the external surfaces were scraped clean. Afterward, the samples were fragmented down to <1 cm in size (Wheat et al, 1994;Expedition 315 Scientists, 2009a). About 40 or 80 g of the sample (depending on the availability of sample weight) was carefully weighed, placed inside an agate ball-mill cylinder together with five agate balls, and 5 or 10 g (for 40 or 80 g samples, respectively) of Milli-Q water or dilute HNO 3 solution with pH adjusted to 3 was added after accurate weighing.…”

“…An alternate method of pore water extraction from sedimentwith porosity <40% was used during Expedition 315. This method was initially developed by Cranston (1991) and later used by Wheat et al (1994) to assess pore fluid composition under conditions where it was impossible to extract a sufficient volume of interstitial water by the standard squeezing method for chemical analysis (Expedition 315 Scientists, 2009a). In this method, an appropriate aliquot of indium standard solution is added to the sediment and ground in a ball mill to dilute the interstitial water.…”

Methods

“…Samples were not collected from Core 348-C0002P-1R due to low core recovery. Squeezed IW and ground rock interstitial normative determination (GRIND) pore water (GW) (Wheat et al, 1994) were sampled for the typical suite of shipboard measurements. In addition, IW was sampled from cores collected during testing of the SD-RCB in Hole C0002M for an experiment examination of the effect of high-pressure squeezing of clay minerals on Clconcentrations and stable isotopes.…”

Development of a New Method of Extraction of Interstitial Water from Low‐Porosity Consolidated Sediments Recovered During Super‐Deep Drilling Projects

“…Sediment samples were ground in a ball mill mixed with distilled water purged with oxygen-free air, and the solution was extracted by the squeezing method described above, but in a helium atmosphere and cooled at the in situ temperature by circulating iced water. Wheat et al (1994) applied this method to consolidated sediments collected during IODP Expedition 315, but did not purge the distilled water. These authors also modified the method by adding indium as a spike to allow calculation of the solute concentration by using the recovery ratio (Expedition 315 Scientists 2009).…”