Authigenic phases

MacKenzie and Garrels equilibrium models. Most marine clays appear to be detrital and derived from the continents by river or atmospheric transport. Authigenic phases (formed in place) are found in marine sediments (e.g. Michalopoulos and Aller, 1995), however, they are nowhere near abundant enough to satisfy the requirements of the river balance. For example, Kastner (1974) calculated that less than 1% of the Na and 2% of the K transported by rivers is taken up by authigenic feldspars. 

The equations governing the age of secondary carbonate deposits stated above assume that all °Th or Pa present in the mineral is formed in situ by radioactive decay of co-precipitated U. Thorium and Pa content at time of formation can often be considered to be negligible in the pure authigenic phase of calcite or aragonite... 

The assumption that montmorillonite of reported composition is being produced and is the only authigenic phase is probably only an approximation. The composition may be in error or may vary areally within Rainier Mesa as a result of areal variations in water quality. Although no zeolites or other clay minerals were reported in the bulk of the Paintbrush Tuff, very small amounts may remain undetected by either x-ray diffraction analysis or thin-sectlon petrography and may affect the aqueous composition by precipitation or by ion exchange. 

Figure 4 Ankerite (a) localized as overgrowths on grains of detrital dolomite (d). Similar high degrees of substrate control on nucleation are observed for several of the common authigenic phases in late diagenesis. Weber Formation, Pennsylvanian, western Colorado.

Figure 7 Replacement driven by force-of-crystal-Uzation is characterized by authigenic phases that develop euhedral faces that are not plausibly constmed as crystal growth within pore spaces (a) sphalerite replaces albitized detrital feldspar and adjacent portions of clay-rich matrix, Frio Formation, Oligocene, South Texas and (b) siderite crystal (s) attacks a detrital K-feldspar (K) in sandstone, Breathitt Formation, Pennsylvanian, eastern Kentucky.

Alternative constructions of this reaction are conservative with respect to aluminum (Boles and Eranks, 1979b Land et ai, 1987). Such reactions require less input of potassium from minerals other than smectite, but result in a diminution of the total amount of clay by —25% and the generation of excess silicon, iron, and magnesium in amounts (near 15% of the total shale volume) that are not readily sequestered in known authigenic phases in either shales or sandstones (Awwiller, 1993). 

At the scale of thin sections, dissolution, cementation, and replacement are all prima facie evidence of elemental mobility. It is clear that late diagenesis involves complex history of material transfer at the scale of a few millimeters because detrital materials immediately adjacent to cements and grain replacements may show no evidence of dissolution, or contain no elements in common with the authigenic minerals. For minor elements that are concentrated in authigenic phases in amounts far in excess of their concentration in the bulk rock (e.g., barite cement), extraction from some relatively larger rock volume is necessary. 

The fi O-Pj system has also recently been applied to phosphates associated with ferric iron oxyhydroxide precipitates in submarine ocean ridge sediments (Blake et al., 2000, 2001). The gi O-Pj signature of phosphate associated with these authigenic Fe-oxyhydroxide precipitates indicates microbial phosphate turnover at elevated temperatures. The latter observation suggests that phosphate oxygen isotopes may be useful biomarkers for fossil hydrothermal vent systems. On the basis of this work, Blake et al. (2001) also hypothesize that authigenic phases extant on other planets may retain imprints of primitive biospheres, in the form of detectable and diagnostic fi O-Pj composition, imparted by biochemical, enzymatic processes. 

Reimers C. E., Ruttenberg K. C., Canfield D. E., Christiansen M. B., and Martin J. B. (1996) Pore water pH and authigenic phases formed in the uppermost sediments of the Santa Barbara Basin. Beoehim. Cosmoehim. Acta 60(21),... 

Siderite has a highly localized distribution at the thin section scale, rendering temporal assessment of its formation relative to other authigenic phases somewhat problematic. The persistence of small amounts of siderite in all the various combinations... 

The Cretaceous fields are all similar in their framework grain composition and in the authigenic phases present (Table 2). There is one important difference to note, however, in comparing the two Dakota Formation producing fields. At Crooks... 

Ferrous iron oxidation by manganese oxide was found to be especially fast as long as no iron oxyhydroxide precipitates, which presumably blocks reactive sites on the manganese oxide surface (Postma 1985). The oxidation of ferrous iron by manganese oxide has been proven to be important for the interpretation of pore water profiles and the precipitation of authigenic phases (Canfield et al. 1993a Haese et al. 2000 van der Zee 2005). In Fig. 7.16 pore water profiles of iron and manganese reveal concurrent liberation of the... 

One of the main uncertainties related to the application of petroleum inclusions as indicators of earlier reservoir crude oil concerns the possibility that the growing authigenic phase has trapped not only oil but also immature kerogen which may mature in-situ in the progressively buried structure. This material could potentially contribute to the oil inclusion biomarker composition, making it less mature. 

Figure 9. Geochemistry of P with age at Sites 1033 (a) and 1034 (b). The geochemical fractionation was performed via a sequential extraction technique (Ruttenberg 1992, Filippelli and Delaney 1996, Anderson and Delaney 2000), which separates reducible and adsorbed phases (P red), authigenic phases (likely CFA-P auth), organically-bound P (P org) and detrital P (P detr). Note an overall decrease in the reducible and organic fractions and an increase in the authigenic fraction in the older sediments.

Figure 10. Corg Porg ratio (molar) with age at Sites 1033 ( ) and 1034 (A) in the Saanich Inlet. This ratio increases sharply over the first -2,000 years of the record, driven by the redistribution of P to authigenic phases with no net loss from the sediment column.

Figure 11. Percent of total reactive P in this authigenic phases (filled circles) and in the reducible and organic phases (open circles) for Site 1033. This comparison reveals the sink-switching that occurs during sedimentary P cycling, with the ingrowth of an authigenic P phase and a loss of P from other reactive phases with age and depth.

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