Abstract
Understanding the authigenesis of carbon ate fluorapatite through isotopic geochem istry can yield important information on fundamental geologic processes occurring on continental margins around the world. This is particularly true for phosphatic hardgrounds, which are often found in regions of upwell ing, but of which questions remain about the initial formation and subsequent diagen esis. Here, we apply standard isotopes (δ 13 C, δ 18 O) alongside the novel clumped isotope (Δ47) used in this study for the first time to reconstruct the temperature of formation of carbonate ions within the lattice of sedimen tary carbonate fluorapatite. We investigated phosphatic hardgrounds of Miocene age (12.7–10.8 Ma) sampled at El Capitan State Beach in the Monterey Formation. The range of isotopic signatures observed is between +1.5‰ and +8.0‰ for δ 13 C relative to the Vienna Peedee belemnite (VPDB) standard and –9.5‰ and –6.0‰ VPDB for δ 18 O, and val ues range between 0.599‰ and 0.615‰ for Δ47. The enriched δ 13 C and depleted δ 18 O signa tures are suggestive of recrystallization within the methanogenic zone. Clumped isotope geo chemistry further constrains this transforma tion as having taken place at a temperature of 61–66 °C ± 5 °C, in line with previous esti mates for maximum burial of the Monterey Formation based on the silica phase transi tion. The calculated δ 18 O for the connate fluid shows an expected range for seawater composition for the Miocene, suggesting only minor contribution of silicaderived oxygen to the δ 18 O of carbonate fluorapatite. The combined conventional and clumped isotope data set also points out that methanogenesis took place deeper within the sediment in the middle Miocene than at present day within the Santa Barbara Basin. This study fur thers our understanding of phosphogenesis and potential links to burial processes in the Monterey Formation, and it shows for the first time that the clumped isotope paleother mometer could be used to under stand funda mental geochemical processes in authigenic sedimentary phosphates.
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