Freitas, M.1*, Magalhães, V. 1,2, Azevedo, M.R. 3, Pinheiro, L. 3,4, Salgueiro, E. 1,5, Abrantes, F. 1,5
1 Instituto Português do Mar e da Atmosfera (IPMA), Av. Doutor Alfredo Magalhães Ramalho, 6, 1495-165 Algés, Portugal 2 Instituto Dom Luiz (IDL, LA), FCUL - Campo Grande Edifício C1, Piso 1, 1749-016 Lisboa, Portugal
3 Departamento de Geociências, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal 4 Centro de Estudos do Ambiente e do Mar (CESAM, LA), Campus Universitário de Santiago, 3810-193 Aveiro, Portugal 5 Centro de Ciências do Mar, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal
* mafalda.freitas@ipma.pt
Resumo: O vulcão de lama serpentinizada Yinazao, localizado no forearc das Marianas, foi amostrado durante a Expedição 366 do programa IODP (Sites U1491 e U1492). Verificou-se a presença de aragonite e calcite predominantemente nos primeiros metros do topo da coluna sedimentar, onde a oxidação e a circulação de água do mar mais se fazem sentir. Os objetivos deste trabalho são entender os processos envolvidos na formação destes carbonatos autigénicos no vulcão de lama Yinazao e inferir sobre a fonte de carbono na formação dos mesmos, comparando estes processos com os da formação dos carbonatos autigénicos nos vulcões de lama sedimentares e pockmarks do Golfo de Cádis, cuja origem é resultante da oxidação anaeróbica de metano. No vulcão de lama serpentinizada Yinazao, os resultados indicam que a principal fonte de carbono na formação dos carbonatos é a água do mar e não o metano. A precipitação destes carbonatos autigénicos será, então, o resultado da reação entre os fluidos com influência da água do mar com os fluidos que sofreram a influência de processos de serpentinização, com elevada alcalinidade, e enriquecidos em Ca e Sr, que ascendem no vulcão de lama Yinazao.
Palavras-chave: Vulcão de Lama Serpentinizada, Carbonatos Autigénicos, Forearc das Marianas, IODP – Expedição 366
Abstract: The Yinazao serpentinite mud volcano, located at the Marianas forearc, was sampled during the IODP Exp. 366 (Sites U1491 and U1492). Authigenic aragonite and calcite were found predominantly within the top meters of the cores where both oxidation and seawater circulation in the sedimentary column are higher. The aims of this work are to understand the processes involved in the formation of these authigenic carbonates within the Yinazao serpentinite mud volcano, infer the major carbon source and compare these processes with those responsible for the precipitation of the carbonates that occur on sedimentary mud volcanoes and pockmarks at Gulf of Cadiz that are methane- derived. At the Yinazao serpentinite mud volcano, the results indicate that the major carbon source for carbonate formation is not methane, but seawater. The precipitation of these authigenic carbonates can be the result of the reaction between the seawater-sourced fluids with the serpentinization sourced fluids, highly alkaline and enriched in Ca and Sr, ascending at the Yinazao mud volcano.
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1. Background
Large seamounts and mud volcanoes occur at the Marianas forearc, an active non-accretionary convergent margin. These geologic structures provide a direct window to the processes occurring at the subduction zone and at the boundary between Pacific and Philippine plates (Fryer et al., 2017). The understanding of such systems is of great importance since they are responsible for frequent and intense seismic activity, submarine slides and slope destabilization, potential triggering natural catastrophic risks with major human impact. Furthermore, marine mud volcanism has a potencial role on climate change through gas release such as methane, besides being a gateway to potencial energy resources such as gas hydrates, oil and gas.
1.1. Yinazao Serpentinite Mud Volcano
During IODP 366 expedition three serpentinite mud volcanoes were sampled from the Marianas forearc (Fig. 1). Authigenic carbonates (aragonite and calcite) were found in serpentinite dominated sediment samples from the Yinazao serpentinite mud volcano (Sites U1491 - flank and U1492 - summit), the closest mud volcano to the Marianas trench.
The top of the cores is composed of pelagic muds with clastic materials, which overlie seawater-altered serpentinite mud breccias, typically oxidized with a yellow- orange color. Below this layer of seawater- altered serpentinite mud breccias the cores gradually pass to non-oxidized serpentinite mud breccias. The major authigenic minerals are aragonite and calcite that are found in the top meters of the cores. Aragonite occurs as single acicular needles or as radiating clusters of acicular crystals, up to several cm in size. Calcite occurs as single rhombohedra or as individual needles up to 3 cm in length.
1.2. Objectives
The aims of the present work are to understand the active processes that promote the precipitation of these authigenic carbonates and infer the major Carbon source in this system.
A model for the formation of Yinazao authigenic carbonates is proposed by comparing them to authigenic carbonates from sedimentary mud volcanoes and pockmarks at Gulf of Cadiz (GoC).
Fig. 1 – Location map of IODP 366 Expedition (Fryer et al., 2017) with a zoom on Site U1491 and
U1492 at Yinazao Mud Volcano (MV).
2. Methodology
A mineralogic and geochemical study of the Yinazao samples containing authigenic carbonates was performed. Samples collected from cores U1491, U1492A and U1492B were sieved and clasts with more than 63 µm in diameter were separated and described. Larger clasts (> 2mm) were powdered, and their mineralogy analysed by X-ray diffraction. Carbon content and Total Organic Carbon content was determined with a LECO elemental analyser. The samples dominated by authigenic carbonates were then analysed for C and O stable isotopes and for Sr isotopic composition (Fig. 2).
3. Results and Discussion
The major authigenic carbonate phases present in these cores are aragonite (flank sourced), calcite (summit sourced) and a mixture of both (summit and flank of the MV). The authigenic calcite and aragonite
samples from Yinazao have constant δ13C
values, ranging from -1.2 up to 3.2‰ VPDB (Fig. 2). These values clearly indicate a non-methane carbon source and most probably indicate that the carbon source for the precipitation of these carbonates is seawater and not methane related.
Oxygen isotope values show a larger compositional variability than carbon isotopes, with δ18O values ranging from 0.5
up to 5.5‰ VPDB. These oxygen isotopic compositional values are close to seawater composition, indicating that the precipitation of the authigenic carbonates occurs from a seawater dominated fluid. The samples with aragonite have very constant 18O compositions with δ18O values
varying from 4.9 up to 5.5‰ VPDB, whilst the samples with calcite are lighter, with δ18O values ranging from 0.5 to 1.9‰
VPDB. On average, the difference between the two carbonate phases is of about 4‰ VPDB, a value that is clearly higher than it would be expected if both phases were precipitated in isotopic equilibrium. Assuming the fractionation equation of Kim and O'Neil (1997) for calcite and the fractionation equation of Bohm et al (2000) for aragonite, the expected difference in the isotopic composition of calcite and aragonite, precipitated in the same conditions and from the same pore-fluid, would be of about 1.4‰ VPDB.
Fig. 2 – Carbon and oxygen isotopic composition of the different authigenic carbonates. All the samples from GoC show typical values of methane derived authigenic carbonates (MDAC), being the result of
anaerobic oxidation of methane (AOM). All the samples from Yinazao Serpentinite MV show carbon isotopic values typical of seawater. Therefore, the authigenic calcite and aragonite were formed from slightly different pore fluids or at different
temperatures. If we assume a temperature range between 4 and 7°C the calcite would have been formed from pore-fluids with compositions ranging from -1 up to 0‰ SMOW (assuming the Kim and O'Neil (1997) fractionation equation). At this same temperature range the aragonite would have to be formed from heavier pore-fluids (2-2.5‰ SMOW).
Fig. 3 – Isotopic 87Sr/86Sr from Yinazao MV carbonate samples.
Isotopic 87Sr/86Sr from Yinazao MV
carbonate samples vary from 0.708791 (flank) to 0.706259 (summit). Samples containing aragonite have a 87Sr/86Sr ration
closer to the value accepted for fluids resultant from serpentinization processes [∼0.705 (Albers et al., 2018)]. In contrast, samples containing calcite have an isotopic signature closer to seawater [0.7092 (Paytan et al., 1993)]. Therefore, the isotopic 87Sr/86Sr is decreasing with depth
along the central conduit.
4. Conclusions
Authigenic aragonite and calcite were predominantly found within the top meters of the cores where both oxidation and seawater circulation in the sedimentary column are higher.
While authigenic carbonates from GoC have δ13C carbonate values as low as -
56.2‰ VPDB, and oxygen isotope values ranging from 0.8 to 6.8‰ VPDB (Magalhães et al., 2012), authigenic calcite and aragonite samples from the Yinazao MV have constant δ13C values, ranging
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87Sr/86Sr values suggest that the
carbonates at the summit of Yinazao mud volcano (calcite) are more influenced by seawater during its formation, while aragonite carbonates, located at the flank, are more affected by the fluids resulting from serpentinization reactions.
The different authigenic phases found indicate distinct formation temperature and/or pore fluids composition, differences clearly reflected on the mineralogy, possibly resulting of distinct setting: flank or summit of the mud volcano.
The results point out that the major carbon source for the authigenic carbonate precipitation is seawater-related (in opposition to GoC authigenic carbonates, which are methane-related). Therefore, the precipitation of the authigenic carbonates at Yinazao serpentinite mud volcano is the outcome of the reaction between seawater sourced fluids with the serpentinization sourced fluids that ascend at the Yinazao mud volcano, highly alkaline and Ca and Sr enriched.
Acknowledgements
This study used samples provided by the Integrated Ocean Drilling Program (IODP). We acknowledge the support from the PES project – Pockmarks and fluid seepage in the Estremadura Spur: implications for regional geology, biology, and petroleum systems (PTDC/GEOFIQ/5162/2014) financed by the Portuguese Foundation for Science and Technology (FCT).
Thanks are also due for the financial support to CESAM (UID/AMB/50017/2019), to FCT/MCTES through national funds; and to Instituto Dom Luiz (UID/GEO/50019/2019).
References
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