IODP Expedition 357 used two seabed drills to core 17 shallow holes at 9 sites across Atlantis Massif ocean core complex (Mid-Atlantic Ridge 30°N). The goals of this expedition were to investigate ...serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. More than 57 m of core were recovered, with borehole penetration ranging from 1.3 to 16.4 meters below seafloor, and core recovery as high as 75% of total penetration in one borehole. The cores show highly heterogeneous rock types and alteration associated with changes in bulk rock chemistry that reflect multiple phases of magmatism, fluid-rock interaction and mass transfer within the detachment fault zone. Recovered ultramafic rocks are dominated by pervasively serpentinized harzburgite with intervals of serpentinized dunite and minor pyroxenite veins; gabbroic rocks occur as melt impregnations and veins. Dolerite intrusions and basaltic rocks represent the latest magmatic activity. The proportion of mafic rocks is volumetrically less than the amount of mafic rocks recovered previously by drilling the central dome of Atlantis Massif at IODP Site U1309. This suggests a different mode of melt accumulation in the mantle peridotites at the ridge-transform intersection and/or a tectonic transposition of rock types within a complex detachment fault zone. The cores revealed a high degree of serpentinization and metasomatic alteration dominated by talc-amphibole-chlorite overprinting. Metasomatism is most prevalent at contacts between ultramafic and mafic domains (gabbroic and/or doleritic intrusions) and points to channeled fluid flow and silica mobility during exhumation along the detachment fault. The presence of the mafic lenses within the serpentinites and their alteration to mechanically weak talc, serpentine and chlorite may also be critical in the development of the detachment fault zone and may aid in continued unroofing of the upper mantle peridotite/gabbro sequences.
New technologies were also developed for the seabed drills to enable biogeochemical and microbiological characterization of the environment. An in situ sensor package and water sampling system recorded real-time variations in dissolved methane, oxygen, pH, oxidation reduction potential (Eh), and temperature and during drilling and sampled bottom water after drilling. Systematic excursions in these parameters together with elevated hydrogen and methane concentrations in post-drilling fluids provide evidence for active serpentinization at all sites. In addition, chemical tracers were delivered into the drilling fluids for contamination testing, and a borehole plug system was successfully deployed at some sites for future fluid sampling. A major achievement of IODP Expedition 357 was to obtain microbiological samples along a west–east profile, which will provide a better understanding of how microbial communities evolve as ultramafic and mafic rocks are altered and emplaced on the seafloor. Strict sampling handling protocols allowed for very low limits of microbial cell detection, and our results show that the Atlantis Massif subsurface contains a relatively low density of microbial life.
•Seabed rock drills and real-time fluid monitoring for first time in ocean drilling•First time recovery of continuous sequences along oceanic detachment fault zone•Highly heterogeneous rock type and alteration in shallow detachment fault zone•High methane and hydrogen concentrations in Atlantis Massif shallow basement•Oceanic serpentinites potentially provide important niches for microbial life
Abyssal spinel peridotites from Hess Deep, East Pacific Rise (ODP Leg 147) were investigated concerning their major, minor, and trace element mineral chemistry and the incorporation of structural ...water in orthopyroxene.
The rocks are partially serpentinized harzburgites containing primary minerals of olivine, orthopyroxene, clinopyroxene, and spinel. Orthopyroxene is enstatitic with Mg# (Mg/(Mg+Fe)) between 0.90 and 0.92 and Al2O3 from 0.5 to 2.9wt.%. The residual harzburgite experienced high degrees of melt removal in the spinel peridotite stability field. The average degree of partial melting was calculated to be 17.5% (range: 16.4–17.8%). Trace element data of ortho- and clinopyroxenes reflect this strong depletion, characteristic for the restitic nature of abyssal peridotites. Mantle re-equilibration temperatures around 1000°C indicate that, after melt extraction and before exhumation to the ocean floor, the rocks experienced significant cooling in the spinel peridotite facies. Water contents of orthopyroxene range from 86 to 233wt. ppm H2O with an average concentration of 142wt. ppm H2O. These results represent the first data on water contents in the sub-pacific mantle obtained by direct measurements of sub-oceanic peridotite. The water contents are not related to mineral chemistry, stratigraphy, melting degree, mantle equilibrium conditions or oxidation state. Calculated post-melt peridotite water contents vary between 40 and 100wt. ppm H2O.
Compared to Mid-Atlantic Ridge peridotites, the East Pacific Rise samples of Leg 147 contain somewhat lower water concentrations than samples from Leg 153 and considerably higher contents than those of Leg 209 (Gose et al., 2009; Schmädicke et al., 2011). In Leg 147, the strongest OH absorbtion band occurs at 3420cm−1, wheras orthopyroxene from MAR peridotite (Legs 153 and 209) has its strongest absorbtion band at 3566 and 3522cm−1. The mantle equilibrium temperature of Leg 147 peridotites is lower than that of Leg 209 (~1250°C) but close to that of Leg 153 samples (950–1000°C). The high degree of partial melting of Leg 147 peridotite samples overlaps with the Leg 209 samples. In accordance to this data we conclude that in order to obtain relatively high water contents of up to 233wt. ppm in depleted peridotite, after melt removal water must have re-entered the orthopyroxene structure. We suggest that re-equilibration of water contents took place under spinel-facies conditions before exhumation of the tectonite, since elevated temperatures enhance diffusion and elevated pressures facilitate hydrogen uptake. The extended time span between melt removal and uplift inferred from significant isobaric cooling of at least 200K at spinel-facies depth facilitates re-equilibration of water contents and may explain the high concentrations. Exhumation from spinel-facies depth was fast and accompanied by further cooling such that re-equilibration to lower pressure assemblages and decompression-induced water loss were prevented.
•Water contents of orthopyroxene from EPR spinel peridotites range from 86 to 233wt. ppm H2O.•First data for sub-pacific mantle obtained by direct measurements of peridotite.•Water contents not related to mineral chemistry, melting degree, or P-T conditions.•Re-equilibration temperatures are around 1000°C.•Between melt extraction and exhumation rocks cooled in spinel peridotite facies.
Abstract
Serpentinized and metasomatized peridotites intruded by gabbros and dolerites have been drilled on the southern wall of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) during International ...Ocean Discovery Program (IODP) Expedition 357. They occur in seven holes from five sites making up an east–west-trending, spreading-parallel profile that crosscuts this exhumed detachment footwall. Here we have taken advantage of this sampling to study heterogeneities of alteration at scales less than a kilometer. We combine textural and mineralogical observations made on 77 samples with in situ major and trace element analyses in primary and serpentine minerals to provide a conceptual model for the development of alteration heterogeneities at the Atlantis Massif. Textural sequences and mineralogical assemblages reveal a transition between an initial pervasive phase of serpentinization and subsequent serpentinization and metasomatism focused along localized pathways preferentially used by hydrothermal fluids. We propose that these localized pathways are interconnected and form 100 m- to 1 km-sized cells in the detachment footwall. This change in fluid pathway distribution is accompanied by variable trace element enrichments in the serpentine textures: deep, syn-serpentinization fluid–peridotite interactions are considered the source of Cu, As, and Sb enrichments, whereas U and Sr enrichments are interpreted as markers of later, shallower fluid–serpentinized peridotite interaction. Alteration of gabbros and dolerites emplaced in the peridotite at different lithospheric levels leads to the development of amphibole-, chlorite- and/or talc-bearing textures as well as enrichments in light rare earth elements, Nb, Y, Th, and Ta in the serpentine textures of the surrounding peridotites. Combining these observations, we propose a model that places the drill holes in a conceptual frame involving mafic intrusions in the peridotites and heterogeneities during progressive alteration and emplacement on the seafloor.
Aim: The aim of the present study was to investigate the association between childhood family structures, including the presence or absence of problematic parental substance use (PPSU), and adverse ...outcomes during adolescence/young adulthood. Methods: The study population included 9,770 young people (aged 15–25 years) from samples drawn for two national surveys in Denmark during 2014–2015. By combining surveys with national register data, five types of childhood family structures were constructed based on whether the child experienced PPSU and/or family separation and the number of years the child lived with a parent with substance use problems. Using binary logistic regression models, the relationships between family structure and adverse outcomes in young adulthood (i.e., hospital admissions, mental disorders and criminality) were investigated. Results: Young people who experienced PPSU and did not live with both parents had higher odds of the different long-term adverse outcomes compared with young people who did not experience PPSU, and similar odds of the outcomes compared to youth who had not experienced PPSU and did not live with both parents. The highest odds of adverse outcomes were found among young people who experienced PPSU and lived with the parent with substance use problems for less than five years. Conclusions: Living with both parents protected against adverse outcomes in young adulthood, and if PPSU was present, the odds of adverse outcomes increased. The hypothesis that there would be a positive association between years living with a parent with substance use problems and adverse outcomes in young adulthood was not supported. Awareness should be raised in health service, educational and legal institutions about the risk for young people from families with PPSU who do not live with both parents.
•Examined magnitude of parental substance use disorder (PSUD) in general population.•Young people whose parents had PSUD were less likely to complete the survey.•Adjusted the estimation of ...self-reported PSUD for non-participation through the use of register data.•Survey-based self-reports likely underestimate number of youth experiencing PSUD.
To estimate the prevalence of parental substance use disorder (PSUD) in the general population based on young adults’ reports adjusted for non-participation using register-based indicators of PSUD.
A national sample survey study combined with a retrospective register-based study. Setting Denmark. Participants 10,414 young people (aged 15–25 years) invited to two national sample surveys in 2014 and 2015 (5,755 participants and 4,659 non-participants).
A crude prevalence of PSUD was calculated based on participants’ reports. Parental data from medical, mortality, prescription, and treatment registers (from the young adults’ birth until the time of the surveys) were used to estimate a register-based prevalence of PSUD for both participants and non-participants. Differences between participants and non-participants were analysed using bivariate comparisons. Inverse probability weighting was used to adjust for bias due to non-participation. The crude prevalence of PSUD based on survey data was adjusted using the ratio of incidence proportion of the register-based PSUD compared with the survey-based PSUD.
A total of 731 (12.7%) of the 5,755 survey participants reported PSUD. Register-based PSUD was more common among non-participants (856/4,659; 18.4%) compared with participants (738/5,755; 12.8%, OR = 1.53, 95% CI 1.38–1.70). The adjusted estimate of the survey-based PSUD increased by 2.5 percentage points, from 12.7% to 15.2%.
In the absence of register data, youth-reported PSUD is likely to underestimate the number of young people experiencing PSUD.
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