Boron has become the principle proxy for the release of seawater-derived fluids into arc volcanics, linked to cross-arc variations in boron content and isotopic ratio. Because all ocean floor ...serpentinites so far analysed are strongly enriched in boron, it is generally assumed that if the uppermost slab mantle is hydrated, it will also be enriched in boron. Here we present the first measurements of boron and boron isotopes in fast-spread oceanic gabbros in the Pacific, showing strong take-up of seawater-derived boron during alteration. We show that in one-pass hydration of the upper mantle, as proposed for bend fault serpentinisation, boron will not reach the hydrated slab mantle. Only prolonged hydrothermal circulation, for example in a long-lived transform fault, can add significant boron to the slab mantle. We conclude that hydrated mantle in subducting slabs will only rarely contribute to boron enrichment in arc volcanics, or to deep mantle recycling.
Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and ...crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.
An expedition to confirm the presence of underwater hazards was carried out in Halmahera waters, Indonesia, to the west of Halmahera Island from August to September 2021. The expedition carried out a ...multibeam survey, surface-towed magnetic survey, and seafloor sampling. A ~ 615-m-tall conical feature with traces of hydrothermal activity was discovered. The feature is bounded on the southeastern (SE) side by a series of normal faults at the peak, with possible dextral strike-slip faults traced west of the feature. The feature displays the potential presence of volcanic rocks based on the observed contrasting magnetic anomaly signature of down to − 100 nT, which at the magnetic equator corresponds to the presence of highly magnetised material. Four 2.5-D magnetic models were built to test various scenarios on the subsurface structure of the feature, mainly focusing on the presence of volcanic rocks at different epochs and a possible presence of serpentinisation. X-ray diffraction (XRD) of the silt and clay sediments sampled confirms traces of late-stage hydrothermal activity, indicated by a high percentage of quartz (53.87%), followed by calcite (34.56%), kaolinite (6.54%), and illite minerals (5.04%). Non-carbonate materials are yet to be found in the sampled sand and gravel sediments, which mainly consist of shell and coral fragments. The discovery of the conical feature, now termed the Yudo Sagoro Hill, provides new information on the structure and activities on the seafloor of Halmahera waters.
In the detachment mode of slow seafloor spreading, convex‐upward detachment faults take up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the ...seafloor. Large, long‐lived hydrothermal systems such as TAG are situated off axis and may be controlled by fluid flow up a detachment fault, with the source of magmatic heat being as deep as 7 kmbsf. The consequences of such deep circulation for the evolution of fluid temperature and salinity have not previously been investigated. Microthermometry on fluid inclusions trapped in diabase, gabbro, and trondjhemite, recovered at the Atlantis Massif Oceanic Core Complex (30°N, Mid‐Atlantic Ridge), reveals evidence for magmatic exsolution, phase separation, and mixing between hydrothermal fluids and previously phase‐separated fluids. Four types of fluid inclusions were identified, ranging in salinity from 1.4 to 35 wt % NaCl, although the most common inclusions have salinities close to seawater (3.4 wt % NaCl). Homogenization temperatures range from 160 to >400°C, with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low‐salinity inclusions trapped in quartz veins. The fluid history of the Atlantis Massif is interpreted in the context of published thermochronometric data from the Massif, and a comparison with the inferred circulation pattern beneath the TAG hydrothermal field, to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by IODP Hole U1309D fluids have been trapped.
Key Points
The TAG model is a good analog for fluid evolution at Atlantis Massif
Fluid inclusion analyses reveal evidence for magmatic exsolution
Fluid inclusion analyses reveal evidence for phase separation
Serpentinite in the Earth system McCaig, Andrew M; Früh-Green, Gretchen L; Kelemen, Peter ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
02/2020, Letnik:
378, Številka:
2165
Journal Article
The shapes and directionality of the oceanic crust at slow-spreading ridges are key to understanding its magmatic or tectonic emplacement. At slow-spreading ridges, magmatic terrain is marked by ...linearly fault-bounded abyssal hills, while a more tectonic emplacement termed detachment terrain is marked by long-lived detachment faults forming Oceanic Core Complexes (OCCs). However, the quantitative description of the magmatic and detachment regimes is still limited. We develop a novel geomorphometric technique to automate terrain classification based on the parameterisation of the shape, directionality, and curvature of the seafloor. The algorithm consists of two steps: (1) characterising the pattern observed in the horizontal axes by computing the horizontal eigenvalues of the slope vectors at each multibeam cells and (2) building a weight matrix derived from the computed slopes. The eccentricity of the horizontal eigenvalues defines the dipping pattern in the horizontal axes, hence the term slope-weighted eccentricity (SWE). The technique is applied through a moving window and is tested at 12.5°–15.5° N on the Mid-Atlantic Ridge (MAR), where the two distinct modes of spreading occur. The application of this novel geomorphometric technique yields results consistent with published qualitative interpretation and the distribution of seismicity observed from the peak amplitudes of the tertiary waves (T-waves) in the study area. Using the established algorithm, we found that 41% of the seafloor in our study area experienced detachment faulting (up to 28% are identified as OCCs), 25% experienced typical magmatic accretion, and a buffer zone termed extended terrain affects 34% of the seafloor, where the morphology shows a transition from detachment to magmatic spreading or vice versa. These findings provide new insights into seafloor classification based on the observed morphology and the potential to automate such mapping at other slow-spreading ridge regions.
•Grid-based algorithm applicable to a shipboard multibeam bathymetry data set.•Automates detachment and magmatic terrain discrimination by exploiting the eigenvalues of the multibeam slope vectors.•Automates oceanic core complexes (OCCs) identification typically formed at slow-spreading ridges.
•Laterally Confined Volcanic Successions record rift-jumps during SDR emplacement.•2 km of subsidence can be generated during SDR emplacement without faulting.•Rift-jumps may occur to establish a ...laterally continuous magmatic spreading centre.
Seaward Dipping Reflectors (SDRs) are a characteristic feature of magma-rich margins, and represent the generation of large volumes of flood basalts at the point of continental breakup. A number of recent studies provide new insights into the emplacement and tilting of SDRs and conclude that the majority of SDRs are contained within new magmatic crust that has a close affinity to oceanic crust. However, the process by which these initial magmatic systems evolve into a fully established spreading centre remains poorly understood. Several characteristic features of magma-rich margins may be explained by the occurrence of rift-jumps during SDR emplacement, yet the cause and prevalence of such rift-jumps remain unknown.
Here we constrain the 3D geometry of the continent–ocean transition in the Orange Basin, offshore South Africa. This allows us to test if, where and why such rift jumps occur. Our results demonstrate an order of along-strike segmentation previously unobserved in these settings. We demonstrate that the SDR belt is disrupted by the occurrence of a volcanic-stratigraphic package, defined as the Laterally Confined Volcanic Succession (LCVS), not previously identified on a rifted margin. We interpret this as a magmatic spreading centre that was abandoned by a subsequent rift-jump. Identification of LCVSs is important for two reasons. First, we argue that the LCVS formed via the same process as SDRs, and hence provides a unique example of SDR geometry prior to their separation onto conjugate plates. Second, as we can map out the 3D geometry of the LCVS and SDRs, we propose that rift-jumps during magma-rich margin formation may be fundamental to the establishment of a laterally continuous incipient spreading centre.
Serpentinite in the Earth system McCaig, Andrew M.; Früh-Green, Gretchen L.; Kelemen, Peter ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
02/2020, Letnik:
378, Številka:
2165
Journal Article
In the detachment mode of slow seafloor spreading, convex-upward detachment faults take up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the ...seafloor. Large, long-lived hydrothermal systems such as TAG are situated off axis and may be controlled by fluid flow up a detachment fault, with the source of magmatic heat being as deep as 7 kmbsf. The consequences of such deep circulation for the evolution of fluid temperature and salinity have not previously been investigated. Microthermometry on fluid inclusions trapped in diabase, gabbro, and trondjhemite, recovered at the Atlantis Massif Oceanic Core Complex (30 degree N, Mid-Atlantic Ridge), reveals evidence for magmatic exsolution, phase separation, and mixing between hydrothermal fluids and previously phase-separated fluids. Four types of fluid inclusions were identified, ranging in salinity from 1.4 to 35 wt % NaCl, although the most common inclusions have salinities close to seawater (3.4 wt % NaCl). Homogenization temperatures range from 160 to >400 degree C, with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low-salinity inclusions trapped in quartz veins. The fluid history of the Atlantis Massif is interpreted in the context of published thermochronometric data from the Massif, and a comparison with the inferred circulation pattern beneath the TAG hydrothermal field, to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by IODP Hole U1309D fluids have been trapped. Key Points * The TAG model is a good analog for fluid evolution at Atlantis Massif * Fluid inclusion analyses reveal evidence for magmatic exsolution * Fluid inclusion analyses reveal evidence for phase separation
A new graphical method is described for assessing the degrees of alteration of orthoclase and albite feldspars to muscovite. The method uses major element data from whole rock samples, and given a ...range of samples of varying degrees of alteration, pathways of the alteration sequences may be determined. The method may be used quantitatively if likely starting compositions are known and aluminium immobility is assumed. Molar proportions of K, Na and Al are plotted in the form of an Na/(Na+K) versus (Na+K)/Al graph. Modelled plots of muscovitisation of an orthoclase/albite system are presented, for comparison with actual data, so that actual compositions may be assessed in terms of feldspar muscovitisation. An example of the application of this technique to muscovitised granitic fault rocks in the external western Alps is presented, illustrating how the method is best used in conjunction with microstructural and field constraints on the fault rock evolution. This yields an evolution pathway for muscovitisation of the feldspars during fault zone deformation, and links specific alteration steps with particular microstructural changes within the fault zone. We also apply this technique to a previously published data set on basement shear zone geochemical changes from the Pyrenees where orthoclase albitisation provides an additional interest. Given that these reactions are fluid-induced, this work provides further information on the effects of fluid flow in such fault zones in terms of basement fault zone rheologies.
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