Abigail Barker | Uppsala University (original) (raw)

Papers by Abigail Barker

Research paper thumbnail of Oxidation of ocean crust and the fate of seawater sulphate in axial hydrothermal systems

Research paper thumbnail of Vertical and Lateral Variations In Hydrothermal Alteration and Fluid Flow of EPR Ocean Crust from Pito Deep

Super fast spreading EPR (ca. 3 Ma) ocean crust has been sampled from two NW-SE striking escarpme... more Super fast spreading EPR (ca. 3 Ma) ocean crust has been sampled from two NW-SE striking escarpments at Pito Deep. The escarpments expose lavas, sheeted dikes and plutonics and provide a unique opportunity to investigate the lateral variations in hydrothermal alteration in the ocean crust. Dense sampling allows investigation of variations on the order of a few to 100's m and the two escarpments extend the sampling area over 35 km. We investigate the trace element and Sr isotopic variations with depth and laterally within the sheeted dikes, which we will integrate with petrographic indications of alteration, in order to consider the hydrothermal alteration and nature of fluid flow. The vertical profile shows a general decrease in 87Sr/86Sr with depth (> 1.2 km) from 0.703826 in the lavas through 0.703131 at the lava-dike transition to 0.702894 at the base of the sheeted dikes. One sample shows anomalously high 87Sr/86Sr (0.703489) for its depth. This sample was collected adjacent to a fault zone of approximately meter scale width, suggesting that the higher 87Sr/86Sr may be due to exchange with fluids that migrated through this fault. This high 87Sr/86Sr perturbation can be traced laterally between dive transects over ca. 640 m. Correlation of high 87Sr/86Sr with fluid mobile elements such as Cs, Rb and U suggest involvement of low temperature alteration. Comparison of Pito Deep with Hess Deep shows that the lowest 87Sr/86Sr from Pito Deep correspond to the highest 87Sr/86Sr from Hess Deep at any given depth. In general Pito Deep has higher 87Sr/86Sr than Hess Deep and ODP Site 504B, suggesting higher fluid flux. Implications of meter to km scale lateral variability sampled at Pito Deep will be discussed.

Research paper thumbnail of Ti-in-quartz thermometry of hydrothermal veins in ocean crust

Research paper thumbnail of Insights into the behaviour of sulphur in mid-ocean ridge axial hydrothermal systems from the composition of the sheeted dyke complex at Pito Deep

Chemical Geology, 2010

The behaviour of seawater sulphate in hydrothermal systems at intermediate- to fast-spreading rid... more The behaviour of seawater sulphate in hydrothermal systems at intermediate- to fast-spreading ridges is investigated using new analyses of the δ34S, sulphur concentration and Fe2O3/Fe2O3total, combined with existing 87Sr/86Sr, of sheeted dykes from the Pito Deep tectonic window. The Pito Deep sheeted dyke complex has a similar composition to the sheeted dykes drilled at ODP Hole 504B suggesting that the measured compositions are representative of sheeted dyke complexes at intermediate- to fast-spreading ridges. The dykes show only small increases in δ34S which, combined with the rock dominated δ34S of vent fluids, requires the majority of seawater sulphate to be precipitated as anhydrite before the fluid reacts with the sheeted dyke complex. This loss of sulphate from the fluid means that a much higher Fe2O3 in the sheeted dyke complex than in fresh MORB glasses cannot be explained by oxidation due to seawater sulphate reduction during fluid–rock reaction. Instead, oxidation probably occurs due to degassing of reduced species, largely H2, during dyke emplacement and solidification. A mass balance model that accounts for anhydrite precipitation and Sr partitioning into the anhydrite, as well as fitting the concentration and isotopic ratios of S and Sr in the sheeted dykes and vent fluids, suggests water/rock ratios of ∼ 1. For a 1 km thick sheeted dyke complex this is equivalent to a fluid flux of ∼ 3 × 106 kg m− 2, sufficient to remove ∼ 60% of the latent heat of crystallization from the lower crust.

Research paper thumbnail of Fault-controlled hydrothermal fluid flow at the EPR

Investigation of an axial mid-ocean ridge fault to determine the character of focused hydrotherma... more Investigation of an axial mid-ocean ridge fault to determine the character of focused hydrothermal fluid flow. Tectonic escarpments at Pito Deep expose ocean crust formed at the super-fast spreading portion of the southern EPR (ca. 3 Ma). Ocean crust from Pito Deep is cut by high angle axial faults striking NE-SW parallel to the sheeted dykes. We focus on trace element and Sr isotopic compositional variations across an approximately 35 m fault zone within the sheeted dyke complex, ca. 100m below the lava-dyke transition, to explore the relationship between fluid flow, alteration and deformation. Samples from across this axial fault can be divided into 1) wall-rock basalts, 2) fault zone basalts and 3) fault zone breccias, ranging from simple cataclasites, veined cataclasites to quartz cataclasites. Breccias and basalts occur closely juxtaposed within the fault zone suggesting heterogeneous, highly localized deformation and fluid flow. Wall-rock basalts have been sampled a few to 30 m away from the fault zone and show extents of alteration typical of dykes away from fault zones with dominant replacement of primary phases by amphibole. The alteration in the fault zone basalts is also amphibole dominated with wider veins than the wall-rocks of quartz, chlorite ± amphibole. The fault zone breccias range in alteration mineralogy from chlorite to quartz dominated assemblages. Wall rock and fault zone basalts have similar trace element compositions to fresh crust. Breccias show large deviations in trace element compositions in comparison to adjacent altered basalts, with depletion of highly mobile elements such as K, Sr and both enrichment and depletion of Cu and Zn. 87Sr/86Sr of the dykes away from the fault zones are slightly elevated over fresh oceanic crust (0.7025- 0.7029) and fault zone samples have similar to higher 87Sr/86Sr. Quartz-epidote veins precipitated from fluids at Pito Deep have 87Sr/86Sr of approximately 0.7040. These new data provide insight into the chemical evolution of axial hydrothermal fluids during focused fluid flow.

Research paper thumbnail of Compositional features of enriched Icelandic mantle components

Geochimica Et Cosmochimica Acta, 2006

Precise in situ analysis of trace-element compositions and isotope ratios has revolutionised geoc... more Precise in situ analysis of trace-element compositions and isotope ratios has revolutionised geochronology and geochemistry over the past decade. This is primarily due to laser-ablation microprobe inductively coupled plasma mass spectrometry (ICP-MS) and the rapid development of the multi-collector (MC-) ICP-MS. The MC-ICP-MS has provided a wider range of isotopic systems (e.g., Li, Mg, Fe, Cu, Hf, and Tl) that can used to constrain the timing and nature of lithospheric processes. The in situ capabilities allow, for the first time, investigation of isotopic variation at the microscopic level and raise questions over the meaning of whole-rock measurements. In situ analysis also allows the isotopic data to be interpreted in a microstructural context and with integration of geochemical data from other microanalytical techniques. Integration of multiple data-sets both constrains the origin of a sample, and can unravel the processes that have subsequently modified it. The in situ isotopic techniques now available not only provide age information, but give new insights on magma genesis, ultimate source rocks and lithospheric tectonic history.

Research paper thumbnail of Direct observation of a fossil high-temperature, fault-hosted, hydrothermal upflow zone in crust formed at the East Pacific Rise

…, 2010

Fault zones in the ocean crust are commonly hypothesized to act as high-permeability conduits tha... more Fault zones in the ocean crust are commonly hypothesized to act as high-permeability conduits that focus fl uid fl ow in oceanic hydrothermal systems. However, there has been little direct study of faults in crust formed at fast-spreading ridges. Here we describe the geology and geochemistry of an ~40-m-wide fault zone within the uppermost sheeted dike complex exposed at Pito Deep (northeastern Easter microplate). Titanium in quartz thermometry gives temperatures of 392 ± 33 °C for quartz precipitation, indicating that this fault zone focused upwelling fl uids at temperatures similar to those of black-smoker vent fl uids. Correlated enrichment in 87 Sr/ 86 Sr and MgO in fault breccias, along with 87 Sr/ 86 Sr ratios higher than in average vent fl uids, provide evidence for mixing between high-temperature upwelling fl uids and a seawater-like fl uid within the fault zone. Large high-temperature fl uid fl uxes are required to maintain high temperatures during mixing. If this fault zone is representative of upfl ow zones beneath hydrothermal vents on the East Pacifi c Rise, then it is possible that vent fl uids evolve thermally and chemically during their ascent and may not record the precise conditions at the base of the hydrothermal system.

Research paper thumbnail of Compositional Characteristics and Spatial Distribution of Enriched Icelandic Mantle Components

Journal of …, 2010

We present compositional data on a suite of 18 primitive neovolcanic alkali basalts from three fl... more We present compositional data on a suite of 18 primitive neovolcanic alkali basalts from three flank-zone regions in Iceland (Vestmannaeyjar in the south, Sn×fell in the east, and Sn×fellsnes in the west) that are peripheral to the main rift zones that are dominated by tholeiitic basalts. This study integrates He isotope data with radiogenic isotope data (Sr^Nd^Pb^Hf), stable isotope data (d 18 O), and trace element data to characterize the compositional features of the trace element enriched components of the Icelandic mantle. We also present high-precision Pb isotope data on an additional 57 lava samples from the flank zones including O « r×fajo« kull in the SE) and the Northern and Eastern rift zones. Most Icelandic lavas have negative Á 207 Pb (^4 to^1), with higher values (^1 to þ4) found only in samples from O « r×fajo« kull, Sn×fell, and parts of the Reykjanes Peninsula. At Sn×fell, this EM1-type component is characterized by a low d 18 O olivine signature (þ4·1ø to þ4·6ø), moderate 206 Pb/ 204 Pb values (18·4^18·6) and mid-ocean ridge basalt (MORB)-like 3 He/ 4 He (6·9^7·5 R/R A ). Samples from Vestmannaeyjar and Sn×fellsnes have mantle-like d 18 O olivine (þ4·9ø to þ5·0ø), and radiogenic 206 Pb/ 204 Pb values (18·9^19·3) that fall on the Northern Hemisphere Reference Line for 208 Pb/ 204 Pb (Á 208 Pb^5 to þ5). Compared with the Vestmannaeyjar lavas, Sn×fellsnes lavas have higher La/Yb N , lower e Nd (5·5^6·5 vs 6·8^7·6) and lower 3 He/ 4 He (6·3^8·6 R/R A vs 11·4^13·5 R/R A ). Therefore, the most trace element enriched components in the Icelandic mantle are not the carriers of the high 3 He/ 4 He values (4 15 R/R A ) found in some lavas on Iceland and the adjacent ridges, and instead are consistent with degassed, recycled components. Even after excluding the EM1-type high Á 207 Pb samples, high-precision Pb isotope data produce a kinked array on a 206 Pb/ 204 Pb vs 208 Pb/ 204 Pb plot, which

Research paper thumbnail of Strontium isotope constraints on fluid flow in the sheeted dike complex of fast spreading crust: Pervasive fluid flow at Pito Deep

Geochemistry Geophysics Geosystems, 2008

Strontium isotopes are useful tracers of fluid-rock interaction in marine hydrothermal systems an... more Strontium isotopes are useful tracers of fluid-rock interaction in marine hydrothermal systems and provide a potential way to quantify the amount of seawater that passes through these systems. We have determined the whole-rock Sr-isotopic compositions of a section of upper oceanic crust that formed at the fast-spreading East Pacific Rise, now exposed at Hess Deep. This dataset provides the first detailed comparison for the much-studied Ocean Drilling Program (ODP) drill core from Site 504B. Whole-rock and mineral Sr concentrations indicate that Sr-exchange between hydrothermal fluids and the oceanic crust is complex, being dependent on the mineralogical reactions occurring; in particular, epidote formation takes up Sr from the fluid increasing the 87 Sr/ 86 Sr of the bulk-rock. Calculating the fluid-flux required to shift the Sr-isotopic composition of the Hess Deep sheeted-dike complex, using the approach of Bickle and Teagle [1] [M.J. Bickle, D.A.H. Teagle, Strontium alteration in the Troodos ophiolite: implications for fluid fluxes and geochemical transport in mid-ocean ridge hydrothermal systems. Earth Planet. Sci. Lett. 113 (1992) 219-237] gives a fluid-flux similar to that determined for ODP Hole 504B. This suggests that the level of isotopic exchange observed in these two regions is probably typical for modern oceanic crust. Unfortunately, uncertainties in the modeling approach do not allow us to determine a fluid-flux that is directly comparable to fluxes calculated by other methods. D

Research paper thumbnail of The Brava seamount, Cape Verde: Beyond the spatial extent of EM1 and petrogenesis of highly evolved alkaline lavas

Alkaline lavas from the Brava seamount, Cape Verde are investigated to establish the spatial dist... more Alkaline lavas from the Brava seamount, Cape Verde are investigated to establish the spatial distribution of compositional heterogeneity in the southwest of the Cape Verde archipelago. Highly evolved lavas provide a record of shallow level magma-crust interaction beneath the Brava seamount. The Brava seamount, located southwest of the island of Brava, Cape Verde was sampled during research cruise 8/85 of the R.R.S. Charles Darwin in 1985. Two groups of highly evolved alkaline volcanics are distinguished from the Brava seamount: 1) pyroxene-phonolites containing clinopyroxene, amphibole, nepheline, ±biotite, and minor sanidine and 2) feldspathoid-phonolites containing nepheline, nausean, minor biotite and leucite. All of the samples have MgO between 0.8 and 2 wt%, comparable to the most evolved volcanics sampled in the Cape Verde archipelago. The feldspathoid-phonolites have NaO2 of 12-13 wt%. Alkaline lavas from the Brava seamount have higher 87Sr/87Sr (0.70337 to 0.70347) at ɛNd of +6 to +7 than previously sampled in Cape Verde. Sr isotopes will be integrated with oxygen isotopes to establish magma and crust interactions in the magmatic plumbing system beneath the Brava seamount. Clinopyroxene-melt thermobarometry will be presented to constrain the depths of equilibrium crystallisation. Sr-O isotopes and thermobarometry will be combined to build a picture of the levels of magma stalling and interaction between magmas and the crust beneath the Brava seamount. The Brava seamount phonolitic lavas have high 206Pb/204Pb of 19.5 to 19.8 with negative ?8/4 and high ɛNd of +6 to +7 in contrast to the positive ?8/4 for lavas from nearby Brava and the southern islands of the Cape Verde archipelago. Lavas from the Brava seamount have Pb-Nd isotope systematics comparable to the northern Cape Verde islands, indicating the southwestern boundary in mantle heterogeneity and thereby the spatial extent of the EM1-like source contributing to the southern islands. The extensive crystallisation and stalling of magma batches at crustal depths shown by thermobarometry will be used in conjunction with geochemistry to constrain the origin of assimilants and implies that an EM1-like source is not found in the mantle source, the shallow lithosphere or crust beneath the Brava seamount.

Research paper thumbnail of A 5 million year record of compositional variations in mantle sources to magmatism on Santiago, southern Cape Verde archipelago

Contributions to Mineralogy …, 2010

High-precision Pb isotope data and Sr-Nd-Hf isotope data are presented together with major and tr... more High-precision Pb isotope data and Sr-Nd-Hf isotope data are presented together with major and trace element data for samples spanning the 4.6 Ma history of volcanism at Santiago, in the southern Cape Verde islands. Pb isotope data confirm the positive Δ8/4 signature of the southern islands indicating that the north-south compositional heterogeneity in the Cape Verde archipelago has persisted for at least 4.6 Ma. The Santiago volcanics show distinct compositional differences between the old, intermediate and young volcanics, and suggest greater involvement of an enriched mantle (EM1)-like source over time. Isotopic variations in the Santiago volcanics indicate convergence towards a homogeneous EM1-like end-member and distinct temporal variations in the FOZO-like end-member. Santiago and Santo Antão (a northern island, Holm et al. 2006), show a simultaneous decrease in 208Pb/204Pb of the high 206Pb/204Pb FOZO-like source with time. Such systematic archipelago-wide variations in the FOZO-like component suggest that this component is more likely to be present as a coherent package of recycled ocean crust rather than as multiple small heterogeneities dispersed in the upwelling mantle. The temporal variations in 208Pb/204Pb reflect minor lateral variations in Th/U of this recycled ocean crust package entering the melting zone beneath the islands. The location of the EM1-like component is more equivocal. A shallow lithospheric location is possible, but this would require a coincidence between spatial compositional variations in the lithosphere (EM1 is spatially restricted to the southern islands) and flow lines in the upwelling mantle revealed by seismic anisotropy. Therefore, we favour a deeper asthenospheric mantle source for the EM1-like source.

Research paper thumbnail of Geochemical Stratigraphy of Submarine Lavas (3-5 Ma) from the Flamengos Valley, Santiago, Southern Cape Verde Islands

Journal of Petrology, 2009

New high-precision Pb^Sr^Nd isotope, major and trace element and mineral chemistry data are prese... more New high-precision Pb^Sr^Nd isotope, major and trace element and mineral chemistry data are presented for the submarine stage of ocean island volcanism on Santiago, one of the southern islands of the Cape Verde archipelago. Pillow basalts and hyaloclastites in the Flamengos Valley are divided into three petrographic and compositional groups; the Flamengos Formation lavas ($4Á6 Ma) dominate the sequence, with the younger Low Si and Coastal groups ($2Á8 Ma) found near the shoreline. Olivine and clinopyroxene compositions and isotopic data for minerals and their host melts indicate disequilibrium between some crystals and the melt. Intra-sample disequilibrium suggests homogenisation of liquids but eruption before complete equilibration between crystals and melt preserves the heterogeneity. Pressures of crystallization for clinopyroxene (0Á4^1Á1 GPa) indicate stalling and crystallization of the magmas over a range of depths in the lithosphere. Major element compositions indicate melting of a carbonated eclogite source. Sr^Nd^Pb isotope data suggest the involvement of FOZO-like and EM1-like components in the mantle source, which are simultaneously available at all depths in the melting column. The Flamengos Valley lavas display large compositional variations, often between stratigraphically adjacent flows; these frequent abrupt changes of magma composition suggest stalling and crystallization of discrete magma batches on transport through the lithosphere.

Research paper thumbnail of Temporal variations in crustal assimilation of magma suites in the East Greenland flood basalt province: Tracking the evolution of magmatic plumbing systems

Lithos, 2008

We review published radiogenic isotope data (> 350 samples in total) on various suites of magmati... more We review published radiogenic isotope data (> 350 samples in total) on various suites of magmatic rocks within the Palaeogene central East Greenland flood basalt province to evaluate the types of crustal assimilants and the extent of crustal assimilation involved in each suite. We use these observations to build a regional picture of how magmatic plumbing systems changed with time and location during the sequential development of the province as magmatism responded to the development of a volcanic rifted margin and eventual plate separation. The earliest phase of magmatic activity (c. 62–57 Ma) is characterised by highly contaminated magmas that show a temporal change in assimilant type from amphibolite to granulite. This transition has been linked to the effects of an increasing magma supply rate which allows the more refractory granulite lithologies to be melted. The voluminous break-up phase of magmatism (c. 56–54 Ma) saw a significant decrease in the extent of assimilation because of the decreasing availability of assimilant material in the mature feeder systems, and many samples have Sr–Nd–Pb isotope compositions that overlap with those of asthenospheric melts (as represented by recent Icelandic basalts and North Atlantic MORB). Detailed study has allowed us to recognise packets of lavas that ponded at different levels in the crust and assimilated material of different compositions. The later stages of break-up magmatism show more diverse and more contaminated compositions that indicate a shift from a few large robust feeder systems to numerous small new conduits as the rifting continued. The post-break-up magmatism (c. 54–13 Ma) is characterised by a return to more highly contaminated magmas, which reflects a change in the style of magmatism: the eruption of small-volume alkalic lava flows from newly established conduits through the thicker inland crust, and the intrusion of mafic and silicic alkalic magmas at shallow levels in the Archaean basement along the present coast.

Research paper thumbnail of Interaction of the rifting East Greenland margin with a zoned ancestral Iceland plume

Geology, 2006

Abstract Neodymium and high-precision lead isotopic data are presented for Paleogene East Greenla... more Abstract Neodymium and high-precision lead isotopic data are presented for Paleogene East Greenland flood basalts that erupted during an early phase of magmatic activity associated with the Iceland hotspot. The 6-km-thick volcanic sequence shows marked chemostratigraphic variations: lavas in the lower half of the sequence (Milne Land and Geikie Plateau Formations) have low 206 Pb/204 Pb values (17.8–18.4), abruptly changing to high 206 Pb/204 Pb values (18.8–19.3) in the overlying Rømer Fjord Formation, ...

Research paper thumbnail of An 40Ar39Ar study of the Cape Verde hot spot: Temporal evolution in a semistationary plate environment

Journal of Geophysical …, 2008

Research paper thumbnail of Oxidation of ocean crust and the fate of seawater sulphate in axial hydrothermal systems

Research paper thumbnail of Vertical and Lateral Variations In Hydrothermal Alteration and Fluid Flow of EPR Ocean Crust from Pito Deep

Super fast spreading EPR (ca. 3 Ma) ocean crust has been sampled from two NW-SE striking escarpme... more Super fast spreading EPR (ca. 3 Ma) ocean crust has been sampled from two NW-SE striking escarpments at Pito Deep. The escarpments expose lavas, sheeted dikes and plutonics and provide a unique opportunity to investigate the lateral variations in hydrothermal alteration in the ocean crust. Dense sampling allows investigation of variations on the order of a few to 100's m and the two escarpments extend the sampling area over 35 km. We investigate the trace element and Sr isotopic variations with depth and laterally within the sheeted dikes, which we will integrate with petrographic indications of alteration, in order to consider the hydrothermal alteration and nature of fluid flow. The vertical profile shows a general decrease in 87Sr/86Sr with depth (> 1.2 km) from 0.703826 in the lavas through 0.703131 at the lava-dike transition to 0.702894 at the base of the sheeted dikes. One sample shows anomalously high 87Sr/86Sr (0.703489) for its depth. This sample was collected adjacent to a fault zone of approximately meter scale width, suggesting that the higher 87Sr/86Sr may be due to exchange with fluids that migrated through this fault. This high 87Sr/86Sr perturbation can be traced laterally between dive transects over ca. 640 m. Correlation of high 87Sr/86Sr with fluid mobile elements such as Cs, Rb and U suggest involvement of low temperature alteration. Comparison of Pito Deep with Hess Deep shows that the lowest 87Sr/86Sr from Pito Deep correspond to the highest 87Sr/86Sr from Hess Deep at any given depth. In general Pito Deep has higher 87Sr/86Sr than Hess Deep and ODP Site 504B, suggesting higher fluid flux. Implications of meter to km scale lateral variability sampled at Pito Deep will be discussed.

Research paper thumbnail of Ti-in-quartz thermometry of hydrothermal veins in ocean crust

Research paper thumbnail of Insights into the behaviour of sulphur in mid-ocean ridge axial hydrothermal systems from the composition of the sheeted dyke complex at Pito Deep

Chemical Geology, 2010

The behaviour of seawater sulphate in hydrothermal systems at intermediate- to fast-spreading rid... more The behaviour of seawater sulphate in hydrothermal systems at intermediate- to fast-spreading ridges is investigated using new analyses of the δ34S, sulphur concentration and Fe2O3/Fe2O3total, combined with existing 87Sr/86Sr, of sheeted dykes from the Pito Deep tectonic window. The Pito Deep sheeted dyke complex has a similar composition to the sheeted dykes drilled at ODP Hole 504B suggesting that the measured compositions are representative of sheeted dyke complexes at intermediate- to fast-spreading ridges. The dykes show only small increases in δ34S which, combined with the rock dominated δ34S of vent fluids, requires the majority of seawater sulphate to be precipitated as anhydrite before the fluid reacts with the sheeted dyke complex. This loss of sulphate from the fluid means that a much higher Fe2O3 in the sheeted dyke complex than in fresh MORB glasses cannot be explained by oxidation due to seawater sulphate reduction during fluid–rock reaction. Instead, oxidation probably occurs due to degassing of reduced species, largely H2, during dyke emplacement and solidification. A mass balance model that accounts for anhydrite precipitation and Sr partitioning into the anhydrite, as well as fitting the concentration and isotopic ratios of S and Sr in the sheeted dykes and vent fluids, suggests water/rock ratios of ∼ 1. For a 1 km thick sheeted dyke complex this is equivalent to a fluid flux of ∼ 3 × 106 kg m− 2, sufficient to remove ∼ 60% of the latent heat of crystallization from the lower crust.

Research paper thumbnail of Fault-controlled hydrothermal fluid flow at the EPR

Investigation of an axial mid-ocean ridge fault to determine the character of focused hydrotherma... more Investigation of an axial mid-ocean ridge fault to determine the character of focused hydrothermal fluid flow. Tectonic escarpments at Pito Deep expose ocean crust formed at the super-fast spreading portion of the southern EPR (ca. 3 Ma). Ocean crust from Pito Deep is cut by high angle axial faults striking NE-SW parallel to the sheeted dykes. We focus on trace element and Sr isotopic compositional variations across an approximately 35 m fault zone within the sheeted dyke complex, ca. 100m below the lava-dyke transition, to explore the relationship between fluid flow, alteration and deformation. Samples from across this axial fault can be divided into 1) wall-rock basalts, 2) fault zone basalts and 3) fault zone breccias, ranging from simple cataclasites, veined cataclasites to quartz cataclasites. Breccias and basalts occur closely juxtaposed within the fault zone suggesting heterogeneous, highly localized deformation and fluid flow. Wall-rock basalts have been sampled a few to 30 m away from the fault zone and show extents of alteration typical of dykes away from fault zones with dominant replacement of primary phases by amphibole. The alteration in the fault zone basalts is also amphibole dominated with wider veins than the wall-rocks of quartz, chlorite ± amphibole. The fault zone breccias range in alteration mineralogy from chlorite to quartz dominated assemblages. Wall rock and fault zone basalts have similar trace element compositions to fresh crust. Breccias show large deviations in trace element compositions in comparison to adjacent altered basalts, with depletion of highly mobile elements such as K, Sr and both enrichment and depletion of Cu and Zn. 87Sr/86Sr of the dykes away from the fault zones are slightly elevated over fresh oceanic crust (0.7025- 0.7029) and fault zone samples have similar to higher 87Sr/86Sr. Quartz-epidote veins precipitated from fluids at Pito Deep have 87Sr/86Sr of approximately 0.7040. These new data provide insight into the chemical evolution of axial hydrothermal fluids during focused fluid flow.

Research paper thumbnail of Compositional features of enriched Icelandic mantle components

Geochimica Et Cosmochimica Acta, 2006

Precise in situ analysis of trace-element compositions and isotope ratios has revolutionised geoc... more Precise in situ analysis of trace-element compositions and isotope ratios has revolutionised geochronology and geochemistry over the past decade. This is primarily due to laser-ablation microprobe inductively coupled plasma mass spectrometry (ICP-MS) and the rapid development of the multi-collector (MC-) ICP-MS. The MC-ICP-MS has provided a wider range of isotopic systems (e.g., Li, Mg, Fe, Cu, Hf, and Tl) that can used to constrain the timing and nature of lithospheric processes. The in situ capabilities allow, for the first time, investigation of isotopic variation at the microscopic level and raise questions over the meaning of whole-rock measurements. In situ analysis also allows the isotopic data to be interpreted in a microstructural context and with integration of geochemical data from other microanalytical techniques. Integration of multiple data-sets both constrains the origin of a sample, and can unravel the processes that have subsequently modified it. The in situ isotopic techniques now available not only provide age information, but give new insights on magma genesis, ultimate source rocks and lithospheric tectonic history.

Research paper thumbnail of Direct observation of a fossil high-temperature, fault-hosted, hydrothermal upflow zone in crust formed at the East Pacific Rise

…, 2010

Fault zones in the ocean crust are commonly hypothesized to act as high-permeability conduits tha... more Fault zones in the ocean crust are commonly hypothesized to act as high-permeability conduits that focus fl uid fl ow in oceanic hydrothermal systems. However, there has been little direct study of faults in crust formed at fast-spreading ridges. Here we describe the geology and geochemistry of an ~40-m-wide fault zone within the uppermost sheeted dike complex exposed at Pito Deep (northeastern Easter microplate). Titanium in quartz thermometry gives temperatures of 392 ± 33 °C for quartz precipitation, indicating that this fault zone focused upwelling fl uids at temperatures similar to those of black-smoker vent fl uids. Correlated enrichment in 87 Sr/ 86 Sr and MgO in fault breccias, along with 87 Sr/ 86 Sr ratios higher than in average vent fl uids, provide evidence for mixing between high-temperature upwelling fl uids and a seawater-like fl uid within the fault zone. Large high-temperature fl uid fl uxes are required to maintain high temperatures during mixing. If this fault zone is representative of upfl ow zones beneath hydrothermal vents on the East Pacifi c Rise, then it is possible that vent fl uids evolve thermally and chemically during their ascent and may not record the precise conditions at the base of the hydrothermal system.

Research paper thumbnail of Compositional Characteristics and Spatial Distribution of Enriched Icelandic Mantle Components

Journal of …, 2010

We present compositional data on a suite of 18 primitive neovolcanic alkali basalts from three fl... more We present compositional data on a suite of 18 primitive neovolcanic alkali basalts from three flank-zone regions in Iceland (Vestmannaeyjar in the south, Sn×fell in the east, and Sn×fellsnes in the west) that are peripheral to the main rift zones that are dominated by tholeiitic basalts. This study integrates He isotope data with radiogenic isotope data (Sr^Nd^Pb^Hf), stable isotope data (d 18 O), and trace element data to characterize the compositional features of the trace element enriched components of the Icelandic mantle. We also present high-precision Pb isotope data on an additional 57 lava samples from the flank zones including O « r×fajo« kull in the SE) and the Northern and Eastern rift zones. Most Icelandic lavas have negative Á 207 Pb (^4 to^1), with higher values (^1 to þ4) found only in samples from O « r×fajo« kull, Sn×fell, and parts of the Reykjanes Peninsula. At Sn×fell, this EM1-type component is characterized by a low d 18 O olivine signature (þ4·1ø to þ4·6ø), moderate 206 Pb/ 204 Pb values (18·4^18·6) and mid-ocean ridge basalt (MORB)-like 3 He/ 4 He (6·9^7·5 R/R A ). Samples from Vestmannaeyjar and Sn×fellsnes have mantle-like d 18 O olivine (þ4·9ø to þ5·0ø), and radiogenic 206 Pb/ 204 Pb values (18·9^19·3) that fall on the Northern Hemisphere Reference Line for 208 Pb/ 204 Pb (Á 208 Pb^5 to þ5). Compared with the Vestmannaeyjar lavas, Sn×fellsnes lavas have higher La/Yb N , lower e Nd (5·5^6·5 vs 6·8^7·6) and lower 3 He/ 4 He (6·3^8·6 R/R A vs 11·4^13·5 R/R A ). Therefore, the most trace element enriched components in the Icelandic mantle are not the carriers of the high 3 He/ 4 He values (4 15 R/R A ) found in some lavas on Iceland and the adjacent ridges, and instead are consistent with degassed, recycled components. Even after excluding the EM1-type high Á 207 Pb samples, high-precision Pb isotope data produce a kinked array on a 206 Pb/ 204 Pb vs 208 Pb/ 204 Pb plot, which

Research paper thumbnail of Strontium isotope constraints on fluid flow in the sheeted dike complex of fast spreading crust: Pervasive fluid flow at Pito Deep

Geochemistry Geophysics Geosystems, 2008

Strontium isotopes are useful tracers of fluid-rock interaction in marine hydrothermal systems an... more Strontium isotopes are useful tracers of fluid-rock interaction in marine hydrothermal systems and provide a potential way to quantify the amount of seawater that passes through these systems. We have determined the whole-rock Sr-isotopic compositions of a section of upper oceanic crust that formed at the fast-spreading East Pacific Rise, now exposed at Hess Deep. This dataset provides the first detailed comparison for the much-studied Ocean Drilling Program (ODP) drill core from Site 504B. Whole-rock and mineral Sr concentrations indicate that Sr-exchange between hydrothermal fluids and the oceanic crust is complex, being dependent on the mineralogical reactions occurring; in particular, epidote formation takes up Sr from the fluid increasing the 87 Sr/ 86 Sr of the bulk-rock. Calculating the fluid-flux required to shift the Sr-isotopic composition of the Hess Deep sheeted-dike complex, using the approach of Bickle and Teagle [1] [M.J. Bickle, D.A.H. Teagle, Strontium alteration in the Troodos ophiolite: implications for fluid fluxes and geochemical transport in mid-ocean ridge hydrothermal systems. Earth Planet. Sci. Lett. 113 (1992) 219-237] gives a fluid-flux similar to that determined for ODP Hole 504B. This suggests that the level of isotopic exchange observed in these two regions is probably typical for modern oceanic crust. Unfortunately, uncertainties in the modeling approach do not allow us to determine a fluid-flux that is directly comparable to fluxes calculated by other methods. D

Research paper thumbnail of The Brava seamount, Cape Verde: Beyond the spatial extent of EM1 and petrogenesis of highly evolved alkaline lavas

Alkaline lavas from the Brava seamount, Cape Verde are investigated to establish the spatial dist... more Alkaline lavas from the Brava seamount, Cape Verde are investigated to establish the spatial distribution of compositional heterogeneity in the southwest of the Cape Verde archipelago. Highly evolved lavas provide a record of shallow level magma-crust interaction beneath the Brava seamount. The Brava seamount, located southwest of the island of Brava, Cape Verde was sampled during research cruise 8/85 of the R.R.S. Charles Darwin in 1985. Two groups of highly evolved alkaline volcanics are distinguished from the Brava seamount: 1) pyroxene-phonolites containing clinopyroxene, amphibole, nepheline, ±biotite, and minor sanidine and 2) feldspathoid-phonolites containing nepheline, nausean, minor biotite and leucite. All of the samples have MgO between 0.8 and 2 wt%, comparable to the most evolved volcanics sampled in the Cape Verde archipelago. The feldspathoid-phonolites have NaO2 of 12-13 wt%. Alkaline lavas from the Brava seamount have higher 87Sr/87Sr (0.70337 to 0.70347) at ɛNd of +6 to +7 than previously sampled in Cape Verde. Sr isotopes will be integrated with oxygen isotopes to establish magma and crust interactions in the magmatic plumbing system beneath the Brava seamount. Clinopyroxene-melt thermobarometry will be presented to constrain the depths of equilibrium crystallisation. Sr-O isotopes and thermobarometry will be combined to build a picture of the levels of magma stalling and interaction between magmas and the crust beneath the Brava seamount. The Brava seamount phonolitic lavas have high 206Pb/204Pb of 19.5 to 19.8 with negative ?8/4 and high ɛNd of +6 to +7 in contrast to the positive ?8/4 for lavas from nearby Brava and the southern islands of the Cape Verde archipelago. Lavas from the Brava seamount have Pb-Nd isotope systematics comparable to the northern Cape Verde islands, indicating the southwestern boundary in mantle heterogeneity and thereby the spatial extent of the EM1-like source contributing to the southern islands. The extensive crystallisation and stalling of magma batches at crustal depths shown by thermobarometry will be used in conjunction with geochemistry to constrain the origin of assimilants and implies that an EM1-like source is not found in the mantle source, the shallow lithosphere or crust beneath the Brava seamount.

Research paper thumbnail of A 5 million year record of compositional variations in mantle sources to magmatism on Santiago, southern Cape Verde archipelago

Contributions to Mineralogy …, 2010

High-precision Pb isotope data and Sr-Nd-Hf isotope data are presented together with major and tr... more High-precision Pb isotope data and Sr-Nd-Hf isotope data are presented together with major and trace element data for samples spanning the 4.6 Ma history of volcanism at Santiago, in the southern Cape Verde islands. Pb isotope data confirm the positive Δ8/4 signature of the southern islands indicating that the north-south compositional heterogeneity in the Cape Verde archipelago has persisted for at least 4.6 Ma. The Santiago volcanics show distinct compositional differences between the old, intermediate and young volcanics, and suggest greater involvement of an enriched mantle (EM1)-like source over time. Isotopic variations in the Santiago volcanics indicate convergence towards a homogeneous EM1-like end-member and distinct temporal variations in the FOZO-like end-member. Santiago and Santo Antão (a northern island, Holm et al. 2006), show a simultaneous decrease in 208Pb/204Pb of the high 206Pb/204Pb FOZO-like source with time. Such systematic archipelago-wide variations in the FOZO-like component suggest that this component is more likely to be present as a coherent package of recycled ocean crust rather than as multiple small heterogeneities dispersed in the upwelling mantle. The temporal variations in 208Pb/204Pb reflect minor lateral variations in Th/U of this recycled ocean crust package entering the melting zone beneath the islands. The location of the EM1-like component is more equivocal. A shallow lithospheric location is possible, but this would require a coincidence between spatial compositional variations in the lithosphere (EM1 is spatially restricted to the southern islands) and flow lines in the upwelling mantle revealed by seismic anisotropy. Therefore, we favour a deeper asthenospheric mantle source for the EM1-like source.

Research paper thumbnail of Geochemical Stratigraphy of Submarine Lavas (3-5 Ma) from the Flamengos Valley, Santiago, Southern Cape Verde Islands

Journal of Petrology, 2009

New high-precision Pb^Sr^Nd isotope, major and trace element and mineral chemistry data are prese... more New high-precision Pb^Sr^Nd isotope, major and trace element and mineral chemistry data are presented for the submarine stage of ocean island volcanism on Santiago, one of the southern islands of the Cape Verde archipelago. Pillow basalts and hyaloclastites in the Flamengos Valley are divided into three petrographic and compositional groups; the Flamengos Formation lavas ($4Á6 Ma) dominate the sequence, with the younger Low Si and Coastal groups ($2Á8 Ma) found near the shoreline. Olivine and clinopyroxene compositions and isotopic data for minerals and their host melts indicate disequilibrium between some crystals and the melt. Intra-sample disequilibrium suggests homogenisation of liquids but eruption before complete equilibration between crystals and melt preserves the heterogeneity. Pressures of crystallization for clinopyroxene (0Á4^1Á1 GPa) indicate stalling and crystallization of the magmas over a range of depths in the lithosphere. Major element compositions indicate melting of a carbonated eclogite source. Sr^Nd^Pb isotope data suggest the involvement of FOZO-like and EM1-like components in the mantle source, which are simultaneously available at all depths in the melting column. The Flamengos Valley lavas display large compositional variations, often between stratigraphically adjacent flows; these frequent abrupt changes of magma composition suggest stalling and crystallization of discrete magma batches on transport through the lithosphere.

Research paper thumbnail of Temporal variations in crustal assimilation of magma suites in the East Greenland flood basalt province: Tracking the evolution of magmatic plumbing systems

Lithos, 2008

We review published radiogenic isotope data (> 350 samples in total) on various suites of magmati... more We review published radiogenic isotope data (> 350 samples in total) on various suites of magmatic rocks within the Palaeogene central East Greenland flood basalt province to evaluate the types of crustal assimilants and the extent of crustal assimilation involved in each suite. We use these observations to build a regional picture of how magmatic plumbing systems changed with time and location during the sequential development of the province as magmatism responded to the development of a volcanic rifted margin and eventual plate separation. The earliest phase of magmatic activity (c. 62–57 Ma) is characterised by highly contaminated magmas that show a temporal change in assimilant type from amphibolite to granulite. This transition has been linked to the effects of an increasing magma supply rate which allows the more refractory granulite lithologies to be melted. The voluminous break-up phase of magmatism (c. 56–54 Ma) saw a significant decrease in the extent of assimilation because of the decreasing availability of assimilant material in the mature feeder systems, and many samples have Sr–Nd–Pb isotope compositions that overlap with those of asthenospheric melts (as represented by recent Icelandic basalts and North Atlantic MORB). Detailed study has allowed us to recognise packets of lavas that ponded at different levels in the crust and assimilated material of different compositions. The later stages of break-up magmatism show more diverse and more contaminated compositions that indicate a shift from a few large robust feeder systems to numerous small new conduits as the rifting continued. The post-break-up magmatism (c. 54–13 Ma) is characterised by a return to more highly contaminated magmas, which reflects a change in the style of magmatism: the eruption of small-volume alkalic lava flows from newly established conduits through the thicker inland crust, and the intrusion of mafic and silicic alkalic magmas at shallow levels in the Archaean basement along the present coast.

Research paper thumbnail of Interaction of the rifting East Greenland margin with a zoned ancestral Iceland plume

Geology, 2006

Abstract Neodymium and high-precision lead isotopic data are presented for Paleogene East Greenla... more Abstract Neodymium and high-precision lead isotopic data are presented for Paleogene East Greenland flood basalts that erupted during an early phase of magmatic activity associated with the Iceland hotspot. The 6-km-thick volcanic sequence shows marked chemostratigraphic variations: lavas in the lower half of the sequence (Milne Land and Geikie Plateau Formations) have low 206 Pb/204 Pb values (17.8–18.4), abruptly changing to high 206 Pb/204 Pb values (18.8–19.3) in the overlying Rømer Fjord Formation, ...

Research paper thumbnail of An 40Ar39Ar study of the Cape Verde hot spot: Temporal evolution in a semistationary plate environment

Journal of Geophysical …, 2008