Randall A. Keller

Discovery of new hydrothermal vent sites in Bransfield Strait, Antarctica

We carried out a search for hydrothermal vents in the Central Basin of Bransfield Strait, Antarctica. The ZAPS (zero angle photon spectrometer) chemical sensor and instrument package (Oregon State University), OFOS (ocean-floor observation system) camera sled and TVG (TV-grab) (GEOMAR) were used to explore the water column and underlying seafloor. These operations were supplemented with a series of dredges. Hydrothermal plumes over Hook Ridge at the eastern end of the basin are confined to the E ridge crest and SE flank. The plumes are complex and sometimes contain two turbidity maxima one widespread feature centered at 1150 m and a smaller, more localized but broad maximum at 600–800 m. We traced the source of the shallower plume to a sunken crater near the ridge crest using sensors on the ZAPS instrument package. Subsequently two TV-grabs from the crater brought back hot, soupy sediment (42–49°C) overlain by hard, siliceous crusts and underlain by a thick layer of volcanic ash. We also recovered chimney fragments whose texture and mineralogy indicate venting temperatures in excess of 250°C. Native sulfur and Fe-sulfides occur in fractures and porous layers in sediment from throughout the area. Pore water data from the crater site are consistent with venting into a thin sediment layer and indicate phase separation of fluids beneath Hook Ridge. The source of the deeper plumes at Hook Ridge has yet to be located. We also explored a series of three parallel volcanic ridges west of Hook Ridge called Three Sisters. We detected water column anomalies indicative of venting with the ZAPS package and recovered hydrothermal barites and sulfides from Middle Sister. We spent considerable time photographing Middle Sister and Hook Ridge but did not identify classic vent fauna at either location. We either missed small areas with our photography or typical MOR vent fauna are absent at these sites.

Bransfield Strait, Antarctic Peninsula Active Extension behind a Dead Arc

Bransfield Strait is a marginal basin landward of the South Shetland Trench. It lies between the South Shetland Islands and the tip of the Antarctic Peninsula and is an example of an extensional basin formed by rifting within a continental volcanic arc. The Antarctic Peninsula is the product of at least 200 m.y. of subduction with the majority of the exposed rocks related to continental arc volcanism older than 20 Ma. Volcanism in Bransfield Strait started by 0.3 Ma and continues today. This new volcanism maintains some of the chemical signatures of the old arc volcanism but also has signatures transitional between arc rocks and backarc basin rocks. On the basis of high heat flow, active volcanism, extensional faulting, and earthquake fault plane mechanisms, Bransfield Strait is an active extensional basin forming within the Antarctic Peninsula. Seismic refraction work in Bransfield Strait indicates some thinning of continental crust, but the basin itself is underlain by as much as 30 km of anomalous crustal material. The observed extension seems to be confined to Bransfield Strait, which is bounded by the landward projections of the Hero and Shackleton fracture zones. The present extension in Bransfield Rift started less than 4 m.y. ago, and possibly less than 1.5 m.y. ago, following the demise of the Antarctic-Phoenix spreading center (ANT-PHO ridge), which ceased spreading about 4 Ma. Apparent, continued subduction at the South Shetland Trench after the ANT-PHO ridge stopped spreading may occur as trench rollback. The amount of trench rollback should be comparable to the amount of extension in Bransfield Strait.

Diamonds and Their Mineral Inclusions, and What They Tell Us: A Detailed “Pull-Apart” of a Diamondiferous Eclogite

For the first time, three-dimensional, high-resolution X-ray computed tomography (HRXCT) of an eclogite xenolith from Yakutia has successfully imaged diamonds and their textural relationships with coexisting minerals. Thirty (30) macrodiamonds (≥1 mm), with a total weight of just over 3 carats, for an ore grade of some 27,000 ct/ton, were found in a small (4 × 5 × 6 cm) eclogite, U51/3, from Udachnaya. Based upon 3-D imaging, the diamonds appear to be associated with zones of secondary alteration of clinopyroxene (Cpx) in the xenolith. The presence of diamonds with secondary minerals strongly suggests that the diamonds formed after the eclogite, in conjunction with meta-somatic input(s) of carbon-rich fluids. Metasomatic processes are also indicated by the non-systematic variations in Cpx inclusion chemistry in the several diamonds. The inclusions in the diamonds vary considerably in major- and trace-element chemistry within and between diamonds, and do not correspond to the minerals of the host eclogite, whose compositions are extremely homogeneous. Some Cpx inclusions possess +Eu anomalies, probably inherited from their crustal source rocks. The only consistent feature for the Cpx crystals in the inclusions is that they have higher K2O than the Cpx grains in the host. The δ13C compositions are relatively constant at −5% both within and between diamonds, whereas δ15N values vary from −2.8% to −15.8%. Within a diamond, the total N varies considerably from 15 to 285 ppm in one diamond to 103 to 1250 ppm in another. Cathodoluminescent imaging reveals extremely contorted zonations and complex growth histories in the diamonds, indicating large variations in growth environments for each diamond. This study directly bears on the concept of diamond inclusions as time capsules for investigating the mantle of the Earth. If diamonds and their inclusions can vary so much within this one small xenolith, the significance of their compositions is a serious question that must be addressed in all diamond-inclusion endeavors.

Isotopic and trace element constraints on mixing and melting models of marginal basin volcanism, Bransfield Strait, Antarctica

Bransfield Strait is a narrow marginal basin separating the South Shetland Islands from the northern end of the Antarctic Peninsula. Quaternary volcanism occurs in the strait as subaerial and submarine volcanoes aligned on the inferred axis of rifting, and as two subaerial, off-axis volcanoes on the northern margin of the strait. The Bransfield Strait lavas are similar to published analyses from other marginal basins, ranging from basalts and basaltic andesites to trachytes. They exhibit moderate enrichments in alkali and alkaline earth elements relative to high-field-strength elements that are typical of many back-arc basin basalts. The seamount basalts have trace element chemistry similar to enriched mid-ocean ridge basalts (E-MORB), but with variously higher alkali and alkaline earth element concentrations and, frequently, lower Nb. Low-pressure fractional crystallization accounts for most of the compositional variation within individual volcanoes, but it does not explain intervolcano differences even though the volcanoes are closely spaced and presently or recently active. Melting of depleted mantle mixed with 0.5–2% crust or sediment explains the isotopic and trace element variations not accounted for by magma chamber processes. The off-axis volcanoes are the products of less partial melting than the on-axis volcanoes. One of the off-axis volcanoes also contains more of an enriched component in its source. Some of the lavas have Rb/Sr too low to account for their 87Sr/86Sr, perhaps due the loss of Rb during mantle metasomatism or interaction between slab-derived fluids and the mantle source of the basalts. Strontium, neodymium, and 207Pb/204Pb isotopic signatures remained constant during the transition from Tertiary island arc volcanism to Quaternary marginal basin volcanism, but 206Pb/204Pb increased. High CeN/SmN of basalts from some volcanoes requires residual garnet in the source. Thus the young (15–25 Ma at the trench) subducted slab is either deeper than 60 km beneath the volcanoes, or it has fractured and is no longer a coherent barrier to melts generated below it.

Isotopic evidence for Late Cretaceous plume-ridge interaction at the Hawaiian hotspot

When a mantle plume interacts with a mid-ocean ridge, both are noticeably affected. The mid-ocean ridge can display anomalously shallow bathymetry, excess volcanism, thickened crust, asymmetric sea-floor spreading and a plume component in the composition of the ridge basalts. The hotspot-related volcanism can be drawn closer to the ridge, and its geochemical composition can also be affected. Here we present Sr–Nd–Pb isotopic analyses of samples from the next-to-oldest seamount in the Hawaiian hotspot track, the Detroit seamount at 51° N, which show that, 81 Myr ago, the Hawaiian hotspot produced volcanism with an isotopic signature indistinguishable from mid-ocean ridge basalt. This composition is unprecedented in the known volcanism from the Hawaiian hotspot, but is consistent with the interpretation from plate reconstructions that the hotspot was located close to a mid-ocean ridge about 80 Myr ago. As the rising mantle plume encountered the hot, low-viscosity asthenosphere and hot, thin lithosphere near the spreading centre, it appears to have entrained enough of the isotopically depleted upper mantle to overwhelm the chemical characteristics of the plume itself. The Hawaiian hotspot thus joins the growing list of hotspots that have interacted with a rift early in their history.

Geochemistry of back arc basin volcanism in Bransfield Strait, Antarctica: Subducted contributions and along-axis variations

[1] Bransfield Strait is a Quaternary, ensialic back arc basin at the transition from rifting to spreading. Fresh volcanic rocks occur on numerous submarine features distributed along the rift axis, including a discontinuous neovolcanic ridge similar to the nascent spreading centers seen in some other back arc basins. Smaller edifices near the northeast end of the rift yielded basalts with the most arc-like compositions (e.g., high large-ion lithophile element/high field strength element and 87 Sr/ 86 Sr). The most mid-ocean ridge basalt (MORB)-like basalts are from a large, caldera-topped seamount and a 30-km-long axial neovolcanic ridge toward the southwest end of the rift, but these two features also yielded andesite and rhyolite, respectively. The volcanic and geochemical variations are not systematic along axis and do not reflect the unidirectional propagation of rifting suggested by geophysical data. The most depleted basalts have major and trace element characteristics indistinguishable from MORB except for slightly higher Cs and Pb concentrations. Pb isotopic ratios show little variation compared to Sr and Nd isotopic ratios and do not extend to the depleted Pb isotopic ratios found in other back arc basins. Either the depleted mantle beneath Bransfield Strait has higher than normal Pb isotopic ratios or the subducted component beneath Bransfield Strait has such high Pb concentrations that it dominates the Pb isotopic composition of the Bransfield Strait mantle without significantly affecting the Sr and Nd isotopic compositions. Metalliferous sediments and fluids extracted from a subducting slab may have the necessary high concentrations of Pb. INDEX TERMS: 1040 Geochemistry: Isotopic composition/chemistry; 3640 Mineralogy and Petrology: Igneous petrology; 3655 Mineralogy and Petrology: Major element composition; 3670 Mineralogy and Petrology: Minor and trace element composition; KEYWORDS: back arc basin, Bransfield Strait, Antarctica, geochemistry, volcanism, recycling

Extreme Chemical Diversity in the Mantle during Eclogitic Diamond Formation: Evidence from 35 Garnet and 5 Pyroxene Inclusions in a Single Diamond

We report major- and trace-element variations in 35 garnet inclusions extracted from a single eclogitic diamond from the Mir kimberlite pipe, Yakutia. The range in CaO and Mg# in these garnets is large-from 3.84 to 9.66 wt% and 45 to 56, respectively. These ranges cover nearly half of the total range in eclogitic garnet compositions from diamonds worldwide. An extremely wide range in trace elements such as Y, Zr, and Sr also covers nearly the total range recorded for garnets included in diamonds, as well as that known for rock-forming minerals of diamondiferous eclogite xenoliths-15.1 to 48.9 ppm Y; 2.2 to 40.8 ppm Zr; and 0.5 to 9.1 ppm Sr. The widest ranges in REE between different grains (by an order of magnitude) are noted for LREE and MREE. Such compositional ranges are the first recorded for garnets available as inclusions in a single eclogitic diamond and cover nearly the complete range of compositions known for all Yakutian eclogites. Five clino-pyroxene inclusions exhibit more moderate ranges in ...

Quaternary marginal basin volcanism in the Bransfield Strait as a modern analogue of the southern Chilean ophiolites

Abstract The Bransfield Strait is an actively-spreading, 65 km wide, Quaternary marginal basin at the northern tip of the Antarctic Peninsula. Across the Drake Passage from the Bransfield Strait are the Tortuga and Sarmiento ophiolites of southern Chile, which have been interpreted as uplifted, Jurassic-Cretaceous, marginal-basin curst that formed in a setting similar to the present Bransfield Strait. We present a comparison between the Bransfield Strait and the Chilean ophiolites, emphasizing new petrological and geochemical data for volcanic rocks dredged from two seamounts at the Bransfield Strait rift axis and published analyses of volcanic rocks from the marginal basin now preserved as the Chilean ophiolites. Volcanic rocks from the seamounts are about 100 000 years old, and major element abundances in these rocks are broadly similar to enriched mid-ocean ridge basalt (E-MORB). The rocks are slightly enriched in light rare earth elements (LREE) relative to heavy rare earth elements (HREE), as in E-MORB, but they are moderately enriched in alkali, and alkali earth elements, as in arc basalts, and concentrations and ratios of diagnostic trace elements show island arc affinities. Geochemical discrimination diagrams suggest that Bransfield Strait lavas represent the initial stages of a petrogenetic transition linked to the tectonic transition from rifting in an ensialic arc to rifting in a marginal basin. Published rare earth element data from dykes and lavas in the Chilean ophiolites range from mildey depleted in LREE in the Tortuga ophiolite (Ce/Yb = 0.5 to 0.8, normalized to chondrites) to mildly enriched in LREE in the Sarmiento ophiolite (normalized Ce/Yb = 1.6 to 2.5). The latter are strikingly similar to data from the seamounts in the Bransfield Strait both in LREE enrichment (normalized Ce/Yb = 2 to 2.5) and total REE concentrations (20–45 times chondrite). We believe these chemical similarities, as well as other geological similarities pointed out by other authors, suggest that the Bransfield Strait marginal basin is at the same stage of development as was the marginal basin now preserved as the Sarmiento ophiolite.

Magmatism in the Bransfield Basin: Rifting of the South Shetland Arc?

Bransfield Basin is an actively extending marginal basin separating the inactive South Shetland arc from the northern Antarctic Peninsula. Rift-related volcanism is widespread throughout the central Bransfield Basin, but the wider eastern Bransfield Basin was previously unsampled. Lavas recovered from the eastern subbasin form three distinct groups: (1) Bransfield Group has moderate large-ion lithophile element (LILE) enrichment relative to normal mid-ocean ridge basalt (NMORB), (2) Gibbs Group has strong LILE enrichment and is restricted to a relic seamount interpreted as part of the South Shetland arc, and (3) fresh alkali basalt was recovered from the NE part of the basin near Spanish Rise. The subduction-related component in Bransfield and Gibbs Group lavas is a LILE-rich fluid with radiogenic Sr, Nd, and Pb isotope compositions derived predominantly from subducting sediment. These lavas can be modeled as melts from Pacific MORB source mantle contaminated by up to 5% of the subduction-related component. They further reveal that Pacific mantle, rather than South Atlantic mantle, has underlain Bransfield Basin since 3 Ma. Magma productivity decreases abruptly east of Bridgeman Rise, and lavas with the least subduction component outcrop at that end. Both the eastward decrease in subduction component and occurrence of young alkali basalts require that subduction-modified mantle generated during the lifetime of the South Shetland arc has been progressively removed from NE to SW. This is inconsistent with previous models suggesting continued slow subduction at the South Shetland Trench but instead favors models in which the South Scotia Ridge fault has propagated westward since 3 Ma generating transtension across the basin.

Cretaceous-to-recent record of elevated 3He/4He along the Hawaiian-Emperor volcanic chain

Helium isotopes are a robust geochemical tracer of a primordial mantle component in hot spot volcanism. The high 3He/4He (up to 35 RA, where RA is the atmospheric 3He/4He ratio of 1.39 × 10−6) of some Hawaiian Island volcanism is perhaps the classic example. New results for picrites and basalts from the Hawaiian-Emperor seamount chain indicate that the hot spot has produced high 3He/4He lavas for at least the last 76 million years. Picrites erupted at 76 Ma have 3He/4He (10–14 RA), which is at the lower end of the range for the Hawaiian Islands but still above the range of modern mid-ocean ridge basalt (MORB; 6–10 RA). This was at a time when hot spot volcanism was occurring on thin lithosphere close to a spreading ridge and producing lava compositions otherwise nearly indistinguishable from MORB. After the hot spot and spreading center diverged during the Late Cretaceous, the hot spot produced lavas with significantly higher 3He/4He (up to 24 RA). Although 3He/4He ratios stabilized at relatively high values by 65 Ma, other chemical characteristics such as La/Yb and 87Sr/86Sr did not reach and stabilize at Hawaiian-Island-like values until ~45 Ma. Our limited 3He/4He record for the Hawaiian hot spot shows a poor correlation with plume flux estimates (calculated from bathymetry and residual gravity anomalies [Van Ark and Lin, 2004]). If 3He is a proxy for the quantity of primordial mantle material within the plume, then the lack of correlation between 3He/4He and calculated plume flux suggests that variation in primordial mantle flux is not the primary factor controlling total plume flux.