Airborne laser altimetry survey of Glaciar Tyndall, Patagonia

The first airborne laser altimetry measurements of a glacier in South America are presented. Data were collected in November of 2001 over Glaciar Tyndall, Torres del Paine National Park, Chilean Patagonia, onboard a Twin Otter airplane of the Chilean Air Force. A laser scanner with a rotating polygon-mirror system together with an Inertial Navigation System (INS) were fixed to the floor of the aircraft, and used in combination with two dual-frequency GPS receivers. Together, the laser–INS–GPS system had a nominal accuracy of 30 cm after data processing. On November 23rd, a total of 235 km were flown over the ablation area of Glaciar Tyndall, with 5 longitudinal tracks with a mean swath width of 300 m, which results in a point spacing of approximately 2 m both along and across track. A digital elevation model (DEM) generated using the laser altimetry data was compared with a DEM produced from a 1975 map (1:50,000 scale — Instituto Geográfico Militar (IGM), Chile). A mean thinning of − 3.1 ± 1.0 m a^− 1 was calculated for the ablation area of Glaciar Tyndall, with a maximum value of − 7.7 ± 1.0 m a^− 1 at the calving front at 50 m a.s.l. and minimum values of between − 1.0 and − 2.0 ± 1.0 m a^− 1 at altitudes close to the equilibrium line altitude (900 m a.s.l.). The thinning rates derived from the airborne survey were similar to the results obtained by means of ground survey carried out at  600 m of altitude on Glaciar Tyndall between 1975 and 2002, yielding a mean thinning of − 3.2 m a^− 1 [Raymond, C., Neumann, T.A., Rignot, E., Echelmeyer, K.A., Rivera, A., Casassa, G., 2005. Retreat of Tyndall Glacier, Patagonia, over the last half century. Journal of Glaciology 173 (51), 239–247.]. A good agreement was also found between ice elevation changes measured with laser data and previous results obtained with Shuttle Radar Topography Mission (SRTM) data. We conclude that airborne laser altimetry is an effective means for accurately detecting glacier elevation changes in Patagonia, where an ice thinning acceleration trend has been observed during recent years, presumably in response to warming and possibly also drier conditions.     

Melt inclusions in high-An plagioclase from the Gorda Ridge: an example of the local diversity of MORB parent magmas

 The use of ocean floor basalt chemistry as a tool to investigate mantle composition and processes requires that we work with basalts that have been modified little since leaving the mantle. One source of such basalts is melt inclusions trapped in primitive crystals. However, obtaining information from these melt inclusions is complicated by the fact that melt inclusions in natural basalts are essentially always altered by post-entrapment crystallization. This requires that we develop techniques for reconstructing the original trapped liquid compositions. We conducted a series of experiments to reverse the effects of post-entrapment crystallization by re-heating the host crystals to their crystallization temperature. For these experiments we used plagioclase crystals separated from a single Gorda Ridge lava. The crystallization temperature for these crystals was determined by a set of incremental re-heating experiments to be ∼1240–1260° C. The inclusions are primitive, high Ca-Al basaltic melts, saturated with plagioclase, olivine and Al-rich chromite at low pressure. The inclusion analyses can be linked to the host lava composition by low pressure fractionation. The major element composition of the re-homogenized melt inclusions within each crystal is relatively constant. However, the incompatible element analyses have extremely wide ranges. The range of La/Sm and Ti/Zr from inclusions analyzed from a single sample from the Gorda Ridge exceeds the range reported for lavas sampled from the entire ridge. The pyroxene compositions predicted to be in equilibrium with the melt inclusion trace element signature cover much of the range represented by pyroxenes from abyssal peridotites. The volumetric proportions of the magmas entering the base of the crust can be evaluated using frequency distribution of melt inclusion compositions. This distribution suggests that the array of magmas was skewed towards the more depleted compositions, with little evidence for an enriched component in this system. This pattern is more consistent with a dynamic flow model of the mantle or of a passive flow model where the melts produced in the peripheral areas of the melting regime were not focused to the ridge. Received: 5 January 1995 / Accepted: 13 June 1995     

Paleomagnetic dating of younger volcanic series

Summary Paleomagnetic dating of younger volcanic series is based on the thermoremanent magnetism of these rocks. This fossilises the periodic reversals of the earth's magnetic field over 180°. The slower a-periodic drifting of the magnetic pole is not considered.Magnetic orientation of younger extrusive rocks, either N(ormal) or R(eversed), can normally be measured in the field with a geologic hand compass. This gives us a simple method, which can be valuable to every field geologist working in volcanic series. It should, however, preferably be used in relation with geologic mapping, as magnetic correlation of sections taken far apart, easily lead to false conclusions.Paleomagnetic stratigraphy has until now been based only on the detailed studies of the french physicist A.Roche. In the Auvergne in Central France he studied the sequence of magnetic periods on geologically dated extrusives. It is expected that paleomagnetic studies of this type will yield many results in the years to come.

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Zusammenfassung Paläomagnetische Datierung junger Ergußgesteine stützt sich auf den thermoremanenten Magnetismus dieser Gesteine. Hierdurch wird der wiederholte Umschlag des erdmagnetischen Feldes über 180° festgelegt. Es bleiben bei dieser Art Untersuchungen die langfristigen Verschiebungen des magnetischen Poles außer Betrachtung.Die magnetische Orientierung junger Ergußgesteine, entweder N(ormal) oder R(eversed), läßt sich im Terrain mit einem geologischen Kompaß meistens festlegen. Hieraus folgt eine einfache Methode zur Charakterisierung junger Ergußgesteine, welche jedem Geologen, der in vulkanischem Gebiet arbeitet, eine Stütze sein kann. Die Methode darf aber nur in Zusammenhang mit geologischen Kartierarbeiten angewandt werden, da sonst leicht Fehlschlüsse gezogen werden können.Die paläomagnetische Stratigraphie stützt sich bis jetzt nur auf genaue Untersuchungen an geologisch datierten Ergußgesteinen in der Auvergne seitens des französischen Physikers A.Roche. Voraussichtlich wird diese Methode in den kommenden Jahren große Erfolge erzielen.

     

Mineralogy of Erromango lavas (New Hebrides): Evidence of an early stage of fractionation in island arc basalts

The orogenic volcanic rocks of Erromango island are divided into a Quaternary theoleiitic group and an older Pliocene one showing calc-alkalic affinity. The microprobe mineralogy of these lavas agrees with this geochemical distinction and indicates a marked iron enrichment trend in the recent tholeiitic lavas, whereas titanomagnetite occurs as early formed phenocrysts (Al, Cr, Mg-rich) only in Pliocene basalts. Some Ni and Mg-rich olivine phenocrysts (Fo88) and unusual chemically zoned clinopyroxene megacrysts are described from Pliocene basic lavas and interpreted as deep-fractionated minerals. The large compositional range of these latter (diopside core to augite rim, through salite intermediate zone) leads us to discuss the physical conditions and the corresponding cation substitutions prevailing during their formation. Diopside cores crystallized at depths of about 20 km from a relatively ‘primitive’ melt. The formation of salite is considered to be the result of concomitant fractionation of Mg-rich phases and increase of f_O2. An early stage of fractionation of Cr-bearing diopside + Ni-bearing olivine would account for the typical Cr and Ni depletion of Erromango lavas.     

Temporal Geochemical Evolution of Neogene Magmatism in the Baguio Gold–Copper Mining District (Northern Luzon, Philippines)

Baguio, in the Central Cordillera of Northern Luzon, is a district that displays porphyry copper and epithermal gold mineralization, associated with Early Miocene–Pliocene–Quaternary calc‐alkaline and adakitic intrusions. Systematic sampling, K‐Ar dating, major and trace elements, and Sr, Nd, Pb isotopic analyses of fresh magmatic rocks indicate three magmatic pulses: an Early Miocene phase (21.2–18.7 Ma), a Middle–Late Miocene phase (15.3–8 Ma) and finally a Pliocene–Quaternary event (3–1 Ma). The first phase emplaced evolved calc‐alkaline magmas, essentially within the Agno Batholith complex, and is thought to be related to the westward‐dipping subduction of the West Philippine Basin. After a quiescence period during which the Kennon limestone was deposited, magmatic activity resumed at 15.3 Ma, in connection with the start of the subduction of the South China Sea along the Manila Trench. It emplaced first petrogenetically related and relatively unradiogenic low‐K calc‐alkaline lavas and intermediate adakites. Temporal geochemical patterns observed from 15.3 to 1 Ma include progressive enrichment in K and other large ion lithophile elements, increase in radiogenic Sr and Pb and corresponding decrease in radiogenic Nd. These features are thought to reflect the progressive addition to the Luzon arc mantle wedge of incompatible elements largely inherited from South China Sea sediments. The origin of the long quiescence period, from 8 to 3 Ma, remains problematic. It might represent a local consequence of the docking of the Zambales ophiolitic terrane to Northern Luzon. Then, magmatic activity resumed at 3 Ma, emplacing chemically diversified rocks ranging from low K to high K and including a large proportion of adakites, especially during the Quaternary (dacitic plugs). The authors tentatively relate this diversity to the development of a slab tear linked with the subduction of the fossil South China Sea ridge beneath the Baguio area.     

Exploring aberrant bivalve shell ultrastructure and geochemistry as proxies for past sea water acidification

Throughout much of Earth's history, marine carbonates have represented one of the most important geological archives of environmental change. Several pivotal events during the Phanerozoic, such as mass extinctions or hyperthermal events have recently been associated with ocean acidification. Nevertheless, well‐defined geological proxies for past ocean acidification events are, at best, scarce. Here, experimental work explores the response of bivalve shell ultrastructure and isotope geochemistry (δ¹³C, δ¹⁸O and δ²⁶Mg) to stressful environments, in particular to sea water acidification. In this study, the common blue mussel, Mytilus edulis, was cultured (from early juvenile stages to one year of age) at four pH regimes (pH_NBS 7·2 to pH 8·0). Shell growth rate and ultrastructure of mainly the calcitic portion of the shells were compared between experimental treatments. Specimens exposed to low‐pH environments show patches of disordered calcitic fibre orientation in otherwise well‐structured shells. Furthermore, the electron backscattered diffraction analyses reveal that, under acidified conditions, the c‐axis of the calcite prisms exhibits a bimodal or multi‐modal distribution pattern. Similar shell disorder patterns have been reported from mytilids kept under naturally acidified sea water conditions. In contrast, this study found no evidence that different pH regimes affect shell carbon, oxygen or magnesium isotope ratios. Based on these observations, it is proposed that: (i) stressful environments, in this case low sea water pH, predictably affect bivalve biomineralization patterns; and (ii) these findings bear potential as a novel (petrographic) proxy for ancient sea water acidification. An assessment of the applicability of these data to well‐preserved fossil shell material from selected time intervals requires additional work.