Räumliche geochemische Variationen des Arc-Vulkanismus in Mittelamerika

Volcanism in Central America results from the subduction of the Cocos plate beneath the Caribbean plate. The resulting central American Volcanic arc (CAVA) extends for ~1500 km parallel to the Middle American Trench from Guatemala in the northwest to Costa Rica in the southeast. Characteristic for the CAVA are systematic variations in tectonic parameters (e.g. subducting angle of the Cocos plate), the composition of the subducting plate and the overlying crust. Various partly still active volcanoes and volcanic centers are located along and across the arc and have been subject to intensive research for the past years. The numerous and often contradicting individual models for the origin of arc volcanics in Central America propose a variety of distinct magmatic sources. This dissertation provides new models for the magmatic sources of northwestern and southeastern CAVA rocks. A new and comprehensive dataset of main and trace elements concentrations and ratios, and isotope ratios (Sr-Nd-Pb and for the first time for Central American volcanics Hf) of mafic volcanics und potential magmatic endmembers from the subducting Cocos plate and the overlying Caribbean Plate has been established. Systematic along and across arc geochemical variations of trace element concentrations and ratios and isotope ratios (in particular of Hf and Pb) clearly show that subducting Cocos plate sediments have no influence on the magmatic source components for NW CAVA rocks, as has been proposed by several already published models. The new dataset provides a system of three magmatic endmember components which can explain the observed systematic geochemical variations of NW CAVA rocks. The northwestern CAVA endmember is an enriched magmatic component in the lithosphere beneath Guatemala. The southeastern part of the arc is characterized by the offshore Costa Rica subducting Cocos and Coiba ridges and a neighboring subducting seamount province, which all show an enriched Galapagos hotspot influenced ocean island basalt (OIB) signature (i.e. radiogenic Pb and Hf but unradiogenic Sr and Nd isotopic ratios). New data from mafic volcanics from Costa Rica and Panama show a strong, though systematically from Costa Rica to Nicaragua decreasing influence of the subducting Galapagos hotspot seamount province on the magmatic source composition of the volcanics.Der Vulkanismus in Mittelamerika ist eine Folge der Subduktion der Cocosplatte unter die Karibische Platte. Die dadurch entstehende mittelamerikanische Vulkankette (im Englischen: Central American Volcanic Arc – CAVA) erstreckt sich über fast 1500 km parallel zum Mittelamerikanischen Graben von Guatemala im Nordwesten bis nach Costa Rica im Südosten. Charakteristisch für die mittelamerikanische Vulkankette ist eine systematische Änderung der tektonischen Parameter (z.B. Subduktionswinkel der Cocosplatte) und der geochemischen Zusammensetzung der subduzierenden Platte und der darüber liegenden Kruste. Entlang und quer zu dieser vulkanischen Front, befinden sich zahlreiche z.T. noch aktive Vulkane und Vulkanzentren, die in den letzten Jahren das Ziel intensiver Forschung waren. Daraus entstanden eine Vielzahl häufig unvereinbarer unterschiedlicher Modelle, die verschiedene magmatische Quellen für den Ursprung der Vulkanite in Mittelamerika postulieren. Im Rahmen dieser Dissertation entstanden neue Modelle, um die magmatischen Quellen der Vulkanite im Nordwesten und Südosten der mittelamerikanischen Vulkankette besser zu bestimmen. Dazu wurde ein neuer, umfassender Datensatz aus Haupt- und Spurenelement Konzentrationen, sowie verschiedener Isotopenverhältnisse (Sr-Nd-Pb und zum ersten Mal für mittelamerikanische Vulkanite Hf) von mafischen vulkanischen Gesteinsproben und verschiedener möglicher magmatischer Endglieder der subduzierenden Cocosplatte und der darüber liegenden Karibischen Platte erstellt. Systematische geochemische Variationen entlang und quer zur Vulkankette von Spurenelement Konzentrationen und Verhältnissen, besonders aber der Hf und Pb Isotopenverhältnisse zeigen deutlich, dass die Sedimente der subduzierenden Cocosplatte nicht zur magmatischen Quellzusammensetzung der Vulkanite im Nordwesten beitragen, wie in mehreren bereits publizierten Modellen angenommen wird. Mit Hilfe der neuen Daten kann vielmehr ein System aus drei magmatischen Endgliedern beschrieben werden, dass die beobachteten systematischen geochemischen Variationen für den Vulkanismus in NW Mittelamerika erklärt. Eine angereicherte (d.h. radiogene Pb und Sr, aber unradiogene Hf und Nd Isotopenverhältnisse) magmatische Quelle in der Lithosphäre unterhalb von Guatemala bildet dabei das nordwestliche magmatische Endglied für die Vulkanite der mittelamerikanischen Vulkankette. Der südöstliche Teil der Vulkankette ist charakterisiert durch die Subduktion des Cocos Rückens und einer angrenzenden submarinen Vulkanprovinz vor Costa Rica. Diese, durch den Galapagos Hotspot geprägten tektonischen Erhebungen, zeigen eine besondere, angereicherte Isotopen-Signatur, die Galapagos Ozeaninseln Basalt (OIB) Signatur (d.h. radiogene Pb und Hf Isotopenverhältnisse). Die im Rahmen dieser Dissertation neugewonnenen Daten von mafischen Vulkaniten aus Costa Rica und Panama zeigen einen von Zentral-Costa Rica nach Nicaragua systematisch abnehmenden aber anfänglich sehr starken Einfluß der subduzierenden Galapagos-Hotspot submarine Vulkanprovinz auf die Quelle für die Vulkanite

Northwestern Central American Volcanic Arc: Increased contribution of enriched lithosphere to lavas along the volcanic front from Nicaragua to Guatemala and behind the volcanic front

The Central American Volcanic Arc (CAVA) has been subject of intensive research over the past decades, leading to a large variety of different models for the origin of CAVA lavas with various source components. Based on a comprehensive new geochemical data set (i.e. major and trace elements and Sr-Nd-Pb-Hf-O isotope ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples from NW CAVA (Nicaragua to Guatemala), we present a new model for the NW Central American Volcanic Arc volcanism. Additional potential source component sample data from subducting Cocos Plate sediments, igneous oceanic crust and Guatemalan granitic and metamorphic continental basement further contributes to our new model. We find systematically increasing Pb isotope ratios and decreasing Nd and Hf isotope ratios along the arc from NW Nicaragua to Guatemala. BVF lavas generally have more radiogenic Pb and less radiogenic Nd and Hf isotopic compositions than related VF lavas, similar to what is observed for trace element ratios going northwards along the VF. Combined isotope and trace element data indicate the presence of three endmembers for the volcanism in NW Central America: (1) NW Nicaraguan VF samples with very high Ba/(La, Th) and U/Th, low La/Yb, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb, (2) NW Guatemalan VF and Guatemalan and Honduran BVF samples with low Ba/(La, Th) and U/Th, high La/Yb, radiogenic Sr and Pb but unradiogenic Nd and Hf, and elevated d18O, and (3) Honduran and Nicaraguan BVF samples with low Ba/(La, Th) and U/Th, high La/Yb, unradiogenic Sr but radiogenic Nd, Hf and Pb. We interpret the NW Nicaragua VF endmember to be dominated by a largely serpentinite-derived fluid flux from the subducting slab, possibly with small amounts (<1 wt. %) of sediment melts, to a depleted N-MORB type of mantle wedge, resulting in large degrees of melting of primarily peridotitic material. Based on combined Hf and Nd and Hf and Pb isotope systematics, the isotopically enriched Guatemala VF and BVF endmember cannot be explained by the addition of subducted pelagic sediments to the source. Instead this endmember could be derived from pyroxenitic cumulates in the lithospheric mantle (and possibly lower crust) that were derived from parental magmas for plutonic rocks in NW Central America, which were melted during the Quaternary subduction-related volcanism. The isotopically depleted Honduras and Caribbean BA endmember could be derived from melting of young, recycled, oceanic crust in the asthenosphere upwelling in the back-arc, based on the OIB-like major and trace element but relatively depleted isotopic compositions of these samples. Mixing between these three endmember types of magmas can explain the observed systematic geochemical variations along and across the NW Central American Arc

Spatial variations in the geochemistry of arc volcanism in Central America

Volcanism in Central America results from the subduction of the Cocos plate beneath the Caribbean plate. The resulting central American Volcanic arc (CAVA) extends for ~1500 km parallel to the Middle American Trench from Guatemala in the northwest to Costa Rica in the southeast. Characteristic for the CAVA are systematic variations in tectonic parameters (e.g. subducting angle of the Cocos plate), the composition of the subducting plate and the overlying crust. Various partly still active volcanoes and volcanic centers are located along and across the arc and have been subject to intensive research for the past years. The numerous and often contradicting individual models for the origin of arc volcanics in Central America propose a variety of distinct magmatic sources. This dissertation provides new models for the magmatic sources of northwestern and southeastern CAVA rocks. A new and comprehensive dataset of main and trace elements concentrations and ratios, and isotope ratios (Sr-Nd-Pb and for the first time for Central American volcanics Hf) of mafic volcanics und potential magmatic endmembers from the subducting Cocos plate and the overlying Caribbean Plate has been established. Systematic along and across arc geochemical variations of trace element concentrations and ratios and isotope ratios (in particular of Hf and Pb) clearly show that subducting Cocos plate sediments have no influence on the magmatic source components for NW CAVA rocks, as has been proposed by several already published models. The new dataset provides a system of three magmatic endmember components which can explain the observed systematic geochemical variations of NW CAVA rocks. The northwestern CAVA endmember is an enriched magmatic component in the lithosphere beneath Guatemala. The southeastern part of the arc is characterized by the offshore Costa Rica subducting Cocos and Coiba ridges and a neighboring subducting seamount province, which all show an enriched Galapagos hotspot influenced ocean island basalt (OIB) signature (i.e. radiogenic Pb and Hf but unradiogenic Sr and Nd isotopic ratios). New data from mafic volcanics from Costa Rica and Panama show a strong, though systematically from Costa Rica to Nicaragua decreasing influence of the subducting Galapagos hotspot seamount province on the magmatic source composition of the volcanics

On the fluid-mobility of molybdenum, tungsten, and antimony in subduction systems

Molybdenum (Mo) and tungsten (W) have long been regarded as being more or less immobile during slab fluid-induced arc magma generation. Here we characterize about 180 samples of young, predominantly mafic to intermediate tephras and lavas for their Mo, W, and antimony (Sb) concentrations, to examine the fluid-mobility of these elements in subduction systems. Samples were taken along the active arcs of the Chilean Southern Volcanic Zone (SVZ) and the Central American Volcanic Arc (CAVA). When relating Mo, W, and Sb to trace element ratios typically used to constrain the involvement of subduction fluids in magma formation, such as Ba/La or U/Th, Mo, W, and Sb are enriched in the most fluid-influenced, highest-degree melts. W/Mo ratios correlate positively with Pb/Ce, which is established to reflect a recent subduction signal or assimilation of crustal material with an ancient subduction signature, suggesting that subduction processes promote enrichment of W over Mo. This is well expressed at the SVZ and most of the CAVA; while few OIB-type rocks from Central Costa Rica form an opposite trend. Moreover, Mo/W ratios co-vary with Cl contents derived from melt inclusions, indicating that the relative degree of mobilization responds to the composition of the subduction fluid. To evaluate the mobility of Mo, W, and Sb during metamorphism in the slab, eclogites with no or minor metasomatic overprint and a fluid-induced overprint in an eclogite-blueschist sequence were investigated. None of the three elements shows a systematic variability related to metasomatism and the minor variations are interpreted to reflect protolith heterogeneity. This suggests that Mo, W and Sb remain relatively immobile up to depths of 70 km in the subduction zone

Along and across arc geochemical variations in NW Central America: Evidence for involvement of lithospheric pyroxenite

The Central American Volcanic Arc (CAVA) has been the subject of intensive research over the past few years, leading to a variety of distinct models for the origin of CAVA lavas with various source components. We present a new model for the NW Central American Volcanic Arc based on a comprehensive new geochemical data set (major and trace element and Sr–Nd–Pb–Hf–O isotope ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples from NW Nicaragua, Honduras, El Salvador and Guatemala. Additionally we present data on subducting Cocos Plate sediments (from DSDP Leg 67 Sites 495 and 499) and igneous oceanic crust (from DSDP Leg 67 Site 495), and Guatemalan (Chortis Block) granitic and metamorphic continental basement. We observe systematic variations in trace element and isotopic compositions both along and across the arc. The data require at least three different endmembers for the volcanism in NW Central America. (1) The NW Nicaragua VF lavas require an endmember with very high Ba/(La, Th) and U/Th, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb and low δ18O, reflecting a largely serpentinite-derived fluid/hydrous melt flux from the subducting slab into a depleted N-MORB type of mantle wedge. (2) The Guatemala VF and BVF mafic lavas require an enriched endmember with low Ba/(La, Th), U/Th, high δ18O and radiogenic Sr and Pb but unradiogenic Nd and Hf isotope ratios. Correlations of Hf with both Nd and Pb isotopic compositions are not consistent with this endmember being subducted sediments. Granitic samples from the Chiquimula Plutonic Complex in Guatemala have the appropriate isotopic composition to serve as this endmember, but the large amounts of assimilation required to explain the isotope data are not consistent with the basaltic compositions of the volcanic rocks. In addition, mixing regressions on Nd vs. Hf and the Sr and O isotope plots do not go through the data. Therefore, we propose that this endmember could represent pyroxenites in the lithosphere (mantle and possibly lower crust), derived from parental magmas for the plutonic rocks. (3) The Honduras and Caribbean BA lavas define an isotopically depleted endmember (with unradiogenic Sr but radiogenic Nd, Hf and Pb isotope ratios), having OIB-like major and trace element compositions (e.g. low Ba/(La, Th) and U/Th, high La/Yb). This endmember is possibly derived from melting of young, recycled oceanic crust in the asthenosphere upwelling in the back-arc. Mixing between these three endmember types of magmas can explain the observed systematic geochemical variations along and across the NW Central American Arc

Geochemistry and Age of Shatsky, Hess, and Ojin Rise seamounts: Implications for a connection between the Shatsky and Hess Rises

Shatsky Rise in the Northwest Pacific is the best example so far of an oceanic plateau with two potential hotspot tracks emanating from it: the linear Papanin volcanic ridge and the seamounts comprising Ojin Rise. Arguably, these hotspot tracks also project toward the direction of Hess Rise, located ∼1200 km away, leading to speculations that the two plateaus are connected. Dredging was conducted on the massifs and seamounts around Shatsky Rise in an effort to understand the relationship between these plateaus and associated seamounts. Here, we present new 40Ar/39Ar ages and trace element and Nd, Pb, and Hf isotopic data for the recovered dredged rocks and new trace elements and isotopic data for a few drill core samples from Hess Rise. Chemically, the samples can be subdivided into plateau basalt-like tholeiites and trachytic to alkalic ocean-island basalt compositions, indicating at least two types of volcanic activity. Tholeiites from the northern Hess Rise (DSDP Site 464) and the trachytes from Toronto Ridge on Shatsky’s TAMU massif have isotopic compositions that overlap with those of the drilled Shatsky Rise plateau basalts, suggesting that both Rises formed from the same mantle source. In contrast, trachytes from the southern Hess Rise (DSDP Site 465A) have more radiogenic Pb isotopic ratios that are shifted toward a high time-integrated U/Pb (HIMU-type mantle) composition. The compositions of the dredged seamount samples show two trends relative to Shatsky Rise data: one toward lower 143Nd/144Nd but similar 206Pb/204Pb ratios, the other toward similar 143Nd/144Nd but more radiogenic 206Pb/204Pb ratios. These trends can be attributed to lower degrees of melting either from lower mantle material during hotspot-related transition to plume tail or from less refractory shallow mantle components tapped during intermittent deformation-related volcanism induced by local tectonic extension between and after the main volcanic-edifice building episodes on Shatsky Rise. The ocean-island-basalt-like chemistry and isotopic composition of the Shatsky and Hess rise seamounts contrast with those formed by purely deformation-related shallow mantle-derived volcanism, favoring the role of a long-lived mantle anomaly in their origin. Finally, new 40Ar/39Ar evidence indicates that Shatsky Rise edifices may have been formed in multiple-stages and over a longer duration than previously believed