343 research outputs found
Rifting of the oceanic Azores Plateau with episodic volcanic activity
Extension of the Azores Plateau along the Terceira Rift exposes a lava sequence on the steep northern flank of the Hirondelle Basin. Unlike typical tholeiitic basalts of oceanic plateaus, the 1.2 km vertical submarine stratigraphic profile reveals two successive compositionally distinct basanitic to alkali basaltic eruptive units. The lower unit is volumetrically more extensive with similar to 1060 m of the crustal profile forming between similar to 2.02 and similar to 1.66 Ma, followed by a second unit erupting the uppermost similar to 30 m of lavas in similar to 100 kyrs. The age of similar to 1.56 Ma of the youngest in-situ sample at the top of the profile implies that the 35 km-wide Hirondelle Basin opened after this time along normal faults. This rifting phase was followed by alkaline volcanism at D. JoAo de Castro seamount in the basin center indicating episodic volcanic activity along the Terceira Rift. The mantle source compositions of the two lava units change towards less radiogenic Nd, Hf, and Pb isotope ratios. A change to less SiO2-undersaturated magmas may indicate increasing degrees of partial melting beneath D. JoAo de Castro seamount, possibly caused by lithospheric thinning within the past 1.5 million years. Our results suggest that rifting of oceanic lithosphere alternates between magmatically and tectonically dominated phases.Peer reviewe
Dating Clinopyroxene Phenocrysts in Submarine Basalts Using ^(40)Ar/^(39)Ar Geochronology
Dating submarine basalts using ^(40)Ar/^(39)Ar geochronology is often hindered by a lack of potassiumābearing phenocrystic phases and severe alteration in the groundmass. Clinopyroxene is a common phenocrystic phase in seafloor basalts and is highly resistive to lowātemperature alteration. Here we show that clinopyroxene phenocrysts separated from marine basalts are a viable phase for ^(40)Ar/^(39)Ar incremental heating age determinations. We provide results from a pilot study comprising 16 age experiments from nine clinopyroxene separates, five of which from samples with dated coeval phases. The clinopyroxene ages range from 11.5 to 112 Ma with relatively high uncertainties (ranging from 0.8% to 7.1%; median of 1.9%) compared to more traditional phases. The clinopyroxene age plateaus form at low to moderate temperature steps and are characterized by relatively elevated K/Ca of 0.002ā0.4, suggesting that other Kābearing phases hosted within the clinopyroxene are likely degassing to yield the ^(40)Ar/^(39)Ar age information. There are three possible origins for the K and corresponding ^(40)Ar* including films of trapped melt/nanomineral inclusions along grain defects, secondary melt inclusion bands, or variations in degassing behaviors between lower and higher crystalline Ca pyroxene phases. Regardless of the source of the K, the age determinations are successful with 75% of the experiments producing long plateaus (>60% ^(39)Ar released) with mean square of the weighted deviations ranging from 0.6 to 1.5 and probability of fit values >0.05. We conclude that clinopyroxene dating by the ^(40)Ar/^(39)Ar method has the potential to provide a wealth of information for previously undated, altered seafloor lithologies and continental equivalents
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High-resolution ā“ā°Ar/Ā³ā¹Ar dating of the oldest oceanic basement basalts in the western Pacific basin
We report new ā“ā°Ar/Ā³ā¹Ar ages for the oldest Pacific oceanic floor at Ocean Drilling Program Site 801C in the Pigafetta basin and Site 1149D close to the Izu-Bonin subduction zone in the Nadezhda basin. These ages were determined by applying high-resolution incremental heating experiments (including 15ā30 heating steps) to better resolve the primary argon signal from interfering alteration signatures in these low-potassium ocean crust basalts. Combined with previous results from Pringle [1992] for Site 801B and 801C, we arrive at a multistage history for the formation of the Pigafetta ocean crust. The oldest part of the Pacific plate was formed at the spreading ridges at 167.4 Ā± 1.4/3.4 Ma (n = 2, 2Ļ internal/absolute error), offering an important calibration point on the Geological Reversal Timescale (GRTS) since it represents the old end of the Mesozoic magnetic anomalies. This mid-ocean ridge basalt sequence, however, is overlain by more tholeiites and alkali basalts that were formed 7.3 Ā± 1.5 Myr later around 160.1 Ā± 0.6 Ma (n = 7, 2Ļ internal error). The older age group is confirmed independently by radiolarian ages ranging from Late Bajocian to Middle Bathonian (167ā173 Ma [Bartolini and Larson, 2001]) and by profound differences in the structural characteristics of this basement section [Pockalny and Larson, 2003]. Thin layers comprising hydrothermal deposits separate these sequences, which in addition to the difference in isotopic age show distinct major and trace element compositions. This indicates that key volcanic and hydrothermal activity took place 400ā600 km away from the spreading ridges, on the basis of a Jurassic ~66 km/Myr half spreading rate in the Pacific. It remains unclear if these processes were active continuously after the initial formation of the Pacific oceanic crust, but all our observations seem to point to an episodic history. Site 1149D gives another important calibration point on the GRTS of 127.0 Ā± 1.5/3.6 Ma (n = 1, 2Ļ internal/absolute error) for anomaly M12 that is slightly younger when compared to current timescale compilations (134.2 Ā± 2.1 Ma [Gradstein et al., 1995]). This might suggest that the dated basalt from Site 1149D does not represent the age of the ocean crust formed at its ridge axis; it may also be part of the Early Cretaceous intraplate events that have produced dolerite sills in the Pacific crust at Sites 800 and 802 around 114ā126 Ma
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Implications of a nonlinear ā“ā°Ar/Ā³ā¹Ar age progression along the Louisville seamount trail for models of fixed and moving hot spots
The Louisville seamount trail has been recognized as one of the key examples of hot spot volcanism, comparable to the classic volcanic Hawaiian-Emperor lineaments. The published total fusion ā“ā°Ar/Ā³ā¹Ar data of Watts et al. [1988] showed an astonishing linear age progression, firmly establishing Louisville as a fixed hot spot in the South Pacific mantle. We report new 40Ar/39Ar ages based on high-resolution incremental heating 40Ar/39Ar dating for the same group of samples, showing a marked increase in both precision and accuracy. One of the key findings in our reexamination is that the age progression is not linear after all. The new data show a significantly decreased āāapparentāā plate velocity for the Louisville seamount trail older than 62 Ma but confirm the linear trend between 47 Ma and the present day (although based on only three samples over 2150 km). The most recent volcanic activity in the Louisville seamount trail has now been dated at 1.11 Ā± 0.04 Ma for the most southeastern seamount located at 50Ā°26āS and 139Ā°09āW. These results indicate that the Louisville age progression should be interpreted on the basis of both plate and hot spot motion. In this paper we examine our new results in conjunction with the numerical mantle flow models of Steinberger et al. [2004] that also predict marked deviations from simple linear age progressions. With these models we can achieve a good fit to the geometry of both the Hawaiian and Louisville seamount trails and their age progressions as well as the ~15Ā° paleolatitudinal shift observed by Tarduno et al. [2003] for the Hawaiian hot spot between 80 and 47 Ma. If the model is restricted to Pacific hot spots only, we can improve the fit to the nonlinear age trend for the Louisville seamount trail by allowing an additional rotation change of the Pacific plate around 62 Ma and by decreasing the initiation age of the Louisville plume from 120 to 90 Ma. This improved model features a significant eastward hot spot motion of ~5Ā° between 80 and 30 Ma for the Louisville hot spot, which is quite dissimilar to the southward motion of the Hawaiian hot spot during the same time interval, followed by a minor ~2Ā° latitudinal shift over the last 30 Myr. If hot spot tracks are considered globally, the age trend observed for the oldest part of the Louisville seamount trail does not entirely follow the numerical model predictions. This may indicate some remaining inaccuracies in the global plate circuit, but it may also indicate that the Louisville hot spot experienced a motion somewhat different than in the numerical model: faster in the interval between 62 and 47 Ma but slower before that.Copyrighted by American Geophysical Union.Keywords: hot spots, seamounts, ā“ā°Ar/Ā³ā¹Ar geochronology, submarine alteration, guyots, Pacific plat
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Intraplate seamounts as a window into deep earth processes
Seamounts are windows into the deep Earth that are helping to
elucidate various deep Earth processes. For example, thermal and mechanical
properties of oceanic lithosphere can be determined from the flexing of oceanic
crust caused by the growth of seamounts on top of it. Seamount trails also are
excellent recorders of absolute plate tectonic motions and provide key insights into
the relationships among plate motion, plume motion, whole-Earth motion, and
mantle convection. And, because seamounts are created from the partial melts of
deep mantle sources, they offer unique glimpses into the chemical development and
heterogeneity of Earthās deepest regions. Current research efforts focus on resolving
the fundamental differences between magmas generated by passive upwelling
from upper mantle regions and deep mantle plumes rising from the core-mantle
boundary, mapping the different modes of mantle plumes and mantle convection,
reconciling fixed and nonfixed mantle plumes, and understanding the prolonged
volcanic evolution of seamounts. The role of intraplate seamounts is pivotal to this
research, and we must collect vast amounts more geochemical and geophysical
data to advance our knowledge. These data needs leave the ocean wide open for
future seamount exploration
"Petit spot" rejuvenated volcanism superimposed on plume-derived Samoan shield volcanoes: Evidence from a 645-m drill core from Tutuila Island, American Samoa
Author Posting. Ā© American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 20(3), (2019): 1485-1507, doi:10.1029/2018GC007985.In 2015 a geothermal exploration well was drilled on the island of Tutuila, American Samoa. The sample suite from the drill core provides 645 m of volcanic stratigraphy from a Samoan volcano, spanning 1.45 million years of volcanic history. In the Tutuila drill core, shield lavas with an EM2 (enriched mantle 2) signature are observed at depth, spanning 1.46 to 1.44 Ma. These are overlain by younger (1.35 to 1.17 Ma) shield lavas with a primordial ācommonā (focus zone) component interlayered with lavas that sample a depleted mantle component. Following ~1.15 Myr of volcanic quiescence, rejuvenated volcanism initiated at 24.3 ka and samples an EM1 (enriched mantle 1) component. The timing of the initiation of rejuvenated volcanism on Tutuila suggests that rejuvenated volcanism may be tectonically driven, as Samoan hotspot volcanoes approach the northern terminus of the Tonga Trench. This is consistent with a model where the timing of rejuvenated volcanism at Tutuila and at other Samoan volcanoes relates to their distance from the Tonga Trench. Notably, the Samoan rejuvenated lavas have EM1 isotopic compositions distinct from shield lavas that are geochemically similar to āpetit spotā lavas erupted outboard of the Japan Trench and late stage lavas erupted at Christmas Island located outboard of the Sunda Trench. Therefore, like the Samoan rejuvenated lavas, petit spot volcanism in general appears to be related to tectonic uplift outboard of subduction zones, and existing geochemical data suggest that petit spots share similar EM1 isotopic signatures.Reviews from Kaj Hoernle and three anonymous reviewers are gratefully acknowledged. M. G. J. acknowledges support from the American Samoa Power Authority and National Science Foundation grants OCEā1736984 and EARā1624840. The Tutuila drill core was the brainchild of Tim Bodell, without whom we would still have no stratigraphic record of Tutuila volcanism. The support of Utu Abe Malae and Matamua Katrina Mariner was instrumental to the project's success. We dedicate this paper to the memory of Abe Malae and his efforts to support science and education in American Samoa. Images of the entire drill core are available online (escholarship.org/uc/item/6gg6p61w). All data presented are either part of this study or previously published and are referenced in text.2019-08-1
Defining the word āseamountā
Author Posting. Ā© Oceanography Society, 2010. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 23, 1 (2010): 20-21.The term seamount has been
defined many times (e.g., Menard, 1964; Wessel, 2001; Schmidt and
Schmincke, 2000; Pitcher et al., 2007; International Hydrographic
Organization, 2008; Wessel et al., 2010) but there is no āgenerally
acceptedā definition. Instead, most definitions serve the particular
needs of a discipline or a specific paper
Vailuluāu Seamount
Author Posting. Ā© Oceanography Society, 2010. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 23, 1 (2010): 164-165.Vailuluāu seamount is an active underwater
volcano that marks the end of
the Samoan hotspot trail
Seamount sciences : quo vadis?
Author Posting. Ā© Oceanography Society, 2010. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 23, 1 (2010): 212-213.Seamounts are fascinating natural
ocean laboratories that inform us about
fundamental planetary and ocean
processes, ocean ecology and fisheries,
and hazards and metal resources. The
more than 100,000 large seamounts
are a defining structure of global
ocean topography and biogeography,
and hundreds of thousands of smaller
ones are distributed throughout every
ocean on Earth
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