Anthony A. P. Koppers

ArArCALC-software for 40 Ar/ 39 Ar age calculations

ArArCALC is a Microsoft Excel® 97-2000-XP application for performing calculations in 40Ar/39Ar geochronology. It is coded in Visual Basic for Applications and can be used under the Windows® 95/98/NT/2000/ME/XP operating systems. ArArCALC provides an easy-to-use graphical interface for the calculation of age plateaus, total fusion ages and isochrons following the regression of 40Ar/39Ar mass spectrometry data. Results are stored in single Excel workbooks including nine different data tables and four different diagrams. Analytical, internal and external errors are calculated based on error propagation of all input parameters, analytical data and applied corrections. Finally, the age calculation results can be recalibrated with reference to the primary K-Ar standards (e.g. GA-1550, MMhb-1) in order to obtain more consistent absolute 40Ar/39Ar age determinations. ArArCALC is distributed as freeware.

ArArCALC—software for 40Ar/39Ar age calculations

ArArCALC is a Microsoft Excel® 97-2000-XP application for performing calculations in 40Ar/39Ar geochronology. It is coded in Visual Basic for Applications and can be used under the Windows® 95/98/NT/2000/ME/XP operating systems. ArArCALC provides an easy-to-use graphical interface for the calculation of age plateaus, total fusion ages and isochrons following the regression of 40Ar/39Ar mass spectrometry data. Results are stored in single Excel workbooks including nine different data tables and four different diagrams. Analytical, internal and external errors are calculated based on error propagation of all input parameters, analytical data and applied corrections. Finally, the age calculation results can be recalibrated with reference to the primary K-Ar standards (e.g. GA-1550, MMhb-1) in order to obtain more consistent absolute 40Ar/39Ar age determinations. ArArCALC is distributed as freeware.

The return of subducted continental crust in Samoan lavas

Substantial quantities of terrigenous sediments are known to enter the mantle at subduction zones, but little is known about their fate in the mantle. Subducted sediment may be entrained in buoyantly upwelling plumes and returned to the Earth’s surface at hotspots, but the proportion of recycled sediment in the mantle is small, and clear examples of recycled sediment in hotspot lavas are rare. Here we report remarkably enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures in Samoan lavas from three dredge locations on the underwater flanks of Savai’i island, Western Samoa. The submarine Savai’i lavas represent the most extreme 87Sr/86Sr isotope compositions reported for ocean island basalts to date. The data are consistent with the presence of a recycled sediment component (with a composition similar to the upper continental crust) in the Samoan mantle. Trace-element data show affinities similar to those of the upper continental crust—including exceptionally low Ce/Pb and Nb/U ratios—that complement the enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures. The geochemical evidence from these Samoan lavas significantly redefines the composition of the EM2 (enriched mantle 2; ref. 9) mantle endmember, and points to the presence of an ancient recycled upper continental crust component in the Samoan mantle plume.

Testing the fixed hotspot hypothesis using Ar-40/Ar-39 age progressions along seamount trails.

Hotspots and their associated intra-plate volcanism producing seamount trails have become an accepted fact in geology from a conceptual theory. The azimuths and age progressions of these seamount trails provide the only means to determine absolute plate motions with respect to an independent reference frame of ‘fixed’ hotspots. However, the presumed fixity of hotspots is in disagreement with recent paleomagnetic studies and global-circuit plate reconstructions for the Hawaiian–Emperor seamount trail. In this study, we provide independent evidence suggesting that hotspots are not fixed relative to each other. We use a straightforward test that compares the observed 40Ar/39Ar age progressions along Pacific seamount trails (0–140 Myr) with the Pacific plate velocities as predicted by their poles of plate rotation (i.e. Euler poles). In most of these comparisons, the age progressions were found incompatible with published Euler poles, or with a new set of Euler poles as derived in this study using discrete seamount locations digitized from the bathymetry maps of Smith and Sandwell [EOS 77 (1996) 315; Science 277 (1997) 1956–1921]. We conclude that the relative motion between hotspots may be required to reconcile the observed age progressions with the predicted plate velocities from their modeled Euler poles. On average, the Pacific hotspots may show motion at 10–60 mm/yr over the last 100 Myr, partly attributed to individual hotspot motion, whereas systematic motion of these hotspots (due to true polar wander) may account for the remainder.

Short-lived and discontinuous intraplate volcanism in the South Pacific: Hot spots or extensional volcanism?

[1] South Pacific intraplate volcanoes have been active since the Early Cretaceous. Their HIMU-EMIEMII mantle sources can be traced back into the West Pacific Seamount Province (WPSP) using plate tectonic reconstructions, implying that these distinctive components are enduring features within the Earth’s mantle for, at least, the last 120 Myr. These correlations are eminent on the scale of the WPSP and the South Pacific Thermal and Isotopic Anomaly (SOPITA), but the evolution of single hot spots emerges notably more complicated. Hot spots in the WPSP and SOPITA mantle regions typically display intermittent volcanic activity, longevities shorter than 40 Myr, superposition of hot spot volcanism, and motion relative to other hot spots. In this review, we use 40 Ar/ 39 Ar seamount ages and Sr-Nd-Pb isotopic signatures to map out Cretaceous volcanism in the WPSP and to characterize its evolution with respect to the currently active hot spots in the SOPITA region. Our plate tectonic reconstructions indicate cessation of volcanism during the Cretaceous for the Typhoon and Japanese hot spots; whereas the currently active Samoan, Society, Pitcairn and Marquesas hot spots lack long-lived counterparts in the WPSP. These hot spots may have become active during the last 20 Myr only. The other WPSP seamount trails can be only ‘‘indirectly’’ reconciled with hot spots in the SOPITA region. Complex age distributions in the Magellan, Anewetak, Ralik and Ratak seamount trails would necessitate the superposition of multiple volcanic trails generated by the Macdonald, Rurutu and Rarotonga hot spots during the Cretaceous; whereas HIMU-type seamounts in the Southern Wake seamount trail would require 350–500 km of hot spot motion over the last 100 Myr following its origination along the Mangaia-Rurutu ‘‘hotline’’ in the Cook-Austral Islands. These observations, however, violate all assumptions of the classical Wilson-Morgan hot spot hypothesis, indicating that long-lived, deep and fixed mantle plumes cannot explain the intraplate volcanism of the South Pacific region. We argue that the observed short-lived and discontinuous intraplate volcanism has been produced by another type of hot spot-related volcanism, as opposed to the strong and continuous Hawaiian-type hot spots. Our results also indicate that other geological processes (plate tension, hotlines, faulting, wetspots, self-propagating volcanoes) may act in conjunction with hot spot volcanism in the South Pacific. In all these scenarios, intraplate volcanism has to be controlled by ‘‘broad-scale’’ events giving rise to multiple closely-spaced mantle plumelets, each with a distinct isotopic signature, but only briefly active and stable over geological time. It seems most likely that these plumelets originate and dissipate at very shallow mantle depths, where they may shoot off as thin plumes from the top of a ‘‘superplume’’ that is present in the South Pacific mantle. The absence of clear age progressions in most

Ages and magnetic structures of the South China Sea constrained by deep tow magnetic surveys and IODP Expedition 349

Combined analyses of deep tow magnetic anomalies and International Ocean Discovery Program Expedition 349 cores show that initial seafloor spreading started around 33 Ma in the northeastern South China Sea (SCS), but varied slightly by 1-2 Myr along the northern continent-ocean boundary (COB). A southward ridge jump of approximate to 20 km occurred around 23.6 Ma in the East Subbasin; this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the Southwest Subbasin, which propagated for about 400 km southwestward from approximate to 23.6 to approximate to 21.5 Ma. The terminal age of seafloor spreading is approximate to 15 Ma in the East Subbasin and approximate to 16 Ma in the Southwest Subbasin. The full spreading rate in the East Subbasin varied largely from approximate to 20 to approximate to 80 km/Myr, but mostly decreased with time except for the period between approximate to 26.0 Ma and the ridge jump (approximate to 23.6 Ma), within which the rate was the fastest at approximate to 70 km/Myr on average. The spreading rates are not correlated, in most cases, to magnetic anomaly amplitudes that reflect basement magnetization contrasts. Shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers, in addition to large vertical intensity variations. These complexities are caused by late-stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion. Deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0.5, which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization. The primary contribution to magnetic anomalies of the SCS is not in the top 100 m of the igneous basement.

Dating crystalline groundmass separates of altered Cretaceous seamount basalts by the Ar-40/Ar-39 incremental heating technique

Abstract Alteration of submarine basalts compromises geochronology using conventional K/Ar and 40 Ar/ 39 Ar techniques. To help overcome these problems, we re-evaluated the potential of groundmass dating techniques. Incremental heating on acid-leached groundmass samples, following an overnight bakeout at 200°C and using high-resolution heating schedules, eliminated most of the low temperature alteration effects in the submarine basalts studied. More than 75% of the groundmass analyses (n=32) display accurate age plateaus consisting of 30–70% of the total amount of 39 Ar K released. More than 50% of the analyses have plateau ages concordant with their total fusion ages implying minor or proportional loss of radiogenic 40 Ar * and 39 Ar K . Overall, we could show a high degree of coherence between ages of groundmass separates and comagmatic phenocrysts. This suggests that the dating of aphyric basalts, which previously has proven problematic, can be accomplished with increasing confidence as well. Adding these rock types to the list of datable submarine basalts significantly enhances our ability to understand the eruptive history of linear submarine volcanic chains.

Samoan hot spot track on a “hot spot highway”: Implications for mantle plumes and a deep Samoan mantle source

We report new geochemical data for submarine lavas from the Samoan region that greatly enhance the geochemical data set for volcanoes from the hot spot. Additionally, two volcanoes dredged in the northern Lau Basin, Futuna Island and Manatu seamount, are young (<5 Ma), appear to be genetically related, and may have been generated by melting a component of Samoan mantle that has been advected into the region. We also find evidence for three seamounts and one atoll along the Samoan hot spot track that are not geochemically related to Samoa. We use a plate motion model to show that three non-Samoan hot spots, currently active in the Cook-Austral Islands, provided volcanism to the Pacific Plate now in the Samoan region approximately 10–40 Ma. The four interloping volcanoes in the Samoan region exhibit geochemical affinities with the three hot spots. All three hot spots would have left a depleted, viscous, refractory keel that is coupled to the base of the Pacific lithosphere that has been “rafted” to the Samoan region. Therefore, the new data also have implications for the origin of the Samoan hot spot as its origin has been suggested to be a result of either a deep-seated mantle plume or a consequence of lithospheric cracking. Without major modification of the current “propagating lithospheric cracks” model, it is not clear how such cracks could yield melts from the refractory keel present under the Samoan lithosphere. Instead, a region of buoyantly upwelling mantle, or plume, is suggested to generate the shield stage volcanism in the Samoan region.

Samoa reinstated as a primary hotspot trail

The classical model for the generation of hotspot tracks maintains that stationary and deep-seated mantle plumes impinge on overriding tectonic plates, thereby generating age-progressive trails of volcanic islands and seamounts. Samoa has played a key role in discrediting this model and the very existence of mantle plumes, because early geochronological work failed to demonstrate a linear age progression along this chain of islands. Specifically on Savai9i Island, the bulk of the subaerial volcanics is younger than 0.39 Ma, much younger than the 5.1 Ma age predicted from the classical hotspot model and a constant 7.1 cm/yr Pacific plate motion. This discrepancy led to alternative magma-producing mechanisms that involve the cracking of the lithosphere beneath the Samoan islands, as a result of the extensional regime generated by the nearby Tonga Trench. Here we report 40 Ar/ 39 Ar ages from the submarine flanks of Savai9i Island showing that its volcanic construction began as early as 5.0 Ma and in a true intraplate setting. This reinstates Samoa as a primary hotspot trail associated with a deep mantle plume and a linear age progression.

PmagPy: Software package for paleomagnetic data analysis and a bridge to the Magnetics Information Consortium (MagIC) Database

Author(s): Tauxe, L; Shaar, R; Jonestrask, L; Swanson-Hysell, NL; Minnett, R; Koppers, AAP; Constable, CG; Jarboe, N; Gaastra, K; Fairchild, L | Abstract: