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| Titel |
Alpha / Mendeleev Ridge and Chukchi Borderland 40Ar/39Ar Geochronology and Geochemistry: Character of the First Submarine Intraplate Lavas Recovered from the Arctic Ocean |
| VerfasserIn |
Samuel B. Mukasa, Larry A. Mayer, Kimberly Aviado, Julie Bryce, Alex Andronikov, Kelley Brumley, Janne Blichert-Toft, Oleg Petrov, Sergey Shokalsky |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250108535
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| Publikation (Nr.) |
 EGU/EGU2015-8291.pdf |
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| Zusammenfassung |
| At least three episodes of magmatic activity have been recognized on the basis of 40Ar/39Ar
age determinations in the submarine basaltic samples dredged, drilled or grabbed with a
manipulation arm from Alpha / Mendeleev Ridge and Chukchi Borderland of the Arctic
Ocean by US Coast Guard Icebreaker Healy, in August-September 2008, and Russian
research vessel Captain Dranitsin in August-October 2012: ca. 112 Ma, ca. 100 Ma and ca.
85-73 Ma. Major-oxide and trace-element concentrations, and Pb, Sr, Nd, and Hf isotopic
ratios of the recovered lavas provide important constraints on the composition and sources for
the original melts. Lavas erupted at ca. 112 Ma (Group 1) have alkali basalt major-oxide
compositions. Their low degree of rare-earth-element (REE) fractionation (CeN/YbN
= 1.7-2.5), combined with high overall HREE (22-24 times chondrite) and Mg#
~54, suggest derivation from a garnet-free source followed by only minimal crystal
fractionation for this group. Pb-Sr-Nd-Hf isotopic systematics of the lavas (206Pb/204Pb =
18.73-18.79; 207Pb/204Pb = 15.54-15.56; 208Pb/204Pb = 38.28-38.35; 143Nd/144Nd =
0.512594-0.512610; 87Sr/86Sr = 0.709458-0.709601; 176Hf/177Hf = 0.283224),
together with ratios of highly incompatible trace elements (Th/Ce = 0.09-0.11;
Ce/Nb = 2.58-3.09; Th/Nb = 0.24-0.33), point toward a lithospheric source for the
magmas.
Eruptions at ca. 100 Ma and 85-73 Ma produced two types of lavas: low-Ti tholeiitic basalts –
LT, and high-Ti alkali basalts – HT, both assigned to Group 2. This distribution of low- and
high-Ti lavas is common in continental flood basalt (CFB) provinces elsewhere, and has been
attributed to plume activity in some studies. The trace-element abundance patterns for these
Group 2 Arctic lavas are also very similar to those of CFBs elsewhere. Their low
degrees of REE fractionation (CeN/YbN = 2.0-3.3) accompanied by progressively
decreasing Mg#s (from 53 to 33) suggest a garnet-free source, with the derivative
magmas experiencing significant crystal fractionation prior to eruption. Both LT and
HT basalts have Sr, Nd and Hf isotope ratios that fall between MORB and Bulk
Silicate Earth (143Nd/144Nd = 0.512669-0.512919; 87Sr/86Sr = 0.703820-0.704764;
176Hf/177Hf = 0.283128-0.283191), and are thus characterized as depleted, most likely
originating in a subcontinental asthenospheric source. Contamination of these lavas
with small amounts of lithospheric components is a distinct possibility. HT basalts,
generally the younger of the two lava types in Group 2, are more depleted in their
Nd and Sr isotopic compositions than the older LT basalts. Measured Pb-isotopic
ratios plot mostly along and above the Northern Hemisphere Reference Line or
NHRL (206Pb/204Pb = 18.59-19.37; 207Pb/204Pb = 15.55-15.60; 208Pb/204Pb =
38.31-38.99). Volume estimates between the Group 1 and Group 2 lavas are not yet possible
to determine. However, the composition-time relationships for the lavas suggest
inception of melting in the Amerasia Basin sub-continental lithospheric mantle
(SCLM) – probably due to rift-related decompression – followed later (at 100 Ma and
85-73 Ma) by asthenospheric melting that may or may not be associated with a
plume. |
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