Geochemical variations in mid-ocean ridge basalts from the Cocos-Nazca Spreading Center
Abstract
The westward propagating Cocos-Nazca Spreading Center (CN) rifts 0.5 Ma ocean crust accreted at the adjacent East Pacific Rise. From its western tip eastward, the CN rift transitions from amagmatic rifting to full magmatic spreading, with shoaling of axial depth (4500 - 2700 m) and increasing spreading rate (16 - 40 mm/yr), segment length (16 - 50 km), and gravity-inferred crustal thickness (Zheng et al abstract). MORB were collected along the first 9 CN segments (360 km), adjacent EPR, and Dietz Volcanic Ridge (RV Sally Ride 1806; 66 dredges).
A striking feature of the CN lava compositions is their unusually depleted incompatible trace element ratios (e.g., La/SmN as low as 0.38) compared to typical EPR MORB. Similarly depleted values have been found in lavas from leaky transforms (eg Garrett, Siqueiros) and EPR seamounts, commonly associated with transitional or enriched compositions, and often interpreted as incomplete mixing of diverse mantle melts. Along the CN rift, however, eruption of only highly depleted to depleted melts suggests an absence of typical source heterogeneity, which may relate to the rift's proximity to the EPR. Melting beneath much of the CN rift may tap a source previously depleted in early melting components (eg, pyroxenite veins) during melt generation beneath the EPR, as suggested for lavas from Garrett. While only depleted samples were recovered from the CN rift, transitional compositions occur along the Dietz VR, the southern boundary of the Galapagos microplate. Along-axis phenocryst systematics differ between western (S2, 3, 4) and eastern segments (S5-9). The size and abundance of plagioclase phenocrysts changes abruptly from western (microlites, < 1 - 4 %) to eastern segments (up to 8 mm, < 1 - 20 %), suggesting that shallow level crustal processes differ between the two regions. Crystal Size Distributions indicate that relatively evolved lavas (MgO ≈ 7.6 wt. %) from western segments experience physical processes that efficiently separate melt from crystals prior to eruption. Greater abundances and crystal sizes of eastern lavas suggests that magma flux is sufficient to entrain crystals during ascent. The contrast in phenocryst systematics between western and eastern segments accords with tectonic interpretations that suggest a stable melt supply has not yet developed beneath S1-4.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V11F0153W
- Keywords:
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- 1033 Intra-plate processes;
- GEOCHEMISTRY;
- 7208 Mantle;
- SEISMOLOGY;
- 8137 Hotspots;
- large igneous provinces;
- and flood basalt volcanism;
- TECTONOPHYSICS;
- 8416 Mid-oceanic ridge processes;
- VOLCANOLOGY