The Oligocene-Miocene Pacific-Australia plate boundary, south of New Zealand: Evolution from oceanic spreading to strike-slip faulting

Despite the key importance of altered oceanic mantle as a repository and carrier of light elements (B, Li, and Be) to depth, its inventory of these elements has hardly been explored and quantified. In order to constrain the systematics and budget of these elements we have studied samples of highly serpentinized (>50%) spinel harzburgite drilled at the Mid-Atlantic Ridge (FifteenTwenty Fracture zone, ODP Leg 209, Sites 1272A and 1274A). In-situ analysis by secondary ion mass spectrometry reveals that the B, Li and Be contents of mantle minerals (olivine, orthopyroxene, and clinopyroxene) remain unchanged during serpentinization. B and Li abundances largely correspond to those of unaltered mantle minerals whereas Be is close to the detection limit. The Li contents of clinopyroxene are slightly higher (0.44-2.8 lg g À1 ) compared to unaltered mantle clinopyroxene, and olivine and clinopyroxene show an inverse Li partitioning compared to literature data. These findings along with textural observations and major element composition obtained from microprobe analysis suggest reaction of the peridotites with a mafic silicate melt before serpentinization. Serpentine minerals are enriched in B (most values between 10 and 100 lg g À1 ), depleted in Li (most values below 1 lg g À1 ) compared to the primary phases, with considerable variation within and between samples. Be is at the detection limit. Analysis of whole rock samples by prompt gamma activation shows that serpentinization tends to increase B (10.4-65.0 lg g À1 ), H 2 O and Cl contents and to lower Li contents (0.07-3.37 lg g À1 ) of peridotites, implying that-contrary to alteration of oceanic crust-B is fractionated from Li and that the B and Li inventory should depend essentially on rock-water ratios. Based on our results and on literature data, we calculate the inventory of B and Li contained in the oceanic lithosphere, and its partitioning between crust and mantle as a function of plate characteristics. We model four cases, an ODP Leg 209-type lithosphere with almost no igneous crust, and a Semail-type lithosphere with a thick igneous crust, both at 1 and 75 Ma, respectively. The results show that the Li contents of the oceanic lithosphere are highly variable (17-307 kg in a column of 1 m Â 1 m Â thickness of the lithosphere (kg/col)). They are controlled by the primary mantle phases and by altered crust, whereas the B contents (25-904 kg/col) depend entirely on serpentinization. In all cases, large quantities of B reside in the uppermost part of the plate and could hence be easily liberated during slab dehydration. The most prominent input of Li into subduction zones is to be expected from Semail-type lithosphere because most of the Li is stored at shallow levels in the plate. Subducting an ODP Leg 209-type lithosphere would mean only very little Li contribution from the slab. Serpentinized mantle thus plays an important role in B recycling in subduction zones, but it is of lesser importance for Li.

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“…Heaton and Hartzell, 1987), or about to be subducted (e.g. Macquarie Island, Sutherland, 1995;Lamarche et al, 1997).…”

Section: Model Parameters and Rationalementioning

confidence: 99%

The Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere

Vils¹,

Pelletier²,

Kalt³

et al. 2008

Geochimica et Cosmochimica Acta

118

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“…At c. 45 Ma, seafloor spreading began in the Emerald Basin, as the Southeast Indian Ridge propagated northeastwards from the Macquarie triple junction into the New Zealand region (Molnar et al 1975;Weissel et al 1977;Kamp 1986a;Sutherland 1995;Wood et al 1996b;Lamarche et al 1997;Veevers 2000). By 40 Ma, spreading to the south of New Zealand was well established, and initial development of the plate boundary system through New Zealand had begun.…”

Section: Reconstructions 40 Ma (Mid Bortonian) (Fig 1a)mentioning

confidence: 99%

Tectonic reconstructions of New Zealand: 40 Ma to the Present

King¹

2000

New Zealand Journal of Geology and Geophysics

340

369

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“…This episode of seafloor spreading generated the oceanic crust in the Emerald Basin and Southeast Tasman Ocean Crust (STOC). Starting around 31 Ma, spreading became more and more oblique until, around 12 Ma, motion along the boundary became essentially strike-slip [Lamarche et al, 1997]. Several recent studies have mapped the fracture zones and anomalies in these basins [Massell et al, 2000;Wood et al, 2000;Keller, 2003).…”

Section: Southeast Tasman Sea and The Emerald Basinmentioning

confidence: 99%

Cenozoic reconstructions of the Australia-New Zealand-South Pacific sector of Antarctica

Cande

Stock

2004

Geophysical Monograph Series

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The Oligocene-Miocene Pacific-Australia plate boundary, south of New Zealand: Evolution from oceanic spreading to strike-slip faulting

Cited by 87 publications

References 18 publications

The Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere

The Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere

Tectonic reconstructions of New Zealand: 40 Ma to the Present

Cenozoic reconstructions of the Australia-New Zealand-South Pacific sector of Antarctica

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