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Structure and geochemistry of Macquarie Island oceanic crust

Entry ID: ASAC_903

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Summary

Abstract: Owing to the fact that the principal investigstor died before data were able to be archived, the only available data are in the form of the referenced paper, which is available as a PDF download.

From the referenced papers:

Macquarie Island is an exposure above sea level of the Macquarie Ridge Complex, on the boundary between the Australian and Pacific plates south of New Zealand. Geodynamic ... reconstructions show that at ca. 12-9.5 Ma, oceanic crust of the Macquarie Island region was created at this plate boundary within a system of short spreading-ridge segments linked by large-offset transform faults. At this time, the spreading rate was slowing (less than 10 mm/yr half-spreading rate) and magmatism was waning. Probably before 5 Ma, and possibly before the extinct spreading ridge had subsided, the plate boundary became obliquely convergent, and crustal blocks were rotated, tilted, and uplifted along the ridge to form the island. Planation by marine erosion has exposed sections through the oceanic crust.
The magmatism that built the oceanic crust produced melts similar in composition to the widespread normal to enriched mid-oceanic ridge basalt (N- to E-MORB) suite found in many spreading ridges, but the melts ranged beyond E-MORB to primitive, highly enriched, and silica-undersaturated compositions. These compositions form one end member of a continuum from MORB but seem not to have been derived from a MORB-source mantle, despite sharing a Pacific MORB isotopic signature. The survival of these primitive melts may be due to their origin in a slow-spreading system that must have been closing down as extension along the plate boundary gave way to transpression, putting a stop to the upwelling of asthenosphere and decompression melting. In a more energetic, faster-spreading system, mixing would have been more efficient, the presence of this end member could not easily have been inferred from its isotopic composition, and the igneous rocks would have resembled a typical N- to E-MORB suite. Macquarie Island may therefore provide a type example of magmatism at a very slow spreading ridge and a clue to the origins of E-MORB.

Macquarie Island is an exposure above sea-level of part of the crest of the Macquarie Ridge. The ridge marks the Austrlia-Pacific plate boundary south of New Zealand, where the plate boundary has evolved progressively since Eocene times from an oceanic spreading system into a system of long transform faults linked by short spreading segments, and currently into a right-lateral strike-slip plate boundary. The rocks of Macquarie Island were formed during spreading at this plate boundary in Miocene times, and include intrusive rocks (mantle and cumulate periodites, gabbros, sheeted dolerite dyke complexes), volcanic rocks (N- to E-MORB pillow lavas, picrites, breccias, hyaloclastites), and associated sediments. A set of Macquarie Island basaltic glasses has been analysed by electron microphobe for major elements, S, Cl, and F; by Fourier transform infrared spectroscopy for H2O; by laser ablation-inductively coupled plasma mass spectrometry for trace elements; and by secondary ion mass spectrometry for Sr, Nd and Pb isotopes. Macquarie Island basaltic glasses are divided into two compositional groups according to their mg-number-K2O relationships. Near-primitive basaltic glasses (Group I) have the highest mg-number (63-69), and high Al2O3 and CaO contents at a given K2O content, and carry microphenocrysts of primitive olivine (Fo86-89.5). Their bulk compositions are used to calculate primary melt compositions in equilibrium with the most magnesian Macquarie Island olivines (Fo90.5). Fractionated, Group II, basaltic glasses are saturated with olivine + plagioclase + or - clinopyroxene, and have lower mg-number (57-67), and relatively low Al2O3 and CaO contents. Group I glasses define a seriate variation within the compositional spectrum of MORB, and extend the compositional range from N-MORB compositions to enriched compositions that represent a new primitive enriched MORB end-member. Compared with N-MORB, this new end-member is characterised by reltively low contents of MgO, FeO, SiO2 and CaO, coupled with high contents of Al2O3, TiO2, NaO2, P2O5, K2O and incompatible trace elements, and has the most radiogenic Sr and Pb regional isotope composition. These unusual melt compositions could have been generated by low-degree partial melting of an enriched mantle peridotite source, and were erupted without significant mixing with common -MORB magmas. The mantle in the Macquarie Island region must have been enriched and heterogenous on a very fine scale. We uggest that the mantle enrichment implicated in this study is more likely to be a regional signature that is shared by the Balleny Islands magmatism than directly related to the hypothetical Balleny plume itself.

Related URL

Link: VIEW PROJECT HOME PAGE

Description: Public information for ASAC project 903

Geographic Coverage

(Click for Interactive Map)

Spatial coordinates

N: -52.0

S: -55.0

E: 165.0

W: 150.0

Temporal Coverage

Start Date: 1995-09-30

Stop Date: 1996-03-31

Location Keywords

OCEAN > SOUTHERN OCEAN

OCEAN > SOUTHERN OCEAN > MACQUARIE ISLAND

GEOGRAPHIC REGION > POLAR

Science Keywords

SOLID EARTH > ROCKS/MINERALS/CRYSTALS > ELEMENTS > MAJOR ELEMENTS

SOLID EARTH > ROCKS/MINERALS/CRYSTALS > ELEMENTS > MINOR ELEMENTS

ISO Topic Category

GEOSCIENTIFIC INFORMATION

OCEANS

Instrument

MASS SPECTROMETERS

SPECTROMETERS

Quality Values provided in temporal coverage are approximate only.

Access Constraints A PDF copy of the referenced papers are available for download from the provided URL to AAD staff only.

Use Constraints This data set conforms to the PICCCBY Attribution License
(http://creativecommons.org/licenses/by/3.0/).

Please follow instructions listed in the citation reference provided at http://data.aad.gov.au/aadc/metadata/citation.cfm?entry_id=ASAC_904 when using these data.

Ancillary Keywords

E-MORB

GEOCHEMISTRY

Data Set Progress

COMPLETE

Data Center

Australian Antarctic Data Centre, Australia

Data Center URL: http://data.aad.gov.au

Data Center Personnel
Name: DATA OFFICER AADC Phone: +61 3 6232 3244 Fax: +61 3 6232 3351 Email: metadata at aad.gov.au Contact Address: Australian Antarctic Division 203 Channel Highway City: Kingston Province or State: Tasmania Postal Code: 7050 Country: Australia

Distribution

Distribution Media: HTTP

Distribution Size: 9562 kb

Distribution Format: PDF

Fees: free

Personnel

RICHARD VARNE (DECEASED)

Role: INVESTIGATOR

Role: TECHNICAL CONTACT

Role: DIF AUTHOR

GARRY DAVIDSON

Role: TECHNICAL CONTACT

Phone: +61 3 6226 2815

Fax: +61 3 6223 2547

Email: Garry.Davidson at utas.edu.au

Contact Address:

DEPARTMENT OF GEOLOGY

GPO BOX 252-79

University of Tasmania

City: Hobart

Province or State: Tasmania

Postal Code: 7001

Country: Australia

Publications/References

Varne R., Brown A.V., Falloon T. (2000), Macquarie Island: its geology and structural history, and the timing and tectonic setting of its N-MORB to E-MORB magmatism., Geological Society of America, Special Publication. Ophiolites and oceanic crust: new insights from field studies and the Ocean Drilling Program.

Creation and Review Dates

DIF Creation Date: 2000-08-08

Last DIF Revision Date: 2011-11-29

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