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doi:10.2204/iodp.proc.309312.205.2009

Shipboard observations of chilled margins

During Expedition 312, the structure team compiled detailed observations of hand samples and thin sections (with polarized light microscopy). We summarize those observations here, with links to the relevant figures in the “Site 1256” chapter of this volume. Kempton (1985) compiled similar observations from Deep Sea Drilling Project/​Ocean Drilling Program (DSDP/ODP) Hole 504B.

Chilled margins are the fine-grained intrusive contacts where aphanitic edges of (younger) dikes are in direct contact with the coarser-grained (older) dikes (see Fig. F299 in the “Site 1256” chapter).

Variations in grain size, abundance of spherules, and alteration cause weak color banding parallel to the chilled margins. In aphanitic to finer grained parts of chilled margins, “rheomorphs”—indicators of flow age when the rock was in a molten state such as flow banding and stretched spherules—are inclined (~10° from parallelism) toward the chilled margin. Where grain size is coarse enough, plagioclase laths are generally oriented parallel to the chilled margins (see Fig. F294 in the “Site 1256” chapter).

In other samples there are few flow indicators, and in one instance the spherulitic texture forms a wavy pattern along the dike margin, potentially indicating static cooling; that is, cooling after magma ceased flowing through the dike conduit (see Fig. F299F, F299G in the “Site 1256” chapter).

A feature near many chilled margins is thin (<1 cm) intrusions of glassy to aphanitic basalt. In some samples these intrusions are apophyses that enclose clasts derived from the adjacent coarser grained basalt (see Fig. F300A–F300E in the “Site 1256” chapter). Elsewhere, intrusions are aphanitic to glassy basalt in veinlets or en echelon cracks adjacent to the main chilled margin (see Figs. F299H, F300B in the “Site 1256” chapter).

Chilled margins in many samples are riddled with chlorite and actinolite veins. In some samples veins are crosscut and offset by laminations in the chilled margin, whereas other veins crosscut the chilled margins (see Fig. F302 in the “Site 1256” chapter).

Where the crosscutting relations between veins and chilled margins are especially chaotic, the chilled margin was classified as a “dike-margin breccia” (see Fig. F301 in the “Site 1256” chapter).

Backscatter electron images and electron dispersive spectroscopy of chilled margins

Structures shown previously within and around chilled margins are difficult to study in hand sample and optical microscopy alone; grains can be a few micrometers to submicrometer in size, and certain phases are intermixed and indistinguishable in transmitted light. To better document these important features, we present a collection of BSEI in Figures F2, F3, F4, F5, and F6. The images were taken from areas either within, adjacent to, or at the contact of chilled margins. The thin sections studied are indicated in Figure F1 at their approximate depth. For a detailed view of thin section images, refer to “Core descriptions.”

All images were taken with the University of Texas Department of Geological Sciences JEOL-8200 electron microprobe in backscatter mode at a voltage of 15 kV and a current of 15 nA.

The host rock or wallrock immediately adjacent to the chilled margins is texturally similar to the interior of the dikes with subophitic textures defined by feldspar and pyroxene or amphibole. At shallower depths, relatively unaltered clinopyroxene partially surrounds a mixture of albite and anorthite, with interstitial calcite and small grains of sphalerite locally present (Fig. F2A); we note that calcium carbonate is otherwise relatively scarce in Hole 1256D below the lavas (Coggon et al., 2006). In deeper sections, clinopyroxene is completely replaced by actinolite (and/or actinolitic hornblende) and feldspars are a mixture of albite and anorthite (Fig. F2B).

The chilled margins define relatively sharp and clean contacts with the wallrocks, cutting phenocrysts of feldspar and pyroxene (Fig. F2C). At relatively shallow levels of the SDC, the chilled margins comprise an aphanitic mixture of clinopyroxene and albite (replacing plagioclase) (Fig. F2D). Locally and at deeper levels, micrometer-scale mixtures of clinopyroxene and feldspar define the texture of relatively unaltered chilled margins, possibly an original “quench” texture (Fig. F2E).

In hand sample and thin section, the defining quality of many chilled margins is color banding from dark gray to dark to medium green. There is no obvious change in grain size across these bands. In BSEI (with EDS), banding is due to variations in Fe content of aphantitic actinolite (Fig. F2F).

A variety of detailed microstructures are preferentially preserved in chilled margins that are indicative of magmatic flow, such as feldspar laths with preferred orientations (Fig. F3A). In Figure F3A, aphanitic (possibly submicrometer) material of chlorite composition surrounds the feldspar laths. Other flow and quench microstructures include widely disseminated micrometer-scale magnetite grains (Fig. F3B) likely formed during devitrification of glassy basalt; microfolds defined by aphanitic, finely mixed albite and chlorite (and adjacent vugs filled with actinolite) (Fig. F3C, F3D); spherules composed of actinolite and albite-anorthite (Fig. F3E); and clasts of feldspar suspended in the chilled margin (Fig. F3F).

One of the more extraordinary microstructures in hand sample, thin section, and BSEI are lenses of material oriented parallel to and stretched along the chilled margins. In a sample from Section 312-1256D-176R-2 a lens of titanite (sphene) is deformed along a chilled margin (Fig. F4A) (a similar example from Pacific crust exposed in the Hess Deep Rift is presented in Hayman and Karson, 2007). A chlorite-actinolite vein cuts the lens, though the composition changes along the trace of the vein. In other chilled margins veins are more homogeneous, in many places with actinolite (and/or chlorite) surrounded by albite (Fig. F4B). In Figure F4B, the composite actinolite-albite vein crosscuts stretched spherules and other rheomorphs but has relatively irregular vein walls. Elsewhere, actinolite fills vugs within albite veins, in turn crosscutting actinolite-rich areas of the chilled margin (Fig. F4C).

Some additional microstructures resulting from hydrothermal alteration include lenses of aphantic aggregates of chlorite that “wrap” around the chilled margin (Fig. F5A), nodules of quartz and chlorite within otherwise unaltered host rock (Fig. F5B), nodules of quartz suspended within chlorite-actinolite and anhydrite (Fig. F5C), and concentrated nodules of pyrite surrounded by quartz (Fig. F5D). Anhydrite is also found within the chilled margins, either along actinolite-chlorite veins (Fig. F5E) or as interstitial nodules surrounded by quartz and needles of iron oxides (Fig. F5F); initial thermomagnetic analyses confirm that magnetite is the main iron oxide phase in most dike-rock samples.

The last suite of BSEIs of note are shown in Figure F6, where feldspars adjacent to the chilled margins show a “mottled” alteration texture, in places enriched in potassium. Such orthoclase (or its low-temperature equivalent, adularia) enrichment appears to be localized to the host rock adjacent to the chilled margins or clasts suspended within the chilled margins (Fig. F6A–F6C, F6E, F6F). In places, orthoclase-rich grains are found adjacent to chlorite veins (Fig. F6D), but here, too, the orthoclase enrichment is spatially associated with the chilled margin.

Wavelength dispersive spectroscopy of feldspar grains

Intrigued by the orthoclase in feldspars along chilled margins, we collected WDS data from eight grains in two different thin sections. Analytical standards included anorthoclase, orthoclase, and albite. We analyzed for Na₂O, K₂O, CaO, SiO₂, and Al₂O₃ weight percent, along with SrO and BaO (Table T1). All analyses have a precision of <0.5%, with the exception of BaO (3.71%). We did not analyze for FeO and MgO, which are presumably present in trace amounts. Probe conditions for quantitative analysis were the same as for imaging, with a beam diameter set at ~1 µm. Such a narrow beam diameter allowed analysis of spatially heterogeneous alteration patterns (as shown in Fig. F6), though it unfortunately led to several discarded analyses (owing to beam damage and/or enhanced cation diffusion); analyses with totals of 98–102 wt% were considered acceptable.

Plagioclase (anorthite-albite) compositions range from ~25% to 100% albite, whereas the alkalai feldspars (albite-orthoclase) compositions range from ~0% to 100% orthoclase (Fig. F7A). Two alkalai feldspars have unusually high CaO concentrations. There is no obvious dependency on feldspar composition and microstructural position (see Table T1, Structure column), depth, or grain habit.

Lastly, we compare SrO and BaO concentrations (Fig. F7B) and K₂O and BaO concentrations; there is a weak, positive correlation between K₂O and BaO. These were the only possible visual correlations after systematically plotting SrO and BaO against the other oxides.

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