GSA Foundation

JinnyjadeGuatemala's Olmec Jade

 

Geologists and archaeologists searching the mountains of Guatemala have discovered rare, translucent blue and blue-green jadeitite, thought to be the long-lost source material for the gems of ancient Mesoamerica. With their find they are also breaking some long-held beliefs about the region’s geology.

 

Some 3,000 years ago, the first recorded culture of Mesoamerica, known as the Olmec, produced sculptures and figurines carved from a hard, translucent blue-green stone. Jadeite rock, or jadeitite, is the rarer and more sought-after sodium and aluminum silicate form of jade. Jadeitite comes in all colors depending on its trace elements and mineral inclusions, but the Olmecs favored a bluish color. The discovery of this type of jade in Guatemala is now providing scientists a chance to unravel many mysteries about the rock and the people who treasured it.

 

Virginia “Jinny” Sisson reclines on a pile of Guatemala jadeitite, some of which she will use to study how the gem rock originates at depth and travels to Earth’s surface. Courtesy of V. Sisson

“Early results from microprobe analysis indicate that the bluish color is correlated with titanium and iron content,” says George Harlow of the American Museum of Natural History in New York. Harlow spoke at a Gemological Institute of America alumni meeting in Washington on June 19. While all colors occur from the ions in the jade playing Frisbee with an electron, Harlow says his findings support the idea that the ions involved in the bluish color are iron and titanium rather than two valences of iron.

 

For decades archaeologists have debated whether the Olmec derived their jade from a mysterious source close to their empire along the Gulf of Mexico or close to Costa Rica where many jade artifacts are found. The Olmec source of blue jade was lost with the disappearance of their culture around 500 B.C. The rising Mayan culture of the time favored green jadeitite, and with the Spanish conquistadors searching only for gold, the blue jadeitite became obsolete.

 

“The market for blue jadeitite in the New World was erased,” Harlow says.”Today, Asian markets favor green or mauve jadeitite. And in the New World, where there should be a market for blue jadeitite there is none.” With no source material accessible, the only bluish jadeitite collected have been ancient artifacts.

 

In 1954, rancher Robert Leslie showed the late jade-hunter William Foshag where jadeitite occurred in outcrops in the Motagua River Valley of Guatemala. The outcrops did not host the translucent bright blues of the Olmecs, but they did inspire archaeologists and geologists to search for the missing blue-green stones. Alas, the geology of the region misled them. The geologists concentrated their search for the blue-green jadeitite north of the Motagua river where serpentinite, jadeitite’s host rock, is plentiful.

 

The river parallels the left-lateral, strike-slip Motagua fault that offsets the rocks of the region by 1,200 kilometers. This displacement led the geologists to believe that south of the fault lay different basement and surface rocks and hence, “there was no reason to look there,” Harlow says.

 

But erosion over the years, followed by the onslaught of Hurricane Mitch in 1998, freed blue-green jadeite boulders the size of tanks from serpentinite outcrops 16 kilometers south of the Motagua valley. Local farmers began collecting the material, but didn’t consider it marketable for tourists on the lookout for the green Mayan jade.

But the scientists still wanted it. Geophysicist Russell Seitz of Cambridge, Mass., began searching Central America for a potential source of Olmec jade in the 1970s as field director of the Harvard Peabody Museum’s Mesoamerican Jade Project. Since then he has continued to keep a keen eye on the local jade shops. His perseverance paid off. While on vacation in 1999, he discovered a rock in one shop’s lapidary scrap heap that looked suspiciously like Olmec jade.

 

He reached the first of the mountain outcrops the following January and spent the next two years investigating the region with a team of jade researchers including Harlow and Virginia Sisson of Rice University. Seitz and his colleagues announced their discovery of Olmec blue jade in the December 2001 issue of Antiquity. In late May this year, a New York Times article described their findings of a “Rhode Island-Size jade lode.”

 

Exploring the mountain region by foot and horseback, the group expanded the known jade-bearing zone 10 kilometers north, 18 kilometers east and 18 kilometers south, to about the size of Rhode Island. The presence of the jadeitite south of the Motagua fault indicates that while the surface collisional rocks on either side are different, the basement rock may be the same, Harlow says.

 

Guatemala sits over the subduction zone of the Cocos plate, with the North American plate grinding along the Caribbean plate at the Motagua fault. The recent discovery indicates the Motagua fault is more than just a single fault, and a new geological map of the region is needed. “Serpentinite is buoyant, like a cork,” Harlow says. He suggests the additional faults provided fractures and openings that allowed the serpentinite to carry precipitated jadeitite from the subduction zone to the surface. But the details of the trip to the surface are fuzzy.

 

To better understand the mineral’s journey, Sisson will study microscopic bubbles of fluid inclusions within the jadeitites to determine if the right amount of salt was indeed trapped in fluids at the low-temperature and high-pressure conditions found at subduction zones. “People think they know jadeitite is precipitated from a fluid, possibly seawater, and that it forms in subduction zones, but that’s what I’m going to confirm,” she says.

 

“I’ll also be working with a structural geologist to map the bodies we find and figure out how the jade came up to the surface,” Sisson says. This transport is the greatest mystery for all colors of jadeitite. Even for the rare places where jadeitites are known to exist, understanding how it arrives at the surface is largely unknown, Sisson says. “Most of the jadeitites found in strike-slip faults we suspect somehow relate to how it got to the surface from the subduction zone but we don’t really know how yet. In Guatemala it occurs along the Motagua fault zone boundary between the North American plate and the Caribbean plate. In Burma it occurs on the strike-slip fault between the India and Asia plates. And California and Japan have strike-slip faults. It’s not proven at all that strike-slip faults play a significant role, just an observation so far, a hypothesis we’re going to work on.”

 

The mineral is difficult to access, sometimes for political reasons. For example, an outcrop in Japan is a nature reserve off-limits to probing geologists. But after some 3,000 years of hiding, the Olmec blue jade in Guatemala is once again a treasured resource to be studied.

Written by Christina Reed

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