A Cool Early Earth? the Textbook View That the Earth Spent Its First Half a Billion Years Drenched in Magma Could Be Wrong

Home , Jack Hills

NEW VIEW of the young earth covered in oceans of liquid water as early as 4.4 billion years ago contrasts sharply with the hot, hostile world typically depicted in textbooks.

58 SCIENTIFIC AMERICAN COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. A Cool Early Earth? The textbook view that the earth spent its first half a billion years drenched in magma could be wrong. The surface may have cooled quickly—with oceans, nascent continents and the opportunity for life to form much earlier By John W. Valley

SCIENTIFIC AMERICAN 59 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. In its infancy, beginning about 4.5 billion years ago, the earth glowed like a faint star. Incan- ed for as long as 500 million years, an hardened over, the thickening layer of descent yellow-orange oceans of magma era thus named the Hadean. Major sup- consolidated rock would have insulated roiled the surface following repeated col- port for this view comes from the appar- the exterior from the high temperatures lisions with immense boulders, some the ent absence of any intact rocks older deep within the interior. If there were size of small planets, orbiting the new- than four billion years—and from the suitably quiescent periods between ma- ly formed sun. Averaging 75 times the fi rst fossilized signs of life, which are jor meteorite impacts, if the crust was speed of sound, each impactor scorched much younger still. stable, and if the early hothouse atmo- the surface—shattering, melting and In the past fi ve years, however, ge- sphere did not trap too much heat, sur- even vaporizing on contact. ologists—including my group at the face temperatures could have quickly Early on, dense iron sank out of the University of Wisconsin–Madison— fallen below the boiling point of water. magma oceans to form the metallic have discovered dozens of ancient crys- Furthermore, the primitive sun was core, liberating enough tals of the mineral zircon with chemical fainter and contributed less energy. gravitational energy compositions that are changing our Still, for most geologists, an undis- to melt the entire pla n- thinking about the earth’s beginnings. puted fi ery birth and scant clues in the et. Massive meteorite The unusual properties of these durable geologic record seemed to point instead strikes continued for minerals—each the size of the period in to a prolonged ultrahot climate. The old- hundreds of millions of this sentence— enable the crystals to est known intact rock is the four-billion- years, some blasting preserve surprisingly robust clues about year-old Acasta gneiss in Canada’s craters more than 1,000 what the environment was like when Northwest Territories. This rock formed kilometers in diameter. they formed. These tiny time capsules deep underground and bears no infor- At the same time, deep bear evidence that oceans habitable to mation about conditions on the surface. OLD VIEW of underground, the de- primitive life and perhaps continents Most investigators assumed hellish con- a hot young cay of radioactive ele- could have appeared 400 million years ditions at the planet’s surface must have earth: Life ments produced heat at earlier than generally thought. obliterated any rocks that formed earli- magazine, rates more than six er. The oldest rocks known to have orig- December 8, 1952. times greater than they Cooling Down inated underwater (and thus in relatively do today. since the 19th century, scien- cool environs) did not form until 3.8 bil- These fi ery conditions had to subside tists have attempted to calculate how lion years ago. Those sediments, which before molten rock could harden into a quickly the earth cooled, but few expect- are exposed at Isua in southwestern crust, before continents could form, be- ed to fi nd solid evidence. Although mag- Greenland, also contain the earliest evi-

fore the dense, steamy atmosphere could ma oceans initially glowed at tempera- dence of life [see “Questioning the Old- ) pool as liquid water, and before the tures exceeding 1,000 degrees Celsius, a est Signs of Life,” by Sarah Simpson;

earth’s fi rst primitive life could evolve tantalizing suggestion of a more temper- Scientifi c American, April 2003]. this page and survive. But just how quickly did ate early earth came from thermody- Single crystals of zircon began to add the surface of the earth cool after its lu- namic calculations showing that crust new information about the early earth in minous birth? Most scientists have as- could have solidified on the surface the 1980s, when a few rare grains from sumed that the hellish environment last- within 10 million years. As the planet the Jack Hills and Mount Narryer regions of Western Australia became the Zircon Time Capsules most ancient terrestrial material known Overview/ at that time—the oldest dating back al- ■ Geologists have long thought that the fi ery conditions of our planet’s most 4.3 billion years. But the informa-

birth 4.5 billion years ago gave way to a more hospitable climate by about tion these zircons carried seemed am- ( PICTURES/GETTY IMAGES LIFE TIME ); 3.8 billion years ago. biguous, in part because geologists were ■ Now tiny crystals of the mineral zircon, which retain clear evidence of when unsure of the identity of their parent and how they formed, suggest that the earth cooled far sooner—perhaps as rock. Once formed, zircon crystals are

early as 4.4 billion years ago. so durable that they can persist even if pages preceding ■ Some ancient zircons even bear chemical compositions inherited from the their parent rock is exposed at the sur- cooler, wet surroundings necessary for life to evolve. face and destroyed by weathering and

erosion. Wind or water can then trans- ( DIXON DON

Isua sediments (oldest evidence for life, 3.8 billion years old)

Acasta Gneiss Fossilized gravel bed in the Jack Hills (oldest intact rock, (above) contained the world’s oldest zir- 4 billion years old) cons yet discovered. Geologists crushed and sorted hundreds of kilograms of this below Rocks older than rock ( ) to ﬁ nd the 20 crystals that 2.5 billion years bear signs of cool conditions more than four billion years ago. Inferred Jack Hills zircons Exposed (oldest earth material, 4.4 billion years old)

Ancient rocks older than 2.5 billion years crop out or lie just underneath the soil in many spots around the globe (red) and are probably hidden below younger rocks across even broader regions (pink). Zircon crystals as old as those discovered in the Jack Hills of Western Australia may eventually be discovered at another of these locations.

port the surviving grains great distances The Jack Hills conglomerate was depos- to lead. When a zircon forms from a so-

) before they become incorporated into ited three billion years ago and marks lidifying magma, atoms of zirconium, deposits of sand and gravel that may the northwestern edge of a widespread silicon and oxygen combine in exact later solidify into sedimentary rock. In- assembly of rock formations that are all proportions (ZrSiO4) to create a crystal deed, the Jack Hills zircons—separated older than 2.6 billion years. To recover structure unique to zircon; uranium oc- photographs by perhaps thousands of kilometers from less than a thimbleful of zircons, my col- casionally substitutes as a trace impu- their source—were found embedded in a leagues and I collected hundreds of kilo- rity. Atoms of lead, on the other hand, fossilized gravel bar called the Jack Hills grams of rock from these remote out- are too large to comfortably replace any conglomerate. crops and hauled them back to the labo- of the elements in the lattice, so zircons

); JOHN W. VALLEY ( So, despite the excitement of ﬁ nding ratory for crushing and sorting, similar start out virtually lead-free. The urani- map

( such primeval pieces of the earth, most to searching for a few special grains of um-lead clock starts ticking as soon as scientists, including me, continued to sand on a beach. the zircon crystallizes. Thus, the ratio of accept the view that the climate of our Once extracted from their source lead to uranium increases with the age young planet was Hadean. It was not rock, individual crystals could be dated of the crystal. Scientists can reliably de- until 1999 that technological advances because zircons make ideal timekeep- termine the age of an undamaged zircon

Colgate University University Colgate allowed further study of the ancient zir- ers. In addition to their longevity, they within 1 percent accuracy, which for the con crystals from Western Australia— contain trace amounts of radioactive early earth is about plus or minus 40 and challenged conventional wisdom uranium, which decays at a known rate million years. about the earth’s earliest history. JOHN W. VALLEY received his Ph.D. in 1980 from the University of Michigan at Ann Arbor, Digging Deep where he ﬁ rst became interested in the early earth. He and his students have since ex- the australian zircons did not plored the ancient rock record throughout North America and in Western Australia, give up their secrets easily. For one thing, Greenland and Scotland. Currently Valley is president of the Mineralogical Society of the Jack Hills and their surroundings are America and Charles R. Van Hise Professor of Geology at the University of Wisconsin–

dusty barrens at the edge of vast sheep THE AUTHOR Madison, where he founded a multimillion-dollar laboratory called WiscSIMS. The cut- stations, called Berringarra and Mileu- ting-edge capabilities of the lab’s new CAMECA IMS 1280 ion microprobe will enable a ra, situated some 800 kilometers north diverse range of research; besides zircons, Valley and his colleagues will probe many

LUCY READING-IKKANDA; SOURCE: WILLIAM PECK H. of Perth, Australia’s most isolated city. rare or extremely small materials ranging from stardust to cancer cells.

Scientists extract multiple clues about the earth’s ancient con’s atomic structure. Simply put, the constant radioac- environment from a single crystal of the mineral zircon (main tive decay of uranium to lead means that the more lead cutaway below). They ﬁ rst embed the zircon in epoxy, then present relative to uranium, the older the crystal. grind and polish the crystal to expose a pristine surface. Investigators then grind down the surface to expose a A scanning electron microscope identiﬁ es the zircon’s deeper layer of the crystal and make a second microprobe growth patterns and any smaller fragments of minerals pit in precisely the same location as the first one to enclosed as it grew. Inclusions of quartz, for instance, occur measure atoms of oxygen, one of three elements that make most commonly in zircons that came from granite, a type of up a zircon. The ratio of certain oxygen isotopes—atoms of rock characteristic of continents. oxygen with different masses—reveals whether the crystal An ion microprobe creates a small pit by sputtering records hot or cool conditions. atoms off this polished area using a narrow beam of ions Researchers create a third pit to measure the abundance and identifies those atoms by comparing their masses. To of certain trace elements—impurities known to make up less determine the age of the crystal, scientists measure atoms than 1 percent of the crystal’s structure. Some of these ele- of uranium and lead, two impurities trapped within the zir- ments are more common in continental crust.

Polished Quartz inclusions zircon surface

Crystal growth patterns

Uranium-lead microprobe pits Oxygen-isotope pit )

Location of oldest Second uranium-lead date polished surface zircons red and and

Trace-element pit

Outer surface rounded ); from abrasion zircon schematic Original rectilinear ( oldest zircon oldest

shape of crystal ( The earth’s oldest zircon, viewed as a cathodoluminescent image, dates back 4.4 billion years.

A CLOSER LOOK Red zircons (right), photographed near University of Wisconsin–Madison Wisconsin–Madison of University

Roosevelt’s nose on Technology of University Curtin a U.S. dime for scale, come from the same rock sample that yielded the earth’s oldest crystal. An

ion microprobe, WILDE A. SIMON ); such as this one in the author’s laboratory at the University of laboratory Wisconsin–Madison (far right), can analyze isotope ratios or 1 trace elements from spots about /15 the diameter of the crystals themselves. ALFRED T. KAMAJIAN; SOURCE: AARON J. CAVOSIE CAVOSIE J. AARON SOURCE: KAMAJIAN; T. ALFRED JOHN W. VALLEY ( W. JOHN

62 SCIENTIFIC AMERICAN OCTOBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. Dating specifi c parts of a single crys- We knew that a zircon could retain evi- one millionth the size of those then pos- tal first became possible in the early dence not only of when its host rock sible in my laboratory in Wisconsin. Af- 1980s, when William Compston and formed but also of how. In particular, ter 11 days of round-the-clock analysis his colleagues at the Australian Nation- we were using the ratios of different iso- and little sleep (typical conditions for al University in Canberra invented a topes of oxygen to estimate the temper- this diffi cult procedure), we completed special kind of ion microprobe, a very atures of processes leading to the forma- the measurements—and found that our large instrument they playfully named tion of magmas and rocks. predictions were wrong. The zircons’ SHRIMP, short for Sensitive High-Res- Geochemists measure the ratio of ␦18O values ranged up to 7.4. olution Ion Micro Probe. Although oxygen 18 (18O, a rare isotope with We were stunned. What could these most zircons are nearly invisible to the eight protons and 10 neutrons, which high oxygen isotope ratios mean? In naked eye, the ion microprobe fi res a represents about 0.2 percent of all oxy- younger rocks the answer would be ob- beam of ions so narrowly focused that it gen on the earth) to oxygen 16 (16O, the vious, because such samples are com- can blast a small number of atoms off common oxygen isotope with eight pro- mon. A typical scenario is that rocks at any targeted part of a zircon’s surface. tons and eight neutrons, which compris- low temperature on the earth’s surface A mass spectrometer then measures the es about 99.8 percent of all oxygen). can acquire a high oxygen isotope ratio composition of those atoms by compar- These atoms are called stable isotopes if they chemically interact with rain or ing their masses. It was Compston’s because they do not undergo radioactive ocean water. Those high-␦18O rocks, if group—working with Robert T. Pid- decay and thus do not spontaneously buried and melted, form magma that re- The tiny zircons from Western Australia did not GIVE UP THEIR SECRETS easily.

geon, Simon A. Wilde and John Baxter, change with time; however, the propor- tains the high value, which is then all then at Curtin University of Technol- tions of 18O and 16O incorporated into passed on to zircons during crystalliza- ogy, also in Australia—that fi rst dated a crystal as it forms differ depending on tion. Thus, liquid water and low tem- the Jack Hills zircons in 1986. the ambient temperature at the time the peratures are required on the surface of Knowing this history, I approached crystal formed. the earth to form zircons and magmas Wilde. He agreed to reinvestigate the The 18O/16O ratio is well known for with high ␦18O; no other process is uranium-lead dates of Jack Hills zircons the earth’s mantle (the 2,800-kilometer- known to do so. as part of the doctoral thesis of my stu- thick layer immediately below the thin, Finding high oxygen isotope ratios in dent William H. Peck, who is now an five- to 40-kilometer-thick veneer of the Jack Hills zircons implied that liquid assistant professor at Colgate Univer- continents and ocean crust). Magmas water must have existed on the surface sity. In May 1999 Wilde analyzed 56 that form in the mantle always have of the earth at least 400 million years undated crystals using an improved about the same oxygen isotope ratio. For earlier than the oldest known sedimen- SHRIMP at Curtin and found fi ve that simplicity, geochemists calibrate these tary rocks, those at Isua, Greenland. If exceeded four billion years. To our great ratios relative to that of seawater and ex- correct, entire oceans probably existed, surprise, the oldest dated back to 4.4 press them in what is called delta (␦) no- making the earth’s early climate more billion years ago. Some samples from tation. The ␦18O of the ocean is 0 by like a sauna than a Hadean fi reball. the moon and Mars have similar ages, defi nition, and the ␦18O of zircon from and meteorites are generally older, but the mantle is 5.3, meaning that it has a Continental Clues nothing of this vintage had been found greater 18O/16O ratio than seawater. could we really base such far- (or expected) from our planet. Almost That is why Peck and I expected to reaching conclusions about the history everyone assumed that if such ancient fi nd a primitive mantle value of around of the earth on a few tiny crystals? We zircons had ever existed, the dynamic 5.3 when we took Wilde’s Jack Hills zir- delayed publishing our fi ndings for more Hadean conditions destroyed them. Lit- cons, including the five oldest, to than a year so we could double-check tle did we know that the most exciting the University of Edinburgh in Scotland our analyses. Meanwhile other groups discovery was yet to come. that same summer. There John Craven were conducting their own research in and Colin Graham helped us use a dif- the Jack Hills. Stephen J. Mojzsis of the University of Wisconsin–Madison of University Evidence of Ancient Oceans ferent kind of ion microprobe specially University of Colorado and his col- peck and i sought Wilde’s zircons suited to measure oxygen isotope ratios. leagues at the University of California at from Western Australia because we We had worked together many times Los Angeles confi rmed our results, and were looking for a well-preserved sam- over the preceding decade to perfect the we published back-to-back technical ar-

AARON J. CAVOSIE ple of the oldest oxygen from the earth. technique and could analyze samples ticles describing our ﬁ ndings in 2001.

www.sciam.com SCIENTIFIC AMERICAN 63 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. THE TALES THEY TELL Zircons from the Jack Hills of Western Australia have changed four billion years ago. Many of these tiny timekeepers also the way scientists think about the early history of the earth. bear clear chemical signs that oceans of liquid water and These crystals are the oldest terrestrial materials yet possibly even continents existed on the earth’s surface at a discovered—hundreds of those identiﬁ ed formed more than time once thought to be molten and ﬁ ery.

ANCIENT AGE COOL OCEANS The oldest age for a Jack Hills zircon—4.4 billion years (red)—is an Oxygen isotope ratios in Jack Hills zircon samples (blue), exact match of two geologic “clocks.” Two pairs of isotopes — which range up to 7.5, are possible only if their source rock uranium 235–lead 207 (vertical axis) and uranium 238–lead 206 formed in a relatively cool, water-rich environment near the (horizontal axis)—form two radioactive time keep ers that start earth’s surface. Had magma oceans covered the planet when ticking when a zircon forms. If they are well preserved, their ﬁ nal these zircons formed, their values would have clustered near ratios plot along a single line (yellow). Dates from other parts of 5.3, as do those of all crystals from hot rock that originates in the zircon (pink) fall off this line because some lead was lost from the planet’s deep interior (red). these areas, but scientists can correct for this damage. Expected value for Expected value for cool, the earth’s hot mantle wet environments 1.20 20- Dates in billions 4.4 of years ago 4.2 15 - 0.80 Typical mantle zircons Oldest zircon Jack Hills zircons 10 - (includes all

date from earth ) >4 billion-year-old 0.40 zircons analyzed so far)

All dates in this region of Samples Number fall between 4.2 billion 5 - zircon grains

and 4.4 billion years ago ( Ratio of Lead 207 to Uranium 235 235 Uranium to 207 Lead of Ratio 0.00 0 - 10 20 30 40 50 60 70 80 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Ratio of Lead 206 to Uranium 238 Oxygen Isotope Ratio

FIRST CONTINENTS? Rounded surfaces of some Jack Hills zircons under

a scanning electron microscope show that wind and );

possibly running water buffeted these crystals over Wisconsin–Madison of University long distances—possibly across a large continental landmass—before they were ﬁ nally laid to rest (right).

Zircons found near their place of origin retain their uranium-lead original sharp edges ( far right). The large number of ( ancient, rounded Jack Hills zircons suggests their original source rocks were widespread. ); AARON J. CAVOSIE CAVOSIE AARON J. );

As the possible implications of the other materials that were encapsulated as granitic rock, that evidence would sup- zircon discoveries spread through the sci- the zircon grew around them. Such zir- port the hypothesis that they are sam- entiﬁ c community, the excitement was con inclusions can reveal much about ples of the world’s ﬁ rst continent. But oxygen isotope ratios isotope oxygen

palpable. In the superheated violence of where the crystal came from, as can the caution is warranted. Quartz can form Curtin University Technology of a Hadean world, no samples would have crystal’s growth patterns and the compo- in the last stages of magma crystalliza- survived for geologists to study. But these sition of trace elements. When Peck and tion even if the parent rock is not gra- zircons pointed to a more clement and I studied the 4.4-billion-year-old zircon, nitic, although such quartz is much less familiar world and provided a means to for instance, we found that it contained abundant. For instance, zircons and a unravel its secrets. If the earth’s climate pieces of other minerals, including few grains of quartz have been found on was cool enough for oceans of water ear- quartz. That was surprising because the moon, which never developed a gra- ly on, then maybe zircons could tell us if quartz is rare in primitive rocks and was nitic, continental-style crust. Some sci- continents and other features of modern probably absent from the very ﬁ rst crust entists have also wondered if the earth’s earth also existed. To ﬁ nd out, we had to on the earth. Most quartz comes from earliest zircons formed an environment look more closely into the interiors of granitic rocks, which are common in more like the early moon or by some single crystals. more evolved continental crust. other means that is no longer common, LUCY READING-IKKANDA; SOURCE: A. SIMON WILDE Even the smallest zircon contains If the Jack Hills zircons came from a perhaps related to giant meteorite im- LUCY READING-IKKANDA; SOURCE: VALLEY W. JOHN (

64 SCIENTIFIC AMERICAN OCTOBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. pacts or deep-sourced volcanism, but no vosie, now an assistant professor at the ﬁ ve Jack Hills zircons in 1999, the data one has found convincing evidence. University of Puerto Rico, found such supporting our conclusions have grown Meanwhile other clues for continen- evidence even within single zoned zir- rapidly. Investigators in Perth, Canber- tal crust came from trace elements (those cons where the core formed early, say, ra, Beijing, Los Angeles, Edinburgh, elements substituting in zircon at levels 4.3 billion years ago, with a surround- Stockholm and Nancy, France, have below 1 percent). Jack Hills zircons have ing overgrowth that formed later, be- now put tens of thousands of Jack Hills elevated concentrations of these ele- tween 3.7 billion and 3.3 billion years zircons into ion microprobes, searching ments as well as patterns of europium ago. That the zircons get younger from for the relatively few that are older than and cerium that are most commonly core to rim is expected because zircon four billion years, and other dating created during the crystallization of crystals grow concentrically by adding techniques have been applied as well. crust, which means the zircons formed material to their grain boundary. But Hundreds of newly discovered zir- near the earth’s surface rather than in the great age difference with time gaps cons have been reported from several the mantle. Furthermore, the ratios of between the cores and rims of these par- localities with ages from 4.4 billion to radioactive isotopes of neodymium and ticular zircons indicates that two dis- four billion years old. David R. Nelson hafnium—two elements used to deter- tinct events took place, separated by a and his colleagues at the Geological Sur- mine the timing of continental-crust- major hiatus. In more commonly avail- vey of Western Australia have found forming events—suggest that signiﬁ cant able, younger zircons, this kind of core- similarly ancient zircons as far as 300 amounts of continental crust formed as to-rim age relation results from tectonic kilometers south of the Jack Hills. Geo- early as 4.4 billion years ago. processes that melt continental crust chemists are scrutinizing other ancient The Jack Hills zircons may be samples of the world’s FIRST CONTINENT.

The distribution of ancient zircons and recycle the zircons within it. Many regions of the earth, hoping to fi nd the provided additional evidence. The pro- scientists are trying to test whether sim- first pre-4.1-billion-year-old zircons portion of zircons older than four billion ilar conditions produced the ancient outside Australia. years exceeds 10 percent in some samples Jack Hills zircons. And the intensifying search is spur- from Jack Hills. Also, the zircon surfaces Most recently, E. Bruce Watson of ring improved technology. Cavosie has are highly abraded and the originally an- the Rensselaer Polytechnic Institute and demonstrated much better accuracy of gular crystal faces are rounded, suggest- T. Mark Harrison of the Australian Na- analysis and reported more than 20 Jack ing the crystals were blown long distanc- tional University have reported lower- Hills zircons with the high oxygen iso- es from their source rock. How could than-expected levels of titanium in these tope ratios that fi ngerprint cool surface these zircons travel hundreds or thou- ancient zircons, suggesting that the tem- temperatures and ancient oceans as ear- sands of kilometers as windblown dust peratures of their parent magmas must ly as 4.2 billion years ago. My colleagues and still be concentrated together unless have been between 650 and 800 degrees and I are continuing the search using the there had once been a lot of them? And C. Such low temperatures would be pos- fi rst model of the newest generation ion how could these zircons escape burial sible only if the parent rocks were gra- microprobe, called the CAMECA IMS and melting in the hot mantle unless nitic; most nongranitic rocks melt at 1280, which was installed in my labora- thick continental-type crust was stable higher temperatures, and so their zir- tory this past March. so as to preserve them? cons should contain more titanium. Many questions will be answered if These fi ndings imply that the zircons pieces of the original zircon-forming rock were once plentiful and came from a Zircons Are Forever can be identifi ed. But even if we never widespread source region, possibly a since my colleagues and I ana- fi nd that rock, there is still much to learn continental landmass. If so, it is quite lyzed the oxygen isotope ratios in those from the tiny zircon time capsules. likely that rocks from this earliest time still exist, an exciting prospect because MORE TO EXPLORE much could be learned from an intact A Cool Early Earth. John W. Valley, William H. Peck, Elizabeth M. King and Simon A. Wilde in rock of this age. Geology, Vol. 30, No. 4, pages 351–354; April 2002. Furthermore, the age distribution of Magmatic ␦18O in 4400–3900 Ma Detrital Zircons: A Record of the Alteration and Recycling Curtin University of Technology of University Curtin ancient zircons is uneven. Ages cluster of Crust in the Early Archean. Aaron J. Cavosie, John W. Valley, Simon A. Wilde and the Edinburgh Ion Microprobe Facility in Earth and Planetary Science Letters, Vol. 235, No. 3, pages 663–681; in certain time periods, and no zircons July 15, 2005. have been found from other eras. My The author’s “Zircons Are Forever” Web site is at

SIMON A. WILDE former graduate student Aaron J. Ca- www.geology.wisc.edu/zircon/zircon–home.html